Title: Fishery biology and stock assessment of Blueskin seabream (Polystagnus coeruleopunctatus) in Mozambique

Author(s): Rui Mutombene
Final project
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stock assessment; blueskin seabrim; Mozambique;


A revision and an update of some important aspects of the fisheries biology of blueskin seabream in Mozambique were made in this study. The principal objective was to assess the status of the stock of this relatively deep water species caught primarily by hook and line in Mozambican coastal waters. The exploitation of the resource showed to be highly variable in response with frequency of the fleet in relatively deep waters. It seems that depletion of the shallow water species is the driving factor for the increased fishing intensity over deepwater reefs observed in last two years. As such, catch of blueskin has increased from less than 20 tons per year to approximate 200 tons making blueskin the major demersal species caught in the semi-industrial and industrial line fishery in the last two years with 20% of the total catch. Biological evidences of the population indicate that blueskin is a hermaphrodite protogynous species. The sex ratio is biased in favour of females (1:3 M:F) and no males are found in the small size classes. The results show also that blueskin is a slow growing and long-lived species with the maximum age estimated at l22 years. Estimated von Bertalanffy parameters were L∞= 40.8., k=0.104 and t0=-1.64. The size at 50% maturity estimated was 22 cm and the corresponding age at maturity was 6 years appointing blueskin as a late maturity species. Both life span and size at maturity were much higher than estimated in previous studies. The reproduction and recruitment are continuous with fish in all maturity development stages being found throughout all year. The current instantaneous fishing mortality rate was estimated by a production model (Fcurrent = 0.15 year-1) that is below the reference point F0.1 (0.24 year-1). The Fcurrent is likely estimated too low. The fishing effort in the last two years has already increased, more than threefold of the level observed from 2003 to 2010. A further increase it is not recommended. The uncertainties involved in the estimation of current F and the risk of growing overfishing in this species leads to necessity of assuming this more conservative position. A maximum effort should not exceed the actual 1500 fishing days for all operations in deep fishing grounds. Massive concentration of the effort in deep reefs in response to the depletion of shallow water species is not sustainable and may lead to negative impacts in both areas in a short midterm. So the managers must avoid it by promoting a redistribution of the fishing effort of the semi-industrial and industrial line fishery in a wide range area. 

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