Stress Responses to Transportation and Fitness for Marine Survival in Coho Salmon (Oncorhynchus kisutch) Smolts

Abstract

Smolting coho salmon (Oncorhynchus kisutch) were transported experimentally at low and high densities (12 and 120 g/L) for short and long periods (4 and 12 h). Because smolts can be transplanted directly to seawater, half of the fish in each treatment were transported to tanks containing seawater and half to tanks containing freshwater. Plasma corticosteroids and glucose were elevated at unloading in all groups, and corticosteroids were still above the resting level 24 h later. Potential smoltification indicators such as plasma thyroxin concentration and gill (Na⁺/K⁺)–ATPase activity were not affected by transportation. Increased corticosteroids were correlated to increased mortality in transported salmon compared to acclimated control fish when subjected to a bioassay of stress — severe confinement. The various transport regimens did not cause increased expression of latent bacterial kidney disease. In a further experiment, smolting coho salmon were released into a stream after transport for different durations. The number of fish migrating on the 1st d appeared to be inversely related to the degree of "stress" en route, but the movements of all groups were similar thereafter. We concluded that transportation caused stress in the fish regardless of hauling regimen; that increased corticosteroids may have potential as indicators of reduced performance capacity; that the greatest stress occurred during loading and the first few hours en route; and that transported smolting coho salmon seem to be equally fit for entry into freshwater or seawater.Key words: coho salmon, stress, transportation, seawater entry, corticosteroids