Responses of sockeye salmon (Oncorhynchus nerka) embryos to intragravel incubation environments in selected streams within the Stuart - Takla watershed
Creators: name: Cope, R. Scott
Publisher: name: University of British Columbia
Year: 2009

description: Before impacts of forest harvesting can be identified, the natural physical and biological influences on incubation processes must be understood within interior British Columbia watersheds. The early Stuart stock of sockeye salmon {Oncorhynchus nerka) utilize the most northerly nursery habitat of the Fraser River sockeye stocks. This has led to speculation that production may be limited by high overwinter incubation mortality. An in situ incubation study was conducted on four adjacent tributaries of the Stuart Takla watershed (Kynock, Forfar, Gluskie, Bivouac creeks), during the 1993 and 1994 broodyears. The study objective was to estimate overwinter survival of sockeye salmon embryos within various redd micro-environments. It was hypothesized that spawning salmon select incubation sites based on environmental cues to optimize egg to fry survival. Egg to pre-emergent , fry bioassays, in conjunction with microhabitat environmental monitoring, were implemented to define a range of natural spawning conditions and their relative contribution to fry recruitment. Results demonstrate that high quality, relatively invariant incubation environment resulted in the lack of classical relations observed in previous studies between incubation parameters and survival.. Physical processes (i.e. hydraulic regime, bedload characteristics) and biological processes (i.e. mass cleaning by high densities of spawning adults) result in uniformly high quality gravel conditions with permeabilities, surface water interchange, and intragravel dissolved oxygen levels associated with high incubation success. Alternative hypotheses of random egg deposition and unlimited high quality habitat were rejected due to; 1) observed spatial preferences and, 2) expansion/contraction of range under different annual population sizes. Sockeye salmon successfully spawned over a wide range of habitats. High density spawning habitat was the downstream end of pools at the pool riffle interface. Habitats utilized to a lesser degree included; riffles, stream margins, intermittent side channels and portions of the off-channel habitat. Survival rates between these habitat types were not significantly different in contrast to predictions generated from optimality models. This was due to the definition of "marginal" habitat. In situ redd simulations showed similar intragravel conditions, in both low density (i.e. assumed marginal) and high density (i.e. assumed preferred) areas. Spawning adults avoided truly marginal areas with intragravel dissolved oxygen levels below 3.0 mg/1. A number of adaptations which would optimize incubation success in northern environments were identified within the early Stuart stock of sockeye salmon. Early Stuart sockeye risk energy depletion and seasonal maximum temperatures during migration and spawning. By spawning early in the season (Jul. - Aug.), early Stuart sockeye enjoy advanced embryological development prior to the onset of low water temperatures. Embryos rapidly accumulate the thermal units necessary to hatch, thereby becoming mobile in time to avoid freezing and desiccation as water-levels decline and reach seasonal minima. Embryos and alevins of the early Stuart stock can apparently tolerate temperature conditions previously considered lethal. Fry successfully emerge in the spring after accumulating less thermal units than any other Fraser river stock. The trade off against this strategy is the effect of unusually stressful migration conditions on the quality and viability of the gametes. Evidence of this trade off was obtained in 1994, when egg survival rates were very low for spawners that arrived late and had suffered severe thermal stress during migration.