Size-based assessment of data-limited inland fish stocks – Review and applications

Book excerpt: Assessment of data-limited fish stocks is a rapidly evolving topic in marine fisheries, and is supported by an increasing focus on the socio-economic and ecological importance of small-scale fisheries. The challenges in such systems can be compounded in inland fisheries, which are often complex, spatially dispersed and difficult to monitor. This publication reviews the application of empirical indicators and simple size-based models usually used in marine fisheries, but also applicable in inland systems. It presents case study applications for important fisheries in the Amazon River (Brazil), Tonlé Sap River (Cambodia), Paraná River (Argentina) and Lago Bayano (Panama). These studies consider issues including spatial separation of life-history stages, strong modality in population size structure, and fishing gear selectivity. Local scientific experts interpreted trends in stock state. Empirical indicators showed strong decline in size structure and relative abundance for one of the four assessed Tonlé Sap stocks. The length-based spawning potential ratio model suggested that two of the three assessed Amazon Goliath catfish stocks, and the sábalo stock in the Paraná River, were below sustainable spawning potential ratio reference points. The Lago Bayano tilapia stock appeared healthy. The review concludes that data-limited assessment methods developed for marine stocks may provide guidance for the sustainable management of important target species in inland fisheries. The methods tested are probably less applicable in non-selective fisheries where small species are preferred, or in river fisheries with extreme dependence on flood pulses. Important considerations are species life history and spatial distribution, environmental variability, and fishery sampling strategy.