Temporal trends in geographic clines of chum salmon reproductive traits associated with global warming and hatchery programmes.
Geographic clines in life-history traits are often recognized as adaptations to the associated transitional environments. As life-history traits evolve in response to anthropogenic processes, these geographic clines can change over time. The geographic and temporal trends of reproductive traits in Japanese chum salmon Oncorhynchus keta were analysed. Data were collected from 23 rivers located between 36° and 45° north latitude and 136° and 146° east longitude from 1994 to 2010. We confirmed the geographic clines of reproductive traits: relative gonad weight increased in more northeasterly locations, and females had fewer, but larger, eggs in more northeasterly locations after standardization by body size. The geographic clines changed over the years. The northeastward geographic trend of increasing gonad weight became more pronounced over time. Temporal trends towards smaller but more numerous eggs were evident, especially in northeasterly locations. Under natural and sexual selection, gonadal investment should be constrained by the energetic demands of the cost of migration, particularly in southwesterly 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 a constraint on growth opportunities for their offspring in more northeasterly regions of Japan, which are colder and have less available food. However, global warming may mitigate this constraint on growth opportunities in northeastern Japan by increasing river water temperatures. Moreover, we consider that relaxation of the effects of natural and sexual selection on intense breeding competition and early growth conditions has occurred through domestication selection by hatchery programmes. These may have caused temporal shifts in geographic clines. 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-reared fry.