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Western Georges Bank Cod Habitat Study

We had the following objectives for the project: 1) Determine if the observed differences in spawning season between the western and eastern cod indicate two genetically distinct spawning populations or if all the cod on Georges Bank intermingle during spawning. 2) Locate and genetically sample the larvae and juvenile cod as they drift in the water currents and determine if we can genetically identify the parent population. 3) Map the locations of sampled adults, larvae, and juveniles to determine if the sampled cod are located in previously identified essential fish habitat (EFH)s.

The well known counter-clockwise circulation of Eastern Georges Bank cod larvae and juveniles around the Bank has resulted in important habitat areas being identified as EFH and closed to certain types of fishing. These closures are critical to provide protection to young cod as they mature to adulthood. If the cod spawning in the fall on Western Georges Bank are a separate population and do not share juvenile habitat with the Eastern Georges Bank population, it is important to identify the habitat they use as EFH.


With the cooperation of 14 fishing vessels and 7 industry technicians, reproductively mature cod DNA samples were collected in the waters off Cape Cod (Western Georges Bank) and Eastern Georges Bank. Looking at historic tidal flow, currents, wind and water temperatures, larval cod from the fall/winter spawning event were estimated to have drifted onto Nantucket Shoals and in Nantucket Sound. Using a bongo net to collect larvae and a specially designed small mesh otter trawl to collect juveniles, DNA samples were collected and sent to MBL for genetic analysis.

There was a significant difference in the genetics between Western and Eastern Georges Bank cod, indicating that the seasonal spawning differences between the areas may have resulted in separate sub-populations. Larvae sampled around Nantucket were genetically linked to the adult Western Georges Bank cod that spawned in the fall. The juvenile results were less conclusive. Some of the juveniles genetically matched both the Western and Eastern Georges Bank adults; however, the majority of the juveniles were not matched to any of the sampled adults. This signifies that there may be a third spawning population that has yet to be located.

The sampled locations of Atlantic cod as spawning adults, larvae, and juveniles overlaps with many of the EFH areas previously identified by NMFS. All reproductively mature cod were caught within areas already designated as adult EFH. The majority of the larvae collected were also caught within areas already designated as larvae EFH (65%); however, there are areas on the map in Nantucket Sound and south- east of Nantucket Island where we found larvae that are not included in the current EFH designation. A small portion of the juveniles caught (8%) were collected within areas designated as juvenile EFH.

Final Report Submitted to Cooperative Research Partners Initiative can be found clicking here.

Last updated: May 16, 2006

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