Fisheries Research Laboratory and Department of Zoology
Southern Illinois University
Carbondale, Illinois, U.S.A.
Alewife, Alosa pseudoharengus, is currently being transplanted into large reservoirs in southeastern United States to serve as a pelagic forage species. With regards to landlocked populations, the anadromous clupeid had previously been limited to the Great Lakes and small glacial lakes in northeastern United States. One of the earlier (1968) transplantations of alewife occurred in Claytor Lake, Virginia, and this population is the subject of the present paper. Several trophic and population studies were conducted from 1977 to 1979 to assess the impact of alewife on the Claytor Lake fishery. Alewife were found to have several undesirable characteristics, the most serious of which included predation on larval sportfishes, alteration of the zooplankton species and size composition by selective predation and rapid growth beyond a size vulnerable to most predators. In terms of growth, only pelagic predators such as white bass (Morone chrysops) and walleye (Stizostedion vitreum vitreum) benefited from the introduction, although recruitment of walleye coincidentally declined following alewife establishment. Alewife have emigrated from Claytor Lake to at least one other reservoir and this, coupled with several recent transplantations, has given them potential access to the Ohio and Mississippi River drainages; in effect, to nearly half of the continental United States. This example demonstrates the need for decision-makers to conduct benefit-risk analyses prior to widespread stocking of fishes not indigenous to a region.
Le gaspareau, Alosa pseudoharengus, est actuellement transplanté dans de grands réservoirs du sud-est des Etats-Unis pour y servir de proie aux poissons pélagiques. Jusqu'à présent, ce clupéidé anadrome ne se rencontrait à l'intérieur des Etats-Unis que dans les Grands Lacs et les petits lacs glaciaires du nord-est. L'une des premières transplantations (1968) a été faite dans le lac Claytor (Virginie); c'est aux conséquences de cette transplantation que s'intéresse l'auteur. Plusleurs études trophiques et études de populations ont été réalisées de 1977 à 1980 pour évaluer l'impact du gaspareau sur les pêcheries du lac Claytor. On a constaté que ce poisson présentait plusieurs défauts; les principaux sont les suivants: il se nourrit de larves de poissons destinés à la pêche sportive; il a une prédilection pour le gros zooplancton, d'où une altération de la composition de la faune zooplanctonique; il a une croissance rapide et atteint en peu de temps une taille qui le met à l'abri de la plupart des prédateurs. En termes de croissance, seuls des prédateurs pélagiques tels que le bar d'Amérique (Morone saxatilis) et le doré jaune (Stizostedion vitreum vitreum) ont bénéficié de l'introduction du gaspareau, on constate toutefois que le recrutement du doré jaune a diminué à la suite de son installation. Du lac Claytor, le gaspareau est passé dans un autre réservoir au moins et, compte tenu de plusieurs autres transplantations récentes, il a désormais accès à tout le bassin de l'Ohio et du Mississippi, soit près de la moitié de la partie continentale des Etats-Unis. L'auteur souligne que les responsables devraient procéder à des analyses advantages risques avant toute introduction sur une grande échelle.
A common fisheries management objective is to augment growth and production of piscivorous game fish through expansion of the forage base. Expansion is usually attempted by transplantation of forage, often without full consideration of the ecological implications. The introduction of a forage fish may not produce the desired result due to competitive interactions with the resident species complex. Nevertheless, there is often considerable justification for expanding a forage base, particularly in reservoirs. After impoundment, reservoirs rarely contain adequate numbers of forage fish to support large populations of game fish because endemic riverine forage species are often unable to expand in the new lacustrine habitat (Kimsey, 1957; Shields, 1957; Fritz, 1968). Introduction of potential forage species into reservoirs has consequently become common practice in North American fisheries management (Ney, 1981). Unfortunately, most of these stockings have been made on a trial-and-error basis (Lackey, 1974; Pritchard et al., 1978; McCammon and von Geldern, 1979), and as with introductions in general, have often been undertaken without adequate information on the possible consequences (Courtenay and Robins, 1975).
In the southeastern United States, the principal reservoir forage species are the gizzard shad (Dorosoma cepedianum) and the threadfin shad (Dorosoma petenense). These clupeids feed primarily upon algae, detritus and benthos (Kutkuhn, 1957; Miller, 1967; Baker and Schmitz, 1971). The trophic status of shad would appear to make them ideal forage fish since they convert underutilized food material into biomass that can be consumed by piscivores. Unfortunately, both shad species have often proved inadequate; gizzard shad rapidly grow beyond a size vulnerable to most predators (Dendy, 1946; Rathum, 1967; Jester and Jensen, 1972), whereas the threadfin shad is limited in its range due to an inability to withstand temperatures below 9°C for prolonged periods (Strawn, 1963). The desire for an ideal forage fish in the southeast has led to the consideration of other species such as the anadromous alewife, Alosa pseudoharengus (Wilson). Much of the impetus for transplanting alewives is due to the highly successful salmonid stocking programme of the Great Lakes; the salmonid populations are almost entirely supported by the existence of an abundant alewife forage base.
