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Modelling for fishery management, utilising data for selected species in Western Australia

Hall, Norman George (2000) Modelling for fishery management, utilising data for selected species in Western Australia. PhD thesis, Murdoch University.

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The work submitted in this thesis applies fisheries modelling techniques to three of the most important fisheries in Western Australia in order to assess the status of their stocks and to improve our understanding of their response to fishing.

A model of the tiger prawn stock in Shark Bay assumed that the varying proportions of prawns that recruited to the overall fishery in the different calendar months remained the same in each year and that the different contributions to recruitment at each of the six fishing grounds in each month also remained the same in each year. Based on field studies, constant proportions of the prawns were assumed to migrate northward each month between adjacent fishing grounds, but with no migration northward from the most northern ground. The parameter estimates from the model indicated that most prawns recruited to the fishery between January and April, with recruitment peaking in March. Recruitment was greatest into the Peron fishing ground and least into the Western Ground. The proportions migrating increased from the southern grounds to the northern grounds. The time series of estimates of annual recruitment indicated that recruitment underwent a marked decline in 1980, which is consistent with the recruitment failure experienced by this stock in the early 1980s. Annual recruitment gradually recovered to reach a peak in 1995, but then declined with the result that, by 1998, it was only 56% of the 1995 level. The temporal and spatial resolution of the model is now at a level that is consistent with the regulations that have been applied by fishery managers to reduce the level of exploitation on tiger prawns and to allow the stock to rebuild.

An age-structured model of the pilchard fisheries off the southern coast of Western Australia was fitted separately to each of the three assemblages that comprise the pilchard stock. Estimates of spawning biomass derived from egg production surveys and samples of the age compositions of the annual catch were used to calibrate the model. The model produced estimates of natural mortality, vulnerability at age, annual recruitment and annual biomass. The natural mortality estimate that had been used in earlier assessments of the pilchard fishery was found to produce estimates of the spawning biomass that were inconsistent with the observed data, and that a higher level of natural mortality was required to produce estimates that were appropriate. The natural mortality of pilchards was estimated for each region, together with the profile likelihood distribution for the natural mortality estimate. The current biomass within each region was estimated to be at a very low level, and a fishery closure from July 1999 was expected to result in little recovery by 2000.

In 1992-93 and 1993-94, managers of the western rock lobster fishery of Western Australia introduced strong measures to reduce the exploitation of female rock lobsters. This manipulation of the fishery by its managers offered an opportunity, through the resulting contrast in the fishery data, to explore the fishery’s response to these measures and thereby extract information on the current status of the rock lobster stock within each of the management zones of the fishery. It also enabled an evaluation to be made of the success of the regulations in achieving the objectives for which the regulations had been introduced. Using an age-structured model of the fishery, egg production in 1992-93 was estimated to have been 12% (95% C.L. 6 to 20%) of the original unfished egg production, but to have increased to 21% (95% C.L. 13 to 29%) by 1999-2000, when the target was 17%. The consequence of a change in management in 2001-2002 was examined. The risk of egg production in 2002-2003 falling below the target level of 17% increased from 10 to 33% with the proposed change.

Item Type: Thesis (PhD)
Murdoch Affiliation(s): School of Biological Sciences and Biotechnology
Notes: Note to the author: If you would like to make your thesis openly available on Murdoch University Library's Research Repository, please contact: Thank you.
Supervisor(s): Potter, Ian and Caputi, Nick
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