Bluefin Tuna Internal Temperature Study Reveals Feeding and Migration Patterns

When you eat, stomach muscles and digestive organs go to work to break down food in a process that releases heat and causes internal body temperatures to increase. The same is true for bluefin tuna, and now scientists have seized upon this thermal characteristic to measure the amount of energy bluefin tuna consume. The results will ultimately help conservationists design better policies to protect this endangered species.

Pacific bluefin tuna (Thunnus orientalis) rely on foraging to survive and to create the energy necessary for migration. Bluefin tuna are equipped with powerful muscles for long transoceanic travels and – unlike most bony fish – are warm bodied, according to a news release. This means that they are able to maintain warmer-than-water body temperatures which researchers from Stanford University, Monterey Bay Aquarium and the National Oceanic and Atmospheric Administration (NOAA) recently developed a way to measure.

Using the thermal data collected, researchers were also able to determine when the tunas ate, how much energy they consumed while swimming and how temperature changes impacated energy intake.

As bluefin tuna swim, their muscles contract and produce metabolic heat that warms their bodies via specialized blood vessels and digestive organs that trap heat and prevent heat loss through their gills. This thermal characteristic allows the fish to swim more efficiently, adapt to various marine ecosystems, and use less energy when digesting food.

For their study, researchers implanted tags into more than 500 tunas off the coast of Southern California and Mexico. They also observed the fishes' body temperature, the ocean water temperature, and the location and diving patterns as they foraged. The data collected from the tags indicated that the fish engaged in seasonal migrations from Mexico to Oregon, according to researchers.

"We've been able to follow what Pacific bluefin tuna do in the open sea and record their feeding and meal size, every day for up to three years," Rebecca Whitlock, a postdoctoral scholar at Stanford and lead author, said in a statement. "Combining laboratory observations with electronic tagging can provide amazingly rich data and insights into the life of a wild marine predator."

When mapping the position data collected from the tags in correlation with nutrient-rich feeding grounds, researchers discovered the fish didn't always take advantage of ample food sources.

"Foraging success was correlated to environmental features," Elliott Hazen, co-author and a research ecologist with NOAA's Southwest Fisheries Science Center, explained in the release. "Tuna may use the oceanography as a roadmap to move from hotspot to hotspot, and temperature appears to be the most important environmental cue."

Temperature changes play a key role because cold waters slow the fish's heart rate and warm waters can be energetically taxing. Therefore, they prefer to feed in areas where the water temperatures will allow them to rapidly digest their food. This helps the researchers better understand the bluefin's range and feeding hotspots.

"Our results add to our understanding of predator-prey dynamics in the California Current," Barbara Block, senior author of the study and a professor of marine sciences at Stanford's Hopkins Marine Station, added in the statement. "By understanding where bluefin forage most, we can help protect these places and improve efforts to rebuild Pacific bluefin tuna stocks."

Their study was recently published in Science Advances.

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