289 



Abstract— This study examines gene- 

 tic variation at five microsatellite loci 

 and at the vesicle membrane protein 

 locus, pantophysin, of Atlantic cod 

 {Gadus morhua) from Browns Bank, 

 Georges Bank, and Nantucket Shoals. 

 The Nantucket Shoals sample rep- 

 resents the first time cod south of 

 Georges Bank have been genetically 

 evaluated. Heterogeneity of allelic dis- 

 tribution was not observed (P>0.05) 

 between two temporally separated 

 Georges Bank samples indicating 

 potential genetic stability of Georges 

 Bank cod. When Bonferroni correc- 

 tions («=0.05, P<0.017) were applied 

 to pairwise measures of population 

 differentiation and estimates of F ST , 

 significance was observed between 

 Nantucket Shoals and Georges Bank 

 cod and also between Nantucket Shoals 

 and Browns Bank cod. However, nei- 

 ther significant differentiation nor sig- 

 nificant estimates ofF ST were observed 

 between Georges Bank and the Browns 

 Bank cod. Our research suggests that 

 the cod spawning on Nantucket Shoals 

 are genetically differentiated from 

 cod spawning on Browns Bank and 

 Georges Bank. Managers may wish to 

 consider Nantucket Shoals cod a sepa- 

 rate stock for assessment and manage- 

 ment purposes in the future. 



Genetic differentiation among Atlantic cod 

 (.Gadus morhua) from Browns Bank, 

 Georges Bank, and Nantucket Shoals 



Christopher Lage 



Department of Biological Sciences 

 Murray Hall 

 University of Maine 

 Orono, Maine 04469 



Kristen Kuhn 



Irv Kornfield 



School of Marine Sciences 



Murray Hall 



University of Maine 



Orono, Maine 04469 



E-mail address (for I Kornfield, contact author): irvk@maine.edu 



Manuscript approved for publication 

 5 November 2003 by Scientific Editor. 



Manuscript received 20 January 2004 

 at NMFS Scientific Publications Office. 



Fish. Bull. 102:289-297 (2004). 



The Atlantic cod (Gadus morhua) is a 

 migratory gadid found on both sides of 

 the North Atlantic. In the Northwest 

 Atlantic, cod are distributed nearly 

 continuously along the continental 

 shelf from Greenland to North Caro- 

 lina, spawning in relatively discrete, 

 temporally stable areas, and differ- 

 ent regions are regarded as different 

 management units defined primarily 

 by latitude and bathymetry ( Ruzzante 

 et al, 1998). Atlantic cod historically 

 supported economically important 

 fisheries in the Northwest Atlantic 

 (Halliday and Pinhorn, 1996). In U.S. 

 waters, cod are assessed and managed 

 as two stocks: 1) Gulf of Maine and 2) 

 Georges Bank and southward (includ- 

 ing Nantucket Shoals). Growth rates 

 differ between the two stocks; growth 

 is slower in the Gulf of Maine compared 

 to growth in Georges Bank (Pentilla et 

 al., 1989 ); each stock is exploited by the 

 same gear type and may show similar 

 biological responses towards such gear 

 selection. Although both stocks sup- 

 port important commercial and recre- 

 ational fisheries, each is overexploited 

 and remains at a low biomass level 

 (Mayo and O'Brien, 1998; O'Brien and 

 Munroe, 2001; Mayo et al„ 2002). Over- 

 exploitation may result in significant 

 life-history changes such as a decline 

 in time to reproductive maturity which 

 has been observed in Georges Bank cod 

 (O'Brien, 1998); such changes maybe a 



compensatory response to overfishing 

 but may also be influenced by shifts in 

 underlying genetic control (Policansky. 

 1993). 



Commercial fisheries are conduct- 

 ed year round, using primarily otter 

 trawls and gill nets. The Canadian 

 fishery on Georges Bank is managed 

 under an individual quota system. 

 United States cod fisheries are man- 

 aged under the New England Fishery 

 Management Council's Northeast Mul- 

 tispecies Fishery Management Plan 

 (FMP) 1 as implemented by the U.S. 

 Federal Register, 50 CFR Part 648 

 (U.S. Federal Register, 2003). Under 

 this FMP, cod are included in a com- 

 plex of 15 groundfish species managed 

 by time and area closures, trip limits, 

 gear restrictions, minimum size limits, 

 days-at-sea restrictions, and a permit 

 moratorium. The FMP's goal is to re- 

 duce fishing mortality to levels that 

 will allow stocks within the complex 

 to initially rebuild above minimum 

 biomass thresholds, and, ultimately, to 

 remain at or near target levels. 



When ecological and evolutionary 

 processes are responsible for stock 

 structuring, it is necessary to incorpo- 



1 New England Fishery Management Coun- 

 cil. 2003. Northeast Multispecies 

 Fishery Management Plan. NEFMC. 

 50 Water St., Mill 2, Newburyport. MA, 

 01950 



