Developing tagging models and validating assumptions for estimating key fishery assessment parameters in rock lobster fisheries
There is a common need in the assessment of all fisheries to obtain precise estimates of exploitation rates and/or biomass. In the Tasmanian rock lobster fishery, biomass is a key performance indicator and change in biomass is the primary driver for recommendations of future Total Allowable Commercial Catch (TACC) allocations by the Fisheries Advisory Committee. In recognition of this need, both the Tasmanian
Government (1992-1997) and FRDC (1997-2001) have funded projects to investigate methods to estimate biomass independent of the fishery.
Fishery independent estimates are essential because estimates derived from fisher’s catch and effort are often biased as exemplified by the change in the link between catch rate and abundance following the introduction of the ITQ management system in Tasmania (FRDC 1999/140). Although initially promising, exploitation rate and biomass estimates based on changes in the population during the fishing season proved unreliable (FRDC 1997/101). Both the Industry and Managers have identified the need to explore new methods to obtain precise estimates of these important performance indicators.
Multi-year tagging models have been identified as a promising way of estimating these parameters because they address the failings (variable catchability and recruitment) in the population derived estimators. Initial trials in northwestern Tasmania proved very successful and support the potential of this approach. However, tag returns from fishers are unpredictable and there is an immediate need to
determine ways of improving tag reporting rate to optimise the output of tag based models. Furthermore, the models applied to northwestern Tasmania relied on a minimum of two tagging events occurring each fishing season. As each tagging event occurred during a different period of the moult cycle, there is a need to validate the tag retention and tag induced mortality rates associated with these different tagging times. Other factors relating to size, sex, damage and their interactions with different tagging times also need further investigation. A careful evaluation of other means of improving the precision of estimates from tagging models is seen as necessary, prior to the implementation of a large scale tagging program.
Validating the assumptions associated with the use of tagging models, demonstrating ways to test for these assumptions and the precision of fishery assessment outputs is needed prior to recommending these models for use in other fisheries.
