Selective Mortality in Esocid Populations – Northern Pike and Muskellunge Case Histories

In fisheries, exploitation and management can produce both unintentional and intentional selective harvest and mortalities. These and other factors affect fish populations and often are not adequately understood or taken into consideration. Depending upon ecology, physiology, and environmental conditions, selective mortalities can result from oxygen depletion, seasonal activity patterns, growth rate differences, and epizootics, greatly altering esocid population structure and dynamics. Winterkill and oxygen depression select against older, larger, faster-growing female pike and result in structurally different populations. Seasonal growth and activity can make female pike vulnerable to exploitation in summer and, especially, late fall and winter. Increased angling pressure during gonadal development can result in selective mortality of maturing females, reducing reproductive potential in both pike and muskellunge populations. Passive fishing techniques, as well as angling, which results in selective exploitation of more active, faster-growing pike, can reduce overall growth rate of the population. By contrast, a viral hemorrhagic septicemia die-off of half the mature muskellunge in the upper St. Lawrence River resulted in selective mortality of slower-growing fish, leaving a population that, independent of density, is significantly faster-growing, reaching a larger ultimate size. Selective mortality may have important and unexpected effects and should be considered in managing esocid populations.