Columbia River northern pike: Investigating the ecology of British Columbia's new apex invasive freshwater predator

Northern pike (Esox lucius) is a widely distributed freshwater fish in the northern hemisphere, holding both recreational and commercial value in many countries including Canada. Pike are apex predators and are well regarded as a sport fish due to their size and voraciousness, leading to a long history of introductions outside their native range in North America. Invasive pike populations can impose significant top-down pressure on native fish community structure through predation and competition for resources, and have been linked with significant reductions of a variety of native species. Pike have recently invaded the Columbia River in British Columbia downstream of the Hugh L. Keenleyside Dam, threatening important sport fisheries, local ecosystems, and Species at Risk recovery work currently being conducted in the Columbia and its tributaries. In response to the invasion, the province implemented a pike suppression program, yet several questions remain regarding the ecology and behaviour of the recently colonized population. This research aimed to answer these questions and aid in the development of a long term management strategy for the Columbia River. Four years of data collected through the suppression program were analyzed to investigate the potential risk the invasion poses to native fish communities in B.C., the status of pike in the B.C. Columbia River, and the efficacy and impacts of current efforts. A total of 352 northern pike were captured through the suppression program by gill-netting (n=323) and angler incentives (n=29), ranging in length from 260 mm to 1050 mm. Cleithra age estimates indicate pike from within this population are achieving sizes in excess of 1 m in five to six years, and are in better than average body condition (relative weight Wr used) when compared to other North American populations. A total of 584, 659, 407, and 676 gill-net hours were set in 2014, 2015, 2016, and 2017 respectively, and an overall reduction in pike catch per unit effort (CPUE) was observed from the initial year (0.19 pike/hr) to the final year (0.05 pike/hr). This suggests gill-netting has been effective at reducing the number of adult pike in B.C. and should be conducted annually. The capture of several juvenile pike (n = 14) near Castlegar in 2017 indicates spawning is occurring in B.C. and future years of suppression should include controlling the juvenile cohort. Salmonids, particularly whitefish spp., appear to be at the greatest risk from pike predation making up approximately 50% of pike diet overall. Acoustic tracking of pike indicated the majority of individuals from within this population remain relatively sedentary, yet one individual migrated in excess of 100 km between B.C. and Washington. Microchemical analysis of pike otoliths revealed significant differences in ratios of Sr and Ba to Ca between pike caught in the Columbia and Pend d’Oreille Rivers, and that a small portion of the current Columbia population in B.C. appears to have originated in the Pend d’Oreille River. Fluctuations in otolith elemental Mg concentrations also appear to provide information that can be used to estimate the age of northern pike and potentially other esocid species. Lastly, the use of environmental DNA (eDNA) sampling as a tool to monitor the presence and spread of northern pike in B.C. was explored with minimal success; the protocol used and results obtained are discussed.