Preliminary estimates of some stock assessment parameters for the mesopelagic species Benthosoma pterotum inhabiting the Western and Northern Arabian Sea
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This report provides preliminary estimates of the stock assessment parameters for Benthosoma pterotum from data collected onboard the acoustic survey vessel Dr. Nansen. The von Bertalanaffy growth estimates are K=2.0, L=5.2 cm (total length) and t =-0.09, the latter being related to assumed birthday dates of 30 April and 31 December. The constants in the power curve relationship (w = aLᵇ) of total weight (g) to total length (cm) are shown as a = 0.02, and b = 2.79. Four alternative estimates for the natural mortality coefficient M provide values of 3.6, 5, 4.8 and 4.3 for the 1975 and 1976 years. The mean numbers recruiting into the stock each half year is estimated to be 123x10¹². Recruitment is indicated as being confined to the summer to autumn and winter to spring periods, which suggests an explanation for the observed differences between biomass estimates from previous acoustic surveys; the stock numbers peak in spring and autumn from the effect of 'net' recruitment and decline to wards summer and winter from the effect of natural mortality. With the assistance of Yield per Recruit Tables, annual yields for a range of levels of exploitation and lengths at first capture are described. For example, using trawls providing a length at first capture of about 2 cm, the annual yield is maximized at about 17.5x10⁶ tones, at a fishing mortality F of almost double M. It is suggested that the most appropriate levels of future exploitation are likely to be considerably lower than in the example with the identification of the best levels requiring additional knowledge, such as concerning commercial operating costs, availability of markets etc.