SEASONAL VARIATION IN SURVIVAL OF LARVAL 



NORTHERN ANCHOVY, ENGRAULIS MORDAX, ESTIMATED 



FROM THE AGE DISTRIBUTION OF JUVENILES 



Richard D. Methot, Jr. 1 

 ABSTRACT 



Juvenile northern anchovy, Engraulis mordax, collected during autumn of 1978 and 1979 were aged using 

 daily increments in their otoliths. Neither year class was dominated by individuals born during some short 

 period, but March and April had the highest frequency of births in each year. Monthly ichythoplankton sur- 

 veys indicated that significant spawning occurred from January through May of each year and peaked in early 

 March. Comparison of the temporal distribution of birth dates with larval abundance indicated that larval 

 survival was similar in the first half of each spawning season and greater during April to May of the 1978 

 spawning season than the same period in 1979. This difference in seasonal pattern of survival was nearly suf- 

 ficient to account for the observed greater recruitment in 1978 and is consistent with the hypothesis that 

 offshore transport of larvae influences recruitment. 



One goal of fish population dynamics is to under- 

 stand the processes responsible for annual variation 

 in recruitment. The variation can be more than an 

 order of magnitude and is poorly correlated with 

 abundance of spawners (Cushing and Harris 1973). 

 The concept of a critical period during the early larval 

 stage (Hjort 1926; Marr 1956; May 1974) has struc- 

 tured much of the research. Recent work has focused 

 on the importance of temporal and spatial coin- 

 cidence of first feeding larvae and concentrations of 

 prey (Beyer and Laurence 1981; Lasker 1978; 

 Vlymen 1977). However, transport of larvae away 

 from juvenile nursery areas can influence recruit- 

 ment (Nelson et al. 1977; Parrishetal. 1981) and the 

 role of predation is unknown. 



Seasonal variation in factors that cause annual 

 variation in recruitment probably influences the 

 average timing of spawning. Support for this 

 hypothesis comes from the latitudinal correlation 

 between duration of the spawning season and the 

 plankton bloom (Wyatt 1980). More direct evidence 

 is found within the North Sea where the short spawn- 

 ing season of each herring population bears a fixed 

 phase relation to the mean date of the local plankton 

 bloom (Cushing 1975). The timing of the most favor- 

 able environmental conditions may not be predict- 

 able in each year. The match- mismatch hypothesis 

 (Cushing 1975) suggests that variation in the relative 

 timing of spawning and the seasonal plankton bloom 

 contributes to variation in recruitment. 



1 Southwest Fisheries Center La Jolla Laboratory, National Marine 

 Fisheries Service, NOAA, 8604 La Jolla Shores Drive, La Jolla, 

 CA 92038. 



Collection of sufficient years of data to test any re- 

 cruitment hypothesis is difficult. However, a testable 

 corollary of Cushing's hypothesis is that, in any year, 

 larvae born during favorable environmental periods 

 constitute most of the year class. The age distribu- 

 tion of juveniles — the survivors of the larval stage — is 

 a function of the seasonal distribution of spawning 

 and seasonal changes in larval survival. To test the 

 match-mismatch hypothesis, the birth dates of 

 juvenile northern anchovy, Engraulis mordax, were 

 determined from daily increments in otoliths 

 (Brothers et al. 1976; MethotandKramer 1979; Pan- 

 nella 1971) and compared with the seasonal distribu- 

 tion of spawning determined from ichthyoplankton 

 surveys conducted during the 1978 and 1979 spawn- 

 ing seasons. 



METHODS 

 Larval Abundance 



The seasonal distributions of the northern an- 

 chovy larval abundance were estimated from ichthyo- 

 plankton surveys (Kramer et aL 1972) on the 

 sampling grid (Fig. 1) of the California Cooperative 

 Oceanic Fisheries Investigations (CalCOFI). Seven 

 surveys were conducted between December 1977 

 and August 1978 and four surveys between January 

 1979 and May 1979. Only larvae from 2.6 mm (live 

 standard length at hatch) to 5.1 mm (few days after 

 yolk absorption and onset of feeding) were used in 

 the analysis. 



Each cruise's larval census was the summed abun- 



Manuscript accepted Februaiv 1983. 

 FISHERY BULLETIN: VOL.' 81, NO. 4. 1983. 



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