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.
Adaptation to novel habitats and phenotypic plasticity can be counteracting forces in evolution, but both are key characteristics of the life history of steelhead/rainbow trout (Oncorhynchus mykiss). Anadromous steelhead reproduce in freshwater river systems and small coastal streams but grow and mature in the ocean. Resident rainbow trout, either sympatric with steelhead or isolated above barrier dams or waterfalls, represent an alternative life-history form that lives entirely in freshwater. We analyzed population genetic data from 1486 anadromous and resident O. mykiss from a small stream in coastal California with multiple barrier waterfalls. Based on data from 18 highly variable microsatellite loci (He = 0.68), we conclude that the resident population above one barrier, Big Creek Falls, is the result of a recent anthropogenic introduction from the anadromous population of O. mykiss below the falls. Furthermore, fish from this above-barrier population occasionally descend over the falls and have established a genetically differentiated below-barrier subpopulation at the base of the falls, which appears to remain reproductively isolated from their now-sympatric anadromous ancestors. These results support a hypothesis of rapid evolution of a purely resident life history in the above-barrier population in response to strong selection against downstream movement.
We monitored growth and life history pathways of juvenile steelhead Oncorhynchus mykiss and compared growth rates between the upper watershed and estuary in Scott Creek, a typical California coastal stream. Growth in the upper watershed was approximately linear from May to December for age-0 fish. For passive integrated transponder (PIT) tagged, age-1þ fish, growth transitioned to a cyclic pattern, peaking at 0.2% per day during February-April, when maximum flows and temperatures of 7-128C occurred. Growth of PIT-tagged fish then slowed during August-September (0.01% per day), when temperatures were 14-188C and flows were low. During each spring, smolts (mean fork length [FL] 6 SE ¼ 98.0 6 1.2 mm) and fry migrated to the estuary; some fish remained there during summer-fall as low flows and waves resulted in seasonal sandbar formation, which created a warm lagoon and restricted access to the ocean. Growth in the estuary-lagoon was much higher (0.2-0.8% per day at 15-248C). Our data suggest the existence of three juvenile life history pathways: upper-watershed rearing, estuary-lagoon rearing, and combined upperwatershed and estuary-lagoon rearing. We present a model based upon the above data that reports size at age for each juvenile life history type. The majority of fish reaching typical steelhead ocean entry sizes (;150-250 mm FL; age 0.8-3.0) were estuary-lagoon reared, which indicates a disproportionate contribution of this habitat type to survival of Scott Creek steelhead. In contrast, steelhead from higher latitudes rear in tributaries during summer, taking several years to attain ocean entry size.
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