The biology of pygmy whitefish, Prosopium coulterii,in a closed sub-boreal lake: spatial distribution and diel movements

Using gillnets and trap nets, we examined the spatial distribution, diel movements, and environmental tolerances of pygmy whitefish, Prosopium coulterii, in a small boreal lake in north-central British Columbia. Most gillnets were set below the thermocline but we also fished a shore net in the littoral zone. During the ice-free season (May to November) there was a strong diel onshore–offshore movement: during the day pygmy whitefish were offshore and below the thermocline (water temperatures of 4–6°C) but at night they were inshore and above the thermocline (water temperatures of 12–18°C). This onshore–offshore movement occurred close to the bottom and, regardless of where they were caught, most fish were <4 m off the bottom. Oxygen concentrations in most of the hypolimnion dropped to <5.0 mg l⁻¹ in June and by late August to <1.0 mg l⁻¹; indicating pygmy whitefish can tolerate low oxygen conditions. The catch of pygmy whitefish in gillnets set below the thermocline was highly skewed: 53% of the nets were empty, 37% caught 18 or less fish, and 10% caught 70% of the total catch (742 fish). Trap nets produced similarly skewed results: most trap net sets caught no pygmy whitefish but one set caught over 2,000 individuals. Our catch data suggest that in Dina Lake #1 some pygmy whitefish aggregate.