Getting the Scoop on Pelagic Forage Fish in Lakes

The common smelt (Retropinna retropinna), a schooling pelagic native, is the main forage fish for introduced rainbow trout (Oncorhynchus mykiss) in many New Zealand lakes. Conventional boat electrofishing using hand-held dip netting may underestimate the density of smelt due to the difficulty of seeing dispersed schools of small fish. We have developed a method for catching small pelagic fish in the surface waters of lakes using bow-mounted scoop nets. Two fine-mesh nets (mouth area approximately 1.4 m² each) were mounted behind each anode of an electrofishing boat. The fishing field was powered by a Smith-Root Inc. model 5.0 pulsator. The net length was 1.6 m, tapering to a double cod end closed with a drawstring. The nets were constructed of polyester hexagonal mesh with a mesh size of 2 mm. The boat was driven at a constant speed (about 1-2 km/h) for 10 min to give a sampled length of 120 to 340 m. Fished width was approximately 4 m wide, which gave a total area fished of 480 to 1360 m² in each 10-min shot. Fish within about 1 m of the surface were entrained in the electric current, immobilised, and caught in the nets. The catch rates in the littoral zone were approximately 30 times higher at night than during the day, and catches of non-target species were negligible. Mean smelt densities at night in the littoral zone caught using the boat-mounted scoop nets (0.58 smelt/m²) were around three times that of beach seining or purse seining (both 0.19 smelt/m²). Smelt densities caught at night with boat-mounted scoop nets were almost 20 times higher in the littoral zone than in open water and varied seasonally, with catches of up to a maximum of 2.3 smelt/m² in the littoral zone.