A MODEL OF THE DRIFT OF NORTHERN ANCHOVY, 

 ENGRAULIS MORDAX, LARVAE IN THE CALIFORNIA CURRENT 



James H. Power 1 



ABSTRACT 



The drift of northern anchovy, Engraulis mordax, larvae in the California Current to unfavorable offshore 

 areas may be an important factor contributing to larval mortality, and hence it may affect recruitment 

 and subsequent adult population size. A simulation model based on a finite-difference approximation to 

 the advection-diffusion equation was developed to aid in the study of larval anchovy drift. Model com- 

 ponents included the long-term mean geostrophic and wind-driven current velocities to 50 m depth, and 

 turbulent diffusion. The model predicted larval distributions in the Southern California Bight and off- 

 shore regions after 30 days of drift, and these distributions were used to assess the extent of cross-shore 

 and alongshore larval transport that occurs when spawning takes place at different locations, seasons, 

 and during times of increased offshore-directed Ekman transport. 



Offshore transport was minimal in most simulations. Simulations of drift starting from the location 

 of peak spawning showed strongest seasonal effects, with currents during the season of peak northern 

 anchovy spawning (March) resulting in reduced offshore dispersal when compared with currents at other 

 times of the year. March currents also produced the greatest downshore (southeasterly) transport of 

 larvae, and strong seasonal currents, such as the nearshore, northwesterly flowing California Counter- 

 current, can greatly affect the alongshore 30-day larval distributions. Offshore directed Ekman trans- 

 port, associated with upwelling, does not strongly affect the drift of larvae in the nearshore region, but 

 large increases in overall Ekman transport, or extension of spawning into offshore regions, can result 

 in significant seaward transport of larvae out of the Southern California Bight. 



The total population of northern anchovy, Engraulis 

 mordax, a common pelagic fish off the west coast 

 of North America, is comprised of three subpopula- 

 tions (Vrooman et al. 1981): northern (found north 

 of lat. 36°30'N); central (between lat. 29° and 38°N); 

 and southern (south of lat. 29 °N). The central sub- 

 population inhabits the Southern California Bight 

 region, and in recent times has exhibited substan- 

 tial changes in population biomass (e.g., Smith 1972). 

 Analysis of northern anchovy scales deposited in 

 sediments indicates that large northern anchovy 

 population fluctuations have also occurred in the 

 past few centuries (Soutar and Isaacs 1974). Histor- 

 ically the central subpopulation of northern anchovy 

 has supported a significant fishery (Messersmith and 

 Associates 1969; Sunada 1975; Stauffer and Charter 

 1982), and although the U.S. fishery has recently 

 declined, there is still a significant Mexican fishery. 

 The northern anchovy fishery, the recent and histor- 

 ical changes in anchovy population size, and the 

 fish's important role in the marine ecosystem all pro- 

 vide the motivation for studying the mechanisms 



'Southwest Fisheries Center La Jolla Laboratory, National 

 Marine Fisheries Service, NOAA, P.O. Box 271, La Jolla, CA 

 92038; present address: Coastal Fisheries Institute, Center for 

 Wetland Resources, Louisiana State University, Baton Rouge, LA 

 70803-7503. 



that may cause interannual variations in northern 

 anchovy stock size. 



Such changes in stock size may be a consequence 

 of variations in the previous spawning stock size, 

 or they can also arise as a result of interannual dif- 

 ferences in mortality during prerecruit life history 

 stages (Rothschild et al. 1982). Because the egg and 

 larval stages have the highest mortalities, it seems 

 possible that processes affecting the relative mor- 

 tality during these stages can have a significant ef- 

 fect on subsequent recruitment. Two major causes 

 of larval mortality are starvation and predation 

 (Smith and Lasker 1978; Hunter 1981). A factor that 

 may contribute to these is larval drift. The northern 

 anchovy eggs and larvae, lacking adequate motil- 

 ity, can be involuntarily transported away from 

 nearshore spawning areas. It is the nearshore 

 regions in the Southern California Bight that most 

 frequently contain adequate food concentrations for 

 growth and survival of first feeding northern an- 

 chovy larvae (Lasker 1978, 1981). 



Although eddies and other short-term mesoscale 

 features are important in the Southern California 

 Bight (Mooers and Robinson 1984; Simpson et al. 

 1984), the broad and relatively slow equatorward 

 flow termed the California Current is the dominant 

 feature in the region that persists on evolutionary 



Manuscript accepted September 1985. 

 FISHERY BULLETIN: VOL. 84, NO. 3, 1986. 



585 



