Temporal trends in geographic clines of salmon eggs associated with global warming and hatchery programs

Koh Hasegawa and colleagues describe their latest research examining the geographic clines in Japanese chum salmon (Oncorhynchus keta).

Many animal and plant species show geographic clines which are often recognized as adaptations to the associated transitional environments. A well-known example is that the body size of bears increase in more northern regions. Even people may notice clines in their daily lives, such as people in Japan who love cherry blossoms looking forward to the bloom date by checking the newspaper every day in Spring.

As life-history traits evolve in response to anthropogenic processes however, these geographic clines can change over time. In our study, we analysed the geographic and temporal trends of reproductive traits in female Japanese chum salmon (Oncorhynchus keta).

Chum salmon originate from various natal rivers of the Japanese archipelago and migrate to the North Pacific Ocean for feeding. They are semelparous (have single reproductive cycle), and migrate from the ocean to their natal rivers in autumn and winter for spawning.

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Left: Adult chum salmon migrating back to their natal river for spawning. Right: Chum salmon fry migrating to the ocean from their natal river

As a popular food source around the world, many chum salmon are now raised in fisheries to meet the increasing demand that can’t be maintained without such interventions. As part of a sophisticated hatchery programme, wild chum salmon are rounded up by weirs (traps) in rivers (spoiling their chance for spawning in natural environments) and their eggs are surgically extracted for artificial fertilization. The spawned fry are then reared in ponds safe from predators and fed sufficiently before being released into the wild.

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A weir (trap) settled in Nishibetsu River, eastern Hokkaido, northern Japan.

In our study, we used data from salmon caught in 23 rivers across the Japanese archipelago between 1994 and 2010 to find geographic clines of their reproductive traits: relative gonad weight increased in more north-easterly locations and females had fewer but larger eggs in more north-easterly locations (after standardization by body size). Further, we found that the geographic clines have changed over the years where the north-eastward geographic trend of increasing gonad weight became more pronounced over time. There were also evident temporal trends toward smaller but more numerous eggs, especially in north-easterly locations.

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Images of salmon fry (left) reared in hatcheries and (right) survive in natural environment

Under natural and sexual selection, gonadal investment should be constrained by the energetic demands of the cost of migration, particularly in south-westerly locations (which are farthest from the feeding grounds), and by breeding competition during natural reproduction. In addition, females should have fewer but larger eggs owing to constraint on growth opportunities for their offspring in more north-easterly locations, which are colder and have less available food.

However, global warming may mitigate this constraint on growth opportunities in north-eastern Japan by increasing river water temperatures. Moreover, we hypothesise that relaxation of the effects of natural and sexual selection on spawning behaviour (e.g. redd construction and breeding competition) and of early growth conditions has occurred through domestication selection by hatchery programmes; this relaxation effects may be larger in more north-easterly locations due to high abundance of salmon and severe growth condition in natural environments. A combination of these factors may have caused the observed temporal shifts in geographic clines.

Based on the results of our study, we should consider several co-occurring anthropogenic impacts on natural and sexual selection when evaluating the life-history traits of organisms. For the sustainable use of biological resources, maintaining geographically adapted life-history traits during adaptation to climate change is essential. Therefore, the conservation of wild salmon populations formed by natural selection is preferable to the stocking of hatchery fry.

Read the full research article: “Temporal trends in geographic clines of chum salmon reproductive traits associated with global warming and hatchery programmes” in Issue 2:4 of Ecological Solutions and Evidence.

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