SHENKER: OCEANOGRAPHIC ASSOCIATION OF NEUSTONIC MEROPLANKTON 



during sunny days, when upwelling wind stress 

 and offshore Ekman transport is generally 

 strongest, can move the larvae into the slow on- 

 shore flow below the surface Ekman layer (Peter- 

 son et al. 1979). In contrast to the usual northerly 

 upwelling winds, occasional storms blow from the 

 southwest, driving the surface layer onshore. The 

 occurrence of megalopae at the surface during the 

 day was most pronounced on these dark stormy 

 days (especially on the second May cruise), and 

 thus may facilitate their onshore transport. 



Observations of megalopae entering into em- 

 bayments and nearshore areas from Washington 

 to northern California in 1984 and 1985 indicated 

 a dramatic increase in the abundance of megalo- 

 pae and an extended seasonal occurrence in the 

 plankton, as compared to previous years (Shenker 

 and Botsford fn. 6; Armstrong^). This high abun- 

 dance of crab larvae may presage an upswing in 

 the cyclical crab fishery along these coasts. 



Numerous mechanisms have been proposed as 

 causes of the 10-yr cycles in crab abundance. Al- 

 though some hypotheses have been discounted, 

 several models have survived scrutiny as possible 

 causes of the cycles (see Botsford 1986 for review). 

 Potential mechanisms of environmental forcing 

 of the cycles focus on larval transport and sur- 

 vival. Johnson et al. (1986) detected periodic 10- 

 yr cycles in the occurrence and strength of south- 

 ward stress during the late larval period that 

 significantly correlated with commercial crab 

 catch 4 and 5 years later. This lag corresponds to 

 the time between larval settlement and growth 

 into the adult fishery (Botsford 1984). 



Model simulations by Botsford (1986) indicated 

 that nonlinear effects of wind on larval transport 

 can produce the cyclical swings in crab abun- 

 dance. However, the models do not preclude the 

 possibility that density-dependent phenomena 

 (such as cannibalism by adult crabs on newly set- 

 tled juveniles, and predation by nemertean 

 worms on egg masses) may act in concert with the 

 environmental forcing to produce the observed 

 cycles. 



The water's surface is the only oceanic habitat 

 that is easily accessible to observation using tech- 

 niques ranging from satellite and aerial scanning 

 to shipboard visual sighting of targets and contin- 

 uous monitoring of environmental parameters. 

 Because of this accessibility, micro- and meso- 

 scale patterns in distribution of neustonic taxa 



'^D. A. Armstrong, School of Fisheries, WH-10, University of 

 Washington, Seattle, WA 98195, pers. commun. October 1984. 



and their associations with hydrographic and bio- 

 logical characteristics of the surface zone can be 

 determined more easily than in other environ- 

 ments. The neustonic realm thus offers an excel- 

 lent opportunity to investigate the mechanisms of 

 transport of the early stages into appropriate 

 nursery habitats, and the availability of food for 

 growth that are required for successful recruit- 

 ment into adult stocks (Hjort 1914; Lasker 1975, 

 1981; Frank and Leggett 1982; Sinclair et al. 

 1984). 



ACKNOWLEDGMENTS 



This research was supported by the Northwest 

 and Alaska Fisheries Center of the National 

 Marine Fisheries Service. 1 thank Captain Leland 

 Oldenberg and his crew on the FV Cumberland 

 Trail, and my colleagues R. Brodeur, A. Chung, 

 J. Fisher, J. Hennessey, L. Krasnow, B. Mundy, 

 C. Paczkowski, and E. Rexstad for their assis- 

 tance on the sampling cruises. W. Laroche helped 

 identify juvenile Sebastes. W. G. Pearcy provided 

 valuable advise on the design and operation of 

 this project, and comments on the manuscript. 

 Helpful suggestions on this paper were also given 

 by R. Brodeur, A. W. Kendall, and two anony- 

 mous referees. 



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