Climate change and pre-spawn mortality linked by genetics

An analysis of 32 years of scientific data suggests that because of warming ocean temperatures, salmon are evolving to return to river systems to spawn earlier than ever before.

The New Scientist reported today:

In the 1970s, Anthony Gharrett was studying why some salmon migrate up Alaska’s Auke Creek a month later than the rest of the population. Gharrett selectively bred some late-migrating fish so that they would share a rare genetic mutation that had no impact on the salmons’ survival prospects. This allowed him to identify the late migrators even before the migration began.

By the time Gharrett ended these salmon-breeding investigations in 1985, 26 per cent of the late-migrating salmon population had the genetic marker, compared to 3 per cent of the rest of the population. He continued to collect DNA from the migrating salmon population every few years.

Fast forward to 2011, when Ryan Kovach and David Tallmon analysed Gharrett’s 32-year-long fish DNA record. They found that the number of salmon with the genetic marker – more likely to be late-migrating fish – was relatively stable throughout the 1980s.

Disappearing marker

But between 1989 and 1993 there was a steep decline in the marker’s frequency. It is now found in about 3 per cent of all fish, and it is no longer possible to distinguish early and late-migrating salmon by studying the frequency of the genetic marker in the population.

The speed with which the marker has disappeared from the population suggests that something in the salmon’s environment changed between 1989 and 1993, making late-migrating fish less fit and increasing the number of fish in the general population with an early-migrating heritage. Stream temperatures in 1989, during the peak migration time, were the second highest on record, the researchers note.

This is a very interesting connection, with strong data to back it up. And it could provide some insight into what is happening with Fraser River sockeye.

Climate change may be forcing salmon to evolve new survival strategies, such as returning to rivers earlier or later to spawn when the temperatures are just right.

Pre-spawn (en route) mortality has increased in some stocks for the past 20 years. The report on climate change prepared for the Cohen Commission strongly suggests warming ocean and river temperatures are affecting the fish, possibly forcing similar evolutionary changes as we are seeing in the Auke Creek salmon in Alaska.

There is good evidence that the  among-stock patterns in en route loss are indicative of stock-specific abilities to cope with warming rivers and high river temperatures.

It seems likely natural selection is favouring the Fraser River sockeye which have developed the best strategies to cope with changes in the ocean.

And with such massive forces at work driving evolution from Alaska to California, which are all seeing similar fluctuations in salmon populations, it seems highly unlikely that a few salmon farms in B.C. have anything to do with it.


2 thoughts on “Climate change and pre-spawn mortality linked by genetics”

  1. Genetic or not I don’t believe that early entry has been beneficial for Fraser River Sockeye. Another study just came out last week which suggests that overall productivity of Sockeye is widespread – even in parts of Alaska.

    I mean if that evolutionary change were beneficial for Fraser Sockeye i wouldn’t think that we should be seeing reduced productivity as suggested in the report.

    1. We totally agree with you. If anything it seems late entry has been more beneficial but the science of predicting fish stocks is incredibly complex and we barely understand it. And not all evolutionary changes are beneficial. Especially when changes in the ocean are quicker than natural selection can keep up. Perhaps some stocks of Fraser sockeye are struggling to adapt and their strategy just isn’t working.

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