Age, growth, and population assessment of grass pickerel (Esox americanus vermiculatus) in two northern populations

Abstract

Age assessment was conducted on the at-risk grass pickerel (Esox americanus vermiculatus Lesueur, 1846) in southern Ontario, Canada (the northern edge of the species’ range), to better understand population dynamics. Cleithral age assessment revealed different age distributions between two sample years; the first year characterized by old, large individuals and the second year characterized by mostly young of the year. Back-calculated lengths at age were used to develop the von Bertalanffy growth relationships and estimates of ultimate length. Growth rates were substantially faster in the second year, when fewer and smaller individuals were present, suggesting density-dependent effects on growth. A pseudoannulus in the cleithrum, common to this species, was shown by sequential sampling to form in early July at 70% to 80% of the first growth zone and to occur more frequently in slow-growing fish and strong year classes than in faster-growing individuals and weaker year classes. Mortality rates of females were high (26%–46%) relative to other studies and probably resulted from greater abundance of individuals. Compared with cleithra, scale ages after age 1 were consistently underestimated. Calculated length at cleithral age intervals overlapped minimally, suggesting length could be used as a proxy for assessing age of this species in Ontario to avoid lethal sampling.