McGOWAN: SPAWNING OF NORTHERN ANCHOVY 



OCTOBER 1978 



APRIL 1979 



CHANNEL N=97 



10 15 20 25 30 



STANDARD LENGTH CLASS (mm) 



35 



Figure 5.— Length-class frequencies of larvae and juvenile northern anchovies for October 1978 and April 1979 showing 

 the different sizes caught in the channel versus those in shallow water. 



(Smith and Lasker 1978). The northern subpopula- 

 tion spawns off Oregon and Washington from mid- 

 June to mid-August when 1 m temperatures are 

 14°-17°C (Richardson 1980). These two subpopula- 

 tions overlap at San Francisco (Vrooman et al. 1981) 

 and the spawning season in the Bay overlapped the 

 spawning seasons of both subpopulations. But 

 spawning in the Bay took place at higher tempera- 

 tures than usual for either population in the ocean 

 (13°-18°C, Brewer 1976). Few eggs were taken in 

 the Bay from December 1978 to March 1979 when 

 water temperature was below 13°C. However, at 

 station 3 in March 1979, 477 eggs were taken at a 

 water temperature of 11.5°C. Peak spawning in the 

 Bay was in July, August, and September when the 

 mean water temperature was 19.0°, 19.8°, and 

 19.2°C, respectively. The highest catch of eggs oc- 

 curred at station 2 in July at 21.0°C. Eggs were also 

 plentiful at station 1 in August at 22.5° C. During 

 June, July, and August, eggs were least abundant 

 at stations 4 and 6, where water temperature was 

 relatively low. During September and October, egg 

 densities at stations 4 and 6 peaked, as did water 

 temperature at these stations. Sitts and Knight 

 (1979) found larvae shorter than 4 mm at 18°-22°C 

 in the Sacramento-San Joaquin estuary in July and 

 August. Although much of the northern anchovy 

 spawning took place in the Bay within the previously 

 reported temperature range and some took place at 

 low temperatures, most occurred in water warmer 



than in the coastal spawning regions. The strong 

 correlation of egg abundance with temperature in- 

 cludes potential confounding effects of presumed 

 seasonal influx of adults, apparent "preference" for 

 spawning within the Bay, and differences in dilu- 

 tion due to tidal exchange which affected stations 

 4 and 6 more than the other stations. Therefore the 

 correlations are descriptive, perhaps predictive, but 

 not causal. 



In the California Current, temperature, upwell- 

 ing, and stable stratification of the water column 

 are thought to interact to produce favorable condi- 

 tions for anchovy larvae (Lasker 1975). In San Fran- 

 cisco Bay there is no upwelling, but salinity or fresh- 

 water outflow variability might influence ecological 

 conditions. Freshwater flow may have an indirect 

 effect by promoting blooms of certain phytoplankton 

 or by retaining particles through estuarine circula- 

 tion (Cloern 1979). Relatively high salinity coincided 

 with warm temperatures at the beginning of the 

 spawning season, but spawning ceased in Novem- 

 ber when water temperature decreased to 13 °C, 

 although salinity remained high until February. Sitts 

 and Knight (1979) found larvae shorter than 10 mm 

 at low salinity (<10%o) and relatively high temper- 

 ature (>18°C). They found only large larvae (>10 

 mm) in November when water temperature fell 

 below 13°C. 



In this study, only temperature had a strong direct 

 relationship with abundance of eggs and larvae; 



889 



