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Oxytetracycline (OTC) immersion treatments were evaluated at the Waterville State Fish Hatchery (Waterville, Minnesota) for producing fluorescent marks on larval Northern Pike Esox lucius otoliths. A pilot marking trial conducted with 7‐d‐posthatch (dph) larvae resulted in poor mark efficacy and prompted further investigation on the treatment of younger larvae and modification of the treatment by use of osmotic induction to promote better mark formation. Northern Pike treated within 24 h post‐hatch had significantly higher mark efficacies and marked otolith intensities than did Northern Pike treated at later life stages. Similar to the pilot marking trial, immersion of 7‐dph fry in OTC solution resulted in poor mark efficacy, in which only 38% of the inspected fry possessed visible marks and most of the marks that were visible appeared faint. Simple immersion of newly hatched fry in OTC solution resulted in 91% marking success, and 65% of the visible marks appeared either clear or intense. The poorer mark efficacy on 7‐dph fry suggests that differences in mark formation were more dependent on contemporaneous physiological processes than on otolith size. Osmotic induction prior to OTC immersion had a much weaker effect on mark intensity than did age‐class of treatment in the study. Received November 12, 2013; accepted March 30, 2014 [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]