Fish ecology in Ootsa Lake, British Columbia in relation to submerged timber harvesting

Ootsa Lake is part of a 48-year-old northwestern British Columbia reservoir (Nechako Reservoir) that contains extensive amounts of flooded forests. Recently, logging companies have begun harvesting this standing underwater timber because of its potential as an alternative fibre source. This project was initiated because of the general lack of information on fishes in the reservoir and because the potential impact of submerged timber removal on the fish community had not previously been explored. Catches with experimental gill nets and live traps were monitored between July 7 and October 22, 1998 to estimate the abundance, diversity, size, and condition of fishes in Ootsa Lake. Catch per unit effort (CPUE) was quantified and ranked to evaluate the impact of submerged timber harvesting on the abundance of the dominant fish species. Mean ranks were compared across species; across three near-shore habitats with different levels of structural heterogeneity (treed, harvested, or open); and across the summer and fall seasons. Shannon-Wiener diversity indices were calculated and compared across habitats. Size and condition of the main fish species were compared across habitats. Fish abundance was positively associated with habitat structural complexity. Overall abundance was highest in the treed habitat. In the summer, rainbow trout (Oncorhynchus mykiss) abundance was highest in the treed habitat whereas northern pikeminnow (Pfychochei/us oregonensis) were abundant in both treed and harvested habitats. Rainbow trout and northern pikeminnow abundance in near-shore areas decreased in the fall, but numbers of kokanee (Oncorhynchus nerka) increased. Shannon-Wiener diversity indices did not differ significantly among habitats overall, but in the summer the index was significantly higher in the open than the treed and harvested habitats, whereas it was significantly lower in the fall. Sizes of rainbow trout, kokanee, and northern pikeminnow were related to habitat structural complexity with the smallest fish occupying the treed habitat and the largest rainbow trout and northern pikeminnow occurring in the open habitat. Fulton's condition factor was not consistently related to habitat structural complexity but differed among sites within habitat. Kokanee were found to be exceptionally small (mean fork length = 180 mm ~ 0.95) with over 85% of individuals being age 2+. They also exhibited characteristics similar to "residual" sockeye salmon (Oncorhynchus nerka) including a significantly male-biased sex ratio and olive-black spawning colouration. I hypothesize that kokanee are exhibiting adaptive life history patterns in response to the cold, oligotrophic conditions in this large reservoir. Juvenile fish often seek complex underwater structure for protection from predators, and submerged structure has been shown to increase invertebrate production. Therefore, it is likely that small rainbow trout and northern pikeminnow inhabit areas with submerged timber because it provides refuge from predators and/or because of a high abundance of food. The use of benthic harvested areas by small northern pikeminnow may be for similar reasons because overturned root wads and woody debris remain post-harvest. Diversity of fish species was not positively associated with habitat structural complexity.