An Evaluation of Bull Trout Bycatch from Lake Trout Suppression Netting in Lake Pend Oreille, Idaho

Survival rates for kokanee Oncorhynchus nerka in Lake Pend Oreille reached record lows following increased predation during the past decade.  To increase kokanee survival in Lake Pend Oreille, extensive predator (lake trout Salvelinus namaycush and rainbow trout O. mykiss) reduction efforts were initiated in 2006 and included contracting a commercial fishing company to remove lake trout. During 2006-2010, about 57,000 lake trout were removed by gill and trap netting. Incidental bycatch of native bull trout Salvelinus confluentus was over 4,200 individuals and direct mortality rate was 24%.  To minimize bycatch and understand potential effects of netting on the bull trout population, data collected during netting operations were evaluated.  Seasonal catch rates by gill net mesh size and panel height were analyzed and used to design nets that maximized the lake trout to bull trout catch ratio.  Gill nets with stretch mesh of 5.1-7.0 cm were most effective for targeting juvenile lake trout, while stretch meshes of 11.4 and 12.7 cm were best for targeting mature lake trout.  Further, shorter gill net panels reduced bull trout bycatch, especially during the spring.  Genetics samples were collected from bull trout and used to assign fish to their stream of origin, which allowed bycatch risk to be assessed for individual tributary stocks.  Genetics analyses showed that bull trout from all tributaries distribute throughout the lake and should have been equally vulnerable to capture. However, bull trout assigned to the Lightning Creek drainage were overrepresented in the catch, while bull trout assigned to Trestle Creek (the most abundant stock) were underrepresented. These analyses, combined with lake trout telemetry, have allowed netting operations to most effectively target lake trout while minimizing bull trout bycatch.  Additionally, this evaluation showed that lake trout netting has not been detrimental to the Lake Pend Oreille bull trout population.