FAO.org

In order improve the management of a fishery it is necessary to determine which species are present and in what quantities. Sampling of the fish stock is thus needed. For monitoring the effects of certain management strategies, it will also be necessary to gather information concerning the fish stocks. Gillnets are often used for sampling fish stocks because they are easy to use, relatively cheap, catch a wide variety of species, and their use can be standardized. Gillnets are however, selective towards the species they catch, the size of fish they catch and the amounts of fish they catch. Also outside factors influence the effectiveness of the nets. ALCOM used multimesh gillnets in 34 reservoirs in 5 countries in southern Africa over a period of 5 years to collect information on the fish stocks of these reservoirs. The analysis of the results of these samplings showed that the gears were useful to collect general information of the fish stocks, but also demonstrated that the catch per unit of effort could not be used as a reliable indication of abundance. The reasons for this were that the fluctuation in the catches were too great, not all species could be sampled with the required precision and, although a wide range of mesh sizes were used, the gears were still size selective. Furthermore it was agrued that the effects on the fish population of different management strategies would not be detected by sampling with gillnets alone. The effects of outside factors, and especially of the water level fluctuation of the reservoirs, would easily confound the effects of the fisheries management on the reservoirs. It was recommended to concentrate the studies on the target species, to determine the effects of the water fluctuatiuon of fish production, and to use the existing fishery in the collection of data.