PART I
TECHNICAL AND REVIEW PAPERS (Contd.)

POSSIBILITÉS D'AVENIR DU SAUMON ATLANTIQUE (Salmo salar L.)

Richard Vibert

Association Internationale de Défense du Saumon Atlantique, 52, Avenue Foch, 64200 Biarritz, France

RÉSUMÉ

Sur le plan directement pratique, les connaissances acquises au cours des toutes dernières décades sur la biologie du saumon et les possibilités d'exploitation rationnelle de ses stocks peuvent être schématisées ainsi qu'il suit. 1—Reconnaissance d'une caractéristique essentielle du saumon atlantique: la précision de son “homing” entrainant l'unicité de ses multiples stocks, ou races locales, chacun d'eux fortement consanguin. 2—Multiplication des causes de régression des stocks. Pollutions, barrages, surexploitation dans les eaux territoriales d'origine ne sont plus seuls en cause: (a) l'accroissement des besoins en énergie et en eau d'irrigation entraîne une réduction de l'importance des crues de printemps et de leur vitesse de transport des smolts qui, sur les grands fleuves, arrivent maintenant à la mer passée la courte période où ils sont le plus aptes à passer sans dommage de l'eau douce à l'eau salée; (b) les pêches abusives sur leurs aires d'engraissement maritimes et sur leurs routes de migration, où ils sont maintenant vulnérables, conduisent évidemment à une régression de l'importance des stocks de saumons dans leur ensemble; pouvant être deséquilibrées parce qu'opérant sur des stocks mélangés, ces pêches risquent en outre d'anéantir les stocks de faible importance. 3-Multiplication des possibilités de développement économique de la “Ressource Saumon”: (a) sur le plan de la production, l'amélioration des techniques d'élevage et de libération des jeunes saumons de repeuplement ouvre la voie aux restaurations de rivières à saumon; les résultats déja obtenus en “Salmon Farming” et “Salmon Ranching” autorisent par ailleurs de grands espoirs; (b) sur le plan politique, deux débuts de prise de conscience sont à prendre en considération: d'une part prise de conscience de l'intérêt d'une unique exploitation fluviale évitant les exploitations déséquilibrées en mer; d'autre part, prise de conscience de l'intérêt d'une association “Production d'Énergie + Production de Saumon,” ce qui, pour certains bassins fluviaux, permettrait de rentabiliser des équipements hydroélectriques de hautes vallées qui ne le seraient pas sans cela. Poisson condamné ou poisson de grand avenir? Le sort du saumon, espèce de choix entre toute, dépendra des mesures politiques qui seront prises à son sujet.

ABSTRACT

On an immediately practical level, the knowledge acquired in the course of the most recent decades on salmon biology and the possibilities of rationally exploiting its stocks may be expressed as follows: 1-Recognition of a salient feature of the Atlantic salmon: the precision of its homing which entails the uniqueness of its numerous stocks or local races, each strongly inbred. 2-Increase in the number of causes of stocks regression. Pollution, dams, overexploitation in home waters are no longer the only causes involved: (a) increase in needs for energy and irrigation water leads to a reduction in the size of spring floods and in the speed at which they transport the smolts which, on the large rivers, now attain the ocean once that short period is over in which they are most apt at passing without harm from fresh to salt water; (b) excessive fishing in their feeding grounds in the ocean and along their migratory routes, where they are now vulnerable, evidently leads to a decrease in the size of salmon stocks as a whole, because this fishing affects mixed stocks and therefore may well be unbalanced, it risks, moreover, to wipe out the less numerous stocks. 3-Increase in the possibilities of economic development of the “salmon resource”: (a) as far as production is concerned, improvement in rearing techniques and in techniques for releasing hatchery smolts opens the way to rehabilitation of salmon rivers; moreover, the results obtained to date in “salmon farming” and “salmon ranching” give us great hopes for the future; (b) on a political level, awareness of two points is dawning on people: firstly, of the interest behind having a unique river harvest, thus avoiding unbalanced exploitation in the ocean; second, of the interest of an associated salmon and energy production which, for certain river basins, would make the hydroelectric schemes installed in high valleys profitable, that would otherwise never be. A condemned fish or a fish with a great future? The fate of the salmon, a select species among all others, will depend on political measures taken with regard to it.

INTRODUCTION

Qu'il s'agisse d'espèces marines, d'eau douce, ou d'espèces migratrices amphihalines, l'exploitation des stocks de poissons se trouve constamment en conflits avec d'autres activités: prelèvements d'eau pour les besoins agricoles, urbains, et industriels, érection de barrages pour la production d'électricité, pollution accélérée par l'industrie, transports.…

Au sein même du monde des pêcheurs des conflits existent entre pêcheurs professionnels et pêcheurs amateurs.

Un conflit spécifique au saumon atlantique a pris naissance il y a une vingtaine d'années: celui opposant, particulièrement sur l'Atlantique, la grande majorité des pêcheurs professionnels et amateurs aux responsables de pêches abusives et/ou non équilibrées, pratiquées sur les aires d'engraissement maritimes relativement circonscrites d'un migrateur amphibiotique dont la précision du “homing” est telle qu'elle entraîne une véritable unicité des stocks. Si la surexploitation entraîne une réduction des stocks dans leur ensemble, une exploitation déséquilibrée risque d'anéantir tels ou tels stocks de faible importance relative.

CARACTÉRISTIQUES ESSENTIELLES DU SAUMON ATLANTIQUE

La théorie du retour des saumons à leur rivière d'origine, ou “homing,” fut considérée comme pratiquement confirmée lors du symposium tenu à Ottawa en 1938 sur la migration et la conservation de ces grands migrateurs (Moulton 1939).

Depuis lors, des multiples études effectuées, en particulier, des synthèses de Hasler (1966), Foerster (1968), Harden-Jones (1968) et Mills (1971), nous retiendrons que les saumons sont des migrateurs amphihalins bénéficiant d'un homing précis, en raison de leur odorate et que cela a pour conséquence biologique l'unicité de leurs stocks.

Précision du Homing

Si les taux de retour sont généralement faibles pour les saumons (0,5 à 2%, en général), compte tenu des dangers inhérents au long voyage en mer, surtout pour les jeunes de petite taille, la navigation finale est en générale précise: sur les retours en eau douce, 95% et plus sont enregistrés dans la rivière d'origine (Hasler et al. 1978). Pour le saumon atlantique, Stasko et al. (1973) font état de divagations¹ de 1 à 2% pour la Suède et de 0,1% pour le Canada (résultats portant pour le Canada sur 517 746 smolts marqués et 8 314 saumons repris, dont 6 dans d'autres rivières que leur rivière d'origine). La précision de ce homing est telle que non seulement les saumons, retour de l'océan, remontent dans leur bassin fluvial d'origine, mais qu'ils choisissent sans grande erreur leur propre affluent d'origine. Mieux, si aucun obstacle ne les en empêche, les saumons provenant de smolts d'élevage reviennent jusque dans les bassins de la pisciculture où ils sont parvenus au stade smolt (Rich et Holmes 1929; White 1936; White et Huntsman 1938; Donaldson et Allen 1958; IPSFC 1954).

Le phénomène était déjà connu au début du siècle, mais les taux de retour étaient alors si faibles qu'ils correspondaient à la négation du principe de repeuplements. On n'en fit état que vers le milieu du siècle quand les progrès réalisés dans les techniques d'élevage permirent, en particulier avec les espèces pacifiques, de créer des stocks d'élevage (hatchery runs) importants.

