Population dynamics of introduced flathead catfish in Lake Mitchell, South Dakota

The introduction of flathead catfish (Pylodictis olivaris) into Lake Mitchell presented a unique opportunity to study the population dynamics of this species in a South Dakota impoundment. We collected flathead catfish using daytime, low-frequency electrofishing during June 2013, 2014, and 2015 and July, August and October of 2014 to examine population characteristics including abundance, recruitment, mortality, growth, condition, and diet. The flathead catfish population in Lake Mitchell was estimated at 1348 individuals (95% CI = 459–1455; density = 4.97/ha) in 2014 and 1197 individuals (95% CI = 931–1461; density = 4.42/ha) in 2015. Individuals from 11 year classes ranging from 1 to 13 years old were present. The population exhibited consistent recruitment, and annual mortality was estimated at 39%. Flathead catfish grew quickly exceeding stock length at age 3 and quality length at age 5; however, growth slowed in 2015. Similarly, condition of substock and stock-quality length fish declined in 2014 and 2015, respectively. The decline in growth and condition coincided with the recruitment of a large 2012 year class and may be an early indicator of intraspecific competition. Diets of Lake Mitchell flathead catfish primarily consisted of crayfish (Orconectes spp.) and fish with flathead catfish shifting to piscivory at approximately 400 mm. The high percentage of centrarchids in flathead catfish diets along with an increase in flathead catfish abundance coinciding with a decrease in bluegill abundance may indicate that flathead catfish are negatively impacting the bluegill population in Lake Mitchell

Effects of Simulated Cold Fronts on the Survival and Behaviour of Yellow Perch Perca Flavescens Yolk-sac Fry

Acute reductions in water temperature (i.e. cold fronts) may influence larval fish survival directly via limits on physiological tolerance or indirectly by acting as a sublethal stressor. The primary objective was to quantify survivorship of yellow perch yolk-sac fry exposed to two different temperature declines (4 and 8°C) and compare survivorship to that of perch fry under ambient temperatures representative of natural conditions. Behaviour of yolk-sac fry following temperature declines was also qualitatively assessed. Mean survival in the control, −4, and −8 treatment tanks was 90, 91 and 97%, respectively, and no significant differences in percent survival were observed between the control and the −4 treatment (ts = −0.10; df = 7; P = 0.93), the control and −8 treatment (ts = −1.85; df = 7; P = 0.11) or the −4 and −8 treatments (ts = −1.33; df = 7; P = 0.22). Observations of yellow perch eggs and fry behaviour following temperature declines differed among treatments. Any remaining eggs in the control treatment and −4 treatments continued to hatch during the experiment, and fry were documented swimming throughout the water column in all tanks. However, in the −8 treatment, any eggs that had not hatched remained inactive and all fry within all −8 treatment tanks ceased swimming activity and settled to the bottom of the tanks once the temperature reached 3.9°C. Fry remained at the bottom of the tanks for the entire 48 h simulated cold-front. Fry resumed swimming activity once water temperatures began to increase (by approximately 6°C). Results indicated that drops in temperature (i.e. cold fronts) similar to or greater than those found in small impoundments did not cause direct mortality of yellow perch during the yolk-sac fry (post-hatch larvae) stage. Although an acute drop in temperature may not induce sudden high mortality, it may be a sub-lethal stressor, leading to increased starvation or predation risk