by
A.R. LONGHURST
Bureau of Commercial Fisheries
Fishery-Oceanography Center
La Jolla, California, U.S.A.
Abstract
Five known species of Galatheidae exhibit the phenomenon of mass occurrences, mostly in highly eutrophic regions of the ocean. Two of the species (Munida gregaria and Pleuroncodes planipes) occur in vast pelagic swarms as juveniles or young adults, and both species have the capacity to graze on diatom blooms. These two species and M. subrugosa, Cervimunida johni and P. monodon occur in benthic concentrations on some parts of the continental shelf, often in water of very low oxygen tension. A fishery off Chile for the last two of these species lands 10 × 103 metric tons yearly, and it is postulated that the unexploited populations of Pleuroncodes planipes could produce similar or greater yields. It is probable that a demersal fishery for this species will shortly be developed, and it is possible that the fishery may include exploitation of the pelagic phase, though this seems less practicable. It is tentatively postulated that the five species of galatheids reviewed may have a total annual potential of between 30 × 103 and 300 × 103 tons, under the same type of exploitation as has developed in the Chilean fishery.
LES APPARITIONS MASSIVES DE GALATHEES : BIOLOGIE ET UTILISATION COMME RESSOURCE HALIEUTIQUE
Résumé
Cinq espèces connues de Galatheidae se concentrent en essaims massifs, principalement dans les régions fortement eutrophiques de l'océan. Deux d'entre elles Munida gregaria et Pleuroncodes planipes, forment de grands rassemblements pélagiques aux stades juvénile ou jeune adulte, et toutes deux peuvent brouter les bancs de Diatomées. Ces deux espèces, ainsi que M. subrugosa, Cervimunida johni et P. monodon, se trouvent en concentrations benthiques en certains points du plateau continental, souvent dans les eaux à très faible tension d'oxygène. Au large du Chili, la pêche des deux dernières espèces mentionnées produit annuellement 10 × 103 tonnes, et l'on suppose que les populations inexploitées de Pleuroncodes planipes pourraient fournir un rendement analogue ou supérieur. Il est probable que l'on entreprendra sous peu la pêche démersale de cette espèce, et il se peut que l'on s'efforce d'exploiter aussi la phase pélagique, bien que ceci paraisse moins faisable. On attribue provisoirement aux cinq espèces de Galathées examinées un potentiel annuel global situé entre 30 × 103 et 300 × 103 tonnes, pour un type d'exploitation analogue à celui de la pêcherie chilienne mentionnée plus haut.
LA BIOLOGIA DE LA PRESENCIA EN GRANDES CANTIDADES DE CRUSTACEOS GALATEIDOS Y SU APROVECHAMIENTO COMO RECURSO PESQUERO
Extracto
Existen grandes masas de cinco especies conocidas de Galatheidae sobre todo en las regiones altamente eutróficas del océano. Dos de ellas (Munida gregaria y Pleuroncodes planipes) aparecen en vastos enjambres pelágicos de individuos juveniles o adultos jóvenes, y ambas especies se alimentan en las floraciones de diatomas. Estas dos especies y M. subrugosa, Cervimunida johni y P. monodon aparecen en concentraciones bénticas en algunas partes de la plataforma continental, frecuentemente en aguas de muy baja tensión de oxígeno. Una pesquería frente a las costas chilenas desembarca de estas dos últimas especies 10 × 103 toneladas métricas anuales, y se supone que las poblaciones sin explotar de Pleuroncodes planipes podrían producir un rendimiento parecido o mayor. Es probable que pueda crearse en breve una pesquería demersal de esta especie, aunque esto parece menos practicable. En principio se supone que las cinco especies de galateidos examinadas pueden tener un potencial total al año entre 30 × 103 y 300 × 103 toneladas, con arreglo al mismo tipo de explotación que se ha establecido en la pesquería chilena.
The Galatheidae are a family of generalized Crustacea Anomura which occur widely, from tropical seas to high latitudes and from the littoral zone down to abyssal depths generally as members of benthic species assemblages. It appears that at least five of the species which live on the continental shelf can occur in great concentrations at certain times and places within their geographical ranges, and it is these mass occurrences which appear to have fishery potential and which are the subject of this review. Other species in other parts of the oceans may also exhibit the same phenomenon, but have escaped the notice of the reviewer.
