The application of recent smoltification research to public hatchery releases: An assessment of size/time requirements for Columbia River hatchery coho salmon ( Oncorhynchus kisutch)

Abstract

This paper develops a hypothesis for the variations observed in adult contributions resulting from size/time of release experiments conducted at West Coast hatcheries over the past decade. Subsamples of hatchery coho salmon were transferred to seawater net-pens and observed for up to 6 months, during which time morphologically distinct forms — parr (stunt and revertant), transitional and smolt — were identified. Both parr forms exhibited little or no growth in seawater net-pens, but maintained plasma sodium concentrations slightly above normal. False smolting occurred for 1–2 months after seawater entry, followed by reversions to parr (desmoltification) of the smaller fish in the population. Critical size — the size of the largest parred individual — is a good indicator of the minimum size necessary for survival and growth in seawater, and a critical growth path must be maintained in seawater to avoid reversion to parr. Critical size increased between the summer and winter solstice and was dependent on fish size, time of year, and age at seawater entry. Critical size in fresh water was the best predictor of mortality after transfer to seawater netpens. In May and June, over 79% of downstream migrants from one hatchery recaptured in the Columbia River estuary were above the critical size; in July, only 23% of the migrants were above critical size. Virtually all migrants captured in the ocean were above critical size: those below critical size disappeared between the estuary and the ocean. Fish released in May and June near the peak of gill Na +K +-ATPase and thyroxine surges grew more rapidly in the ocean than those released in July. Fish released in June produced a greater adult contribution to the fishery than fish released before or after. For any given time of release, the greater the mean size, the greater the contribution. Smoltification and desmoltification are initiated by photoperiod and mediated by size, and may be events independent of migration. Reversion to parr may be a delayed and unrecognized source of ocean mortality. It is recommended that fish be released from hatcheries at sizes substantially larger than at present.