Estimating lobster recruitment and exploitation rate from landings by weight and numbers and age-specific weights

Abstract

Aprocedure is presented for incorporating catch totals by both weight and numbers in stock assessment. Their ratio is the weight of an average harvested individual which, in turn, reflects mean mortality rate. The model is age-based and requires, as input, a vector of average age-specific weights in the catch. The model developed for the South Australian rock lobster (Jasus edwardsii) fishery assumes steady state, constant fishing mortality on all age classes and a natural mortality rate equal to 0·1. Also explicit are reduced vulnerability of recruitment-aged lobsters and incidental mortality of lobsters below the legal minimum size. The solution yields estimates of average exploitation rate and yearly recruitment. These give (absolute) age-specific population numbers in the fishable stock. Exploitation rates were estimated for statistical reporting blocks in South Australia using this catch weight–numbers (qR) method and, for comparison, that of Beverton and Holt employing mean length. Each method was also tested with simulated data, the former yielding estimates of lower variance and bias. In practice, counting individuals captured is likely to require less time and to yield lower measurement error than measuring lengths. This method thus offers the possibility for improved precision and accuracy at lower cost.