Cohort dynamics of summer-spawned bluefish as determined by length-frequency and otolith microstructure analyses

Elucidating recruitment processes in marine fish is difficult when species occupy distinct habitats at different ontogenetic stages and when multiple cohorts determine year-class strength. Currently, there is insufficient information on the cohort dynamics of the early life history stages of summer-spawned bluefish Pomatomus saltatrix to critically evaluate each cohort's role in regulating population dynamics. The objective of this study was to provide greater insight into the cohort dynamics of summer-spawned bluefish during the summer and fall on the inner continental shelf off southern New Jersey. Length-frequency and otolith microstructure analyses were used to estimate the age, hatch date, and growth of neustonic individuals collected from shelf surface waters during August-October 1998. Bluefish ranged between 9 and 46 d in age, corresponding to back-calculated hatch dates between 27 June and 13 September. Hatch date frequencies were decomposed into a series of normal curves and identified three cohorts with mean hatch dates occurring in early July (cohort 1), late July to early August (cohort 2), and late August to early September (cohort 3). Cohort-specific growth rates ranged between 0.28 and 0.81 mm standard length/d. Otolith microstructure analysis revealed that growth was significantly faster for cohort 1 than for the other cohorts, most likely because of spatial and temporal variability in the environment (e.g., temperature) and size-selective processes (e.g., mortality). Back-calculated spawning dates indicated that the functional spawning output of bluefish was significantly greater during the first-quarter phase of the lunar cycle. Collectively, these observations suggest that the cohort structure of bluefish is more complex than previously recognized.