Sockeye Salmon Migrating at the Northern Edge of Their Distribution

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The physiological challenge for anadromous fish to migrate upriver to spawn and complete their life cycle is influenced by river temperature.

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The impacts of river temperatures can be difficult to predict due to an incomplete understanding of how temperature influences migration costs, especially in high latitude ecosystems. To assess temperature influences on migrating Pacific salmon in a Subarctic watershed, we measured an indicator of heat stress (heat shock protein 70), energy density of Sockeye Salmon (Oncorhynchus nerka) throughout their upriver migration, and pre-spawning mortality. We suspected Sockeye Salmon energy levels would be reduced by warmer river temperatures; however, we found no evidence of heat stress, similar rates of energetic decline from river entry to the spawning grounds amongst years, and minimal evidence of pre-spawn mortality.  In fact, higher accumulated thermal units (ATU) resulted in higher energy densities in migrating salmon.  This is likely due to how the bell-shaped curve of thermal optimal conditions for Sockeye Salmon interacts with the thermal regime of the river.  The Pilgrim River is at the northern edge of the Sockeye Salmon distribution and often ranges below the optimal temperature for Sockeye migration resulting in a positive effect of warmer temperature pulses on somatic energy.  Hindcasting back to 1907, the predicted river temperatures during the spawning migration historically were not substantially different from the measured river temperatures (2013-2016).  Air temperature predictions for 2040, 2060, and 2090 indicate an increase in ATU, but not to the level that would suggest negative impacts. ATU levels resulting in negative effects would require a combination of increased air temperature with a dramatically prolonged migration. Understanding interactions between environmental drivers and biological responses will help anticipate future changes and will provide insights to make informed management decisions within the watersheds.

Scientists walking up a beach to collect sockeye salmon from subsistence gill nets

Sockeye salmon being collected with gill nets with cooperation of subsistence harvesters in Grantely Harbor, where the Pilgrim River enters the Bering Sea.
(Credit: Mike Carey, USGS. Public domain.)


Two scientists in a boat sampling the water of Pilgrim River

Collecting water quality data on the Pilgrim River. Types of measurements taken were on water temperature, dissolved oxygen, pH, and conductivity.
(Credit: Chris Zimmerman, USGS. Public domain.)

A fish weir on the Pilgrim River to prevent fish from going downstream except when controlled by a gate

The fish weir is operated on the Pilgrim River by the Norton Sound Economic Development Corporation with assistance from the Alaska Department of Fish and Game.
(Credit: Mike Carey, USGS. Public domain.)

Two people pulling a beach seine net in Salmon Lake.

Beach seining on the spawing grounds of sockeye salmon in Salmon Lake. This is the headwater of the Pilgrim River which is the northern edge of the sockeye salmon distribution.
(Credit: Mike Carey, USGS. Public domain.)

Scientist cutting open head of dead sockeye salmon to get otolith (ear bone) for analysis

Chris Zimmerman removing otoliths from sockeye salmon carcasses on the shoreline of Salmon Lake.  Otoliths are used to read age of a fish and other chemical signatures over time.
​​​​​​​(Credit: Mike Carey, USGS. Public domain.)