Spatial distribution and interannual variability of catches for pink salmon of artificial origin in the waters of eastern coast of Sakhalin Island in modern times

Abstract

Mixed herds of pink salmon including the fish of both artificial and natural origin are exploited by fishery in many areas of Sakhalin Island. To separate the fish of different origin, large-scale tagging of the juveniles otoliths is conducted at salmon hatcheries since 2008. The hatcheries contribution to pink salmon catches in certain areas of East Sakhalin (from the Langeri River to Cape Crillon) is assessed for 2011–2017 on the base of previously published and new data on occurrence of the fish with marked otoliths. The specimens were sampled from commercial catches of 5–13 fixed nets and from research catches in 5–9 rivers throughout entire spawning season. In total, 40,992 otoliths of pink salmon were collected. The otoliths were prepared for microstructure analysis using standard methods of processing. All marked otoliths had the tags of hatcheries located on the eastern coast of Sakhalin. The fish of artificial origin were caught in all examined sites between Cape Soymonov and Cape Crillon every year, but in some years their distribution was wider, up to the northern limit of the investigated area (Langeri River). The fish of artificial origin migrated to all rivers within this area. The total annual landing of the pink salmon marked at artificial hatcheries in the East Sakhalin waters was estimated as 300–10600 t, by years. The rest of the fish of artificial origin (0.4–2.3 . 106 ind.) entered to the rivers and mostly reached the spawning grounds. Long-term dynamics of landing was compared for the pink salmon of artificial and natural origin and their independence was concluded. Distribution of pink salmon originated from certain artificial hatcheries by sites of catch was rather constant that allowed to extrapolate the results for marked fish to all released pink salmon. The highest contribution of pink salmon of artificial origin was noted for catches in the Aniva Bay (23.9 %, on average), it was lower for catches at the southeastern coast of Sakhalin (13.2 %), and more lower for catches in the Patience Bay (7.6 %). Total number of pink adults of artificial origin caught on a particular stretch of coast did not depend directly on the number of pink juveniles released from a local hatchery, but was determined mainly by number of fish migrated there from other hatcheries. The landing of pink salmon originated from artificial hatcheries directly in the rivers where these hatcheries were located and at their mouths was < 10 % of the total catch of pink salmon of artificial origin. Schemes of quantitative distribution of pink salmon with marked otoliths in the waters of East Sakhalin are presented.