Assessment of broad-scale exploitation rates and biomass estimates for the Tasmanian southern rock lobster fishery

Project Number:



University of Tasmania (UTAS)

Principal Investigator:

Stewart Frusher

Project Status:


FRDC Expenditure:




Assessment of broad-scale exploitation rates and biomass estimates for the Tasmanian southern rock lobster fishery

Final Report
Author(s):Stewart Frusher
Date Published:March 2003

Exploitation rate is an important fishery assessment parameter linking catch to legal-sized biomass, the portion of the stock available for harvest.  Relative change in legal-sized biomass is a crucial performance indicator for the fishery as it measures the success of management outcomes.  Under the recently introduced Individual Transferable Quota Management System (ITQMS) in the Tasmanian rock lobster fishery, rebuilding of legal-sized biomass is a key management objective.  The assessment model that produces biomass estimates for this fishery is primarily dependent on commercial catch and effort data. 

The use of commercial catch and effort data for stock assessment relies on its de facto relationship with stock abundance.  However, the relationship between catch and effort data and abundance is not always constant or linear.  Improvements in fishing gear and technology can result in greater catch for a given amount of effort, unrelated to changes in the biomass.  Management changes and fishers’ behaviour can also affect the relationship between catch rates and biomass.  Under the new ITQMS introduced in 1998, catch is fixed and improved profits can be made by improving the return per unit of fish caught rather than by increasing the amount of catch through increased effort.  Thus fishing during periods when catch rates are low but price is high can change the catch effort relationship independent of biomass change.

Fishery independent surveys, using established sampling protocols and standardised fishing gear are a way in which catch rates can be standardised irrespective of gear efficiencies or fisher’s behaviour.  If these surveys can also produce accurate estimates of exploitation rate then accurate estimates of biomass can be achieved, provided the exploitation rate estimates are representative of the fishing grounds.  Fishery independent estimates of exploitation rate are thus a valuable way of validating model biomass estimates especially with the introduction of an ITQMS where the relationship between catch rates and legal-sized biomass was likely to change pre- and post-quota.

This project aimed to trial change-in-ratio (CIR) and index-removal (IR) techniques to obtain estimates of exploitation rate and biomass from broad scale regions in the fishery.

Keywords: southern rock lobster, change-in-ratio, index-removal, exploitation rates, tagging.


1. To assess the precision of exploitation rates and biomass estimates derived from broad scale sampling using fisheries independant and fishery dependant sampling.

2. To evaluate both the precision and cost effectiveness of biomass estimation from fisheries dependant and fisheries independent derived exploitation rates and recommend future monitoring methodology for the rock lobster fishery.