Forage Base and Feeding of the Sea of Okhotsk Pink Salmon Oncorhynchus gorbuscha (Salmonidae) During Marine Migrations in the Sea of Okhotsk and Adjacent Waters of the Pacific Ocean
摘要
The forage base of the pink salmon Oncorhynchus gorbuscha during its marine migrations in the Sea of Okhotsk (August–December) is sufficient for feeding and migration of juveniles of this species despite significant changes. According to long-term data, the daily ration of juvenile Sea of Okhotsk pink salmon was 7–10 in summer and 4.0–4.5% of the body weight in autumn. At the beginning of the winter period, in December, the biomass of forage plankton in the epipelagic zone in the Sea of Okhotsk and northwestern Pacific waters off the Kuril Islands decreased by an average of 1.5 times, in the upper epipelagic zone, by more than two times. However, the feeding intensity of juvenile pink salmon decreased slightly in December compared to October–November, mainly due to large interzonal copepods (Copepoda) due to their seasonal migrations to the mesopelagic zone for wintering. The minimal biomass of forage plankton (on average 204.1 mg/m3) and low feeding intensity of pink salmon (daily ration of 3.1%) were observed during the typical winter period, in January. In late February–early April (winter-spring period in most of the studied water area), large interzonal copepods migrate to the epipelagial after wintering, their proportion of the forage plankton was 48.9%, as a result, the average biomass of the latter increased more than twice (up to 459.4 mg/m3). The feeding intensity of pink salmon also increased (daily ration 3.6%). During spring and summer periods, the biomass of forage plankton in the epipelagial increased significantly to 718.7 and 820.2 mg/m3, respectively. Pink salmon fed intensively before the beginning of pre-spawning migrations. In July, despite a favorable forage base, the feeding intensity of migrating pre-spawning pink salmon was minimal: in open waters the daily diet was 3%, closer to the coast, 1–2%.