Biology and Ecological Niches in the Gulf of the Farallones 



Free-Floating Larvae of Crabs, Sea Urchins, and Rockfishes 



Stephen R. Wing 



Free-floating larvae of bottom-dwelling 

 marine organisms play an important role in 

 the ecology of the Gulf of the Farallones and 

 are a major food source for many species, 

 including salmon and seabirds. Free-floating 

 larvae are also the primary means by 

 which several important species are dis- 

 persed along the northern California coast. 

 Most commercially harvested invertebrates 

 (for example, sea urchins and crabs) and 

 some fish populations are made up of a net- 

 work of adult subpopulations spread along 

 the coastline, connected almost solely by 

 larval dispersal. 



In the Gulf of the Farallones many com- 

 mercially important coastal species, such 

 as Dungeness crab (Cancer magister), red 

 sea urchin (Strongylocentrotus francisca- 

 nus), and rockfish (Sebastes spp.), have 

 swimming larval forms that drift as plank- 

 ton during the spring. These larvae are 

 subject to the strong offshore and south- 

 ward flow present in the California Current 

 system during the spring upwelling season 

 (see chapter on Current Patterns Over the 

 Continental Shelf and Slope). Understand- 

 ing how larvae of these species maintain 

 themselves at latitude and are transported 

 inshore into suitable juvenile habitat is 

 important in ensuring the long-term viabil- 

 ity of these fisheries. 



In 1992, a study was begun to understand 

 how coastal oceanographic processes influ- 

 ence the distribution of crab and sea-urchin 

 larvae around Bodega Head, a coastal head- 

 land at the northern margin of the gulf. This 

 required the cooperative efforts of many 

 organizations, including Gulf of the Faral- 

 lones and Cordell Bank National Marine 

 Sanctuaries, U.S. National Marine Fisheries 

 Service, Scripps Institution of Oceanogra- 

 phy, Point Reyes Bird Observatory, Oregon 

 State University, California Department of 

 Fish and Game, and Pacific Fisheries Envi- 

 ronmental Group. 



Sampling extended over several years 

 and was done both from aboard ships and 

 at shore stations. Results indicate that during 

 periods of upwelling, larvae accumulate in 

 the warm water that collects between the 

 Farallon Islands and Point Reyes in the 

 northern gulf. Larval accumulation was also 

 observed in the upwelling shadows, or 

 eddies, that form behind other capes and 

 headlands when the wind is blowing from 

 the north and surface water is moving in 

 a southerly offshore direction. When the 

 northerly winds that cause upwelling relax, 

 this larvae-rich water moves north in buoy- 

 ancy-driven currents that remain coastally 

 trapped. During this time many larvae that 

 have been retained in the gulf are trans- 



ported to the north, and some settle out of 

 the water column and onto the sea floor for 

 their next life stage. 



In both 1994 and 1995, an upwelling jet 

 seaward of Point Reyes and an upwelling 

 shadow downwind (south) of the point were 

 evident from the currents, temperature, and 

 salinity of the surface waters. Information 

 was collected at the boundaries between four 

 distinct water masses: (1) newly upwelled 

 water, (2) oceanic water from offshore, (3) 

 San Francisco Bay outflow, and (4) a mix- 

 ture of these water types that was called 

 "gulf water." The different water masses con- 

 tained different types of larvae. In general, 

 all stages of larval development for crabs of 

 several species could be found within, but 

 not outside, the gulf water. This water mass 

 is found in the wind shadow south of Point 

 Reyes. In contrast, rockfish larvae were 

 found in high concentrations offshore in oce- 

 anic water and at the boundary between 

 newly upwelled water and gulf water. The 

 Gulf of the Farallones provides a pathway 

 along which distinct masses of water con- 

 taining both crab and rockfish larvae are 

 likely to be transported to suitable juvenile 

 habitats within the gulf and to the north. 



Larval settlement becomes more epi- 

 sodic and less frequent with increasing 

 northward movement away from the area of 



larval retention/accumulation south of Point 

 Reyes. Nearly twice as many crab larvae 

 settle at Point Reyes and to the south, where 

 warmer gulf water is constantly present, than 

 to the north, where settlement occurs only 

 when the northerly winds that cause upwell- 

 ing subside and relaxation currents move 

 northward. Sea urchin larvae also settle 

 during specific oceanic conditions, but less 

 predictably than crabs. 



Consistent larval dispersal from any sub- 

 population of adult animals may be limited 

 to the length of an embayment. For example, 

 two such embayments on the northern Cali- 

 fornia coast are from Point Reyes to Point 

 Arena and from Point Arena to Cape Men- 

 docino. Because strong seaward currents 

 form at each end of these embayments 

 during upwelling, larvae caught in these cur- 

 rents would be swept out to sea and have no 

 chance of developing and becoming a part of 

 the adult population. 



The data collected in this study on the 

 distribution of free-floating larvae in the 

 Gulf of the Farallones provide critical infor- 

 mation on the lifecycles of commercially 

 important bottom-dwelling species. Because 

 these larvae are a food source for many 

 marine species, this information is crucial to 

 the effective management and conservation 

 of coastal marine ecosystems in the gulf. 



40 Biology and Ecological Niches in the Gulf of the Farallones 