In talking of these crustaceans to the fishing industry, one is faced with the problem of their nomenclature in the vernacular; they are neither shrimps nor prawns in the English sense, though they may be utilized by the shrimp industry, nor are they crabs, though the name is often applied to them by fishermen. Having no true vernacular name in any language, fishermen and biologists have applied to them epithets derived from other crustacea, unrelated but of approximately similar appearance: thus the ‘red crabs’ of the American tuna fishermen, the ‘langostino’ of the Chilean shrimp fishermen. There is no simple solution to such a problem, except that of being aware of its existence and of taking care to check independently what species or genera are involved.
The five species which are known to show mass occurrences are tabulated below and grouped geographically. It will be noted that all these species have ranges within relatively eutrophic areas of the ocean, in which strong phytoplankton blooms are a seasonal occurrence.
Munida subrugosa (White), Munida gregaria (Fabricius): ‘whale-feed’, ‘lobster-krill’; New Zealand, Tasmania, Sub-Antarctic islands, Patagonia.
Cervimunida johni Porter: ‘langostino azul’, ‘langostino amarillo’; Pleuroncodes monodon (H. Milne Edwards): ‘langostino colorado’, ‘zanahoria’; Humboldt Current.
Pleuroncodes planipes Stimpson: ‘red crab’, ‘pelagic crab’; California Current, Gulf of California.
These species appear to share some biological characteristics which may have some significance in the mass occurrences: (i) Each species shows a wide range of feeding mechanisms, often including the capability of grazing, by filtration, upon blooms of the larger phytoplankton organisms. (ii) The life-history includes a postlarval pelagic phase, which may be rather ephemeral, may be repeated later in the life of an individual, or may extend at least until first maturity. (iii) The mass occurrences of adults in the benthic phase tend to occur both in locations below mass diatom blooms and at depths corresponding to those at which oxygen deficient water meets the continental slope.
The paper examines these factors in relation to the distribution and present and future exploitation of the species.
2.1 Biology
The most complete account of the biology of M. subrugosa and M. gregaria is that of Matthews (1932), who shows clearly that these two similar species are in fact distinct, though their geographical ranges very largely coincide. The most important difference between the two species is that M. gregaria is very commonly found in the pelagic as well as the benthic phase, while M. subrugosa has no prolonged pelagic phase after larval development is completed. The following account of these two species is drawn very largely from that of Matthews, based on material from the South Atlantic. Pelagic M. gregaria from off New Zealand have been examined and found to conform exactly to his descriptions of morphology and stomach contents.
The pelagic phase of M. gregaria is morphologically distinct from the benthic phase, and was described as a separate genus (Grimothea Leach). The principal distinction, apart from a usually lighter and less spinous exoskeleton, is the great development of the third maxillipeds as flattened, foliaceous, filtering appendages in the pelagic phase. In the benthic phase, these limbs remain foliaceous but are relatively much reduced. The young M. subrugosa, on the other hand, closely resemble the adult in all but size from the time of completion of larval development.
The place occupied in the life history of M. gregaria by the pelagic phase (or grimothea stage) has not directly been investigated, but it appears to be of variable duration. Chilton (1909), working on records of the New Zealand population, and Matthews (1932), considering the Patagonian records, were of the opinion that the point in the life history at which settlement occurred was labile and depended upon the circumstances facing the stock of young pelagic individuals. If conditions for settlement into the benthic phase were not suitable (because they found themselves over oceanic depths, for example), they could prolong the pelagic phase, while other stocks would settle early in their lives. It appears possible for the pelagic phase to be extended at least until the time of first maturity, and ovigerous grimothea stages have been recorded for this species. Thus, while the normal pelagic individuals have carapace lengths within the range 7 to 12 mm, and benthic adults 20 to 38 mm, individuals in the grimothea stage may be larger than the smaller benthic adults.
Evidence is given later that an alternation between the pelagic and benthic phases can occur during the life of an individual, but this is apparently not a regular occurrence.
