Estimate of size and interaction of the South African anchovy and pilchard populations
Master Thesis
1977
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University of Cape Town
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Abstract
The size of the South African anchovy and pilchard population was estimated using Virtual Population Analysis (Gulland 1965). These estimates were used to test classical stock recruit models as well as to investigate possible stock interactions. The population biomass of the pilchard indicated a severe decline after 1959 from a level of approximately 2 000 000 metric tons to about 200 000 metric tons in the mid 1970's. This change was in agreement with catch per unit effort estimates of abundance (Newman et al in press). The anchovy abundance remained fairly constant at about 300 000 metric tons during the period 1965 - 1974 for which estimates could be made. During 1976 the pilchard stock showed a partial recovery of its former abundance. Adult and recruit stocks of both anchovy and pilchard were fitted to the Ricker (1954) and Beverton and Holt (1957) stock/recruit curves, but there was insufficient data to allow a decision to be made as to which model was appropriate for either stock. The anchovy and pilchard populations or age groups within them do not appear to interact in a very definite manner. Attempts to quantify interactions using Virtual Population Analysis have not been successful although better estimates of the strength of age-groups would be possible if the variation in natural mortality with age was understood. The fact that elements of the stock of anchovy and pilchard do not appear to be rigorously related indicates the importance of understanding which environmental factors are critical to survival. A difference between the area in which recruitment takes place and the area in which most adults are caught indicates a movement of young fish onto the west coast fishing grounds. The movements were substantiated by observations of catch per unit effort of juveniles on the west coast in each month of the fishing season.
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Bibliography: pages 85-87.
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Centurier-Harris, O. 1977. Estimate of size and interaction of the South African anchovy and pilchard populations. University of Cape Town.