¹ Divagation: rapport entre les entrées dans d'autres rivières que la rivière d'origine et les entrées dans la rivière d'origine.

Mécanismes du Homing: Rôle de l'Odorat

Il est des saumons qui se contentent de courtes migrations maritimes tels certains stocks qui ne sortent pas de la baie de Fundy au Canada ou de la baie de Riga en U.R.S.S.

Il en est d'autres qui accomplissent des voyages en mer de plusieurs milliers de miles, tels ceux des stocks de l'Europe Occidentale tributaires de l'aire d'engraissement maritime de l'Ouest du Groënland.

Quels sont les mécanismes utilisés par les saumons pour s'orienter dans ces “voyages au long cours”? De quel programme disposent-ils au départ pour cette aire d'engraissement que seuls les parents ont connue? Comment trouvent-ils leur route de retour? Bien que nous sachions maintenant les poissons capables de s'orienter par rapport aux astres, quand ils sont visibles (Hasler 1954; Groot 1965, 1967) ou par rapport au champ magnétique terrestre (Rommel et McCleave 1973), ces problèmes ne sont pas encore élucidés.

Par ailleurs, nous savons que les mécanismes de reconnaissance de la rivière de départ, ou des lieux d'élevage et de naissance, autrement dit de la précision finale du “homing,” relèvent essentiellement de l'odorat, extrêmement développé chez les poissons, ainsi que le montre Fontaine (1975) dans une synthèse sur la physiologie des migrations:

• la capacité d'attention et de mémorisation olfactive des jeunes saumons est maximum dans les dernières semaines précédant la migration à la mer (Kastin et al. 1971; Sandman et al. 1969);

• à l'approche de leur zone de fraye, les saumons du Pacifique ont une activité cérébrale maximale au niveau des bulbes olfactifs (Hara et al. 1965);

• la sensibilité des bulbes olfactifs est plus prononcée pour l'eau de leur propre zone de fraye que pour l'eau provenant d'autres zones de fraye pourtant relatives à la même espèce. À chaque zone de fraye correspondrait donc une odeur bien caractéristique et une population de saumons bien définie (Oshima et al. 1969; Ueda et al. 1967).

En fait, si les saumons gardent en mémoire et sont capables de reconnaître les caractéristiques olfactives de la zone de frayères dont ils proviennent ils gardent également en mémoire les caractéristiques olfactives successives des lieux où ils ont passé leur jeunesse, et plus particulièrement des lieux où s'est accompli leur transformation physiologique en “smolts.” Cette particularité explique que malgré toutes les difficultés qu'elles comportent, les transplantations de saumons d'une rivière à une autre ne soient pas impossibles: les saumons gardent en effet en mémoire les caractéristiques olfactives de leur dernier domicile fixe en eau douce; mieux ils peuvent garder en mémoire tel parfum synthétique, tel la morpholine, auquel ils auront été sensibilisés durant leur smoltification (Hasler et al. 1978).

Conséquence Biologique de la Précision du Homing des Saumons: l'Unicité de Leurs Stocks

Comportement sinon absolu, mais très général (de l'ordre de 95%) le homing conduit à l'existence d'une multitude de stocks de saumons bien individualisés et comprenant des individus d'une même espèce qui fraient dans une rivière ou une portion de rivière donnée, sans mélange avec d'autres groupes se reproduisant en d'autres endroits, ou aux mêmes endroits mais à une saison différente (Ricker 1972).

Gardant bien présents à l'esprit:

• cette notion capitale de l'unicité de leurs stocks (Saunders 1978), chacun d'eux étant le résultat d'une longue adaptation à son milieu;

• la notion que les smolts d'élevage ont jusqu'à présent un taux de retour nettement plus faible que les smolts sauvages (7,2‰ contre 22,6‰ d'après les statistiques de marquages effectués de 1963 à 1978 sur plus de 4 millions de smolts de saumons atlantiques).

On ne s'étonnera pas que les transplantations de stocks de saumons en vue d'une acclimatation soient des opérations délicates et que les chances de réussite aient été trouvées d'autant plus faibles que rivières donneuses et rivières réceptrices, et surtout leurs estuaires étaient plus éloignés (Ricker 1972; Ritter 1975).

En fait, malgré tous les efforts faits, McCrimmon et Gots (1979) constate que le saumon atlantique n'a été introduit avec succès que dans l'Est de l'Amérique du Nord, en Argentine, aux Îles Feroë et en Nouvelle-Zélande.

MULTIPLICATION ET AGGRAVATION DES CAUSES DE RÉGRESSION DES STOCKS

Espèce migratrice, dont les zones de reproduction situées sur le cours supérieur de nos rivières peuvent être distantes de plusieurs milliers de kilomètres des zones d'engraissement maritimes, le saumon régresse, voire disparaît de toute artère fluviale ne lui laissant plus le libre accés de ses frayères.

Espèce d'eau pure, fraiche et bien oxygénée, le saumon ne peut que régresser, voire disparaître, de toute artère fluviale souffrant de pollutions chimiques, physiques ou thermiques.

Espèce se déplaçant sur des itinéraires de mieux en mieux connus, donnant lieu à des rassemblements importants tant sur ses zones de reproduction connues depuis les temps anciens que sur ses aires d'engraissement maritimes découvertes depuis une vingtaine d'années (mer de Norvège, Îles Feroë et, surtout, l'Ouest du Groënland), le saumon est exposé à la surexploitation, et cela d'autant plus qu'il est un poisson de choix, tant par la qualité de sa chair que par la qualité de sa pêche sportive.

La saumon ne put, en conséquence, que régresser devant le développement industriel du XVIII et du XIX siècle, régression variable selon les pays et d'autant plus marquée que le développement y fut plus intense et qu'il fut poursuivi sans souci de ses répercussions sur l'environnement et sur la conservation du saumon.

Les pollutions, équipements hydrauliques et surexploitation, cités du temps de nos pères comme adversités classiques du saumon, connaissent à l'heure actuelle de nouveaux développements.

• À la classique pollution chimique industrielle s'ajoutent les pollutions urbaines et agricoles, non moins graves, auxquelles il faut ajouter les importantes pollutions thermiques des centrales nucléaires et les catastrophiques pollutions mécaniques provoquées par les extractions de sable et gravier en rivière.

• Aux obstacles aux migrations constitués par les barrages, dont le nombre risque fort d'augmenter du fait de la crise de l'énergie, s'ajoutent les modifications de débit qui sont loin d'être bénéfiques au saumon: l'écrétage des crues et les prélèvements d'eau pour les besoins urbains et agricoles réduisent en particulier le débit des crues de printemps des cours d'eau et leur vitesse de transport des smolts qui, sur les grands fleuves, arrivent maintenant à la mer passée la courte période où ils sont le plus aptes à passer sans dommage de l'eau douce à l'eau salée (Anonyme 1978).

• À la réduction de l'importance globale des stocks de saumons dans leur ensemble, résultat de pêches abusives sur leurs aires d'engraissement maritimes et sur leur route de migration nouvellement découvertes, s'ajoute enfin une autre menace: opérant à l'aveuglette, sur des stocks mélangés, ces pêches peuvent être déséquilibrées et anéantir les stocks de faible importance.