It has been shown that the stomach contents of the pelagic phase frequently comprise masses of partially digested diatoms (Kemp, in Matthews, 1932), and this was confirmed by examination of specimens in the plankton collections of the Scripps Institution of Oceanography. As shown below for Pleuroncodes planipes, it is the function of the third maxillipeds to act as a filtering mechanism when grazing on phytoplankton blooms. On the whole the species is a very catholic feeder, and in the benthic phase it eats small organisms captured by the chelipeds and organic detritus from the upper deposit levels.
2.2 Range of pelagic phase
Matthews (1932) documents many accounts from early voyagers of the mass occurrences of the pelagic phase of Munida gregaria, beginning with that of Sir Richard Hawkins in 1594 who named ‘Crabby Cove’ in the Straits of Magellan after the fact that the water was ‘full of a small kind of red crabbes’. It is evident from these accounts by Dampier, Banks and other early voyagers, together with the modern accounts from, for example, the METEOR and DISCOVERY expeditions, that mass surface occurrences of the grimothea stage are commonplace, especially in the early summer. They are reported from both coasts of Patagonia, to 41°S on the west, and to 51°S on the east, around the Falkland Islands, the coasts of New Zealand and its sub-Antarctic islands, and the coast of Tasmania. Although it is not possible to quantify these accounts, the repetition within them of statements as to the occurrence of ‘clouds’ of little red lobsters at the surface, of the sea being coloured red with small galatheids, and so on, leaves little doubt as to the magnitude and concentration of the populations.
It is also evident from the published records that these mass occurrences may occur either close to land, over the continental shelf, or over oceanic depths at some hundreds of miles from land. The factors inducing settlement are obscure, but probably are connected with the nutrient cycle of the sea area concerned and the chance of pelagic stocks being carried by horizontal advection off the continental shelf.
There are a few records of adult M. gregaria being taken at the surface, especially along the western coast of Patagonia. These individuals in the DISCOVERY collections do not have grimotheal characters, yet were dip-netted at the surface from enormous concentrations extending from 45°S to 50°S. At the same latitude there are records of windrows of these galatheids being drifted ashore in very great numbers.
2.3 Range of benthic phase
Records of mass benthic occurrences of these two species are less common, perhaps simply because they are not exposed to the view of the curious voyager; nevertheless, there are published records which indicate that they occur. Thompson (1898) records massive summer occurrences of benthic individuals of M. subrugosa in Otago Harbour, New Zealand, and the DISCOVERY collections include adult M. gregaria, trawled in abundance in 25 to 115 m of water off East Falkland Island. Cooked in the ship's galley, these were found to be of very good flavour (Matthews, 1932).
2.4 Exploitation
Neither of these two species has been utilized directly as a fishery resource, though they have been shown to be an important food of several exploited cetaceans including southern right, humpback, sei and blue whales. In addition, they are important in the diet of many rock-fish, seals and several species of sea-birds. It is to be expected that systematic trawling in sub-Antarctic waters will indicate large benthic stocks of both these species. By analogy with other species discussed below, they are unlikely to confine themselves to rocky substrates where trawling is impossible.
3.1 Biology
To the north of the region occupied by the two species of Munida described above, there occur two species which do not appear to have a postlarval pelagic phase. These are the ‘langostino amarillo’, Cervimunida johni, and the ‘langostino colorado’ or ‘zanahoria’, Pleuroncodes monodon, both of which have mass occurrences in the benthos off the Chilean coast. Méneville noted the occurrence of pelagic galatheids in the harbour of Callao in 1830, during the cruise of LA COQUILLE round the world (in H. Milne Edwards, 1837), but there appears to be no other evidence of a pelagic phase in these species. The Callao specimens appeared to belong to the then-valid genus Grimothea though Méneville's illustrations are not very satisfactory. U.S. tuna fishermen, very familiar with the pelagic phase of P. planipes, know of no similar form from the Peru Current, though the benthic phase of P. monodon appears to extend as far north as Central America.
The benthic concentrations off Chile, where they are best known, appear to be comprised of individuals of both species within the same size range. Specimens have a mean weight of about 25 g and carapace lengths ranging from about 20 to 40 mm. This is about the same size as the benthic adults of M. gregaria discussed above (2.1). Shrimps of commercial importance may occur on the same grounds, but usually the mass occurrences of galatheids do not appear to correspond with the most dense populations of penaeids (Mistakidis and Henriquez, 1966).
