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.

In Tasmania, there is a paucity of information on the life-history, population parameters and habitat requirements of fish associated with inshore soft-sediment habitats, particularly seagrasses. Clearly, such information is needed before stock assessment models can be developed, recruitment processes understood, key habitats identified and appropriate management measures developed to minimise impacts on these habitats. In order to examine the structure of fish communities in coastal soft-sediment habitats around Tasmania, the demersal and larger mobile fish fauna were routinely sampled from three areas-Norfolk Bay, Georges Bay and Prosser Bay. In each area, representative unvegetated (mud and sand) and seagrass habitats between 1 and 12 m deep were sampled seasonally.

In Tasmania, there is a paucity of information on the life-history, population parameters and habitat requirements of fish associated with inshore soft-sediment habitats, particularly seagrasses. Clearly, such information is needed before stock assessment models can be developed, recruitment processes understood, key habitats identified and appropriate management measures developed to minimise impacts on these habitats. In order to examine the structure of fish communities in coastal soft-sediment habitats around Tasmania, the demersal and larger mobile fish fauna were routinely sampled from three areas-Norfolk Bay, Georges Bay and Prosser Bay. In each area, representative unvegetated (mud and sand) and seagrass habitats between 1 and 12 m deep were sampled seasonally.

The fish fauna associated with Heterozostera seagrass and unvegetated habitats were compared in all three areas. Seagrass sites had a significantly higher abundance of demersal fish and a distinct community compared to unvegetated sites in Norfolk Bay and Georges Bay. In contrast, neither abundance or community composition differed between habitats in Prosser Bay. This pattern may be attributed to the patchy distribution of seagrass beds that result from the higher degree of exposure of the bay and the significant loss of beds over the past 20-30 years. Demersal fish in seagrass beds were dominated by small resident species, while those in unvegetated habitats were dominated by juveniles of larger species. Few larger more mobile species showed a distinct habitat preference. Unvegetated habitats were found to be more important than seagrass as a nursery area for juveniles of commercially important finfish species.