Despite the apparent benefits of alewife in the Great Lakes, there is substantial evidence that alewives may also have serious deleterious effects. Historically, the alewife has had a major negative impact on the endemic fisheries of several of the Great Lakes. In Lakes Ontario, Huron and Michigan, the shallow-water planktivores declined in the first decade after alewife establishment, the minor piscivores initially increased then declined in the second decade and the deep-water planktivores declined in the third decade (Smith, 1968, 1970). The dominance of alewives resulted in a severe reduction in overall fishery productivity as well as species diversity. Trophic interactions between alewife and young-of-the-year fishes have been suggested as a major cause for the reduction in recruitment of various resident fishes in the Great Lakes (Smith, 1970; Wells and McLain, 1972). Alewife are extremely size-selective zooplanktivores, and have been reported to cause shifts in zooplankton species and size composition toward small forms in several lacustrine systems (Brooks and Dodson, 1965; Brooks, 1968; Wells, 1970; Hutchinson, 1971; Warshaw, 1972). Consequently, alewife are potentially severe competitors with other planktivorous fish species or life stages. Moreover, it has been suggested that alewife directly affected population densities of Great Lakes' fish species by predation on their pelagic young (Smith, 1970; Wells and McLain, 1972).
The evidence that alewife have had positive and negative effects on the fisheries of the Great Lakes, makes it imperative that the impacts of alewife establishment be evaluated prior to widescale transplantation in southeastern United States. To date, alewife have been transplanted to several reservoirs in both Virginia and Tennessee. The first of these transplantations occurred in 1968 at Claytor Lake, Viriginia. The work summarized here was designed to assess the consequences of that transplantation in order to evaluate the benefits and risks of alewife establishment in reservoirs throughout the southeastern portion of North America.
Claytor Lake is a mainstream hydro-electric impoundment of the New River located in southwestern Virginia (U.S.A.), which was filled in 1939. It has a surface area of 1 820 ha at a normal pool elevation of 663 m above mean sea level and drains approximately 3 862 km2. The reservoir includes approximately 161 km of shoreline, contains approximately 2.74 × 103 m3 of water and has a maximum depth of 37.5 m (Roseberry, 1950). Claytor Lake contains several shallow coves but with little rooted aquatic vegetation. The littoral (5 m depth) regions are not otherwise extensive. The marginally eutrophic lake is dimictic, containing a distinct thermocline throughout the summer with spring and fall overturns. Anoxic conditions occur in summer in the hypolimnion. Ice commonly occurs from approximately mid-January through mid-March.
The fishery of Claytor Lake is similar to other impoundments in the region. The primary angler-exploited species are black basses (Micropterus salmoides, M. dolomieui, M. punctulatus); striped bass (Morone saxatilis), which are maintained on a put-grow-take basis; white bass (Morone chrysops); walleye (Stizostedion vitreum vitreum), maintained by natural spawning and supplemented by stocking; channel catfish (Ictalurus punctatus); crappie (Pomoxis nigromaculatus and P. annularis); bluegill (Lepomis machrochirus), and yellow perch (Perca flavescens). Although alewife were introduced to provide forage for all angler-exploited species, they were particularly intended to serve as food for the large, pelagic predators (striped bass, white bass and walleye).
Several trophic and population ecology studies were conducted in Claytor Lake from November 1977 through August 1979 in order to assess the impact of introduced alewife on the reservoir fishery. In the following sections the major findings of those studies are summarized; for fuller detail the reader is referred to Kohler (1980), Kohler and Ney (1980, 1981, 1982, in press) and Kohler et al. (1979).
Alewife established a reproducing population two years after their introduction in 1968 (Boaze, 1972), and subsequently became a major component of the species complex of Claytor Lake (Boaze and Lackey, 1974). The pelagic nature of this clupeid makes it difficult to assess their relative biomass, but it is significant and is roughly estimated to range up to 25 percent of that of the total reservoir.
Alewife were found to be an apparent preferred prey of pelagic predators (striped bass, white bass, walleye) but were of minor importance in diets of littoral-inhabiting black basses and were not found in stomach contents of black and white crappie. Because alewife are primarily pelagic, spatial segregation appears to be the operating mechanism for the disparity in predator utilization. Alternative forage included crayfish, golden shiner (Notemigonus chrysoleucas), sunfishes, crappies and yellow perch.
Alewife growth in Claytor Lake exceeds all documented growth rates for other landlocked populations, and the clupeid rapidly reaches a size not vulnerable to most predators. Maximum total length of alewife found in predator stomachs was 165 mm, the approximate size of age-1 alewife. Consequently, it appears that in Claytor Lake, predator utilization is limited to age 1+ and younger alewife; age 2 and older alewife essentially serve only to tie up biomass, similar to the management problem commonly experienced with gizzard shad.