3.2 Range of benthic phase
The Chilean shrimp fishery, which includes a fishery for these two galatheids, has defined their range as extending approximately from Caldera in the north to Calbuco in the south, though the whole of this zone has not yet been properly explored (Lopez-Capont et al., 1965). Within this zone, the largest concentrations of langostino appear to occur within the depth-range 125 to 200 m. In the same depth-range the abundant shrimp appears to be Heterocarpus reedi, although below about 150 m the galatheids overlap the range of the deep shrimp (‘gamba’) Hymenopenaeus diomedeae. Gallardo (1963), in his study of the benthic communities off the Chilean coast, recognizes a lower sublittoral zone, extending from 50 to 400 m in which the biomass is very low and in which the bottom water is very deficient in oxygen. This is the result of the well-known oxygen minimum layer (e.g. Brandhorst, 1959) of the eastern Pacific Ocean meeting the continental slope, and similar conditions appear to extend along much of Central America. They will be referred to again below in connection with the distribution of the Mexican species of Galatheidae. It appears that P. monodon and C. johni must be capable of supporting mass populations in water with dissolved oxygen values as low as 0.1 ml/l. It is noteworthy that Gallardo describes the deposits in these zones as being of soft, highly organic substrata probably derived from the sinking of diatom cells from surface blooms. These blooms are associated with the upwelling areas so typical of this coastline.
3.3. Exploitation
The fishery for these galatheids has been prosecuted since 1953, and has reached an annual catch of 10 × 103 tons annually (Table I).
TABLE I
The development of the Chilean ‘langostino’ landings (from Lopez-Capont, et al., 1965) Units = metric tons
| Total catch | Consumed fresh | Canned | Frozen tails | |
| 1953 | 0.93 | 0.29 | 0.59 | 0.47 |
| 1954 | 2.64 | 0.85 | 1.20 | 0.58 |
| 1955 | 2.05 | 0.78 | 1.10 | - |
| 1956 | 5.71 | 1.61 | 3.57 | - |
| 1957 | 11.38 | 4.27 | 0.73 | 5.82 |
| 1958 | 12.83 | 5.37 | 0.22 | 6.33 |
| 1959 | 6.75 | 0.93 | 0.80 | 5.02 |
| 1960 | 8.12 | 1.22 | 0.83 | 6.02 |
| 1961 | 8.15 | 0.62 | 0.65 | 6.89 |
| 1962 | 7.99 | 0.36 | 0.94 | 6.69 |
| 1963 | 9.25 | 0.20 | 0.94 | 7.91 |
| 1964 | 10.67 | 0.68 | 0.94 | 9.03 |
In the early years of the fishery the landings were generally utilized in the fresh state or as a canned product, but at the present day almost the entire landings are frozen for export. The landings comprise about 60 percent C. johni (Mistakidis and Henriquez, 1966), but this may not be an accurate reflection of relative abundance in the sea. C. johni has a slightly higher meat yield (9 to 10 percent) than has P. monodon (6 to 7 percent), and there may well be a preference for it in the fishery.
There is a seasonal fluctuation in the landings, which are normally highest during the first half of the year (Lopez-Capont, et al., 1965). The small trawlers used in the fishery average about 0.6 ton/h, but recent surveys have shown that catch-rates may, on occasion, go much higher than this average. Maximum catch-rates, obtained during a survey to the north of Constitución (Mistakidis and Henriquez, 1966), were 4.16 ton/h at 36 to 37°N and 2.53 ton/h at 35 to 36°N.
It seems likely that this resource extends a good deal further to the north than it is currently exploited, for P. monodon is known to extend along the Peruvian coast as far north as the Lobos de Afurea Islands (Haig, 1955). There is even evidence of its occurrence off the west coast of Mexico, at Acapulco (Boyd, MS), though this record, together with Boyd's discussion of specific characters, suggests that the distinction between P. monodon and P. planipes requires further examination.
