Project Summary
David W Townsend,
Linda J Kling, and
Irving L Kornfield
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.