Life history, growth and production of Neomysis integer in the Westerschelde estuary (S.W.

ABSTRACT: The Neomysis integer (Leach, 1814) (Crustacea, Mysidacea) population of the brackish part of the Westerschelde estuary was sampled on a fortnightly basis from November 1990 to December 1991. Denslty, biomass, population structure and brood size were recorded. The Bhattacharya method was applied to the length-frequency data for the detection and separation of cohorts. Growth is described both by a generalised von Bertalanffy function and by a von Bertalanffy function incorporating seasonal oscdlations in growth. Secondary production was estimated for each cohort using 4 approaches. The seasonal pattern in density and biomass showed 3 peaks: a relatively small, yet distinct, peak in early March (30 ind. m-2, 60 mg AFDW m-*) and 2 main peaks in late spring (160 ind. m-'. 225 mg AFDW m-2) and in summer (140 ind. m-2. 125 mg AFDW m-2). Throughout winter, N. integer density remained well below 30 ind. m-2 Three periods of increased reproductive activity and subsequent input of juveniles were found. Thls suggests that 3 cohorts were produced per year. The overwintering generat~on lived from autumn until the following spring. The spring generation was born in early spring and lived for about 3 mo, while the summer generation lived from summer until early winter. The 3 cohorts showed marked differences in their biology. The overwintering generation showed seasonal growth oscillations, larger brood size and a larger size at maturity. Individuals belonging to the other 2 cohorts generally grew faster, produced less young per female, and attained maturity at a smaller size. Within each cohort, both sexes exhibited different growth characteristics: females generally lived longer, grew faster and consequently became larger than males. The size-frequency, growth summation and removal summation methods yielded comparable production estimates. The annual production was 0 3 g AFDW yr-' with an annual P/B ratio of 6. The average cohort P/B was 3. The size-frequency method gave similar results only when applied to the 3 cohorts and to both sexes separately. The spring cohort accounted for almost half of the annual production. Despite the longer life span of the overwintering generation, it generated only a quarter of the annual production. An independent estimate of product~on using the mortality rate of the different cohorts resulted in values comparable to those obtained by the other methods for the overwintering cohort, while the production of the other 2 cohorts was overestimated