4.1 Biology
The red or pelagic crab, Pleuroncodes planipes, occurs at the southern end of the California Current and in the Gulf of California in both the benthic and pelagic phases. Mass occurrences of the pelagic phase in the surface waters are a well-known phenomenon in this region, and although it occurs at least 1,000 mi to the southwest of Baja California in the extension of the California Current, it does not appear to extend southwards along the coast of Mexico far below the mouth of the Gulf of California.
Until relatively recently the benthic phase was unknown, but it is now established that it occurs widely on the upper parts of the continental slope, both in the Gulf and along the western coast of Baja California (Boyd, MS; Parker, 1964; Perkins, personal communication). Boyd found benthic P. planipes along this coast to 26°N, while Parker found them to 28°N in Sebastian Viscaino Bay, and this appears to be their northern limit in normal years. Parker, however, found them as far as latitude 32 to 33°N, in the area of Ensenada and San Diego, during the warm year 1959 to 1960, a range extension which will be discussed below.
The data on the depths at which P. planipes have been found in the benthic phase by these three authors are consistent, as the following tabulation indicates:
| Depth(m) | T°C | O2 (ml/l) | |
| Boyd | 75–300 | 10.0–14.0 | 0.5 |
| Parker | 64–366 | 9.0–14.0 | 0.5–2.0 |
| Perkins | 73–201 | 12.0–14.6 |
Further ecological data on the associated fauna of stations at which P. planipes occurred, given both by Boyd and Parker, indicate that mass concentrations occur to the virtual exclusion of other forms. Parker writes of enormous numbers of Pleuroncodes (together with an unidentified species of Munida) occasionally occurring in a depth zone otherwise largely devoid of benthic organisms, and in which occurred large quantities of undecayed organic detritus at the surface of the deposits. As with the Chilean species, all benthic records of P. planipes are from depths corresponding to the oxygen minimum layer, and this is confirmed by the actual observations of oxygen concentration tabulated above. It is suggested that the mass occurrences often, or usually, occur on the deposits underneath coastal upwelling regions and other places in which diatom blooms occur. The ‘rain’ of Coscinodiscus and other diatom cells in these areas produces green, unoxidized muds, with a very high organic content, which are probably utilized as food by the benthic galatheids. Aquarium observations of P. planipes show that, as well as feeding as a predator on almost any organism of suitable size, it will readily feed on organic detritus lying on the bottom of the vessel. It sweeps detritus towards the mouth by the action of the fan-shaped third maxilliped.
There do not appear to be any significant morphological differences, apart from size, between pelagic and benthic individuals of this species. The pelagic phase is as regular an occurrence in the life history of P. planipes as is the grimothea stage of M. gregaria. The third maxilliped is foliaceous and the walking legs have a fringe of natatory setae in both the benthic and the pelagic phases. It seems probable that in P. planipes there can be an alternation between pelagic and benthic phases in the life of a single individual.
4.2 Range of pelagic phase
The distribution of the pelagic phase of P. planipes in the California Current is now well known, both as a result of special investigations and of the long-term plankton studies of the California Cooperative Oceanic Fisheries Investigations. Fig. 1 and 2 are derived from the latter data and indicate the frequency of occurrence of pelagic swarms in different regions off the California coast and the oceanic areas in which smaller numbers of specimens have been recorded. The swarms appear in regions of coastal upwelling, and there is some dispersal in the waters of the offshore drift, to the southwest of the California Current.
 |  |
Fig. 1 Distribution of Pleuroncodes planipes in pelagic phase in California Current, expressed as percentage occurrence in plankton hauls at standard CalCOFI stations. Shaded areas (lighter to darker) represent: 1–10%, 11–25%, 26–50%, 51–75%, 76–100% occurrence.
Fig. 2 Distribution of Pleuroncodes planipes in pelagic phase in offshore and oceanic regions of California Current. Coastal rectangle is area illustrated in Fig. 1; outer rectangle was sampled regularly but less frequently (occurrences indicated by dots); other records from irregular or occasional observations.
Fig. 3 Variation in distribution of pelagic Pleuroncodes planipes in 1955–1962
related to sea temperature. Numerals in top left margin represent
station lines lying normal to coast in geographical sequence indicated
at top right margin. Percentages are number of stations in each line
at which plankton sample contained pelagic crabs in month referred to
at top of figure.