MULTIPLICATION DES POSSIBILITÉS DE DÉVELOPPEMENT ÉCONOMIQUE DE LA RESSOURCE SAUMON

Sur le plan économique, la précision du homing, jointe à l'amélioration déjà réalisée et à poursuivre des techniques d'élevage et de libération des jeunes ouvre de nouvelles possibilités:

1. en matière d'élevage de saumon de consommation entièrement intensif, commencé en eau douce et terminé en eau de mer (salmon farming);

2. en matière d'élevage de saumon de repeuplement ou de consommation, commencé en général de façon intensive en eau douce, puis terminé de façon extensive en eau de mer en général (salmon ranching).

L'élevage intensif des saumons du Pacifique, en enceintes d'eau de mer, est essentiellement pratiqué sur la côte pacifique de l'Amérique du Nord, en particulier avec le Coho (Oncorhynchus kisutch) vendu à quelques centaines de grammes.

L'élevage intensif du saumon atlantique est à l'heure actuelle pratiqué en Ecosse, mais surtout en Norvège où les conditions d'environnement sont idéales. Les 4 000 tonnes/an de saumons de consommation d'un ou deux ans d'élevage en mer, sont déjà atteintes et dépassées par ce dernier pays.

L'élevage extensif ou “salmon ranching,” comporte un lâcher de jeunes en eau libre avec recapture ultérieure. Ce lâcher peut être fait avec des oeufs ou des alevins, difficilement marquables, la capture ultérieure étant le fait des pêcheurs. Le rendement de l'opération est difficilement appréciable, réserve faite de situations spéciales, telles celles des stocks de saumons Chum d'Hokkaido au Japon, entretenus à 80% par l'élevage. Leur taux de retour moyen est passé de 1% pour les années 1950 à 2% pour les années 1970 compte tenu des progrès réalisés dans les techniques d'élevage et de libération (Nash 1977), les dépenses correspondantes n'étant que de 6% de la récolte obtenue.

Ce lâcher peut être fait avec des smolts prêts à prendre la mer, éventuellement sensibilisés à l'odeur de la morpholine ce qui facilite leur retour et leur capture à la pisciculture d'où ils sont partis, pour peu que la législation des pêches des états en cause ait été modifiée en conséquence, comme ce fut le cas en Californie, en Orgeon et en Alaska (Nash 1977).

On estime qu'en Baltique, ce type d'élevage qui y bénéficie d'un taux de recapture moyen exceptionnel (10%) procure 50% de captures. Le coût de l'opération est du même ordre de grandeur que celui du supplément de récolte obtenue, compte tenu de l'importance des frais occasionnés pour la production des smolts dont l'élevage dure un ou deux ans.

Sur le plan économique, le développement de cet élevage extensif en plein océan, où les taux de retour sont plus faibles qu'en Baltique, est en conséquence subordonné à plusieurs conditions, entre autres:

1. choix de l'espèce d'élevage: espèce à courte ou longue vie juvénile en eau douce?

2. Amélioration des taux de retour par une meilleure connaissance des problèmes physiologiques en cause dans le passage de l'eau douce à l'eau salée, et par une sélection génétique appropriée (Naevdal 1978).

Un “salmon ranching,” essentiellement avec espèces du Pacifique, se développera-t-il dans l'hémisphère sud au cours de la prochaine décade, selon les programmes américains et japonais, à partir d'élevages situés au Chili, en Argentine, aux Îles Falklands et accessoirement en Nouvelle Zélande et aux Îles Kerguelen? Si tel était le cas, ces saumons prospéreraient aux dépens d'un stock de plus de 100 millions de tonnes de krill antarctique, pratiquement hors de toute chaîne alimentaire utilisée par l'homme depuis la disparition des baleines. Si tel était le cas, les nations signataires de la convention de l'Antarctique pourraient se dispenser de pêche en haute mer et se contenter de capturer les saumons à leur retour au point de départ (Joyner et al. 1974).

Si la réalisation de ce projet grandiose reste encore très problèmatique, les études en cours devraient pour le moins faciliter un meilleur choix des stocks susceptibles de convenir à la restauration de telle ou telle ancienne rivière à saumon. Ce sera d'ailleurs l'une des tâches des centres de recherches de créer au besoin de tels stocks, tant par sélection que par croisement, à partir de stocks dont on a vu le caractère d'unicité.

Sur le plan politique, deux débuts de prise de conscience permettent de ne pas considérer ces possibilités comme purement utopiques. D'une part un début de prise de conscience de l'intérêt d'une unique exploitation fluviale qui éviterait les exploitations déséquilibrées en mer (Buck 1979): d'autre part un début de prise de conscience de l'intérêt d'une association “Production d'Énergie + Production du Saumon,” ce qui, pour certains bassins fluviaux permettrait de rentabiliser des équipements hydroélectriques de hautes vallées qui ne le seraient pas sans cela (Anonyme 1978).

CONCLUSION

Poisson condamné ou de grand avenir? Le sort du saumon, espèce de poisson de choix entre toutes, dépendra des mesures le concernant, lui et son environnement, directement ou indirectement, qui seront prises dans les toutes prochaines années.

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Joyner, T., C.V.W. Mahnken and R.C. Clark, Jr. 1974 Salmon—future harvest from the Antarctic Ocean? Marine Fish. Review, 36(5):20–28.

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COLUMBIA BASIN ANADROMOUS SALMONID FISHERIES

Frederic Vincent

U.S. Fish and Wildlife Service, 500 N.E. Multnomah St., Portland, Oregon 97232 USA

ABSTRACT

For the past 50 years, runs of anadromous fish within the Columbia River Basin have declined steadily. Only one-half of the original habitat of the basin remains accessible to migratory salmonids. Fishing efforts by sport and commercial fishermen have increased where now two-thirds of the yield of Columbia stocks are taken for the commercial industry. Adult losses attributable to mainstem dams result mainly from injury or extended delays in migration. Gas supersaturation has been a major cause of fish mortality until the installation of spilling deflectors. Turbine mortality of juveniles attempting to pass downstream remains high. Major fish passage research has culminated in inter-related technologies at tremendous costs. Mitigation of development-related losses of salmon stocks has been mainly in the form of increased hatchery capacity. Recent court rulings have guaranteed fish allocations to treaty Indian tribes. This has increased the complexity of the management problem faced by fishery resource agencies. Conflicts considered critical to restoring or maintaining present runs have been identified.

RÉSUMÉ

Depuis les 50 dernières années, les migrations de poissons anadromes dans le bassin de la Columbia River ont décliné de manière constante. Seule une moitié de l'habitat original du bassin reste accessible aux salmonidés migrateurs. Les efforts de pêche des pêcheurs sportifs et commerciaux ont augmenté à tel point qu'à présent les deux-tiers de la production des stocks de la Columbia sont capturés pour le secteur commercial. Les pertes en adultes attribuables à des barrages résultent principalement de blessures ou de délais prolongés à la migration. La supersaturation en gaz dissous a été une cause majeure de mortalité en poisson jusqú à l'installation de brise-courants. Les mortalités de juvéniles tentant de passer en aval des turbines restent élevées. La recherche principale concernant le passage des poissons a culminé par l'application de technologies interdisciplinaires et à des coûts faramineux. Un adoucissement des pertes en stocks de saumons lié au développement des barrages a été realisé principalement par une augmentation de la capacité des écloseries. De récentes ordonnances de justice ont garanti les allocations en poissons pour des tribus indiennes liées par des traités. Ceci a augmenté la complexité du problème de gestion auquel sont confrontées les offices de ressource de la pêche. Des sources de conflit considérées comme critiques en ce qui concerne le rétablissement ou la conservation des migrations actuelles ont été identifiées.