Growth rates of white bass and walleye significantly (P 0.05; Wilcoxon's signed rank test) increased following establishment of the alewife forage base, whereas a general decline was noted for black basses and crappie. Whether alewife were in any way involved in the decline in growth rates of black basses and crappie is not known, but the overall trend is cause for concern.
Although no before-and-after growth data were available for sunfishes, interviews with sport fishermen indicated that sunfish growth dramatically declined following alewife introduction. Conceivably, sunfish populations stunted due to reduced predator pressure that resulted when pelagic predators switched to alewife as their primary source of prey.
Severe die-offs of landlocked alewife are a common occurrence and such a major die-off occurred in Claytor Lake during the winter of 1977–78. However, catch-per-unit effort and piscivore stomach analyses indicated that the alewife population rapidly recovered due to highly successful reproduction in 1978. Accordingly, pelagic predators were able to return to an alewife diet within a year of the die-off, probably minimizing the effects of the increased predator pressure that was exerted on the alternative forage supply in the interim. However, a predator may not always be able to successfully utilize alternative prey in the absence of their primary forage. Such a situation could occur if other potential forage species were less available or vulnerable, or if the predator had established an inflexible search image (Krebs et al., 1974) for its primary prey. For example, striped bass in Santee-Cooper Reservoir, South Carolina, did not exploit abundant alternative prey following a clupeid forage base collapse, and many starved (Stevens, 1979).
Alewife in Claytor Lake consumed the young (maximum 26 mm total length) of at least five fish species (largemouth bass, white bass Lepomis sp., yellow perch, golden shiner) as well as their own young. As previously noted, alewife predation on larval fish has been suspected as a causative factor for the collapse of several resident fish populations of the Great Lakes. The observance of alewife piscivory in Claytor Lake adds much support to that hypothesis.
Direct dietary overlaps (Levins, 1968) were not significant between alewife and most larval sport fish. However, alewife may have an indirect adverse impact on other species and/or life stages of corresponding trophic level by altering the zooplankton species complex. Alewife in Claytor Lake were found to be highly size-selective planktivores, as has been demonstrated in northern lakes of the United States (Brooks, 1968; Hutchinson, 1971; Janssen, 1976; Janssen and Brandt, 1980). Electivity (Ivlev, 1961; Strauss, 1979) indices and statistical analyses indicated that alewife selectively prey on zooplankers 1.0 mm length, and such selective predation apparently exerts a major influence on species and size composition of the zooplankton community. The disparity in alewife abundance between 1978 (following die-off) and 1979 (population recovered) provided the opportunity to assess the response of the zooplankton community to alewife size-selective predation in Claytor Lake. Major limnetic zooplankters (Daphnia, Diaphanosoma, Cyclops, Diaptomus) were significantly (P 0.05; Wilcoxon's rank-sum test) smaller when alewife abundance was high. Alewife-induced shifts in zooplankton composition toward smaller forms represent a potentially significant adverse impact on cohabiting planktivores, including young-of-the-year sportfishes.
The potential for inadvertent establishment of alewife populations is exemplified by their rapid and unplanned spread throughout the Great Lakes (Miller, 1957). In a main-stream reservoir situation, alewives obviously have two avenues for escapement, upriver and downriver. Alewife have, in fact, emigrated to and have become established in Bluestone Lake, West Virginia, an impoundment over 100 km downriver from Claytor Lake. From Bluestone Lake, it is possible for alewife to emigrate to the Ohio River and from there into much of the Mississippi drainage. Thus, the transplantation of alewife to Virginia, as well as to Tennessee, has given them potential access to nearly half of the continental United States.
Alewife in Claytor Lake were found to have several undesirable characteristics for a forage species. These included predation on larval sportfishes, alteration of zooplankton species and size composition by selective predation, and rapid growth beyond a size vulnerable to most predators. Increased growth rates of white bass and walleye following the alewife introduction indicated they benefited from the expanded forage base. However, it must be noted that walleye recruitment has coincidentally declined following alewife establishment, and supplemental stockings of walleye have been necessary to maintain their population levels.
The results of studies summarized in this paper raise serious questions about the advisability of further alewife transplantations. Unless future studies can prove otherwise, the risks of alewife establishment in waters of southeastern United States appear to far out-weigh the benefits, and consequently further transplantations should be halted. Unfortunately, the alewife may already have access to most of the southeast, and it may only be a matter of time until they are a common component of the species assemblages of the region.
It is hoped that the case history of alewife in Claytor Lake will serve to create an awareness among decision-makers as to the importance of conducting benefit-risk analyses prior to widespread stockings of non-indigenous fish. It should be noted that alewife were transplanted to Claytor Lake before most of the literature concerning alewife interspecific impacts in northern waters was available. However, such has not been the case for several transplantations recently made elsewhere.
Transplantations should be viewed by fisheries managers with the same concern as exotic introductions. A proposed protocol (Kohler and Stanley, two papers in press) for evaluating exotic fish introductions would have equal utility for evaluating transplantations, and this or a similar protocol should be adopted as a standard fisheries management tool.
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