Yoklavich et al.: Larval rockfishes and their physical environment off central California 



779 



dance of several potential larval fish predators 

 can increase nearshore during these events. 

 During the 1958 El Nino event, the California 

 Current moved shoreward, and abundance of 

 potential larval fish predators (siphonophores 

 and chaetognaths) increased in inshore coastal 

 waters (Smith, 1985). Mean zooplankton biom- 

 ass, dominated by copepods, euphausiids, and 

 chaetognaths, was significantly greater in near- 

 shore than in offshore areas of Monterey Bay 

 during the first year of the 1991-93 El Nino 

 event (Baduini, 1995); this trend reversed af- 

 ter upwelling commenced in early April 1993. 

 Limited information on chaetognaths (largely 

 Sagitta bipu aetata) collected in our ichthyo- 

 plankton samples (Bridges 4 ) indicated ex- 

 tremely high numbers (e.g. up to 20,000 indi- 

 viduals/10 m 2 at 3 km from shore, which repre- 

 sented five times the greatest larval rockfish 

 abundance) in early February 1992, coincident 

 with the wind reversal, onshore transport of wa- 

 ter, and peak abundance of larval rockfish. At 

 least 20% of these animals were > 11 mm in length, 

 a size at which chaetognaths are capable of in- 

 gesting a larval fish (senior author, unpubl. data). 

 The lack of upwelling and associated offshore 

 transport during subsequent months could pro- 

 long nearshore retention and further reduce re- 

 cruitment success. 



Although predation on fish eggs and larvae has been 

 generally characterized as a density-independent con- 

 trol on fish populations (Bailey and Houde, 1989), ex- 

 ceptions include co-occurrence of predators and prey 

 concentrated in small areas (e.g. high density of her- 

 ring larvae and scyphomedusae [Moller, 1984]) and 

 prey-switching when preferred diet is not available 

 (Pepin, 1987). Although small copepods are the pre- 

 ferred prey of larval fish and many of their inverte- 

 brate predators, reduction in this food source during 

 El Nihos (McGowan, 1985; Miller et al., 1985) could 

 increase the likelihood of predation on small fish lar- 

 vae. Several species of chaetognaths prey secondarily 

 on young fish larvae (Kuhlmann, 1977), and an in- 

 verse relationship between larval fish abundance and 

 chaetognath densities has been described from 

 CalCOFI samples collected during the 1958 El Nino 

 off California (Alvariho, 1980). 



In contrast, increased upwelling intensity and off- 

 shore transport during March and April 1993 may 

 have facilitated survival of larval stages for those 

 species with broad periods of parturition by advect- 

 ing them from nearshore areas. Although there is a 



-2 -1.5 -1 -0.5 0.5 1 1.5 2 



Standardized monthly abundance anomaly 



Figure 8 



Comparison of mean monthly standardized anomalies of larval 

 rockfish abundance in January (J) and February (F) of various 

 years (indicated by last two digits of each year), with those of 

 upwelling indices estimated at 36° and 39°N and averaged over 

 a 4-day period prior to and including day of ichthyoplankton sam- 

 pling. Estimates of larval abundance prior to our study are from 

 CalCOFI ichthyoplankton surveys conducted inshore of station 

 60 along lines 63, 67, and 70 (Moser et al., 1993). Standardized 

 anomalies were calculated by using data from our study and the 

 CalCOFI surveys. 



significantly negative relationship (^=0.63; P<0.001 ) 

 between larval rockfish abundance and offshore Ek- 

 man transport (i.e. positive upwelling indices) dur- 

 ing peak production in January and February (Fig. 

 8), dispersal mechanisms like upwelling seem to be 

 important in determining recruitment to the juve- 

 nile stage (Ralston and Howard, 1995). Breaker 

 (1983) characterized the spring transition from win- 

 ter ocean conditions to the onset of upwelling and 

 offshore Ekman transport as an abrupt decrease in 

 SST (e.g. 0.38-0.59°C per day) off central California 

 (36°26'N ); dramatic spring transitions occurred in six 

 of the twelve years (1971-83), with suppression of 

 transition during El Nino events. According to ex- 

 aminations of daily SST's from the same station dur- 

 ing the 1991-93 El Nino event (Schwing 5 ), a spring 

 transition did not occur at all in 1992 and a modest 

 transition (0.34°C per day) was initiated in early April 

 1993, coinciding with the birthdate distribution of 

 the surviving cohort of shortbelly rockfish juveniles. 

 In conclusion, the extended El Nino event of 1991- 

 93 provided a unique opportunity to assess abun- 

 dance and distribution of larval rockfishes during a 



Bridges, J. 1994. Department of Biology, University of Cali- 

 fornia, Santa Cruz. CA 95064. Unpubl. data. 



5 Schwing, F. 1995. Pacific Fisheries Environmental Group, 

 Natl. Mar. Fish. Serv., Pacific Grove, CA. Unpubl. data. 



