Estimating the occurrence of chum salmon Oncorhynchus keta of artificial origin on spawning grounds in the rivers and lakes of Iturup Island

Artificial reproduction of pacific salmon has been repeatedly criticized due to negative impact on natural populations. One of its aspects is replacement of wild spawners by spawners originated from hatchery on natural spawning grounds. This problem is relevant to Iturup Island, where the chum salmon stock is formed by both natural reproduction and artificial breeding. To evaluate their ratio, otolith marking of juveniles has been massively implemented at Iturup salmon hatcheries since 2009. The otoliths of chum salmon were collected for analysis from the spawning grounds in 6 rivers and lake-river systems of the Okhotsk Sea coast in the northern part of Iturup Island and at the slaughter point of salmon hatcheries from 2012 to 2022. Microstructure of otoliths from 8229 fish individuals, including 2709 specimens of сhum salmon, was examined to identify the tagged fish. The chum salmon of artificial origin were presented on all examined spawning grounds. Their number was calculated taking into account the percentage of tagged juveniles released from the hatcheries. Low level of straying was concluded for chum salmon on Iturup Island. Long-term dynamics of the ratio between wild and artificial spawners on spawning grounds was determined by changes in volume of juveniles release. Until 2014, the Reidovy and Kurilsky hatcheries released 50.10 6 chum juveniles annually, and the spawners of artificial origin were found sporadically on the spawning grounds in “wild” rivers. New fish hatcheries constructed after 2010 provided increase of their portion in 2014–2022 to 9.6–13.6 % on the spawning grounds unaffected to artificial reproduction and up to 93.3 % on the spawning grounds in the rivers with hatcheries. These high returns were supported by release of 74.4–191.8.10 6 juvenile fish annually. In the Reidovaya and Kurilka Rivers with a branched network of tributaries, the highest occurrence of artificial spawners was observed on the spawning grounds neighbored with hatcheries, whereas their portion decreased to 20 % on the spawning grounds in large tributaries of lower reaches; possibly, spawning grounds where the spawners of artificial origin are absent still exist in the basins of these rivers. Some increase of straying should be probably expected in future years because of huge amount of juvenile chum salmon released into the rivers of Prostor and Kurilsky Bays after 2020, at least 175.10 6 ind.