SUFB 069: How to Manage Genetically Different Tuna Populations

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In our first installment of Research Thursdays, Andrew and I bring you a study on populations genetics of yellowfin (YF) tuna in the Western and Central Pacific Ocean (WCPO). Traditionally, the YF tuna populations off the coast of the Philippines and in the Bismarck Sea north of Papua New Guinea have been managed as a single stock. This usually assumes that for all intents and purposes, these tuna could all theoretically have babies with each other. A research team out of the Philippines, led by Roselyn Aguila, examined this assumption by analyzing regions of the tuna’s DNA called microsatellites.

What they discovered is that while individual tuna from the Philippines are genetically similar, and thus interbreed frequently, there are distinct genetic differences between tuna from the Philippines and the Bismarck Sea. These two populations therefore exhibit what are called reproductive barriers. These barriers could be physical, like an oceanographic eddy that prevents drifting larvae from each population from reaching the other, or ecological, like temperature or salinity differences that make each respective habitat unsuitable for the alternate population. Either way, the study shows that WCPO tuna populations are comprised of two distinct subpopulations rather than one cohesive one.

Now, why does it matter whether YF tuna from the Philippines are infuriating their parents and making babies with tuna from the Bismarck Sea? Well, it all comes down to stock management. We manage fish stocks according to reproductively-isolated populations. This means that, ideally, two tuna populations that are not breeding with each other on the reg are managed separately. There’s good reason for this; if one stock gets depleted, the only way another stock could help replenish and sustain the impacted population is if the two stocks can produce viable offspring together. It’s all well and good to bring in reinforcements from another location, but if those reinforcements live out their lives and die without ever mingling with the target population, then you’re right back where you started from a management perspective. Therefore, it’s extremely important for fisheries managers to understand the population structure of a selected species, especially in a region like the WCPO where tuna fishing is a huge industry. Proper management is reliant on accurate data, and working from an improper assumption can affect the ability of managers to protect their livelihoods.

Preserving genetic diversity through stock management is also important for the target species as a whole. Species rely on genetic variability to adapt to future environmental conditions. The chance that a single catastrophic event could wipe out a given species is greater if the genetic variability of that species is low than if it’s high. Genes are like the tool belts of a species. If you only give the species a couple different tools, it may not be able to survive under a wide range of conditions. However, if you give the species plenty of different types of tools, it’s more likely to successfully adapt to future conditions (like those brought about by climate change). By protecting and managing genetically distinct populations separately, managers are ensuring that as much genetic diversity exists within a species as possible.

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