Marine survival of steelhead (Oncorhynchus mykiss) enhanced by a seasonally closed estuary

• Published in: Canadian journal of fisheries and aquatic sciences 2008-10, Vol.65 (10), p.2242-2252
• Main Authors: Bond, Morgan H, Hayes, Sean A, Hanson, Chad V, MacFarlane, R. Bruce
• Format: Article
• Language: English
• Subjects: Animal and plant ecology; Animal, plant and microbial ecology; Applied ecology; Aquatic sciences; Biological and medical sciences; Brackish; Canada; Coastal streams; Emigration; Environmental aspects; Estuaries; Evaluation; Exploitation and management of natural biological resources (hunting, fishing and exploited populations survey, etc.); Fish migration; Fundamental and applied biological sciences. Psychology; Health aspects; Lagoons; Management; Mortality; Oncorhynchus mykiss; Rivers; Salmon; Sand bars; Sea water ecosystems; Spring; Steelhead (Fish); Survival analysis; Synecology; Trout
• ISSN: 0706-652X; 1205-7533
• DOI: 10.1139/F08-131

To investigate the role that estuaries play in the survival of steelhead, Oncorhynchus mykiss , we compared juvenile size at ocean entry with back-calculated measures of size at ocean entry for returning adults in Scott Creek, a representative California coastal stream. During the annual spring emigration, the largest smolts (>150 mm fork length (FL)) move directly to sea, while some smaller smolts remain in the estuary until sandbar formation creates a closed freshwater lagoon. High growth rates in the estuary throughout the summer result in a near doubling of fork length from the time of estuary entry (mean FL of spring migrants = 102.2 mm;; mean FL of fall lagoon resident = 195.9 mm). Analysis of the scale morphology of returning adult steelhead indicates that there is strong size-dependent mortality at sea, with estuary-reared steelhead showing a large survival advantage, comprising between 87% and 95.5% (based on tag returns and scale analysis, respectively) of the returning adult population despite being between 8% and 48% of the annual downstream migrating population. Although the estuary forms less than 5% of the watershed area, it is critical nursery habitat, and steelhead population persistence in southern margin ecosystems may well depend upon healthy estuaries.