HISTORY

The Columbia River is one of the world's great rivers. It drains nearly 673 400 km² of the Pacific Northwest, including most of the State of Washington, more than half of Oregon, virtually all of Idaho, small portions of Wyoming, Utah and Nevada, and 103 600 km² of British Columbia, Canada.

Fishery habitat has changed dramatically in the Columbia River Basin during the past 50 years. Today, less than 188 600 km² remain accessible to anadromous fish. Of this accessible area much is unsuitable to salmon and anadromous steelhead trout (Fig. 1). Dams, pollution, waterflow manipulations, flow depletions, irrigation, various watershed management practices of forestry and agriculture, urbanization, industrialization, and development of transportation networks have resulted in significant adverse changes to the ecosystem (Thompson 1976).

The culture of the Pacific Northwest in part has developed because of the salmon and steel-head trout resources of the region. Early explorers and settlers of the region found runs of anadromous fish that exceeded seven million annually, some migrating more than 1 450 km to upper tributaries of the vast river basin.

Fig. 1. Columbia Basin anadromous salmon and steelhead habitat (Chaney and Perry 1976).

For hundreds of years prior to colonial settlement salmon and steelhead trout that entered the Columbia River and its tributaries were a source of food for Pacific Northwest Indians (Craig and Hacker 1940). The numbers of fish landed by Indians is unknown but probably exceeded one million annually.

Emulating their Indian predecessors, non-Indians quickly developed intensive subsistence and commercial fisheries on what then appeared to be inexhaustible salmon and steelhead runs. By the late 1800's, non-Indian commercial catches on the lower Columbia River alone were averaging 13 600 tons per year and often exceeded 18 000 tons (Chaney and Perry 1978).

Columbia Basin catch includes five species of the Pacific salmon: chinook (Oncorhynchus tshawytscha), coho (Oncorhynchus kisutch), sockeye (Oncorhynchus nerka), chum (Oncorhynchus keta), and pink (Oncorhynchus gorbuscha) plus the steelhead trout (Salmo gairdneri). The catch of these species has significantly declined since the early 1900's. Pink salmon have never been major contributors to the fishery. Three races that comprise the chinook salmon stock are identified as the spring, summer, and fall runs. Both winter and summer races of steelhead occur in the basin.

The strongest runs of salmon and steelhead are found in the lower part of the Columbia Basin, below Bonneville Dam. It is the lower river stocks that contribute most to the commerical and recreational fisheries.

Approximately two-thirds of the yield of all Columbia Basin stocks are taken in the ocean commercial and sport fisheries. The Columbia Basin chinook and coho salmon stocks account for one-third of the catch along the Pacific Coast.

The upper river stocks of salmon and steel-head, especially those destined for the mid-Columbia River and Snake River, have suffered marked reduction in numbers during recent years. Habitat reductions have been severe, both in quantity and quality.

Only one-half of the original habitat of the basin remains accessible to migratory salmonids. The Columbia and Snake Rivers largely have been converted into a series of reservoirs by construction of hydroelectric dams (Fig. 2). The reductions in run sizes can be attributed largely to the high mortality rates suffered by adults and juveniles as they attempt to migrate past these structures.

Fig. 2. Mainstem Columbia and Snake river dams with impact on anadromous salmon and steelhead (Chaney and Perry 1976).

Upriver spring chinook runs once supported large commercial and sport fisheries in the basin. Documentation of run sizes was possible only after construction of Bonneville dam in 1938. Runs which had been depleted exhibited some increases during the period preceding construction of McNary Dam in 1953, but during the 1960's the runs generally declined. Increased hatchery production since the 1960's has resulted in some gains, but since 1972 runs have decreased and remained at low levels.

The Snake River supports the bulk of the upriver spring chinook run. Recent run sizes have been small and variable. Only occasionally in recent years has a small sport fishery been permitted on this stock. Runs into the mid-Columbia have been small but relatively stable.

The summer chinook run supported a harvest of 2,3 million fish late in the 19th century. Overexploitation and environmental change caused great reductions in run size. Runs recovered somewhat during the 1950's but have subsequently declined to very low levels. Trends in the size of summer chinook runs into both the mid-Columbia and Snake Rivers are downward since the mid-1960's. Commercial and sport fishing have not been permitted for many years.

Fall chinook runs supported an annual harvest of more than 900 000 fish as late as 1941. The runs decreased after the 1940's. Much of this decrease was attributed to the great increase in the ocean troll fishery following World War II. Only in the lower river below Bonneville pool where runs have been supplemented by hatchery production has there been any recent increase in run size.

Runs into the mid-Columbia and Snake rivers of fall chinook have declined greatly since the early 1960's. Fewer than 3 000 fish have returned annually to the Snake River since 1974. The fall run which totaled 600 returning adults in 1978, along with other upriver stocks, is being reviewed for possible listing under the Endangered Species Act of 1973.

In common with other salmon stocks, the sockeye run once supported a large commerical fishery. The catch in 1883 was more than 1,3 million fish. Annual commercial catches of 300 000 were recorded for fisheries in the Pay-ette and Salmon River Basins of the Snake River drainage. Only recently has the sockeye entered the sport fisheries.

Run sizes of sockeye were reasonably large as late as the 1950's—the period that preceded construction of most of the Columbia and Snake River dams. The runs have declined greatly since 1959. Only about 18 000 fish were counted past Bonneville Dam in 1978.

Degradation of spawning and rearing habitat along with poor survival of juveniles past mainstream Columbia dams appear to be the main causes of the decline in sockeye run sizes.

The best estimates of historic run sizes of steelhead trout derive from commercial catch records. The harvest during 1892 reached 674 000. Run sizes equaling only one-half this number were recorded in the early 1940's. Except for the increases that steelhead, like other stocks, achieved during the 1950's the trend in numbers has generally been downward. Some recovery occurred in 1977, with run sizes totaling 192 000 fish. However, during the 1978 migration, the numbers of steelhead that stay for 1 year in the ocean, reached only 60 000; about one-half that of the previous year.

The Snake River summer steelhead run was predominant over other Columbia Basin upriver steelhead runs but it has declined greatly since the early 1960's. Non-Indian commercial fishing ceased in 1975 and sport fisheries have been prohibited or limited to hook-and-release regulations in recent years. A modest sport fishery was permitted in 1978–79 on the stock of steel-head that rears for 2 years in the ocean.

Mid-Columbia River runs of summer steel-head have remained relatively stable at low levels for many years.

RIVER MORTALITY

Adult losses attributable to mainstem dams result mainly from injury or extended delays in migration. Effects are most severe during the periods of high river discharge. Damage occurs when the fish leap against concrete structures or are caught in high velocity flows which injure them directly or force them against obstacles. Delay at dams prevents timely arrival at spawning areas and causes depletion of the fish's energy resources. Further, up to 30% of adults, mainly spring/summer chinook may ascend ladders only to become disoriented and fall back over a spillway. Fish that survive may be counted repeatedly causing discrepancies in counts between mainstem dam counting stations.

Gas embolism resulting from exposure of fish to supersaturated levels of dissolved gases created during heavy spilling of water at upstream dams has been an important cause of mortality. Spillway deflectors installed at some of the dams have reduced gas supersaturation in recent years. Both adults and juveniles have benefited greatly.

Juveniles attempting to pass dams in their down-river migration to the sea are injured or killed during passage through turbines. Power peaking operations will essentially eliminate spilling of water in future years and all juveniles will be forced to pass through the turbines. Screening of turbines and collection of juveniles for safe transport may provide important benefits. Losses resulting from passage through turbines average 15% at each dam. Trucking and barging of juveniles from upstream dams and hatcheries may prove beneficial, but imprinting of smolts for successful adult return to home-stream waters as yet cannot be guaranteed.

