Estimating biomass and production of pelagic larvae of brachyuran decapods in western European shelf waters

Abstract

Methods are described for estimating the dry weight biomass, production due to growth, production due to moults, flux to the plankton from the benthos of hatching zoeae and from the plankton to the benthos of settling megalopae of common Brachyura from north-western European waters. These methods are applied to brachyuran larvae taken in Continuous Plankton Records in the Irish Sea during 1981–1983 and from the Longhurst Hardy Plankton Recorder over Great Sole Bank in June 1983. The former provided estimates of input of hatching zoeae to the plankton at 10 m depth from the benthos of 0.41 mg dry weight · m −3 · a −1 with a peak in April, a mean standing stock of 0.19 mg · m −3 with a maximum in May. The peak in production was in July and the annual ratio of production to biomass ( P: B) was 23.2:1 excluding moults, or 26.1:1 including moults. The estimated flux of settling megalopae from the plankton at 10 m depth to the benthos was 1.33 mg · m −3 · a −1 with a peak in July. The data from the LHPR haul gave instantaneous daily values of an input of 0.004 mg · m −2 · day −1 to the plankton and a mean biomass 6.696 mg · m −2 of brachyuran larvae in the plankton. The daily P: B ratio was 0.067:1 excluding moults and 0.073:1 including moults and an estimated 0.07 mg · m −2 · day −1 was returning to the benthos.