Catchability of Larval and Juvenile Northern Pike in Quatrefoil Light Traps

Abstract

New approaches are needed for sampling northern pike Esox lucius in the complex habitats they use as nursery areas. We evaluated the potential of Quatrefoil light traps to measure differences in density of larval and juvenile northern pike, and we monitored changes in capture probability as the fish grew in size and their swimming capabilities improved. In hatchery raceways, light traps were effective for collecting northern pike at densities that seemed most realistic relative to natural nursery habitat (4–20 fish/m² surface area). Light-trap catch rates differentiated among different densities in raceways, but efficiency decreased as density increased. Light traps effectively caught all sizes of fish, ranging from the stage when larvae first became active (12–13 mm total length [TL]) until the fish became too large to fit through the trap entrance (>66 mm TL). We also tried to apply the technique for sampling managed wetlands that simulated a more natural environment than the raceways. Light-trap catches detected both increases and decreases that were attributed to different stocking rates used over 2 years in individual wetlands. Light-trap catches showed patchy distribution of small northern pike and illustrated growth and differential survival of the fish among wetlands. Light trapping should be considered a potential tool for sampling both temporal and spatial variation in density and growth of larval and juvenile northern pike.