Size-based estimates of survivorship for the South African rock lobster Jasus lalandii

Abstract

Natural survivorship estimates ( s ̂ ) for the South African west coast rock lobster, Jasus lalandii, are obtained by regressing a size-based population model to pristine population size structure information. The model assumes that juvenile settlement and somatic growth rates are invariant from one year to the next. The pristine size structure information that is used is obtained from unfished protected populations (sanctuary populations), and from catches made at the beginning of the fishery. A total of ten different size structure datasets are investigated. The age of the sanctuary at the time the data were collected is shown to be critical when estimating the natural survivorship. A dynamic analysis is carried out to adjust for the bias due to the age of the sanctuary. Once the age of sanctuary effect is taken into account, the estimates of survivorship vary from 0.88 to 0.95. The precision of the natural survivorship estimates is calculated, and the sensitivity of the estimates to somatic growth levels, as well as to other parameters and model assumptions is determined.