Ebook Fisheries biology, assessment and management (2/E): Part 2

Part 2 book "Fisheries biology, assessment and management" includes content: Stock structure and abundance, stock assessment, fisheries management, fisheries symbols and formulae; standard deviation and confidence limits; correlation and regression; spreadsheet models; collection of length–frequency data, bhattacharya plots, statistical tables.
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Ebook Fisheries biology, assessment and management (2/E): Part 2 VetBooks.ir 4 Stock structure and abundance purpose of fisheries science is to provide fisheries 4.1 Introduction managers with advice on the relative merits of Fisheries biologists contribute to fisheries science alternative management strategies. in two main areas: first, by studying the basic It is instructive to consider a fish population biology and distribution of resource species; and or stock as a simple biological system in which second, by studying the dynamics of fish stocks. the forces acting to control stock numbers and The first will lead to the elucidation of the life weight are shown (Fig. 4.1). Ignoring environ- cycle of the species, including its place in marine mental factors that impose an upper limit, stock ecosystems and the habitats that are crucial for numbers are being increased by reproduction and its survival. The second will provide the basis the eventual addition, or recruitment, of small for stock assessment – for the quantitative stud- fish. In addition, the weight, or biomass, of the ies that lead to predictions of how stocks will fish stock is increased by the growth of individ- respond to management actions. The ultimate uals – in the figure, three consecutive age groups, Fig. 4.1 An exploited fish stock viewed as a simple biological system. The stock biomass (top elongate shape) is increased by recruitment and growth (three age groups are shown), and is reduced by natural mortality and fishing mortality. 172 Stock structure and abundance 173 VetBooks.ir Box 4.1 individuals, in the expectation that a small number Sampling the catch of these are later recaptured, has the potential to be used, with some assumptions, in the estimation Besides collecting catch and effort data, sampling of abundance, movement, growth and mortality. the catch either on vessels, at landing places or in • Hard part analyses. Hard parts including shells processing factories can be used to obtain the of molluscs, scales and otoliths of fish and the following types of information. vertebrae of sharks have been used to assess • Morphometrics and condition indices. the age of individuals. Relationships between length and weight can be • Reproductive condition. The development of used to assess the well being of individuals and to gonads (usually ovaries) can be used to trace determine possible differences between separate development and suggest spawning times. unit stocks of the same species. Electrophoretic • Stomach contents. The analysis of stomach and DNA analyses may also be used for the latter contents may be possible in some species and purpose. those caught by some fishing methods (fish • Length–frequency data. The number or brought up from deeper water often disgorge their frequency of individuals in different size classes stomach contents). There is renewed interest in may be suitable for the determination of stock such analyses and species interactions because structure and the estimation of growth. of multispecies and ecosystem-based stock • Tagging or marking. Tagging or marking assessments. or year classes, are shown. At the same time, the 4.2 Structure and abundance stock is being reduced by natural mortality and, in exploited species, by fishing mortality as well. Questions of abundance and geographical limits Fishing mortality refers to fish caught by fishers, of stocks arise at the start of all fisheries. At this and natural mortality refers to fish which die by stage, both fishers and fisheries managers require other means, most commonly by predation. some order of magnitude assessment of the stock. In a species that is unexploited or fished at a Fishers and investors need some idea of the extent low level, losses due to mortality are balanced, of the resource to justify some level of investment on the average, by gains through recruitment, in boats, gear and shore facilities. Fisheries and stock abundance will fluctuate, often greatly, managers, well aware of the difficulty of having around a mean level. If exploitation is high, the to reduce fishing effort in an overcapitalized or number of mature fish may be reduced to a level overexploited fishery, have to make some judge- where reproduction and recruitment are unable ment on the level of fishing that should be encour- ...