Bottom part of figure shows temperature anomalies at a number of
selected stations.
Also indicated (Fig. 3) is the manner in which the range of the pelagic phase off the western coast of Baja California changed in response to the changing climate in the years 1955 to 1962. Pelagic crabs were recorded abundantly as far north along the Californian coast as the Monterey Peninsula in 1960. Both the range extension to the north at the start of the warming period and the regression at the end of the period can be explained (Longhurst, in press) by the changing patterns of flow in the California Current at these times, particularly the changing form of the large permanent gyre which normally exists to the south of Point Conception. During this northerly range extension, there were numerous records of windrows of pelagic P. planipes being swept ashore at points along the coast of California, a phenomenon which is of common occurrence along Baja California and the Gulf coasts, and which recalls the reports of strandings of M. gregaria described above (Glynn, 1961; Boyd, MS; Radovich, 1961; Longhurst, in press).
The problem of the relationship between the pelagic swarms and the benthic populations along the continental slope is an interesting one and hard to resolve. It is evident from the general pattern of flow at the southern end of the California Current that there is an offshore drift to the southwest in the surface waters. The distribution of the pelagic phase in oceanic areas suggests that pelagic specimens from the coastal areas of Baja California are swept offshore by this drift. The flow of the California Current is far from laminar (Reid, Schwartzlose and Brown, 1963) and sufficient eddying occurs to maintain a spat-fall of planktonic larvae of benthic organisms along the continental shelf. There is evidence, however, that the larval forms of P. planipes are flushed out of the Baja California centers of spawning (Fig. 4) into oceanic areas to the southwest. It has also been demonstrated (Longhurst, in press) that the pelagic postlarval forms in oceanic areas are generally smaller than those encountered in massive swarms over the continental shelf, and both these groups are generally larger than the individuals encountered in the benthic phase (Boyd, MS; Long-burst, in press).
The question to be resolved, therefore, is whether or not the oceanic individuals in the pelagic phase are lost to the parent stock on the continental slope, or whether they (or a proportion of them) return to coastal areas. The evidence is conflicting: a possible mechanism for their return exists in the sub-surface currents flowing to the northeast under the California Current, and closing-net tows have shown concentrations of crabs at the right depths for such transport (Longhurst, in press), but there is recent evidence that maturity is reached by some pelagic individuals in the oceanic region and that breeding occurs there (Blackburn, personal communication). Perhaps both possibilities are partly true. It is particularly difficult to determine whether or not an alternation occurs between benthic and pelagic phases over the continental shelf, but the fact that benthic individuals retain their natatory setae is evidence for such an alternation. Individuals captured at sea in the pelagic phase are very ready to adopt a benthic existence again, and can be seen clinging to any substrate which they encounter; it is not unusual to bring one up clinging to an oceanographic instrument, they have been observed clinging to large salps in the upper few metres of water, and they settle at once in an aquarium.
4.3 Food relationships
The ability of P. planipes to graze directly upon blooms of phytoplankton with a major diameter in excess of 25 to 30 μ has recently been demonstrated and the filtering mechanism described (Longhurst, Lorenzen and Thomas, in press). The aperture size of the filter formed by the exopodites of the third maxillipeds appears to correspond with the minimum particle size retained during experimental grazing on natural phytoplankton, and is of the order of 25 μ in Pleuroncodes of approximately 20 mm carapace length.
Pelagic individuals can be watched at sea from an underwater observation chamber, and it can be seen that they graze by using their fan-shaped maxillipeds as a cast-net while they are sinking slowly with their legs spread out like a parachute. A grazing population comprises many individuals slowly sinking in this way, interspersed with others swimming rapidly upwards by abdominal flexure. Animals that have become accustomed to an aquarium can be induced to graze even when they are resting on the bottom.
Fig. 4 Distribution of larval (dotted line) and megalopa (solid line) forms of Pleuroncodes planipes off Baja California in June 1964.
