Browsing by Author "Newman , G G"

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    A computer simulation of the population dynamics of the Cape hakes (Merluccius capensis and M. paradoxus)
    (1978) Mertens, Robert George Stephen; Field, John G; Newman , G G
    Catch and effort statistics for the Cape hakes (Merluccius capensis and M. paradoxus) have indicated a substantial decrease in the stock size over the period 1955-74; consequently, a catch projection model was designed to investigate the hake population. Where appropriate, available data have been included in the model; however, some further research was required: 1. The monthly pattern of availability for 1974-76 was examined using South African data; it was found to be similar to that reported for earlier years, and the later figures were used. 2. Assuming that selection curves for different mesh sizes are identical except for their position on the X-axis, a method to calculate selection values for combinations of mesh size and fish length has been outlined, and used with data presented by Bohl et al. (1971). 3. An investigation was made of two methods of estimating natural mortality (M), utilizing the results of Virtual Population Analysis (V.P.A.). In both cases the criteria used to judge M were found to be insensitive, and therefore neither can be used. 4. The stock estimates obtained using V.P.A. were applied to two stock/recruit curves. The goodness of fit in both cases was poor, and nearly identical.
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    Estimate of size and interaction of the South African anchovy and pilchard populations
    (1977) Centurier-Harris, Oliver Michael; Field, John G; Newman , G G
    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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    Implications of recruitment, distribution and availability of stocks for management of South Africa's Western Cape purse-seine fishery
    (1979) Crawford, R J M; Field, John G; Newman , G G
    South Africa's Western Cape purse-seine fishery is situated off the country's western seaboard in the highly productive waters of the southern Benguela Current system (Cushing 1969). Management of the multi-species resource is the responsibility of the government's Department of Industries and since 1950 a large volume of data relating to performances of contributing species has been collected. Recent analyses have indicated an oversubscription of effort, a sequential depletion of the more valuable stocks and, consequently, a present reliance upon less favourable species (Newman and Crawford 1979, Crawford et al. in press). These adverse trends have been precipitated, in part at least, by ineffective legislation, which has resulted from a past failure to appreciate that exceptionally good year classes occur infrequently (Newman and Crawford in- press). Optimal ways of harvesting strong cohorts, or of avoiding overexploitation in the event of recruitment failure, remain to be explored.