The application of fecundity estimates to determine the spawning stock biomass of Irish Sea Nephrops norvegicus (L.) using the annual larval production method

Ovigerous female Nephrops were collected by trawl and creel and maintained in individual containers over the nine-month incubation period to investigate aspects of fecundity. Females which extruded eggs in captivity shortly after capture provided an estimate of mean realised fecundity of 104.3 eggs g(-1) live weight (s.e.=2.7). Egg loss during incubation, estimated by sampling ovigerous females from research trawl hauls at different times of year, was 36.7 eggs g(-1) (s.e.=3.3). A similar value was obtained by monitoring individuals kept in the hatchery. Mean effective fecundity at the time of hatching was estimated as 67.6 eggs g(-1) (s.e.=4.3) and is the difference between the number of eggs extruded (realised fecundity) and egg losses during incubation. The abundance of larvae at development stages I, II, and III was estimated over a series of surveys using high-speed plankton samplers. Abundance values were converted to daily production values using relationships between temperature and stage-duration. Annual larval production by stage was estimated by fitting Gaussian curves to the survey estimates. The mortality rate of larvae (Z=0.033 d(-1); s.e.=0.006) was estimated from the values of annual production by stage, using a maximum likelihood method. Annual production at hatching, estimated as the intercept of the mortality curve, was 440 x 10(9) larvae (s.e.=62 x 10(9)). The biomass of mature female Nephrops in the western Irish Sea in 1995 was estimated from annual larval production and mean fecundity to be 6290 t (CV=0.17). The current ICES estimate of female SSB in 1995 of 7750 t, obtained from analysis of commercial catch data, lies within the 95% confidence limits for the ALP estimate. This indicates that current estimates of fishing mortality for female Nephrops in the Irish Sea may be robust.