Experimental data show that a large volume of water can be filtered during grazing. The maximum daily production of faecal pellets recorded for a freshly-caught grazing crab amounted to 102 mg dry weight and consisted of greenish material in which 5 × 105 whole undigested cells of Coscinodiscus were counted. Under the natural standing crop conditions observed, this would correspond to 540 1/day filtered, assuming a filtration efficiency of 100 percent and a digestive coefficient of 50 percent. Calculations based on close observation of the rate of beat of the filtering appendages produce an estimate of the maximum filtration rate of 432 1/day. The two figures are in general agreement.
The above observations were made off Baja California, and the grazing budget of the same location was monitored over a five-day period. The results suggest that herbivorous zooplankton was responsible for only about 10 to 15 percent of the observed excess of phytoplankton production over the observed standing crop, and that the stock of pelagic crabs was sufficient to be responsible for most of the remaining apparent grazing. Evidently the pelagic phase of P. planipes may play a very important role in the grazing of diatom blooms in coastal upwelling areas in the California Current.
The importance of Pleuroncodes in the diet of many predatory animals in this area is well known. The fish Eschrichtius, Neothunnus, Thunnus, Katsuwonus, Coryphaena, Seriola, and the birds Larus, Phalacrocorax and Pelecanus all feed actively on mass occurrences of pelagic crabs (Longhurst, Lorenzen and Thomas, in press). Alverson (1963) has demonstrated that the stomach contents of yellowfin tuna (Neothunnus albacares) from the region to the west of Baja California were dominated by pelagic Pleuroncodes; the mean volume of crabs contained in each yellowfin stomach was about 0.5 l (or about 100 crabs). This figure must be less than the average daily intake of each member of the population of tuna, and 3.99 × 106 tuna were taken by the United States fishery alone in 1960 off Baja California. This represents a daily cropping at the beginning of the season of 2 × 103 tons of Pleuroncodes by those fish which would be caught by the fishery in the subsequent six months or so. The quantity utilized by this single stock of predatory fish is thus very high indeed over the year as a whole.
The main ecological significance of the utilization of pelagic Pleuroncodes in this manner is that the food-chain is thus rendered unusually short and direct between the primary producers and the climax predators.
In the offshore oceanic environment, the pelagic crabs maintain themselves on a more varied diet than over the continental shelf. Food is obtained both by the filtration apparatus and also by selective particulate feeding by use of the chelipeds; it consists mainly of small Copepoda (Microsetella, Clausocalanus, etc) and large Protozoa (radiolarians, tintinnids), together with a few large phytoplankters such as dinoflagellates. A survey of stomach contents from a grid of stations to the west of Baja California in June 1964 showed that it was only in a relatively few localities that phytoplankton dominated the stomach contents and that these stations were situated in upwelling areas.
4.4 Breeding and growth
Breeding occurs in the California Current during the winter months, especially in the first three months of the year; this is evident from data of Boyd and Johnson (1963) on the percentage of ovigerous females in the CalCOFI plankton samples, and is confirmed by work now in progress on the distribution of larval forms in the same plankton samples. The five larval stages described by Boyd (1960) are passed through in 3 or 4 months in the laboratory (Boyd and Johnson, 1963), and this is essentially confirmed by studies on net-caught larvae.
The work in progress indicates that breeding is concentrated at a few centers over the continental shelf, and it is presumed that these correspond with the centers of distribution of the adult crabs as a whole, including both benthic and pelagic stocks.
Growth rates of the postlarval forms have been studied both by Petersen's method on net-caught samples and in the laboratory (Boyd, MS). In general terms, a carapace length of about 20 mm is attained at the end of the first year, a little less than 30 mm at the end of the second year, and about 35 mm at the end of the third year. There are reasons to suppose that growth rates are not constant over all the ecological situations in which the species occurs. The approximate growth rates just given, however, suggest that the benthic stocks are in their second and third years of life (the deeper, the older), the pelagic stocks over the continental shelf are between 9 and 15 mo old, and the offshore oceanic stocks are within their first year of life. There may be an underestimation of the age of offshore stocks as these will be living at a lower nutritional level than the inshore stocks.
4.5 Exploitation
There has been no exploitation of this species directly, but the possibility of such exploitation is clear, and food-processing interests in California are currently studying the viability of a fishery for P. planipes.
