Assessment of fish by-catch species from coastal artisanal shrimp beam trawl fisheries in Nigeria

An observer programme that incorporated both fisheries-dependent and -independent surveys with fishers, was conducted to identify and quantify by-catch species from artisanal and nearshore shrimp beam trawl fisheries off Lagos, Nigeria. Catches were sampled from 62 replicate landings by five boats from January to December 2002. Twenty-five species belonging to 20 families of fish targeted in other inshore/offshore commercial fisheries constituted the by-catch species. Target shrimps Nematopalaemon hastatus (0.5-1.5 cm CL) are smaller in size than important commercial species (4-30 cm TL), which occur as juveniles on their nursery grounds. The percent of weight compositions of commercial by-catch species are: Pseudotolithus elongatus (19.89%), P. senegalensis (13.69%), Ilisha africana (8.85%), Pentanemus quinquarius (4.95%), Callinectes amnicola (3.27%), Pseudotolithus typus (2.75%), Galeoides decadactylus (2.26%), Cynoglossus senegalensis (1.43%) and Chloroscombrus chysurus (1.05%). By-catch species having total length of 11-30 cm are marketable and consumed, while a length range of 4-10 cm, which constitute 75%, is discarded ashore, thus resulting in high biodiversity loss. A t-test comparison of by-catch species weight, with weight of target shrimps was significant (P < 0.05). The results are discussed in terms of: (1) the contrast meaning of by-catch in this fishery and (2) possibilities of technological changes that involve the introduction of a suitable and simple by-catch reduction device to mitigate by-catch problem.