Otolith microstructure of three larval gadids in the Gulf of Maine, with inferences on early life history

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Abstract
A detailed analysis of the assumptions underlying the application of otolith microstructure was undertaken for three gadid species from the Gulf of Maine. Daily increment counts provided a precise and accurate index of age in cod (Gadus morhua) and appeared to do so in haddock (Melanogrammus aeglefinus). However, the widths of increments formed shortly after hatch in cod and haddock otoliths approached the resolution limit of light microscopy, resulting in an underestimation of age by 2–3 d for all fish. Daily increments were also formed in the otoliths of pollock (Pollachius virens) but, perhaps because of more severe resolution problems, the microstructure associated with the early larval stage was unclear. Factors that could confound growth back-calculation efforts included differing otolith length – larval length relationships among samples and an inverse correlation between hatch check diameter and temperature. Estimates of hatch date and growth rate developed in this study were not affected by slight deviations from the underlying assumptions, but might be affected by such deviations under different environmental conditions. Larval pollock growth was comparable to that of cod and haddock, despite a later hatch date. Pollock hatched primarily in November, while peak dates for cod and haddock hatching occurred in March–April and May, respectively. Cod on Georges Bank hatched significantly earlier than those on Browns Bank, although the latter included a less abundant component that hatched in the fall.