One of the first decisions to be made in planning such a fishery is whether the exploitation should be based upon the pelagic or the benthic phase of the species, or upon both. Unfortunately, the data for an entirely rational decision do not at present exist, for no comprehensive trawling survey of the region has been completed to determine the real extent of the benthic stocks, nor their seasonal variation. One of the major problems to be solved currently is the fate of the pelagic stocks over the continental shelf which are generally considerably more abundant early in the year. Have they been flushed out to sea, have they been grazed down by the migratory tuna which arrive in the area in the spring, or have they descended and been recruited to the benthic stocks?
Blackburn (personal communication) has observed the density of pelagic swarms to be very variable, from situations where the crabs are exceedingly sparse to those in which they appear to be piled up on top of one another. During several cruises to the region of Baja California in which standard tows were made with a nekton net on a standard grid system, a total of 33 hauls collected pelagic Pleuroncodes, at a mean density of 101.7 ml of crabs per 103m3 of water filtered. At 32 of these stations the volume of crabs was from 1 to 355 ml/103m3, while at one station it was 1539 ml/103m3; such a distribution confirms subjective observations at sea, which suggest that the most dense mass occurrences are denser by almost an order of magnitude than the ‘background’ occurrences of pelagic individuals. It can be supposed, therefore, that by searching it would be possible to find concentrations that would yield 250 kg/h with a small pelagic trawl; this figure is based on a mouth aperture of 30m2, a towing speed of 5.4 km/h, and a filtration efficiency of 100 percent. This figure is likely to be an underestimate, since the density of individuals on which it is based was taken from a net worked on a standard oblique haul to 200 m depth and which was not held at the depth at which crabs were seen or thought to be aggregated. My observations at sea suggest that, at times, it would be possible to exceed this figure by a factor of ten, by selective fishing with such a net.
The data of Boyd (MS), Parker (1964) and Perkins (personal communication) on the benthic phase include some estimations of the population density of individuals. Using a swept area method based on the catch of a trawl having an aperture approximately 3 m wide, Boyd suggested that the average density at one station was of the order of 10 Pleuroncodes per m2 on the deposits. Extrapolating to a commercial otter trawl of 10 m across the mouth and a towing speed of 5.4 km/h, this implies a catch rate of 13.5 × 103 kg/h, assuming a catching efficiency similar to that of Boyd's 3 m trawl. At such a high catch rate it would be impossible to tow a demersal trawl for as long as one hour without it so filling with crabs as to be torn or lost altogether. This has, in fact, been the experience of a trawling survey along the west coast of Baja California. Perkins (personal communication) found that during a trawling survey for demersal fish along the continental shelf it was impracticable to trawl deeper than about 150 m along much of the coastline since the trawl rapidly filled with benthic Pleuroncodes and became unmanageable. This first estimate of likely catch rates in a demersal fishery for Pleuroncodes cannot be more than preliminary, and may be considerably modified by a properly conducted survey of the benthic crab resources, and such a survey is urgently required before any rational planning of a fishery can take place. In any case, the available evidence suggests that catch rates at least as high as those which prove to be economically feasible in the Chilean fishery may be obtained off Baja California.
As part of the FAO participation in the Indicative World Plan for Agricultural Development, estimates of the total potential yield from fisheries resources are required, and it is interesting to postulate what might be the long-term annual yield from the utilization of mass occurrences of galatheids. The estimate is restricted to the three regions discussed here, and ignores any further stocks which may in the future be identified in other eutrophic ocean areas. It leans rather heavily on the figure of 10 × 103 tons for the present annual landings of the Chilean fishery and assumes approximately equivalent biology and size of individuals in the various species concerned. It is suggested that the potential yield is most unlikely to be less than 30 × 103 tons per annum. It may be higher than this figure: a factor of ten times seems subjectively just possible, while a factor of one hundred seems impossible. It is suggested therefore that the ultimate yield from the stocks considered here will be found to lie between 30 × 103 and 300 × 103 tons per annum. To attempt a narrower estimate than this would be quite unrealistic, in our present state of ignorance about the real extent of the benthic stocks of two of the five species under consideration.
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