Formula for predicting climate change impact on salmon stocks

University of British Columbia researchers have found a way to accurately predict the impact of climate change on imperilled Pacific salmon stocks that could result in better management strategies.

The findings, among the first to quantify a relationship between river temperature and salmon mortality rate, are published in the current issue of the journal Physiological and Biochemical Zoology.

While climate change and rising river temperatures have been linked to dwindling salmon stocks, other factors have made it difficult to measure the exact impact – these including diseases, fisheries and man-made structures such as dams and fish ladders.

"Calculating the affect of climate change on animal fitness has been particularly challenging for scientists," says lead author Tony Farrell, who is jointly appointed in the Dept. of Zoology and the Faculty of Land and Food Systems.

"Animals have a thermal window, or high and low temperatures between which they are at their best for aerobic activities. Our study has shown that high temperatures push certain sockeye salmon stocks beyond their thermal window, resulting in cardiovascular failure and death," says Farrell.

Led by Farrell and Prof. Scott Hinch in the Dept. of Forest Sciences and the Institute for Resources, Environment and Sustainability, the UBC team has been studying Pacific salmon using biotelemetry trackers for a decade. They have identified the optimal thermal windows for several species of Pacific salmon, which coincide with the historic temperatures the fish would have encountered while migrating in the river.

In 2004, unusually warm river temperatures and earlier entry into the Fraser River system contributed to the "disappearance" of 70 per cent of the Weaver Creek sockeye stock.

"We analyzed river temperature data and the sockeye's migration dates and found that almost half of the population likely experienced temperatures that would cause a complete collapse of aerobic scope," says Farrell.

"In contrast, the Gates Creek sockeye stock, which have a higher thermal window, experienced few problems with the same high river temperatures that year."

In further investigations, the UBC team captured and placed individual fish in holding tanks of varying temperatures to simulate traversing different river temperatures before releasing them simultaneously back to the migratory run. Fish released from a high holding temperature were half as successful as those from colder environments at reaching their spawning grounds.

In a separate study, fish were intercepted during migration and implanted with biotelemetry trackers. None of the tracked salmon survived after release at river temperatures above the thermal window (at 19.5 degrees Celsius). Fish released at a cooler river temperature – one within the thermal window – later in the summer had much greater survival rates.

"This study shows that an increase over the past 50 years of 1.8 degrees Celsius in the Fraser River's peak summer temperatures is too much too fast for some salmon stocks," says Farrell.

"It also shows that climate change affects even the same species differently because individual populations may have adapted to their respective environments. As a result, we must develop strategies based on population – or even watershed – to predict, manage and conserve stocks."

Farrell adds that the same concepts may be applied beyond salmon management as another recent study co-authored by Farrell and published in the journal Science has revealed similar findings for fish and squid from the Atlantic Ocean.

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Formula for predicting climate change impact on salmon stocks