Trends in the fishery and population traits of the Namibian stock of Cape horse mackerel Trachurus trachurus capensis (Castelnau, 1861)

Abstract

This study explored possible factors linked to the instability of the stock biomass of Namibian horse mackerel Trachurus trachurus capensis (Castelnau 1861) in recent years (2001-2007). The major focus of the analysis was the contrast of stock traits in years of low and high stock biomass, as perceived from research cruises performed annually between February and March. Stock traits analyzed included: recruitment and year class composition; spatial distribution of the stock; variations in fish body-mass condition and reflections on stock biomass; and maturation patterns and influences on spawning stock biomass. An additional analysis, also based on secondary data, explored the changes in effort capacity, activity and efficiency of the fishing fleet over a longer period of time. Fish 0-3 years old dominate the stock at present, and the modes of the fish size composition appeared to be fairly stable along the years. Thus, length-growth seemed to proceed unaffected by changes in stock biomass. There was, however, a marked affluence of pre-recruits in the years of high stock biomass. Thus, stock biomass was related to recruitment (measured as fish <12cm), which varied by a factor of 20 between the years of high and low stock biomass. Study of the fish allometry showed a trend of decreasing b (exponent) and increasing a (proportionality constant), and this trend seemed to be temporal rather than density-dependent. This seemingly resulted in better condition of the fish, and increased stock biomass by up to 14%. Most of the fish during the survey period were found to be maturing. But, there was a strong declining trend in the calculated size at maturity, which did not seem to be immediately related to stock density. This resulted in a compensatory increase in the calculated spawning stock biomass with time. The spatial distribution of the stock has largely expanded after a reflux in 2002. This does not seem to be directly related to stock biomass, and may conceal migratory fluxes to/from a southern stock. The fishing fleets may have become more efficient at catching the fish, as the reduction in the fishing capacity is not reflected in the catches. Further, the nominal catch per unit effort showed stability and failed to reflect the decline in stock size, particularly in the last years of the series. The surveys seem to underestimate the occurrence of bigger sized fish, as compared to the midwater trawler fleet that more efficiently target the spawning stock. Contrastingly, purse-seiners efficiently target 1-year old fish. The catches of the different life-stages of horse-mackerel by both fleets seem to form a large phase-cycle that is partly conditioned by the occurrence of another species, the pilchard, and, thereby, by transference of effort of seiners between stocks. Rather than a single-species approach, a more strategic approach to management would probably be to consider a fishery system composed by two-prey and two-competing fleets, linked by strong economic and technological interactions.