Migration Timing, Size, and Salinity Tolerance of Sea-Type Sockeye Salmon (Oncorhynchus nerka) in an Alaska Estuary

Abstract

Migration timing, size, and salinity tolerance were determined for sea-type sockeye salmon (Oncorhynchus nerka), which migrate to sea as underyearlings (age-0), in the Situk River estuary, Southeast Alaska. Ten sites in three habitat types were seined monthly from Sate April through August, 1987, and age-0 sockeye from the estuary were tested for salinity tolerance. Age-0 sockeye were most abundant (up to 13∙m−2) from late April through June, and by late July, most sockeye had left the estuary. Mean fork length (FL) was 31 mm in April and increased 0.4 mm∙d−1 to 70 mm in late July when most (72%) remaining sockeye had grown to about the same size as age-1 smolts (69–95 mm) emigrating in May and June. Mean FL of age-0 sockeye in the estuary in July was 23 mm greater than in freshwater areas of the river. Salinity tolerance was directly related to fish size, and a size of at least 50 mm was required for 100% survival in seawater. Because salinity in the estuary ranged 0–30‰, sockeye of all sizes could survive in the estuary. Thus, in 3–4 mo, sea-type sockeye attained a size large enough to adapt to seawater and migrate to sea.