Recruitment implications of larval herring overwintering distributions in the Gulf of Maine, inferred using a new otolith technique

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Abstract
We have applied a Sr/Ca ratio technique of otolith analysis to reconstruct the temperature histories of fall-spawned herring larvae Clupea harengus L. captured in inshore waters of the Gulf of Maine, USA, in order to infer their overwintering distributions. Winter survival of herring larvae is believed to be a determinant of recruitment in the Gulf of Maine, though very little is known about this important phase in their life history. Shortly after hatching in coastal spawning areas, larvae congregate inshore along the Maine coast during fall (September to November). Following this abundance peak, inshore larval densities drop to very low levels in mid-winter (late December and January), before a second group of fall-spawned larvae appears inshore in spring (February through April), thus giving a bimodal abundance distribution with time. We have inferred the overwintering distributions of larvae captured in the spring in the Sheepscot River estuary, Maine, by analyzing changes in the concentrations ratios of strontium to calcium in the otoliths. These ratios vary as a function of temperature and thus act as a biological recorder of water temperatures experienced by an individual larva throughout its life history. Results suggest that the spring peak of herring larvae inshore represents larvae which overwintered offshore in the Gulf of Maine, implying that larvae which enter the inshore nursery areas in the fall do not survive.