Evaluating Harvest Strategies for a Rapidly Expanding Fishery: The Australian Broadbill Swordfish Fishery

Evaluating Harvest Strategies for a Rapidly Expanding Fishery: The Australian Broadbill Swordfish Fishery

Robert A. Campbell and Natalie A. Dowling

Evaluating Harvest Strategies for a Rapidly Expanding Fishery: The Australian Broadbill Swordfish FisheryThis is part of Fisheries Assessment and Management in Data-Limited Situations
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Description

The Australian longline fishery, operating off eastern Australia, expanded rapidly in the mid- to late 1990s with swordfish catches increasing from around 50 t to over 3,000 t. Combined with New Zealand catches, the present swordfish harvest in the southwest Pacific is several times greater than historical catches by Japanese longliners that fished in the region. While comprehensive catch and effort data exist, uncertainty remains about the biology and productivity of broadbill swordfish in the region. With declines in swordfish catch rates in recent years, the Australian Fisheries Management Authority has sought advice on sustainable harvest strategies for the fishery, including total allowable effort levels.

Given the limitations and uncertainties in the available information, a management strategy evaluation (MSE) framework has been developed for swordfish in the southwest Pacific to evaluate alternative future harvest strategies. The operating models incorporate multiple fleets and areas to account for differences in targeting practices and hypotheses about seasonal swordfish movements. Catchabilities are fleet and area specific, with parameters describing changes in targeting practices over time. The model is conditioned on historical information, which includes catch and size frequency data.

The results indicate that large increases in the combined effort of both the Australian and foreign longline fleets would decrease the proportion of large fish in the catch and place the stock at a high risk of being overfished. The results were most sensitive to the assumed level of present depletion and the degree of spatial movement, the latter result highlighting the need to develop area-specific performance indicators if movement is limited. The use of an empirical decision rule to adjust effort levels slowed stock depletion, but may not allow the stock to rebuild if it is already depleted.

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