Coastal Cutthroat Trout as Sentinels of Lower Mainland Watershed ...

25spawners can also be affected by mortality from sport fishing as well as predation. Thesefactors may be difficult to separate except in lightly fished areas. Local coastal marineconditions are likely to have more influence on survival than high-seas conditions.Cutthroat diets in the ocean include amphipod shrimps, isopods and marine fishesincluding northern anchovy, kelp greenling, cabezon, and rockfish, and occasionallyyoung salmonids. This is similar to chinook and coho salmon for which there is a 60 %overlap in marine diets (Pearcy 1997). Cutthroat diets in the Columbia River estuary areprimarily comprised of threespine stickleback, shrimp and herring. Growth rates of searuncutthroat in the ocean are about 25 mm per month (Trotter 1997).2.1.4.2 Lake PopulationsLake populations of coastal cutthroat can be residents of small lakes, where mature adultsmove to inlet or outlet streams to spawn, and juveniles rear for varying periods untilmovement into the lake. In smaller lakes, the diet of allopatric cutthroat is zooplanktonand aquatic insects, but under sympatry with Dolly Varden char there is much greater useof terrestrial insects (Andrusak and Northcote 1971). Under these conditions there islittle use of fish in the diet of cutthroat in coastal BC lakes, and these stocks are largelynon- piscivorous. Regardless, because cutthroat are documented to be cannibalistic, evenin the small allopatric lakes, there is selection for young-of-the-year to remain in nurserystreams until the parr stage. In sympatry with char, there is diet-based spatial segregationof the two species, with cutthroat feeding nearer to the surface and char feeding more inthe benthic zone (Andrusak and Northcote 1971).Overall, coastal cutthroat trout are piscivorous in most coastal lakes, especially largerlakes. Through piscivory, adults can attain sizes of 35-40 cm at maturity, with fish up to52 cm documented from the most productive Powell River lakes (Global FisheriesConsultants Ltd 1993). Cutthroat attain sizes up to 60 cm in Ruby Lake on the SunshineCoast (Wightman and Taylor 1979). Juvenile cutthroat migrate into Ruby Lake at age 1to 3, as determined from interpretations of samples of adult scales. As adults they feedon threespine stickleback, peamouth chub and kokanee in Ruby Lake (Wightman andTaylor 1979). However, 29 % of 208 cutthroat trout tagged at this lake were recoveredfrom Sakinaw Lake, located downstream below a barrier in the same drainage. Althoughthere is some evidence of between-lake interchanges, migrations to external areas arelikely unnecessary for lake cutthroat populations if there are adequate nursery streamsand foraging resources. An exception would be large oligotrophic lakes such as Harrisonand Pitt Lakes in the Lower Fraser basin, where seasonal movements to riverinespawning areas of salmon appear to be important as feeding migrations (N. Basok pers.comm. 2004), although these migrations have not been confirmed by tagging studies.2.2. Population GeneticsIt is beyond the scope of this report to provide a comprehensive review of the populationgenetics of cutthroat trout at the species level because fisheries management in the LowerMainland Region is directed primarily at the local population level. Primary