Review of northern pikeminnow (Ptychocheilus oregonensis) control programs in western North America with special reference to sockeye salmon (Oncorhynchus nerka) production in Cultus Lake, British Columbia Brent Mossop 415 W. 26th St. North Vancouver, B.C. V7N 2G9 Tel: (604) 988-8516 Email: [email protected] Michael J. Bradford Fisheries and Oceans Canada Cooperative Resource Management Institute Simon Fraser University, Burnaby, B.C. V5A 1S6 Jeremy Hume Fisheries and Oceans Canada Cultus Lake Salmon Research Laboratory 4222 Columbia Valley Highway Cultus Lake, B.C. V2R 5B6 Prepared for: Fisheries and Ocean Canada Fraser River Stock Assessment Draft #4 June 2, 2004 Acknowledgements This project was funded by the Cultus Lake Sockeye Recovery Team. We thank Neil Schubert, Don Hall, Bob Gregory, and Jeff Grout for advice on this project. 2 Table of Contents Acknowledgements ....................................................................................................................2 Table of Contents .......................................................................................................................3 Executive Summary ...................................................................................................................4 1. Pikeminnow life history ...........................................................................................6 2. Sockeye life history in Cultus Lake .......................................................................10 3. The Cultus Lake fish community ...........................................................................11 4. Previous pikeminnow work at Cultus ....................................................................12 4.1 Summary of predator removals at Cultus Lake....................................13 4.2 Diet studies...........................................................................................15 4.3 Predator abundance estimates ..............................................................17 4.4 Did removals affect predator populations? ..........................................20 4.5 Did removals affect sockeye survival?.................................................20 5. Pikeminnow research and control in other systems ...............................................27 5.1 Pikeminnow research in rivers .............................................................27 5.4 Pikeminnow research in lakes..............................................................29 6. Feeding ecology of Pikeminnow............................................................................32 6.1 Timing of pikeminnow predation.........................................................32 6.2 Approaches to quantify pikeminnow predation ...................................33 7. Predator responses..................................................................................................36 8. Considerations for designing predator control programs.......................................38 9. Potential pikeminnow control methods..................................................................40 10. Summary, potential management actions, and recommendations .......................41 10.1 Summary ............................................................................................41 10.2 Potential management actions............................................................42 10.1 Future research ...................................................................................43 10.2 Recommendations ..............................................................................44 References ................................................................................................................................46 3 Executive Summary Pikeminnow predation in Cultus Lake has been investigated during a predator control program in 1932-42, a pikeminnow study in 1969, and a pikeminnow control program in 1990-92 (see section 4). Interpretations vary as to the benefits of these programs to sockeye production in the lake. To help provide an informed assessment and evaluate the likelihood that future pikeminnow control can improve juvenile sockeye survival in Cultus Lake, we summarize existing information on northern pikeminnow ecology and predation on salmonids at Cultus Lake and in other systems (Sections 1 –7). The following six points summarize the evaluation. Data that would support pikeminnow removal: 1) At Cultus Lake, pikeminnow eat juvenile sockeye, especially when sockeye are abundant (Section 4.2). 2) Juvenile sockeye survival was much higher during three of the intensive predator control years during the 1930s (Section 4.5). 3) Pikeminnow are abundant in Cultus Lake, likely more abundant now than historic estimates (Section 4.3). Data that would not support pikeminnow removal: 1) Sustained, intensive predator removals (1935-42), while initially successful in reducing the pikeminnow population, may not have been effective at controlling the pikeminnow population over the duration of the control program (Section 4.4). Future pikeminnow control may have similar challenges. 2) The initial increases in sockeye survival following predator control (1935-38 broods) were not sustained over the duration of the control program (1939-41 broods), although this interpretation is confounded by high density-dependent mortality in some broods. Increases in survival pikeminnow removals in the 1990s were less striking and are confounded by data quality issues (Section 4.5). 3) Pikeminnow switch to feed on other prey and eat few sockeye when sockeye are not abundant (Sections 7 and 4.2). 4) The current pikeminnow abundance and likely acceptable means of removal would only result in a small decrease in the population. 4 Exceptional sockeye survival during three of the predator control years in the 1930s provides the only empirical evidence for benefits to sockeye. However, removals could potentially have an incremental benefit to sockeye survival, provided that pikeminnow continue to feed on sockeye when sockeye abundance is very low. Given the constraints of future pikeminnow removals and the potential for complex ecosystem responses following intensive pikeminnow removals, directed efforts at suspected predation ‘hot-spots’ may provide a beneficial management action over the short-term, while Eurasian watermilfoil control may be effective at controlling the pikeminnow population over the longer term. Trapnets can be used to target pikeminnow that aggregate at the outlet of the lake during the smolt outmigration (mid-March to mid-June). Steps to control milfoil are described in a companion report on milfoil control. 5 Introduction Cultus Lake is a small (6.3 km2) lake in the lower Fraser Valley of British Columbia that supports a run of sockeye salmon (Oncorhynchus nerka) that are genetically distinct from other Fraser River stocks (summarized in Schubert et al. 2002). Sockeye salmon escapements to Cultus Lake have declined precipitously in recent years due to a number of factors, and the stock is at a risk of extinction (Schubert et al. 2002). Harvest restrictions and other measures designed to conserve and rebuild this run will have significant socio-economic costs. Predation on juvenile sockeye salmon by other fish species is believed to affect the survival of juvenile sockeye in Cultus Lake (e.g., Foerster and Ricker 1941) and predator control programs have occurred in the past. Of particular concern is predation by an abundant predator, the northern pikeminnow (Ptychocheilus oregonensis), and a renewed effort to reduce the pikeminnow population has been suggested to help the recovery of Cultus sockeye (Schubert et al. 2002). To help provide an informed assessment and evaluate the likelihood that future pikeminnow control can improve juvenile sockeye survival in Cultus Lake, we summarize existing information on northern pikeminnow ecology and predation on salmonids at Cultus Lake and in other systems (Sections 1 –7). We evaluate previous removal program at Cultus Lake (Section 4), discuss potential pikeminnow management options (Section 9), and provide recommendations for their implementation (Section 10). 1. Pikeminnow life history Northern pikeminnow have broad habitat requirements and are found in lakes, reservoirs, small streams, and large rivers. They are an opportunistic species that can adapt to habitat changes (Parker et al. 1995). In this section, we highlight life history characteristics as they relate to pikeminnow control. We focus on lake populations and include results from fluvial populations1 where appropriate. Pikeminnow are long-lived and slow growing with a life expectancy between 15 and 20 years (McPhail and Lindsey 1986). Pikeminnow can grow to 635 mm (Scott and Crossman 1973) and 2.5kg (Beamesderfer et al. 1996). Females are larger than males at a given age (Jeppson and Platts 1959). Pikeminnow are difficult to age and scales, otoliths and other structures are used. Hall (1992) aged over 1000 pikeminnow from Cultus Lake using otoliths and found that most were less 1 Note: Results from the Columbia River are for reservoirs and the river. 6 than 16 years old (mode 5-7 years old), though some were up to 30 years old. The resulting length- at-age data were similar to other populations (summarized by Rieman and Beamesderfer 1990). Steigenberger (1972) aged pikeminnow in Cultus Lake using markings on pharyngeal teeth, though Hall (1992) suspects that these ages are underestimated. Pikeminnow reach sexual maturity by age five