Collaborative Research: Origins of Cods on Georges Bank: Contributions of Early Developmental Stages for the Scotian Shelf

Recent work in the Georges Bank-Gulf of Maine area has documented significant, and apparently episodic, fluxes of Scotian Shelf Water (SSW) from the Nova Scotian continental shelf to Georges Bank. SSW is a relatively cold and fresh water mass with a significant component from the St. Lawrence River, and is commonly identifiable with temperature-salinity analyses of hydrographic data and in satellite images of sea surface temperature. One such flux episode was observed last March (1997) in satellite imagery and from shipboard hydrographic sampling on Georges Bank. Qualitative at-sea analyses of ichthyoplankton sampled on the March cruise revealed a remarkably tight association between abundances of gadid eggs and the distribution of SSW suggesting, along with other lines of evidence, that most of those eggs were spawned on the Scotian Shelf and were advected with the SSW water mass to Georges Bank. The fundamental question thus arises: to what extent are cod on Georges Bank imported to the Bank as early development stages by advection from Canadian waters to the east? The goal of this research is to answer the above question. The approach will be two tiered: (1) Drs. Townsend and Radtke will perform retrospective elemental analyses of otoliths from archived larval cod samples, as well as of ichthyoplankton samples to be collected in 1998 and 1999 as part of the continuing Georges Bank GLOBEC project, analyzing them for Sr/Ca ratios, using an X-ray electron microprobe, and elemental fingerprints , using UV lazer ablation inductively coupled plasma mass spectroscopy (ICPMS). (2) They will also assess the genetic identity of the larvae relative to larval and adult populations from Georges Bank and from the Scotian Shelf using nuclear DNA microsatellite techniques. They will first verify the elemental composition of otoliths from cod larvae known to have been spawned in the two locations. This elemental analyses will be combined with laboratory rearing experiments of larvae to determine the effects of temperature, salinity, feeding, and growth rates on the incorporation of elements in the otoliths. DNA based techniques will be used to identify individuals in these common-garden rearing experiments. The second step will be to identify the origin of larvae based on individual DNA profiles generated by characterizing nuclear DNA microsatellites, a new class of genetic markers that they have used to differentiate Georges Bank cod from those of Scotian Shelf waters.The intent in using the two different, independent approaches is to be able to identify the spawning locations of the larvae and track their transport in relation to hydrographic characteristics of water masses in the region