There’s always a bigger fish -- Scientists detail findings of Chinook salmon trackers

Researchers getting ready to tag the fish. The "cradle" is filled with water, so the fish's gills are submerged during the tag attachment process. Also, the fish is blindfolded, which reduces stress and typically prevents the fish from struggling. (Courtesy Andrew Seitz)

ANCHORAGE (KTUU) - On the whole, it’s been accepted that smaller, younger salmon are at more risk in the ocean than the full-grown Chinook. A recent study says those big fish also need to keep watch.

A research study by Andy Seitz, Associate Professor and Fish Ecology researcher with the University of Alaska Fairbanks, tested the suitability of tags on adult Chinook salmon, and it showed some interesting data.

“I think it points out that the ocean is a dangerous place,” said Seitz.

Seitz and his group tagged 45 late-stage king salmon. The tags measure temperature and depth and are designed to release after a certain time period. The tag then floats to the surface and transmits its data to a satellite. But one tag reported early.

“It has this failsafe mechanism where if the tag reads a constant depth then it undergoes this programming to start transmitting data because the assumption is either the fish died or the tag came off the fish and is floating on the surface of the ocean,” Seitz said.

Seitz looked at the data and noticed a spike in temperature, going from about 40 degrees Fahrenheit to the upper 70s.

“So, I immediately thought somebody captured this tagged Chinook salmon. They removed the tag from the fish and it's riding around in the bridge of a fishing vessel,” said Seitz. “But then I looked at the depth record and the tag was reading 78, 79 degrees Fahrenheit even when the tag was down at 350 meters depth.

“I scratched my head and said where in the world, or where in the Bering Sea I should say, is it almost 80 degrees at almost 350 meters depth?” Clearly, it was inside another fish.

“And it's not a marine mammal. They are much warmer, about the same body temperature as a human,” Seitz said. “So what else is warm-blooded in the Bering Sea? And it's highly likely it's a salmon shark.”

Salmon sharks live in the Bering Sea and as Seitz points out, “salmon sharks are called salmon sharks for a reason and that's because they eat salmon.”

Of the 43 tags the researchers put on salmon, they received data from 35 of them. Nineteen of those 35 indicate they were eaten by predators. The study is too small to show whether this is a primary impact to salmon numbers but Seitz says it gives researchers more targeted areas to study.

Seitz said they are running models to examine the potential effects of predators on late-stage Chinook salmon in the ocean. “Usually late stage marine mortality is relatively low in these population dynamics models, so what we're trying to simulate is high rates of marine mortality to see if it can account for trends in abundance and changes in age structure.”

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