The IUCN/SSC Shark Specialist Group
Shark News 12: November 1998
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Australia's Southern Shark Fishery
Goes Spatial
André Punt, CSIRO Division of Marine Research, and Terry Walker,
Marine and Freshwater Research Institute, Australia
The fishery for school shark Galeorhinus galeus off southern Australia
has operated for about 70 years. During that time, it has developed
from a fishery based in Bass Strait and off eastern Tasmania to one that
now extends through South Australia to about 100 miles into southern
Western Australia (Figure 1). Trends in catch rate for different parts of
the fishery differ substantially, even though tagging studies have
shown that some school sharks are capable of long distance migrations.
An assessment of the school shark stocks in southern Australia is
currently being revised using a model that makes the spatial-structure
of this fishery explicit. The Southern Shark Fishery Assessment Group
(SharkFAG) has developed the model and is considering a variety of
alternative hypotheses in a cooperative manner. SharkFAG consists of
biologists, modellers, shark fishers, an economist and the manager of
the Southern Shark Fishery. This cooperation has meant that the
assessments and their results have considerably greater support than
would have been the case if scientists did the analyses without
external input.
Overview of the Model
Like previous assessments of these stocks (e.g. Punt and Walker
1998), the model is designed to capture the underlying peculiarities
of shark populations and fisheries by considering multiple gear-types
(hooks, and various sizes of gill-nets) and by explicitly modelling the
pupping and recruitment processes.
For modelling purposes, the fishery has been divided into eight
"regions" (Figure 1) based on the size/age-structure of the population
and the history of exploitation. The model allows for multiple stocks
to enable consideration of a wide range of hypotheses regarding
stock-structure and movement. A recent extension to the model
allows sharks from New Zealand to move to Australian waters. In this
extension, New Zealand sharks therefore form part of the Australian
catch. This extension was motivated by the large number of
recaptures in Australia of school sharks tagged in New Zealand.
The model uses a monthly time-step to mimic shark movement
dynamics effectively. Movement is modelled as being the
probability of a shark of a given age and stock in a given region
moving to another region. The movement probabilities are currently
selected to represent both large-scale pupping and feeding migrations
as well as random movement. The initial choices for the movement
probabilities are based on the output from an individual-based
movement model that operates on a daily time-step and considers
movements among 1° square blocks (Taylor 1997a, b). The
assessment model incorporates features that permit these initial
probabilities to be modified to fit the data better.
Data Included in the Model
Previous assessments of school shark have been based on either
tagging data (e.g. Olsen 1954, Grant et al. 1979) or on trends in
catches and catch-rates (e.g. Punt and Walker 1998). However, the
current assessment incorporates all of these sources of information
and, because of its spatial nature, fits the model to trends in catch-rate
by region rather than to the trend in catch-rate aggregated over the
whole fishery. Although not currently included in the model, the
possibility exists for incorporating data on the size-/age-structure of
the catch and trends in the mean mass of sharks in the catch.
Results and Further Work
Given the model's complexity, it is important to select ways in which
to summarise the model output succinctly yet in an easy to follow
manner. Currently, the results are presented as tabular or graphical
summaries, but work is underway to develop a graphical interface to
the model along the lines suggested by Walters (1995).
Figure 2 shows fits of the model to trends in catch-rate for the four
regions for which reliable effort data are available. The model-estimates
of catch-rate differ among the regions because of differences
in the underlying population structure and the mix of gear-types used
in each region. Plots of observed and model-predicted tag returns (by
year, and by the distribution of recaptures among regions) give further
confidence in the ability of the model to mimic the actual data and
hence make reliable predictions.
This modelling work has been conducted primarily to investigate
the implications of spatial structure for the management of the
resource. Another objective for developing a complicated model of
school shark dynamics is to have a basis for testing the robustness of
simpler assessment methods (Punt 1992) and to assess the value for
management of additional monitoring (McDonald and Smith
1997). The model framework is relatively general and will be used
in the future for an assessment of gummy shark Mustelus antarcticus.
References
Grant, C.J., Sandland, R.L. and A.M. Olsen. 1979. Estimation of
Growth, Mortality and Yield per Recruit of the Australian School
Sharks, Galeorhinus australis (Macleay), from tag recoveries.
Australian Journal of Marine and Freshwater Research 30:
625-637.
McDonald, A.D. and A.D.M. Smith. 1997. A Tutorial on Evaluating
Expected Returns from Research for Fishery Management. Natural
Resource Modelling 10(3): 185-216.
Olsen, A.M. 1954. The Biology, Migration, and Growth Rate of the
School Shark, Galeorhinus australis (Macleay) (Carcharhinidae)
in South-Eastern Australian Waters. Australian Journal of Marine
and Freshwater Research 5: 353-410.
Punt, A.E. and T.I. Walker. 1998. Stock Assessment and Risk Analysis
for the School Shark off Southern Australia. Marine and Freshwater
Research 49(7): 553-572.
Punt, A.E. 1992. Selecting Management Methodologies for Marine
Resources, with an Illustration for Southern African Hake. South
African Journal of Marine Science 12: 943-958.
Taylor, B.L. 1997a. Computer Software Tool for Displaying Tag
Release-Recapture Data from the Australian southern shark fishery.
p. 53-56. In: Fisheries Research and Development Corporation.
"Southern Shark Tagging Project". Walker, T.I., Brown, L.P., and
N.F. Bridge (Eds) (Marine and Freshwater Resources Institute:
Queenscliff, Victoria, Australia).
Taylor, B.L. 1997b. Movement modelling shell for the school shark
(Galeorhinus galeus) in the Australian southern shark fishery. p.57-61. In: Fisheries Research and Development Corporation.
"Southern Shark Tagging Project". Walker, T.I., Brown, L.P., and
N.F. Bridge (Eds) (Marine and Freshwater Resources Institute:
Queenscliff, Victoria, Australia).
Walters, C.[J.] 1995. Use of gaming procedures in evaluation of
management experiments. Canadian Journal of Fisheries and
Aquatic Science. 51: 2705-2714.
André E. Punt, CSIRO Division of Marine Research, GPO Box 1538,
Hobart, Tasmania 7001, Australia
Fax: (+61) 3 6232 5000.
Email: andre.punt@marine.csiro.au
and
Terence I. Walker, Marine & Freshwater Research Institute,
PO Box 114, Queenscliff, Victoria 3225, Australia
Email: terry@msl.oz.au
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