Juvenile survival and successful outmigration through slack water reservoirs requires appropriate augmentation of flows and the spilling of appropriate amounts of water at mainstem dams. Recent studies indicate the combinations of sequential reduction of generator loading across the face of the dam and the subsequent spilling of small quantities of water results in successful movements of juveniles away from generator intakes. The maintenance of minimum instream flows and spill is essential if harvestable runs of salmon and steelhead are to be continued. Provisions of adequate stream flows for fisheries will require reordering priorities for use and regulation of the water of the Columbia River.

The serious implications of cumulative adult and juvenile salmon and steelhead mortalities at mainstem dams and reservoirs have been recognized for many years. State and federal fishery agencies and the Army Corps of Engineers have developed major fish passage research programs aimed at getting upper basin fish to and from the ocean at a capital investment of $500 million (Mains 1977). This research has culminated in five principal inter-related technologies:

1. Spillway flow deflectors to reduce nitrogen supersaturation during periods of high runoff and heavy spill;

2. Screens to divert juvenile downstream migrants from turbine intakes—particularly during periods of low runoff and, therefore, little spill;

3. Improved fishway design to more successfully attract and pass upstream migrating adult salmon and steelhead;

4. Collection and transportation of downstream migrants around mainstem dams and reservoirs to the estuary below Bonneville Dam, thereby avoiding the cumulative, often catastrophic passage mortalities;

5. Adequate mainstem flow and/or flow manipulations to (a) spill juvenile fish over mainstem dams to avoid turbine inflicted mortalities, and (b) flush the young downstream migrants quickly through slack water reservoirs before they lose the urge to migrate (Chaney and Perry 1978).

Table 1. Numbers and weights of salmon and steel-head trout released from Columbia River hatcheries during 1976.

Mitigation of development-related losses of salmonid stocks has been mainly in the form of increased hatchery capacity. It has been estimated that releases from 44 mainstem hatcheries account for more than 50% of the salmon and steelhead in the basin. Releases have been substantial in recent years in both numbers and weights (Table 1).

Proposed expansion of fish culture facilities, mainly those associated with the Lower Snake River Fish and Wildlife Compensation Plan, includes nine new hatcheries, four enlargements of existing stations and 11 satellite rearing stations. Production at these facilities would result in additional releases of 16 500 000 coho salmon, 8 900 000 fall chinook, 10 493 000 spring and summer chinook salmon, 13 247 000 steelhead and 6 000 000 chum salmon.

The average rates of return of adults from juvenile releases differ greatly between species and between upriver and downriver release locations. The numbers of fish captured in the fisheries and numbers of “spawners” that return to the fish hatcheries in the Columbia Basin together are equivalent to 2,6 1,7 and 2,7% of the spring chinook, coho, and steelhead trout respectively, that are released as juveniles from the facilities. Fall chinook and coho contribute most to the commercial fisheries whereas steel-head predominates in the recreational catch (Table 2).

Many of the major problems currently impacting salmon and steelhead are the long-predicted result of equally long-planned exploitation of the basin's valuable water resources (Chaney and Perry 1976). There has been little long-range, basinwide planning to optimize salmon and steelhead values in water resource development.

As Chaney and Perry (1976) explained, it is questionable whether any amount of fishery planning would have had much effect in an era inexorably committed to maximum water resource exploitation. State and federal water laws and politics largely ignored salmon and steel-head values. Warnings were no deterrent to water development and compensation for long-predicted damages has yet to materialize.

Future demands on the Columbia Basin's valuable land and water resources will inevitably increase environmental and socio-political problems which have reduced many once productive salmon and steelhead runs to critically low, even endangered levels.

Table 2. Distribution in percentages of the catch of Columbia River stocks in the major fisheries.

Flows in the Columbia River, fourth largest in North America, are currently inadequate to simultaneously serve all demands. (For example, approximately 40 500 ha of new land are going into irrigation each year in the Pacific North-west).

INDIAN FISHERIES

For hundreds of years, the Columbia River and its tributaries have been a source of fish for Pacific Northwest Indians (Craig and Hacker 1940). Salmon and steelhead were caught for food, ceremonies, and trading purposes. Many Indians lived on the rivers or travelled to fishing sites during fish runs (Beiningen 1977). Conflicts between non-Indian and Indian fishermen over catch allocations developed early and continued for many years in and out of court.

The federal courts recently established that treaties of the U.S. with a number of Pacific Northwest Indian tribes secure to the latter certain rights to take fish, including salmon and steelhead, on their reservations and at their usual and accustomed fishing grounds outside those reservations.

This ruling guaranteed fish allocations to treaty tribes. In 1979, the courts agreed to a 5-year plan between the fishery agencies and tribes that would allow Indian fishermen the opportunity to harvest 60% of the total Columbia River fall chinook run within the basin.

Legal decisions have increased the complexity of the management problems facing fishery agencies. In the course of clarifying basic legal issues, the courts became involved in the entire scope of fisheries management. The results have rarely been satisfactory to either the Indians or the non-Indian fishermen (Beiningen 1976).

MAJOR RESOURCE CONFLICTS

Conflicts considered critical to restoring or maintaining present runs are identified below:

1. Over-harvest: mixed stock harvest and illegal fishing reduce already inadequate spawning escapement.

2. Water development projects (irrigation and hydropower) have greatly reduced or eliminated formerly abundant upriver stocks without adequate restoration or compensation.

3. Hatchery production goals may be incompatible with restoration of some depleted stocks.

4. Operation of hydropower generation plants significantly reduces survival of downstream migrants.

5. Water diversions cause significant smolt mortality.

6. Western (U.S.) water law does not provide for adequate flows for fish.

7. Reduced quality and quantity of spawning and rearing habitat reduces survival and recruitment.

Presently, fishery agencies could best contribute to the solution of salmon/steelhead problems in freshwater by working to secure the joint cooperation of federal agencies whose operations have affected, or are now adversely affecting, salmon production.

It is recognized that salmon and steelhead are a part of the high quality for which the natural environment of the Northwest is recognized. The standard of pure water is derived from the requirement of these fish and similarly they have become synonymous with the quality of life for which the Pacific Northwest states are known. It follows, therefore, that management of anadromous fish of the Columbia Basin must adhere to such high standards.

Demand for the valuable salmon and steel-head resources of the basin has exceeded the supply for many decades. The complex interrelated environmental, social, economic, legal, political, and philosophical conflicts that have continued for years, remain today.

However, despite the overwhelming odds the fish runs of the basin can be saved and increased. The challenges and opportunities are well known by the basin's fishery agencies. Chaney and Perry (1976) have outlined recommendations that must be expedited if the most valuable runs of Columbia Basin anadromous stocks are to be saved.

1. Preserve all natural habitat now available to salmon and steelhead by encouraging management of conflicting use to assure no obstruction to access and maintenance of high standards to protect water quality and quantity for spawning and rearing salmon and steelhead. This involves close liaison with other resource management agencies that deal with environmental quality and land use.

2. Restore lost habitat where reasonably feasible by seeking compensation for damage caused by major water development projects through additional facilities for production equal to that lost or compensation to open up natural areas that have not been in production.

3. Require facilities at new projects that will assure adequate protection and passage of fish and/or compensating artificial propagation.

4. Restore Snake River production to permit an Idaho harvest (estimated by author to be +112 000 salmon +51 000 steelhead).

