Management of human-induced contemporary evolution to maintain and restore genetic diversity in brown trout

Abstract

Maintaining genetic diversity and integrity of animal populations is a key challenge in conservation. While ecological population declines can be reversed, loss of genetic diversity and original trait distribution is often a one-way road. Migratory brown trout is critically endangered in Finland due to extensive modification of river ecosystems and unrestricted fishing on feeding areas. In addition to being too intense in general, fishing creates challenges by being selective for a number of traits in fish. We have shown that recreational angling, for example, is selective for certain personality traits and can cause inherited behavioural changes in the offspring of brown trout. Only a few migratory brown trout stocks representing large watershed-specific areas are maintained in hatcheries and used in large-scale stockings to support intensive recreational fisheries. The hatchery-based broodstocks are under the risk of accumulating genetic changes that improve their survival in the captive environment, but decrease their survival in the wild. While natural reproduction of most migratory populations is marginal, numerous original resident brown trout populations still occur in small brooks. We have studied the potential to use wild resident populations as genetic material to increase the genetic diversity and the adaptability of hatchery broodstocks. Our results show that while interbreeding can improve predation avoidance, survival from predation and even juvenile growth, the migration tendency of hybrid fish can be compromised. In addition, both common garden experiments and population genomic data indicate that migration tendency in brown trout has a genetic underpinning. Thus, the only way to conserve the iconic, fast-growing, late maturing migratory brown trout populations is to restore their natural life-cycle and manage the fisheries at sustainable harvesting levels. The fine-scale genetic structuring, that we have revealed using both traditional microsatellite and genome-wide techniques, provides solid evidence that each brown trout population is genetically distinct and thus intrinsically valuable. Our results imply that extinct local stocks cannot be equivalently replaced with non-native fish. This creates significant societal pressures to change the existing policy and legislation of hydropower functions and compensation measures set for the companies that significantly alter natural river ecosystems and prevent the natural reproduction of most of the migratory brown trout stocks. It has become evident that even the broodstocks used to produce hatchery fish to support fisheries need regular updating with local fish, that have experienced natural selection pressures through a natural life-cycle. Selection induced by fishing still remains a challenge but the most effective solution is simple: fishing mortalities have to be decreased to sustainable levels.