Incorporating numbers harvested in dynamic estimation of yearly recruitment: onshore wind in interannual variation of South Australian rock lobster (Jasus edwardsii)

Abstract

A method for deriving historical recruitment time-series is presented using annual total weight and total numbers landed and, optionally, effort data. The method is age-based and requires a vector of age-specific mean weights. Average weight of harvested individuals (catch weight divided by numbers) conveys information about size structure and thus about mortality rate and recruitment. Annual recruit numbers and fishing mortality were estimated in the Northern Zone fishery of South Australian rock lobster (Jasus edwardsii). Estimation accuracy was tested with simulated data sets from an individual-based lobster model for four patterns of recruitment variation. Recruitment time-series were accurately estimated for all four patterns. In application to the lobster fishery, effort data, in addition to catches by weight and number were employed. Model-derived recruitment time series from 1978–1998 were corroborated by the trend of reported annual numbers of lobsters captured below legal size. The principal outcomes are threefold: (1) catches by weight and number can yield accurate yearly recruitment estimates; (2) a rough 11-year cycle was evident in both recruitment and wind time-series; (3) yearly recruitment is correlated with August (mid-winter, the time of post-larval settlement) western wind strength five to seven years earlier.