5. Maintain runs in the upper Columbia River (estimated by author to be +198 000 salmon +6 000 steelhead).

6. Continue improving passage at main Columbia and Snake river dams to provide natural passage if possible. Artificial transportation will be provided for juvenile downstream migrants until natural passage is adequate.

7. Strive toward an adequate research program to provide the best means of managing the resource, including the following, among others: fish passage, artificial and natural production, estuarine habitat, and stock identification.

8. Enhance production in the lower basin to benefit all user groups.

9. Manage harvest to assure allocation for all user groups as required by law, the courts and treaties as well as preservation of modern accustomed fisheries insofar as possible.

LITERATURE CITED

Beiningen, K.T. 1977 Fish runs investigative report of Columbia River fisheries projects. Vancouver, Wa. Pacific Northwest Regional Commission. 65 p.

Chaney, E., and T. Perry. 1976 Columbia basin salmon and steelhead analysis. Summary Report. Pacific Northwest River Basin Commission. 74 p.

Chaney, E., and T. Perry. 1978 Salmon and steelhead production in the upper Columbia River basin. Summary Report. Pacific Northwest Regional Commission. 29 p.

Craig, J.A., and R.L. Hacker. 1940 The history and development of the fisheries of the Columbia River. U.S. Bureau of Fisheries, Bull. No. 32.

Mains, E. 1977 Corps of Engineers responsibilities and actions to maintain Columbia basin anadromous fish runs, pages 40–43 in E. Schwiebert, ed. Columbia River salmon and steelhead. Am. Fish. Soc. Spec. Pub. 10. Bethesda, Md.

Thompson, K. 1976 Columbia basin fisheries past, present, and future. Investigative Reports of Columbia River Fisheries Project. Pacific Northwest Regional Commission, Vancouver, Wa. p. 41.

EFFECTS OF REDUCED AND PERTURBATED FLOW BELOW DAMS ON FISH FOOD ORGANISMS IN ROCKY MOUNTAIN TROUT STREAMS

James V. Ward

Department of Zoology and Entomology, Colorado State University, Fort Collins, Colorado 80523 USA

Jack A. Stanford

University of Montana Biological Station, Bigfork, Montana 59911 USA

ABSTRACT

The alteration of flow regimes resulting from stream regulation may greatly modify the density, biomass, diversity, and species composition of fish food organisms in the tailwater reaches below dams. Important taxonomic or functional groups of lotic benthos are often rare or absent in the receiving stream. Life cycle phenomena and the trophic structure of the benthic community may also be transformed by direct and indirect effects of flow regulation. Recovery from temporary adverse conditions is greatly delayed since “drift,” a major recolonization mechanism of stream benthos, is blocked by the upstream lentic environment. The degree and type of biotic modifications engendered by flow alterations are a function of (1) the extent of discharge alteration, (2) the periodicity of the altered pattern, (3) the relationship of periodicity and season, (4) synergistic effects (e.g., thermal-flow relationships), and (5) site-specific factors. Data from regulated Rocky Mountain trout streams are presented to elucidate the interrelated factors responsible for qualitative and quantitative alterations of the benthic communities below dams. Flow regime modifications include reduced flow from interbasin diversion, perturbated flow patterns with diel periodicities, enhanced seasonal flow constancy associated with diel fluctuation, and severe arrhythmic fluctuations.

RÉSUMÉ

L'altération des régimes résultant de la régularisation des cours d'eau peut modifier substantiellement la densité, la biomasse, la diversité et la composition par espèces des organismes dont se nourrissent les poissons dans les biefs situés en aval des barrages. D'importants groupes taxonomiques ou fonctionnels du benthos lotique sont souvent rares ou absents dans le cours d'eau récepteur. Les phénomènes relatifs aux cycles biologiques et à la structure trophique de la communauté benthique peuvent également être modifiés par les effets directs et indirects de la régularisation du débit. Le rétablissement faisant suite à des conditions contraires temporaires est souvent retardé, car “la dérive”—principal dispositif de recolonisation du benthos fluvial—est bloquée par l'environnement lentique d'amont. Le type et l'ampleur des modifications biotiques engendrées par les altérations du débit sont fonction de: 1) l'amplitude des altérations; 2) la périodicité du mode d'altération; 3) le rapport entre la périodicité et la saison; 4) les effets synergiques (par exemple, les rapports température-débit); et 5) les facteurs de spécificité du site. Les auteurs présentent des données ayant trait aux rivières à truites régularisées des Montagnes Rocheuses pour mettre en lumière la corrélation entre les facteurs responsables des altérations qualitatives et quantitatives des communautés benthiques en aval des barrages. Les modifications des régimes comportent la réduction du débit par suite de dérivations entre bassins, les perturbations du réseau d'écoulement associées aux périodicités nycthémérales, le raffermissement de la constance du débit saisonnier associé aux fluctuations nycthémérales, et de fortes fluctuations arythmiques.

INTRODUCTION

The myriad hydrological, limnological, geochemical, meteorological, and biological factors affecting ecological conditions in regulated streams have recently been described in the proceedings of the First International Symposium on Regulated Streams (Ward and Stanford 1971a). Since 1972, we have studied the effects of stream regulation on a variety of trout streams in the Rocky Mountains. The lotic systems investigated occur at different elevations and are influenced by impoundments with various operational schemes and managerial objectives.

It is the purpose of this paper to briefly summarize the effects of reduced and perturbated flow on fish food organisms in regulated Rocky Mountain trout streams. The effects of other factors, such as temperature modification, resulting from stream regulation have been dealt with elsewhere (e.g., Ward 1976a; Ward and Stanford 1976b).

FLOW REGIME ALTERATIONS

Reduced flow below dams, compared to historical discharge, may occur for a variety of reasons. Within-basin diversions, such as irrigation withdrawal from reservoirs, may reduce discharge below dams. In some instances, the majority of water released from the reservoir is transported by conduit to a downstream powerhouse, thus bypassing a section of the stream. Modification of the annual flow regime by regulation may result in periods of abnormally low flow even though the yearly discharge remains the same. Annual discharge may, however, be significantly reduced by increased evaporative loss from impoundment.

Perhaps the most dramatic reductions in flow are associated with interbasin diversions. In Colorado, major transmountain diversion schemes carry water through tunnels under the Continental Divide (Stanford and Ward 1979), resulting in great reductions in flow in some stream systems and increases in others.

Tailwaters below hydroelectric dams characteristically undergo severe short-term periodic flow perturbations as a function of temporal variations in power demand. Such short-term flow fluctuations may be associated with enhanced seasonal flow constancy relative to preimpoundment discharge patterns.

Downstream demands for water for agricultural, industrial, and domestic uses may result in great short-term variations in discharge below storage and irrigation reservoirs, although without a well-defined periodicity.

Three regulated Rocky Mountain trout streams will be used in this paper to exemplify three types of flow regulation (Table 1). Impoundment and transmountain diversion of upper Colorado River water reduced annual discharge below Granby Dam, Colorado, to only 11% of the historical flow (Weber 1959). Prior to diversion, mean annual discharge was 9,1 m³/sec with monthly means ranging from 1,0 (February) to 38,6 (June). Following closure of the dam, a stepwise flow schedule was established in which 0,6 m³/sec was released from September through April, 2,1 m³/sec May-July, and 1,1 m³/sec during August. The South Fork of the Flathead River near Glacier National Park, Montana, exhibits severe diel flow fluctuations, associated with enhanced seasonal flow constancy, due to hydroelectric generation. Discharge may range from 7,5–260 m³/sec in a single day (Fig. 1). Joe Wright Creek, a high mountain stream in northern Colorado, exhibits great short-term fluctuations in discharge without a well-defined periodicity, due to erratic releases from an irrigation reservoir (Ward and Short 1978). At low flow, pools and riffles alternated and the stream averaged 5 m in width; at high flow, the stream was about 17 m wide without an apparent pool-riffle sequence. During extreme periods of low discharge, surface flow was barely discernible. Although varying in size and type of regulation, the three lotic systems described above are all cold mountain streams with relatively soft waters and stony bottoms. All were virtually unimpacted except by regulation.

