CalCOFI Reports, Vol. 11, 1967 - California Cooperative Oceanic ...

REPORTS VOLUME SI, 1 JULY 1963 TO 30 JUNE 1966 153dicate that prolonged residence at depths from 100to 300 m, which would be required for such transport,are well within the capacity of the species.Although most of the data on the distribution ofpelagic crabs within the oceanic area come from normaloblique zooplankton hauls, integrated for depth,or from surface observations (e.g., Beklemishev,1960), the only data which contain a depth elementindicate that the highest densities of crabs were atdepths suggestive of such return transport. Duringthe La Pared cruise referred to above no crabs wereobserved at the surface west of about 113' longitude,but between 115 and 120" at the latitude of ClarionIsland many crabs were taken in subsurf ace openingclosingnet tows (Jerde, Berger, pers. comm.) ; theirvertical distribution showed that they were distributedin the depth range 50-300 m even at night and thatsome of the largest concentrations (of more than 200crabs per haul) occurred between 100-150 m, whiledown to 150-300 m concentrations of up to 60 crabsper haul occurred.It is therefore postulated that a larva hatched inthe inshore area off Baja California may either remainwithin the coastal eddies for the duration of itslarval period and be recruited directly to the stockof pelagic sub-adults within the inshore area, or itmay be flushed into the offshore areas while still amegalopa and be recruited to the offshore stocks ofpelagic sub-adults. In the latter case while thereappears to be a mechanism for purposeful return tothe inshore areas, and although the data on the sizestructure of the population suggests that reproductiondoes not occur in the oceanic areas, these postulatescannot be directly proved.Mass mortalities of pelagic Pleuroncodes by coastalstrandings are well known (e.g., Stimpson, 1860;Matthews, 1932; Glynn, 1961; Boyd, 1963) as aremassing of crabs in surface windrows at sea (Shimadain Boyd, 1963) and such observations suggest that attimes pelagic Plezcroncodes may find themselves ininimical oceanographic situations ; Boyd (1963) assumedthat oceanic individuals in the California Currentextension were in this state and were thereforeexpatriates contributing nothing further to the maintenanceof the species.This view and that expressed in the previous postulateare perhaps not entirely conflicting, for it isvery likely that mortality during the offshore excursionis extremely high, and it may be presumed thatthe farther to the south-west a population of subadultsis carried the greater will be the attrition bypelagic fish and other causes of mortality. (It is alsovery likely that even if the postulate of return migrationon the undercurrent is shown to be correct, itwill be found that there is a point of no return beyondwhich the crabs will be, as Boyd suggested for alloceanic individuals, expatriates of no further significanceto the species as R whole.It is now appropriate to consider the relative rolesof the pelagic and benthic phase in the biology of thespecies in the inshore area ; even here the proportionof the pelagic stock which at any time has the possibilityof changing to the benthic phase is probablyrather small, due to the narrow continental shelf tothe west of Baja California, since most of the individualsare over depths greater than those at whichthe benthic phase has been found. Thus, it follows thatindividual residence times in the pelagic phase mustbe of the order of weeks or months, rather than days,if indeed such pelagic individuals have previouslysettled temporarily into the benthos.During the northern movement of 1956-1960 alreadydiscussed the plankton record indicates thatfor limited periods pelagic crabs disappeared fromthe CalCOFI samples (as, for instance, during Juneand July 1959) and this, together with the record ofParker (1963) of benthic crabs off Ensenada duringthis year and the observations of Sund and Quast(Boyd, 1963) of the occurrence of Pleuroncodes inthe stomach contents of many species of demersalfish off San Diego, including some with very slightswimming powers (e.g., Pimelometopon pulchrunz and#corpaena guttata) and which may be presumed tohave taken the crabs on, or very close to, the bottom;this again suggests that the disappearance from theplankton record may well be due to settlement intothe benthic community.The sequence of events during the recession fromthe northern extension of the range again suggests aresidence time in the pelagic phase of some weeks ormonths, and also that the benthic individuals reenteredthe pelagic phase and were swept to the southagain during this period.SUMMARY1. This survey of the ecology of the pelagic phaseof Pleuroncodes planipes in the California Currentindicates that this phase is comparable with theGrimothea stage of Munida gregaria in that it is anextension or recurrence of the larval habit, butdiffers from the Grimothea state in that no morphologicaldifferentiation is involved between the pelagicand benthic phases.2. It is demonstrated that the distribution of thepelagic phase is restricted to water of the range9-28' C., and that the bulk of the pelagic populationoccurs within 100 miles of the coast of Baja California,and that about 7570 of the occurrences were insituations with 10 meter temperatures in the range16-21' C.; the areas in which highest overall frequenciesof occurrence and in which the very denseshoals occurred are those in which the influence ofwater derived from coastal upwelling and hence bearinghigh standing crops of biota is most likely to befelt.3. Lower frequencies of occurrences are demonstratedin oceanic areas to the south-west of BajaCalifornia, and it is shown that these populationshave their origin as larval forms generated over thecontinental shelf of Baja California and subsequentlyflushed out of this area on the offshore trend of theCalifornia Current. It is shown how a proportion ofthese could be returned to the coastal areas on theundercurrent formed by the Pacific Intermediate