UBC Theses and Dissertations
An investigation of fisheries yield equations with particular reference to annual state models and seasonal pulse fishing Wild, Alexander
Traditional methods of stock assessment rely on the calculated value of effective effort (f) and the assumed constancy of the catchability coefficient (q) to provide estimates of abundance and the effects of fishing. It is difficult to account for or to quantify all the variables that affect f and the reliability of this term is often questionable. The uncertainty associated with f and q is reflected in assessments and undermines confidence in management recommendations. Under certain conditions the objective of circumventing these difficulties is realized in the development of the annual state models. At the end of each fishing year, the fishing (F) and natural mortality (M) coefficients and the apparent abundance of each age group (i) are estimated without reference to f, q, the number of recruits, or the entire history of a year class. The data requirements that make this analysis possible are the catch (C[sub i]) and yield in weight (Y[sub w, i]) of each age group and estimates of the growth equation parameters. The derived quantity for the mean weight of fish (Y[sub w, i]) /C[sub i]) is independent of abundance and provides a deterministic solution for the total mortality coefficient, Z[sub i]. Consecutive year class estimates of Z[sub i] and Z[sub i + l], when coupled.with a ratio of catch equations, yield estimates of F[sub i], F[sub i + l], and M. The assumptions of this particular model are that growth and mortality are concurrent and continuous during the fishing period, the stock is closed to immigration and emigration, and M is constant for all age groups. Alternative models are developed that provide a simultaneous solution for the stock using equations for meanlength and weight or mean weight alone when .M is constant or a function of age. The equation for Y[sub w, i] is based on a generalized growth model and integrated by an approximate technique. The effect of seasonal pulse fishing on equilibrium yield is examined for hypothetical species having twenty different patterns of seasonally distributed growth and natural mortality. Each pattern is subjected to ten fishing strategies that vary in seasonal intensity and annual distribution. The effect on yield of increasing values of M, F and the growth parameter K is also explored. Relative to .continuous fishing, the greatest increase in yield is consistently achieved by concentrating the fishing activity into a single season. While the magnitude of this increase varies from three to thirty per cent, in any particular situation the optimal; time and the potential yield improvement is a function of K, M, F and the growth-mortality pattern.
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