Table 1. Regulated Rocky Mountain trout streams which exemplify three types of flow regime alterations (see text).

^a hm³ = cubic hectometers.
^b Maximum daily variation.
^c Annual mean discharge, 11% of historical flow (Weber 1959).
^d July-November, 1975.

Fig. 1. Hourly discharge from Hungry Horse Reservoir on the South Fork of the Flathead River, Montana, 9–15 October 1977.

GENERAL EFFECTS ON BENTHOS

Reduced and perturbated flow below dams have a variety of subtle and direct effects on stream benthos mediated through a complex series of interrelated responses of the stream ecosystem to altered conditions. Following a description of qualitative and quantitative effects of reduced and perturbated flow on stream zoobenthos, causal interrelationships will be examined.

Some general biological responses to different types of stream regulation are shown in Table 2. It is not implied that all biotic alterations below dams are flow related. Some, for example, are clearly the result of thermal alterations (Ward and Stanford 1979b). However, the thermal regime may be significantly influenced by the discharge pattern as discussed later.

Macroinvertebrate density and biomass values were only slightly reduced by arrhythmic flow fluctuations below the reservoir on Joe Wright Creek. Even above the reservoir, the generally harsh conditions in this high elevation, soft water stream are manifested in a low standing crop of benthos. However, macroinvertebrate composition was dramatically different below the dam. Only gross composition changes are indicated in Table 2; more detailed description is necessary to examine the possible reasons for alterations in the community structure. Thus, while mayflies (Ephemeroptera) exhibited similar relative abdundance above and below the reservoir on Joe Wright Creek, there was a shift from a predominantly heptageniid mayfly community (69% of mayflies) to a Baetis dominated (97% of mayflies) assemblage below the dam. Stoneflies (Plecoptera) shifted from nemourids (55%) to chloroperlids (98%). Other workers have also reported a relative enhancement of chloroperlid stoneflies in streams with widely fluctuating flow regimes (Radford and Hartland-Rowe 1971; Trotsky and Gregory 1974). Surprisingly, caddisflies (Trichoptera), although reduced by regulation, did not exhibit any dramatic shifts in composition. The dipteran community changed from one dominated by chironomids to a chironomid/simuliid assemblage. Oligochaetes were only collected in the stream section below the reservoir. The stream below the dam was characterized by fewer species and a less even distribution of taxa.

Table 2. Macroinvertebrate standing crop and taxonomic alterations associated with stream regulation in Rocky Mountain trout streams.

^a Symbols: S = similar, I = increase, D=decrease in relative abundance (numbers) in regulated, compared to unregulated sections.
^b In regulated reaches.

Benthos in the upper Colorado River below Granby Dam was studied by the U.S. Fish and Wildlife Service in 1949 prior to impoundment, and in 1957 (Weber 1959) and 1978–9 (Ward, unpubl.) following impoundment. Data from September (the only month common to all three studies) are shown in Table 2. A progressive increase in numbers is shown since 1949 when the stream was impounded and the majority of the water diverted to the east side of the Continental Divide. Biomass shows a reduction from 1949 to 1957, followed by an increase, but not to preimpoundment levels. A comparison of biomass and numbers indicates a much smaller individual size for the species present after impoundment. Small mayflies (Baetis), chironomids, simuliids, and initially oligochaetes dominated the regulated stream benthos. Stoneflies have been virtually eliminated and caddisflies have been greatly reduced. Food habits of rainbow trout Salmo gairdneri sampled during the summers of 1949 and 1957 (Weber 1959) reflected the alterations in the benthic composition during that period except for two major exceptions. The great increases in dipterans and oligochaetes were not reflected in the food habits of rainbow trout, indicating the lesser value or availability of these invertebrates as food items. Trout collected in 1949 generally had higher condition factors than those collected in 1957. Crisp et al. (1978) found that the stomachs of brown trout Salmo trutta below a dam in England contained more mayflies and chironomids but fewer terrestrial insects than prior to regulation. Baetid mayflies increased in abundance in trout stomachs relative to heptageniid mayflies, which reflected the composition of the regulated river benthos.

The reduced flows below Granby Dam were not sufficient to remove sediment which accumulated during dam construction. Bottom samples taken shortly after impoundment (Eustis and Hillen 1954) revealed more dramatic changes in stream benthos than those shown in Table 2. To ameliorate the sediment accumulation, a large volume of water (1 224677 m³) was released from Granby Dam during 15–18 April 1952 to simulate spring runoff which had been eliminated by regulation. The flushing operation reduced average sediment depth in pool areas from 3,5 to 1,2 cm. Maximum sediment removal occurred in riffle areas. While considered highly successful in improving the habitat for trout and fish food organisms, to our knowledge such simulated runoff conditions have not been repeated, although flood waters have topped the spillway on rare occasions.

A striking feature of the Flathead River system is the phenomenal diversity of aquatic insects in unregulated reaches (Stanford and Gaufin, in press; Hauer and Stanford, unpubl.). Only stoneflies (42 species) and caddisflies (38 species) have been intensively studied. Until additional samples including the total macroinvertebrate community are analyzed, the exact standing crop values in Table 2 should be regarded as preliminary. The general pattern, however, is clear: a considerable reduction of biomass in the regulated South Fork associated with density values not dissimilar to those of the unregulated Middle Fork. Of the 42 species of stoneflies, only 5 small-bodied speices are found (in low numbers) in the South Fork. Caddisflies are virtually eliminated below the dam; the majority of samples collected do not contain a single trichopteran. The mayfly fauna of the South Fork is composed almost exclusively of small Baetis nymphs; 35 species, dominated by heptageniids, occur in the Middle Fork. Riffle beetles have been eliminated below Hungry Horse Dam (and below Chambers Lake). Approximately 130 species of aquatic insects occur in the Middle Fork, of which only 15 have been collected in the regulated South Fork where from 95–99% of the fauna is comprised of chironomids and oligochaetes. Although a few kokanee salmon (Oncorhynchus nerka) migrate into the South Fork from Flathead Lake, other salmonids (Salmo clarki, Salvelinus malma) are absent.

CAUSAL INTERRELATIONSHIPS

The majority of factors responsible for alterations of fish food organisms, which directly or indirectly result from flow modifications in regulated streams, may be conveniently discussed under (1) substrate-flow relationships, (2) current-flow relationships, and (3) temperature-flow relationships.

