Overwinter Growth and Mortality of Young‐of‐the‐Year Windowpanes: Cohort‐specific Responses

We tested the overwintering capability of young‐of‐the‐year (age‐0) windowpanes Scophthalmus aquosus to determine if there are cohort‐specific differences in growth and mortality. We conducted a laboratory study at ambient estuarine winter temperatures from 9 November 1995 to 22 April 1996 with two cohorts (spring‐spawned cohort: 83–140 mm total length [TL]; fall‐spawned cohort: 15–37 mm TL). We hypothesized that (1) among individuals within both cohorts, winter growth rates would be low compared with summer growth rates; and (2) overwinter mortality rates of the smaller, fall‐spawned fish would be higher than those of larger, spring‐spawned fish due to size‐selective mortality. Under ambient winter temperatures (−2.0 to 14.0°C) with daily, ad libitum feeding in the laboratory, growth rates were low but positive for the fall‐ and spring‐spawned fish. Overwinter mortality occurred in both cohorts but was higher for the fall cohort than for the spring cohort (75% and 31%, respectively). Within the fall cohort, mortality was size selective, with reduced survival of individuals smaller than 24 mm TL. Within the spring cohort, there was no evidence of size‐selective mortality. We concluded that the first winter of life may be an important mortality bottleneck for both cohorts of age‐0 windowpanes in the Middle Atlantic Bight but that mortality patterns are cohort specific and may be determined by different combinations of factors, such as fish size at the start of winter, the magnitude of water temperature decline between fall and winter, and the frequency and duration of extremely cold water temperatures. Thus, overwinter mortality during the first winter of life may influence the population dynamics of the windowpane and other multimodal spawners inhabiting the Middle Atlantic Bight.