Chemical Signatures Tell Critical Story About Habitat

A new study by University of Washington fisheries researchers documents how chemical signatures imprinted inside the ears of fish show that two of Alaska’s most productive salmon populations and the fisheries they support depend on the entire watershed.

The study notes that sockeye and Chinook salmon born in Nushagak River in Alaska’s Bristol Bay watershed, and its network of streams and lakes use the whole basin as young fish to search for the best places for prey, shelter and safety from predators. From birth until these young fish migrate to the ocean a year later is a critical period for them to eat and grow.

The study by lead author Sean Brennan, a postdoctoral researcher at the UW School of Aquatic and Fishery Sciences, notes that different parts of the Bristol Bay watershed are hot spots for salmon production and growth, and these favorable locations change annually, depending on how climate conditions interact with local landscape features.

The study, published in the May 24 edition of Science, analyzed the tiny ear stones, known as otolith, that form in the fish.

“Habitat conditions aren’t static, and optimal places shift around,” Brennan said. “If you want to stabilize fish production over the years, the only strategy is to keep all of the options on the table.”

“The overall system is more than just the sum of its parts, and small pieces of habitat can be disproportionately important,” said Daniel Schindler, a UW fisheries professor and senior author of this study. “The arrows point to the need to protect or restore at the entire basin scale if we want rivers to continue to function as they should in nature.”

Release of the study coincides with renewed efforts of Canadian miners to get permits for development of the proposed Pebble mine, to extract copper and gold from near the headwaters of the Nushagak River. The deadline for public comment on the draft environmental impact statement was recently extended to July 1, 2019 by the US Army Corps of Engineers.

As part of the study, researchers reconstructed the likely geographic locations of nearly 1,400 adult salmon, from their birth in a Nushagak stream until their migration to the ocean. By looking at the otolith of each fish, they could tell where the fish lived by matching the chemical signatures imprinted on each growth ring of the otolith with the chemical signatures of the water where they swam.

“Results like those we’re presenting in this paper hopefully will get people to think about what they stand to lose by starting to develop and eliminate habitat in places like the Nushagak River,” Schindler said, “The Pebble mine environmental impact statement, which is supposed to be a mature, state of the science assessment of risks, really does a poor job of assessing risks of this specific project.”