Observations on the predation by squawfish (Ptychocheilus oregonense) on sockeye salmon (Oncorhynchus nerka), with particular reference to Cultus Lake, British Columbia
Creators: name: Steigenberger, Lance W.
Publisher: name: University of British Columbia
Year: 2011

description: This study was aimed at providing information that could be used to estimate the predation effect of squawfish on sockeye salmon. Initial studies at Griffen Lake revealed that the percentage of the population acting as predators, and the average stomach volume, increased with increasing size in non-spawning squawfish. Squawfish in Griffen Lake appeared to be most active at night. For the most part, a force fed volume (2.0 ± .1 gms) was digested by all sizes of squawfish in less than 24 hours. Further studies at Cultus Lake in the laboratory revealed that the rate of digestion was dependent on temperature, volume of food, and size of fish. At 6°C a lag prior to digestion commencing was observed and digestion was not complete for the medium volume in less than 52 hours. On an average the medium fed volume was digested in less than 24 hours for temperatures greater than 15°C. Medium volume corresponded to approximately twice the smallest volumes recorded in stomach contents in the field. A life span of ten years for males and 14 years for females with no differential in the growth rate was determined from the banding patterns and sections of pharyngeal teeth. Consumption rates and periodicity of feeding and activity were within the limits of data from Griffen Lake. Laboratory calculated routine metabolic rate was approximately twice the theoretical rate for pooled species. A tagging experiment at Cultus Lake revealed a population of approximately 20, 000 squawfish greater than 720 cm that, on an average, grew less than 0.36 cm during the winter. Growth during the summer was assessed to be in the form of weight increase of body tissue and gametes. Trap catches and temperature preference experiments indicated that squawfish are found within particular temperature regimes in different phases of the year. Within Cultus Lake, distance did not prevent squawfish aggregation on concentrations of sockeye smolts. There was increased consumption of smolts during smolt migration from the lake. In the field a significant difference in rate of digestion for different sizes of squawfish could not be demonstrated; however, there was a difference in the volume voluntarily consumed. With these findings and other theoretical information, it is possible to determine quantitatively the predation effect of a squawfish population on sockeye. Having established a population estimate, an estimate of annual mortality (55.2 per cent per year), and the temperature specific phases (early smolt phase, peak smolt phase, spawning phase, summer phase, fall phase, winter phase), two methods for the assessment of the predation effect were possible. First, knowing proportion of the size classes acting as predators, the numbers and frequency distribution of squawfish remaining within any phase, the duration and temperature characteristics of the phase, the effect of temperature on the rate of digestion, and the volumes that can be digested in 24 hours, it was possible to get an accumulated volume consumed for the population. For the 20, 000 squawfish greater than 20 cm fork length in Cultus Lake, this represents an approximate consumption of 1.4 million sockeye smolt equivalents. The second estimate of consumption was based on the energy requirements converted to consumption rates using conversion coefficients for the same population. The energy requirements to complete spawning, growth and mean metabolism were summed, then converted to a consumption volume for the population. The findings revealed that approximately 2.8 million sockeye equivalents are required.