A study to evaluate distribution and abundance of rockfish larvae and associated hydrography on small spatial (<20 km) and temporal (biweekly to monthly) scales was conducted in an upwelling center north of Monterey Bay during an extended El Niño. Secondary objectives were to assess hatchdate distributions of early larvae and pelagic juveniles (May-July) for common species, potential larval transport mechanisms, and subsequent strength and timing of nearshore juvenile recruitment. This project was supported by California Sea Grant Program, with collaboration among research biologists from NMFS-Pacific Fisheries Environmental Group (Mary Yoklavich), Moss Landing Marine Laboratories (Valerie Loeb, Mary Nishimoto, Brendan Daly, and Greg Cailliet), and California Department of Fish and Game (David VenTresca).
The prolonged El Niño resulted in anomalously warm, low salinity water to depths over 50 m in nearshore areas, suggesting onshore displacement of the California Current. Upwelling was reduced and delayed, without the usual persistent upwelling plumes in the survey area. Larval rockfish abundance was similar in both years (winter-spring of 1991-92 and 1992-93), peaking in early February, and was among the highest when compared with CalCOFI estimates from central California 1951-1984. Based on larval ages, the dominant species (shortbelly rockfish) hatched from December to April, with median hatchdates in mid-February both years. Growth rates of shortbelly rockfish larvae did not differ among months or between years. Relative to 1993, however, rockfish larvae were more abundant and size of larval shortbelly was significantly larger at onshore stations in 1992. This coincided with onshore advection of water (i.e. reversals in wind direction; warmer and less saline water) during the onset of the El Niño event, suggesting nearshore retention of larvae. Initiation of upwelling in March and April of 1993 and fewer larvae at onshore stations suggest greater offshore transport during the second year.
Extremely low numbers of pelagic juvenile rockfish were encountered in summer months of 1992 (2.3 fish/trawl in 56 midwater trawls). In 1993 pelagic juveniles were 20 times more abundant (75.1 fish/trawl in 64 midwater trawls), comprised twice as many taxa, and were substantially larger than in 1992. Surviving juvenile shortbelly rockfish were born late in the season during both years (median hatchdates of 21 April 1992 and 31 March 1993); upwelling occurred coincidentally during this period in 1993 but not in 1992. Limited information on abundance of chaetognaths, a potential predator, indicated high numbers nearshore in February 1992. We suggest that much higher survival and recruitment of juveniles in 1993 likely was due to increased offshore transport and perhaps lower predation during the larval stages. This paper is scheduled for publication in Fishery Bulletin 94 (4). (M. Yoklavich, [831] 648-9036).