Juvenile supplementation does little to increase wild salmon stocks

Stocking of juvenile salmon in the Torniojoki river and other spawning rivers extending from the Gulf of Bothnia in the last few decades has not markedly increased the salmon population, according to a doctoral dissertation set to be presented at the University of Helsinki on August 29.

The stocked salmon fare poorly due to high natural mortality rates and are more vulnerable to net fishing than wild salmon, meaning few supplemental fish survive long enough to return and spawn. The dissertation reaches the conclusion that stock supplementation should be abandoned as a means to conserve wild stocks in favour of adequate fishing restrictions.

Atso Romakkaniemi is a researcher at the Finnish Game and Fisheries Research Institute. In his study, he examines the development of the Gulf of Bothnia’s salmon stock over a period of twenty years. His data on the salmon stock of the Torniojoki river contain details about the survival and characteristics of the stocked salmon throughout the different phases of their life cycle.

Overfishing in the late 20th century caused the wild salmon stocks of the Baltic Sea to become endangered. Supplementation of hatchery-reared juvenile salmon was introduced in many spawning rivers to minimize the risk of stock depletion and to encourage stock recovery. Supplementation was meant to increase the amount of spawning salmon and thus improve the natural production of young fish. Because the salmon stock in question is genetically distinct, any fish farming and stocking must support the preservation of the stock’s innate biological characteristics. The study appraises the achievement of these goals.

Weak survival rates and genotypic divergence in stocked salmon

Substantial divergences between the characteristics of reared and wild salmon were identified. The divergences grew in tandem with the time the reared fish spent in the hatchery and their rate of growth there. The hatchery’s selection of brood fish may also have resulted in changes in the maturation age of the reared salmon. These factors increase the risk of genotypic divergence in the extant wild population.

The survival rates of wild smolt salmon was twice that of smolts stocked as parr and 2.5 times higher than that of stocked two-year old smolts. This is in part explained by the fact that reared salmon are more vulnerable to sea fishing than wild salmon. This divergence only grew as the fish reached spawning age, however: the survival rate of wild salmon from smolt to spawners was 2.8 times higher than that of salmon stocked as parr and 3.3 times higher than that of salmon stocked as smolts. Due to the lower survival rates of the reared salmon, there were far fewer stocked fish among the returning spawning fish than there had been earlier among the parr.

Fishing restrictions are more effective at reviving stocks than supplementation

Supplementation was not seen as having an effect on the recovery of wild salmon stocks in the Torniojoki river and other rivers off the Gulf of Bothnia in the 1990s. Revival of the stocks was just as rapid in areas with no supplementation. Fishing reductions resulted in manyfold increases in spawning populations, while stock supplementation in the Torniojoki river improved the spawning population by a 20-30 per cent margin at most. Large amounts of supplemental stock appeared to weaken the survival rates of young fish overall, and so the population-increasing effect of the supplements may in reality be even less. The development of salmon stock is also affected by other factors, including anthropogenic activities, stock cycles and environmental conditions.

The doctoral dissertation will be presented in Tornio, Finland on Friday 29August.

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