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The development and application of a length-based method to estimate the spawning potential ratio in data-poor fish stocks

Hordyk, Adrian (2014) The development and application of a length-based method to estimate the spawning potential ratio in data-poor fish stocks. PhD thesis, Murdoch University.

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Although they support many millions of people, the vast majority of the world’s fisheries are small-scale and data-poor, and without the resources or data systems needed for comprehensive stock assessments. There is strong evidence that unmanaged fisheries are a recipe for disaster, with over-exploitation of the stock almost inevitable. Additionally, it is increasingly recognised that the spatial scale of the stocks of many marine species is much smaller than previously thought, which adds another layer of cost to the stock assessment process, as the cost of collecting and analysing such fine-scale data is prohibitive. The overall aim of this thesis was to develop and test novel methods of stock assessment for data-poor and small-scale fisheries, based on the basic biological characteristics of the exploited species.

Knowledge of the basic biological parameters of fish stocks, such as the natural mortality rate (M), the growth parameters (commonly described by the von Bertalanffy equation, L¥ and k), and the length at maturity (Lm), is important for many stock assessment methodologies. However, collecting such information is costly, and usually requires sophisticated ageing studies. I conducted a meta-analysis of over 120 marine species, from a range of taxa including teleosts, chondrichthyans, mammals and invertebrates, and examined the variation and patterns in the life-history ratios, and the relationships between size and spawning potential (Chapter 2). These patterns were examined by standardising the age and size of each species so that the relationship between size and spawning-per-recruit for a large range of diverse species could be compared on the same scale. This meta-analysis demonstrated that species that are often considered to be quite different, essentially have the same life-history strategy when viewed on the same relative scale. For example, tuna can be considered as ‘larger, slower’, anchovies, while prawns are ‘smaller, faster’ versions of fish. Additionally, and somewhat surprisingly, a number of teleosts with low Mk values of _ 0:5 appear to have life-histories similar to marine mammals, and quite different from those expected of fish. The results of this study suggest that there is potential to establish a theoretical framework for ‘borrowing’ knowledge from well-studied species to apply to unstudied species and populations as an initial starting point for management.

The ratios of these parameters _ Mk and Lm L¥ _ are less variable between individual stocks o_f the same species than the individual parameters, and certain values of these ratios Mk= 1 : 5 and Lm L¥ = 0:66 _ , known as the Beverton and Holt Life History Invariants (BH– LHI) have been used commonly to provide preliminary estimates of unknown parameters. However, many species have life-history ratios that vary considerably from the BH–LHI, and in this study I demonstrate the link between variation in the ratios _ Mk and Lm L¥ _ and the life-history strategy of a species. For example, species with low Mk

Publication Type: Thesis (PhD)
Murdoch Affiliation: School of Veterinary and Life Sciences
Supervisor: Loneragan, Neil and Prince, Jeremy
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