Variation in gill-net and angling catchability with changing density of northern pike in a small Minnesota lake

The variation in the catchability coefficients (q) of gill netting and angling was monitored while manipulating the density of a population of northern pike Esox lucius during a series of nettings. The nettings removed nearly one-half of the population of northern pike age 2 and older from a small Minnesota take. Estimates of q differed among age-classes of fish, thereby corroborating the size selectivity of the multimesh experimental gill nets. The results showed that gill-net catch rates tracked the declining population size of northern pike relatively closely. However, daily estimates of q declined during the removals, indicating hyperdepletion in catch rates. The decline in q contrasts with the results of several previous studies, which showed inverse relationships between q and density, and was most likely a result of gear size selectivity and fish behavior. Changes in age and size structure stemming from our intensive nettings helped cause the declining catchability. Angling catch rates were less useful for tracking changes in the northern pike population density. Angling catches and the values of q for a single angler were much more variable than the catches from gill nets. Angling data also showed no apparent relationship between density and q. Our results confirm that gill nets can be used as a tool for monitoring changes in the abundance of northern pike, and knowledge of this type of variation in catchability contributes greatly to our ability to interpret trends in catch rates.