Substrate-Flow Relationships

Reduced and fluctuating flow regimes both drastically alter the substrate with important effects on stream benthos. Siltation resulting from reduced flow, especially if the flushing action of spates and major periods of runoff are lacking, has direct and subtle effects on the benthic community. Quality food organisms such as mayflies, stoneflies, and caddisflies are generally reduced, whereas burrowing species of oligochaetes and chironomids are favored. The density of benthos may increase, although the size of individual organisms and their availability as food items normally decrease. Silt reduces interstitial spaces, current velocity, and oxygen levels in the hyporheic zone, thus decreasing the value of the substrate as an incubation site for salmonid eggs and fry, and as a population reservoir for benthic invertebrates. Reduced flow tends to reduce substrate heterogeneity and the suitability of habitat for erosional zone zoobenthos. Such conditions may, however, favor aquatic angiosperms and allow establishment of organisms (e.g., amphipods, snails, tricorythid mayflies) not normally present in high gradient mountain streams. Algal growth may also be enhanced under conditions of increased water clarity and reduced bank and bed erosion. Algal densities may reach nuisance levels in the Colorado River below Granby Dam during long periods of low, constant discharge. Benthic species requiring clean rock surfaces may be eliminated, even without excessive siltation, by dense populations of epilithic algae, which may partly explain the reduction of heptageniid mayflies below dams (Ward 1976b).

Low, constant flow may allow encroachment of riparian vegetation to the detriment of fishes and benthos or may result in the establishment of nonriparian species along the stream banks. Under certain conditions, the elimination of silts and clays, due to the clarifying effect of an upstream reservoir, may decrease the moisture-holding capacity of newly-formed banks and thus prevent colonization by riparian vegetation (Simons 1979). Streamside vegetation has major thermal, trophic, and hydrologic ramifications for lotic systems. Any modification of the density or composition of the riparian habitat by flow regime alterations (or other factors) will have important effects on the stream ecosystem.

As discharge is reduced, the substrate areas most productive of fish food organisms are the first to be exposed. Below Granby Dam a reduction of discharge from 2,8 to 0,6 m³/sec decreased the most productive areas of stream bed from 59 to 18%, respectively (Weber 1959).

Successive releases of sediment-free water from Hungry Horse Reservoir have resulted in an armored and cemented substrate below the dam. The hyporheic habitat, so extensive in other portions of the Flathead River system (Stanford and Gaufin 1974), has been eliminated by flow regulation. In contrast, the arhythmic flow fluctuations below Chambers Lake, while greatly reducing nemourid stoneflies and heptageniid mayflies, has increased the relative abundance of chloroperlid stoneflies. This latter group consists of small linear species apparently able to tolerate periods of extreme low flow by burrowing into the substrate (Ward and Short 1978). The distributions and abundance of sedimentary detritus, which influences stream productivity (Rabeni and Minshall 1977), must be greatly modified by flow perturbation, but few data are available. The reduction of nemourid stoneflies (large-particle detritivores) below Chambers Lake may be due to the reduction of their habitat (accumulations of leaf litter) by rapidly fluctuating discharge.

Taxonomic variations in susceptibility to stranding and tolerance to exposure also selectively eliminate benthic species in streams with rapid flow fluctuations (Brusven et al. 1974). Chironomids appear most tolerant, whereas mayflies are most severely affected by stranding and exposure. A heterogeneous substrate provides some refuge for benthic species during both low and high water periods.

Current-Flow Relationships

Benthic species may have very specific current velocity requirements associated wih feeding mechanisms of respiratory physiology (Hynes 1970). Short-term flow perturbations may eliminate species restricted to pools as well as those adapted to rapid water. Requirements for a given substrate type may also restrict benthic macroinvertebrates to a specific range of current velocity.

The phenomenon of invertebrate “drift” is an important functional attribute of lotic ecosystems (Waters 1972). Drift is a major recolonization mechanism and drifting invertebrates provide a major source of food for stream salmonids. Catastrophic drift may be induced by increases or decreases in discharge (Anderson and Lehmkuhl 1968; Minshall and Winger 1968), which may conceivably permanently reduce faunal density if recolonization from upstream is blocked by a reservoir. Since propensity to drift is species specific, catastrophic drift witout compensatory recolonization would result in significant shifts in community structure.

Thermal-Flow Relationships

The extent to which the thermal characteristics of water released from a dam influence downstream reaches is directly related to discharge. Small volumes of water more rapidly equilibrate with ambient air temperatures if a thermal differential exists. Reduced flow below dams may therefore result in elevated summer water temperatures and extreme ice conditions in winter.

Fluctuating flow patterns in the South Fork of the Flathead River below a deep-release dam result in an expanded and desynchronized diel thermal pulse in the mainstream river in the summer (Fig. 2). During periods of high discharge associated with power generation, cold water is carried great distances downstream, whereas high summer air temperatures rapidly modify water temperatures during low flow periods. A sudden drawdown during the winter of 1973 caused massive mortality of stonefly nymphs in the mainstream Flathead River (Stanford and Ward 1979). These winter species which had congregated in shoreline areas prior to emergence were killed when reduced flow permitted icing conditions in shallow waters. During the same year the stonefly Claassenia sabulosa failed to emerge because increased late summer discharge from Hungry Horse Reservoir depressed mean daily temperatures below 18°C, thus failing to provide an appropriate thermal cue (Stanford and Gaufin, in press).

Fig. 2. Relationship between discharge and temperature in the Flathead River system, Montana. The upper curve shows discharge in the mainstream river below the confluence of the South Fork. The South Fork, which is regulated by a deep-release hydrogeneration facility, contributes an average of 37% of the mainstream flow. The diel pulses of cold water and intervening periods of low flow greatly modify the summer thermal regime of the mainstream river, which is contrasted with the unregulated North Fork on the lower curve. Diel fluctuations in discharge expand the thermal range and temporally displace the diel temperature pattern.

Thermal factors may provide a major influence on the processing of detritus in regulated streams despite reductions in allochthonous inputs and shredder species (Short and Ward 1980).

CONCLUSIONS

The biota of streams have evolved in response to natural variations in the flow regime. Spates, peak periods of seasonal runoff, and major long-term flow events (floods) have shaped the evolutionary history of lotic organisms and lotic ecosystems. For example, Hayden and Clifford (1974) describe a mayfly which uses seasonal flow variations as migration cues during different life cycle stages. It is, therefore, not surprising that major alterations in discharge patterns are manifested by severe modifications of the stream community. Some types of stream regulation mimic conditions in special lotic habitats. Thermal and flow constancy in tailwaters below some deep-release storage reservoirs present conditions similar to those prevailing in spring-brooks, and the biota respond accordingly (Ward and Short 1978). Other operational schemes have no natural analog. Only certain glacier-fed streams, which each day during summer change from raging torrents to mere trickles, exhibit anything resembling the severe diel flow perturbations characterizing streams below hydroelectric power dams. Species which have “estival” life cycles may be better adapted for conditions in regulated streams with severe short-term flow perturbation than “hiemal” species (Henricson and Mueller 1979).

Although biotic communities are invariably altered by stream regulation, and although the extent of modification is a function of site specific factors, some managerial control is feasible within the stricture of operational objectives. Extremely productive salmonid fisheries have been maintained for long periods in regulated streams under certain conditions (see e.g., reference to Cheesman Canyon in Stanford and Ward 1979). Much depends on maintaining environmental heterogeneity. Simulating spring runoff may greatly benefit some streams as described for the upper Colorado River. Decreasing the abruptness of flow perturbations will lessen stranding losses of fishes and invertebrates. Extant ecological data should be more fully utilized and additional research should be conducted for the purposes of minimizing downstream impacts of stream regulation projects.

ACKNOWLEDGMENTS

Data contained herein are based, in part, upon research supported by the Rocky Mountain Forest and Range Experiment Station, Forest Service, U.S.D.A. through the Eisenhower Consortium for Western Environmental Forestry Research; the U.S. Environmental Protection Agency; and the Montana Department of Game, Fish and Parks. This manuscript was prepared while J.V. Ward was supported by the Colorado Experiment Station.

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