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.

Enhancing fishery assessments for an Australian abalone fishery using commercial weight-grade data

“…This information is rare in most fisheries, including those for abalone. These estimates of mean weight and the total number of abalone harvested could significantly improve any future fishery assessments or numerical modelling in this fishery, and in others where such weight-grade data are routinely available (McGarvey et al, 1997(McGarvey et al, , 2005McGarvey and Matthews, 2001).…”

Evidence of large-scale spatial declines in recruitment patterns of southern rock lobster Jasus edwardsii, across south-eastern Australia

“…These indicators are largely estimated from fishery-dependent data derived from commercial logbooks which became mandatory across south-eastern Australia during the 1970s. In addition, quota management controls are also influenced by outputs from stock assessment models that have been specifically developed for these fisheries McGarvey et al, 1997). More recently, a single length-frequency based model (LenMod) has been developed and adopted by each State for the purpose of management advice (Gardner and Ziegler, 2010;Linnane et al, 2009a,b).…”

Assessing the effectiveness of size limits and escape gaps as management tools in a commercial rock lobster (Jasus edwardsii) fishery

“…In addition, a total allowable commercial catch (TACC), in the form of individual transferable quotas, was introduced into the Southern Zone in 1993 and the Northern Zone in 2003. TACCs are set annually based on commercial catch rate trends as well as outputs from specifically developed stock assessment fishery models (McGarvey et al, 1997;Punt and Kennedy, 1997).…”