Tropical tuna under surveillance

1Any object floating on the surface of the ocean is likely to attract tropical tuna. In the early 1990s, this encouraged fishermen to deploy artificial floating objects (“fish aggregating device” or FAD) to aggregate fish and facilitate their harvest using large nets known as seines, which encircle schools of fish. While this has contributed to improving vessel performance, it has raised concerns about the sustainability of tuna resources and the environmental impact of this practice.

Seine fishing, Seychelles.

••• Data has been collected since the late 1960s to monitor changes in tuna populations •••

2Thousands of FADs are currently adrift on the ocean surface, some of which are crude and consist of bamboo rafts ballasted with a net. Their journey sometimes ends in vulnerable areas such as coral reefs, where they cause substantial damage, which prompted researchers to test new biodegradable models, in conjunction with the industry. Other FADs are equipped with measuring instruments, including GPS and even sounders to remotely inform fishermen of the presence of tuna under the FAD. If this technological upgrading were not regulated by Regional Fisheries Management Organisations (RFMO), it could have serious implications for the sustainability of abundant target populations and species affected by collateral damage.

3Research confirmed that FADs tend to aggregate young (juvenile) yellow-fin tuna and big-eye tuna, who have a lifespan of approximately 15 years, and this could have adverse consequences for these two populations. Lack of knowledge about the natural mortality rate of juveniles makes it difficult however to get a clear picture of the actual impact of this fishing method. FAD fishing also causes the highest number of by-catches of non-target and sensitive species such as sharks, rays and green swordtail fish. Certain individuals are kept on board and others thrown overboard, in most cases dead.

Drifting FADs on the dockside, Seychelles.

4Since the French started fishing for tropical tuna in the late 1960s, IRD’s observatory of exploited tropical pelagic ecosystems has been entrusted to collect and verify the commercial data of French vessels operating in tropical waters, in conjunction with the fishing industry and partners from the Global South. Fishing data is based on the collection of logbooks kept by fishermen, indicating the dates and locations of fishing operations, catch by species and the fishing method used (FAD or free school). This information is corrected and validated by landed catch samples, as well as the frequent presence of scientific observers on board fishing vessels, thereby supplementing said information with observations on by-catches and incidental catches. Electronic observation can sometimes complement human observation. This data is used by scientists within Regional Fisheries Management Organisations to evaluate the stocks of tropical tuna and certain species caught incidentally, and to measure the environmental impact of this form of fishing. They are however still struggling to fully quantify the effect of FADs.   To be continued…

“Insufficient knowledge of the number of FADs deployed by fishing vessels each year leads to significant uncertainties when evaluating stocks. As a result, FADs are a major and topical research area for countries of the Global South, to whom scientists must provide guidance to effectively respond to sustainable development issues.” 
Justin Amande, Ocean Research Centre, Abidjan