Evaluating Age Structure and Foraging Ecology of Yellowfin and Bigeye Tuna in the Northwest Atlantic

To say marine food webs are complex would be an understatement. Research that helps us understand how species interact with each other across their lifespans, though, is essential for determining how marine food webs are constructed, what linkages in the food chain are important for each species, and how that information can be used to assist in the stock assessment and management of exploited fish stocks.

For example, top predators like tuna distribute themselves in water masses with sharp temperature gradients where physical forces aggregate prey. Understanding these dynamics, and how fish distribute themselves, helps us understand shifts in distributions and changes in landings from year to year. The latter is very important as Catch Per Unit Effort indices, which represent the rate at which fish are captured, are often the most influential aspect of fisheries stock assessments. All sorts of things can influence these indices, especially changes in ocean conditions, and prey distribution shifts, but it's important to narrow down causes. Stocks might be stable, but moving in and of areas fishermen can access, or their populations may actually be declining and at risk. Foraging ecology and tagging research can help explain some of these changes.

In our work, we examine stomach contents from these species to improve understanding about tropic relationships. Visual observations of stomach contents can be useful, as many eaten species have definable, diagnostic morphological characteristics. However, due to the rapid digestion rates in many tunas, visual characteristics may not be enough to identify prey to the species level. Some prey have specific digestion resistant tissues, like otoliths and beaks, which can be used to identify digested prey.

Tissue samples can also be analyzed for stable isotopes — chemical signatures that tell us details about groups of species these fish may be consuming. While not as specific as visual observations or otoliths or beak analyses, stable isotopes can provide foraging ecology data over much longer periods of time.

As a last resort, or if all that remains in a tuna's stomach are globs of mush, we can send out some of the highly digested material for genetic barcoding, which very precisely identifies species. Collectively, these techniques allow us to gather a much more detailed look at the foraging ecology of these species.

Additionally, aging is one of the fundamental inputs for stock assessments, yet for many highly migratory species we have little to no age data. This project samples otoliths from these two tunas and uses them to estimate age structure of the landings. In addition to aging, this project focused on validating the annual structures accreted in the otoliths each year through a technique called bomb radiocarbon. This provides confidence that marks on the otoliths used to estimate age do in fact represent annual events.

The gonadal tissues we collect enable us to learn more about tuna reproduction and information gained from the electronic tags allows us to test hypotheses about stock structure and habitat use.