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Defining Juvenile Tuna - Why Size Really Matters Global, September 11, 12

The use of fish aggregating devices (FADs) – man-made structures placed at sea as anchored or floating devices designed to attract schools of fish – has been a hotly debated topic because of their tendency to catch higher rates of juvenile yellowfin and bigeye tunas as by-catch of tuna purse seine fishing.

For unknown reasons, juvenile tunas form schools and swim beside similar-sized adult skipjack tunas – the target of purse seine fishers. Depending on geography and season, the proportion of juvenile yellowfin and bigeye tunas vary between five to 30 percent of total catch.

But what exactly do we mean by juvenile tuna and why is it critical that we set a policy in properly defining them?

From a biological definition, the term juvenile refers to the life stage of an animal that has not reached a size where it is capable of reproduction. For oceanic tuna species, first spawning occurs at the size of 40 centimeters for skipjack, 100 for yellowfin, and 105 for bigeye. Sizes smaller than these are considered juvenile or immature.

Catching juveniles causes “growth overfishing,” a condition where fish is caught before it can reproduce. Without effective regulation on taking juveniles, a lesser number of fish will reach adulthood. The reproductive potential of the tuna population tasked to supply the next generation is therefore reduced, resulting in lower number of tunas produced the next year. If this scenario continues, we will have what science calls the “negative feedback loop” whereby in each cycle, the adult biomass of reproductively capable individuals is reduced every year, producing lesser number of eggs and lesser tuna larvae that will grow to maturity.

Catching juveniles also undermines the economic potential of tunas, because the value of tuna increases with size. There is about a USD 100 - USD 150 price difference between a ton of juvenile from a ton of adult skipjack. Similarly, in the Philippines for example, yellowfin tuna weighing half a kilogram or 30 centimeters long, costs USD 1.20 compared to USD 3.50 for a full kilogram or one meter long.

If catching juvenile tunas undermines environmental sustainability, economic potential, and ultimately, food security, why is there a huge resistance to establishing a science-based limit? Why doesn’t the Western Central Pacific Fisheries Commission (WCPFC) advocate minimum size requirements for countries fishing oceanic tunas?

In some countries where minimum catch sizes are declared, why are the sizes that of immature tuna? If tunas become more valuable as size increases, why are the minimum sizes small? The answers to these questions are simple yet complex.

Setting a common minimum catch size limit for a fishery, say for purse seine fishing, is not that simple because there are several species caught along with the targeted species in a particular fish school. The stock health of varied tuna species within one school varies considerably from very healthy (skipjack) to fully exploited (bigeye). With a single minimum size, the impact of overfishing will push the fully exploited stocks to an even more dangerous level.

Setting a minimum catch size requires an in depth analysis of the complex socio-ecological dimension of the fishery. Governments very often would have a bias for socio-economic issues over ecological arguments when defining juveniles. The 500 gram minimum size limit for the purse seine sector in the Philippines, for instance, was based on the significant volume of juvenile tunas destined for canneries, which if taken out, could decrease productivity and therefore compromise employment. Setting a minimum size that is much bigger also impacts the affordability of poor people to buy fish. Cultural norms also play a role. Filipinos, for example, are more accustomed to buying whole fish instead of filleted ones. A ban on the trade of juvenile fish then becomes an issue of domestic food security.

Tackling ecological issues in fisheries, such as defining minimum catch sizes, cannot simply rely on pure scientific arguments. Catch size limits also need to fit social, cultural, and economic dimensions.