AGE DEPENDENT FECUNDITY, NUMBER OF SPAWNINGS PER YEAR, SEX RATIO, AND MATURATION STAGES IN NORTHERN ANCHOVY, ENGRAULIS MORDAX

Abstract

ABSTRACf Maturity stage data from fishery sampling programs and ovarian histological data from research cruises were used to develop a method for determining the age-specific number ofspawnings per year and annual fecundity of the central stock of northern anchovy, Engraulis 7/lQrdax. The sex ratio was found to be size and age dependent in both the fishery and trawl surveys with females increasingly dominant in the larger and older size and age classes. The overall sex ratio in trawl surveys was nearly 1:1; the fishery data favored females 1.48:1. The magnitude and duration of maturity stages were size and age dependent with peak spawning occurring earlier in the season in younger fish. Maturity stages and histological classes with hydrated eggs showed essentially the same diurnal pattern for nightly spawning activity indicating that the presence of hydrated eggs could be used as an index of daily spawning. The daily spawning incidence and total annual fecundity were found to be heavily age dependent. Females in their first spawning season had an average of 5.3 spawnings, while those in their fourth had an average of 23.5 spawnings. When combined with the higher batch fecundity of larger fish this results in 4 + year-old females producing nearly 5 times as many eggs per unit of weight as l·year-olds. When the age-specific fecundity and sex ratio in the fishery are combined it is apparent that the catch of a ton of 4 + year-old northern anchovy reduces the reproductive potential of the stock 7.3 times as much as the catch of a ton of l-year-olds. It was concluded that age-specific fecundity in multiple spawning fishes is of greater significance for management than previously thought. It is also significant that much of the observed variance in stock-recruitment relationships for multiple·spawning fishes may be due to the fact that spawning biomass is a poor index of the egg production and reproductive potential of the stock. Age-specific variation in life history rates is a major factor in population and management models of ex­ ploited fishes, and variation in reproductive effort is of great significance in such models. Size and age­ specific batch fecundity estimates have been avail­ able for many species for decades, and for species which spawn only once per spawning season these are readily incorporated into models. However it has been impossible to determine the age-specific repro­ ductive effort of species which spawn many times during a spawning season because there has been no way to determine the number of spawnings per year. Recent research on the histology of the ovaries of northern anchovy, Engraul-is mot'd.ax, and an­ choveta, Engraulis ringens, suggest that they spawn approximately once a week during peak spawning months (Hunter and Goldberg 1980;