Detecting critical periods in larval flatfish populations

We evaluate the time-course of deaths and evidence of periods of increased mortality (i.e., critical periods) in laboratory populations of larval flat fish. First, we make the distinction between age-at-death and abundance-at- time data for fish larvae, the latter being typical in studies of natural p opulations. Next, we describe an experimental investigation of age-and temp erature-dependent mortality in larval winter flounder, Pseudopleuronectes a mericanus. The survivorship curves of these populations differed significan tly in both the magnitude and time-course of mortality among the four water temperatures evaluated (7, 10, 13, and 16 degreesC). Mortality was highest in the cooler temperatures and concentrated in the third quarter of larval life, largely concurrent with settlement of surviving members of the cohor t. Among the statistical methods for analysing survival data, the proportio nal-hazards model with time-varying covariates proved best at capturing the patterns of age-specific mortalities. We conclude that fair appraisals of recruitment hypotheses which are predicated on periods of high, age-specifi c mortality that vary with environmental conditions (e.g., Hjort's critical period hypothesis) will require: (1) data that are based on age, not time; (2) data that are of higher temporal resolution than commonly available at present and (3) analytical methods that are sensitive to irregularities in survivorship curves. We suggest four research approaches for evaluating cr itical periods in nature. (C) 2001 Elsevier Science B.V. All rights reserve d.