Predation by Juvenile Hatchery Sea-Run Cutthroat Trout in the Cowlitz River, Washington

The diets of juvenile sea-run cutthroat trout have been documented in small headwater stream reaches and estuaries; however there is little available information for large mainstem rivers systems.  In this study, I examined the diets of hatchery-origin sea-run cutthroat trout Oncorhynchus clarkii from the Cowlitz River, a tributary to the Columbia River in southwestern Washington.  A concern on predation impacts of hatchery-origin sea-run cutthroat trout on juvenile salmonids (specifically natural-origin fall Chinook) has been noted especially for salmon in the lower Columbia River that are listed as threatened under the Endangered Species Act. In the last 30 years, the Cowlitz River has been heavily stocked with hatchery-origin salmon to mitigate for loss of upstream production due to hydroelectric dams.  A hatchery sea-run cutthroat program was also implemented in 1968 to provide recreational opportunities for anglers in addition to production of salmon.  In the Cowlitz River, juvenile cutthroat trout are released from the hatchery in April during the period when natural-origin spring and fall Chinook fry are emerging from the gravel.  This study was therefore designed to assess the diets of juvenile hatchery-origin cutthroat trout in the Cowlitz River from the time they were released until mid-summer (April 15 – August 15).  Additionally, the study was designed to compare the diet of cutthroat trout before and after hatchery-origin fall Chinook fry were released.  A total of 349 cutthroat trout were captured using hook and line sampling and ranged from 90-236 mm in length.  Gastric lavage was used to obtain stomach samples from the cutthroat trout. The majority of cutthroat sampled, (97%) had prey in their stomachs that included invertebrates and fish prey.  Mayfly (Ephemeroptera) and caddisfly (Tricoptera) were the most common invertebrates and 82 cutthroat trout (23.5% of the population samples) had fish prey (i.e. Salmonidea or Cottus sp.) in their diet. Of the cutthroat trout that had fish prey in their diet, 1.7% contained salmonids when only natural-origin fall Chinook were present in the river.  This compares to 33.5% of the cutthroat trout diets that contained fish prey in their diet once hatchery-origin fall Chinook fry were released into the river.  The frequency of juvenile fall Chinook in the diet of cutthroat trout significantly increased once hatchery-origin fall Chinook were available. Cutthroat trout predation on natural-origin Chinook; however was minimal.