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Environment
Environment
Environment

Use of archival tags for studying the movement and swimming behaviour of school sharks

Project number: 1996-128
Project Status:
Completed
Budget expenditure: $257,962.00
Principal Investigator: John Stevens
Organisation: CSIRO Oceans and Atmosphere Hobart
Project start/end date: 16 Oct 1996 - 5 Apr 2001
:

Objectives

1. To determine the frequency and extent of movements of individual school sharks within the fishery leading to an improved understanding of spatial structure required for regional assessment of school shark stocks.
2. To determine whether the total population of mature female school sharks migrate to Tasmania and Victorian waters to pup and consequently whether recruitment is dependent on south-eastern pupping areas. Answering this question will also address the effectiveness of the current rolling fishery closures (aimed at protecting aggregating pregnant females) which are based on an eastward pupping migration.
3. To provide information on the swimming depth of school sharks and to estimate the amount of time they spend off the bottom and unavailable to commercial fishing gear. This information is required in assessing the relative impact of current fishing effort directed at school and gummy shark.

Final report

ISBN: 0-643-06226-2
Author: John Stevens
Final Report • 2001-02-12 • 11.90 MB
1996-128-DLD.pdf

Summary

There has been serious concern over the status of the school shark fishery for many years, and it has been the subject of numerous stock assessments (Punt & Walker 1998). It is a well researched species: Olsen (1954) showed that school shark can make extensive migrations, and that females moved to pupping grounds in the south-east of Australia to give birth. However, despite their apparent ability to move freely through the fishery, there are regional differences in size, age and reproductive condition, together with different catch-rate trends in different regions of the fishery. Originally stock assessment models were based on the assumption of a single freely-mixing stock, in light of the ability of the species to move long distances. However, Prince (1992) argued that the biological and catch data was incompatible with the assumption of a single stock, and was instrumental in having spatial structure in the fishery taken into account. Punt et al. (2000) developed a model that takes account of the spatial structure of the population, allows for multiple stocks and which uses catch-rate data and information from tagging studies to derive values for the parameters of the model.

In this study it was the longitudinal movements of the fish that were of greatest interest, and although daily longitude estimates from the archival tags were highly variable, weekly median estimates appeared reliable. Latitude estimates were based on bathymetry except around Tasmania where none were possible. One fish appeared to move to New Zealand, but the clock in the archival tag was found to be seriously defective when tested upon its return. Although conventional tagging shows movements of school shark between Australia and New Zealand, we conclude that there was no evidence of any movement of archival-tagged fish further east than 149–150° E. No school shark migrated across the entire range of the Australian fishery in the time they were at liberty (<1.5 y), and there was limited mixing between the eastern and western regions of the fishery in this time frame. The restricted movement shown by the archival-tagged sharks provides additional support for the move to spatially-structured stock assessments. Although these results seemed at odds with the impression of wide-ranging movements shown by earlier conventional tagging, re-analysis of tag returns from the 1947–56 tagging program (Olsen 1954) provided support for restricted longitudinal movements. For school sharks ≥ 95 cm total length at release tagged in Victoria and Tasmania, the percentage recaptured in South Australia did not peak until after 4–6 y at liberty for females (~70%) and 12–14 y for males (~40%). These results however depend upon the relative fishing effort in these two areas at the time.

Environment
Environment
PROJECT NUMBER • 1998-109
PROJECT STATUS:
COMPLETED

Risk analysis and sustainability indicators for prawn stocks in the Northern Prawn Fishery

This project has been highly successful at determining factors that affect the outputs and outcomes of the model and the uncertainty underlying the model system. The report has been divided into seven broad sections and two Appendices. The first chapter contains the context of the work;...
ORGANISATION:
CSIRO Oceans and Atmosphere Hobart
Environment
PROJECT NUMBER • 1997-139
PROJECT STATUS:
COMPLETED

Mesoscale oceanographic data analysis and data assimilative modelling with application to Western Australian fisheries

The more that is understood about the factors controlling the abundance of an exploited fish stock, the more optimally it can be harvested for sustainable yield and profit. It has been known for some time that catches of western rock lobster are closely related to the number of larvae surviving...
ORGANISATION:
CSIRO Oceans and Atmosphere Hobart
Environment
Environment
PROJECT NUMBER • 1997-108
PROJECT STATUS:
COMPLETED

Definition of effective spawning stocks of commercial tiger prawns in the NPF and king prawns in the eastern king prawn fishery: behaviour of post-larval prawns

To effectively manage most fisheries, including penaeid prawn fisheries in northern and eastern Australia, it is important to know the relationship between the size of the spawning population and the number of young adults that recruit to a fishery in the next generation. In the tiger prawn fishery...
ORGANISATION:
CSIRO Oceans and Atmosphere Hobart
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