HEWm AND METHOT DISTRIBUTION AND MORTALmY zyxwvuts OF NORTHERN ANCHOVY LARVAE CalCOFI Rep., Vol. XXIII, zyxwvutsrqpon 1982 DISTRIBUTION AND MORTALITY OF NORTHERN ANCHOVY LARVAE IN 1978 AND 1979 ROGER P. HEWITT AND RICHARD D. METHOT. JR National Oceanic and Atmospheric Administration National Marine Fisheries Service Southwest Fisheries Center La Jolla. California 92038 ABSTRACT Eleven ichthyoplankton surveys, conducted during the winter and spring quarters of 1978 and 1979, indi- cate that spawning of the northern anchovy, zyxwvutsrq Engraulis rnordax, was contracted both spatially and temporally in 1978 relative to 1979. Larvae were farther offshore in 1979. Instantaneous daily larval mortality rate esti- mated from slope of the age-frequency distribution (yolk sac through 15 mm, ca. 25-30 days) varied in- significantly between 1978 and 1979 and averaged 0.168. Comparison to a simple model of spawning suggests that seasonal changes in the slopes are due solely to a systematic bias and not to real changes in the mortality rate. Seasonal larval production was compared with birthdate distributions of fish surviving to the juvenile stage. In both years March had the greatest larval pro- duction and was the commonest month of birth among the survivors of the larval stage. Larval survivorship tended to increase within the 1978 spawning season and decrease within the 1979 season. Low survivor- ship in winter 1978 is consistent with the hypothesis that winter storms disrupted aggregations of prey necessary for larval feeding, but does not explain why apparent recruitment was greater in 1978 than 1979. Variation in larval survivorship could not zyxwvutsr be attributed to larval mortality. The offshore distribution of larvae in 1979 may have contributed to the relatively low survival. RESUMEN Once estudios de ictioplancton realizados durante 10s trimestres de invierno y primavera en 1978 y 1979 indican que el desove de la anchoveta zyxwvutsrq Engraulis mor- dax en 1978 se produjo relativo al de 1979, tanto es- pacialmente como temporalmente. Las larvas estaban mas lejos de la costa en 1979. El indice de mortalidad instantanea diaria de larvas que se estimo de la in- clinacion de la distribucion de frecuencias de edades (vitelo hasta 15 mm, alrededor de 25-30 dias) vario insignificativamente entre 1978 y 1979, con un pro- medio de 0.168. Una comparacion con un modelo simple de desove sugiere que cambios temporales en las inclinaciones se deben unicamente a un sesgo sis- tematico y no a cambios verdaderos en el indice de mortalidad. [Manuncnp received March I, 1982 1 226 La produccion larval temporal fue comparada con distribuciones de fechas de nacimiento de peces que sobrevivieron hasta la etapa juvenil. En ambos aiios, la mayor produccion de larvas occurrio en marzo, y el haber nacido en este mes era mas comun entre 10s sobrevivientes de la etapa larval. La supervivencia larval tendia a aumentar en la epoca de desove de 1978 y a descender en la Cpoca de 1979. La supervivencia baja del invierno de 1978 es consistente con la hi~tesis de que tormentas de invierno interrumpieron conjuntos de presas necesarias para alimentar las lar- vas, pero no explica porque el reclutamiento era mayor en 1978 que en 1979. La variacion en la super- vivencia larval no se pudo atribuir a la mortalidad larval. La distribucion de larvas fuera de la costa en 1979 pudo haber contribuido a la supervivencia re- lativamente baja. INTRODUCTION The planktonic phase of a schooling fish’s life his- tory is considered the most amenable to quantitative sampling (Smith and Richardson 1977). Egg and lar- vae surveys have been used to estimate the number of adults responsible for their production (e.g., Sette and Ahlstrom 1948; Saville 1964; Smith 1972). Fish larvae are also of interest because they are the link between the present adult stock and some future recruitment to the adult stock. The lack of a clear relationship be- tween stock and recruitment has focused attention on events during the larval stage and their ultimate effect on survival to the juvenile and adult stages. The literature on the pelagic fishes of the California Current is particularly rich. Smith (1981) sum- marized the influences on northern anchovy larval survival: (I) the availability of suitable prey for larvae exhausting their yolk sacs (Lasker 1978); (2) in- terspecific and intraspecific predation (Hunter 1976; Hunter and Kimbrell 1981); (3) starvation (Hunter 1976; O’Connell 1980); (4) effect of adult nutritional state on quality of eggs and fitness of larvae (Smith and Lasker 1978; Hunter and Leong 1981); (5) pre- schooling dispersal (Smith 1973; Hewitt 1981);and (6) larval transport to or from favorable areas (Sette 1950; Parrish et al. 1981). In this report we describe the results of ichthyo- plankton surveys conducted in 1978 and 1979, and discuss what may be inferred about factors affecting larval anchovy survival during those 2 years. Distri-