418 



Fishery Bulletin 97(3), 1999 



The larger fish in this study are a composite of at 

 least ten year classes (2-11 years); therefore separa- 

 tion or classification would be expected to be less 

 precise. Indeed there was considerable overlap in 

 classification of the larger fish (Table 10). 



From results of the DFA, and in light of the back 

 calculations of Chiarella and Conover ( 1990) and the 

 genetic data of Graves et al. ( 1993 ), it would appear 

 that there is only one stock of bluefish in the mid- 

 Atlantic Bight and that the morphometric differences 

 among bluefish cohorts are a result of phenotypic 

 plasticity. Because the environment at the time of 

 spawning and juvenile development varies geo- 

 graphically and interannually, so too will morpho- 

 metric features. As the fish grow, their plastic mor- 

 phological characters, expressed as an index of char- 

 acter length versus fish length, become less and less 

 reliable: when year classes mix, as in our >400 mm 

 sample, they provide no discriminant characteriza- 

 tion unless separated by individual year class. 



Although a single genetic stock, the two MAB co- 

 horts (spring- and summer-spawned), may exhibit 

 interannual differential recruitment success and sur- 

 vival to yearlings. In addition! the reported differ- 



ences in growth, seasons and location of spawning, 

 migration routes, and variations in fishing mortal- 

 ity along the MAB can complicate management ef- 

 forts if the two cohorts are managed as a single stock. 

 Even for a single mid-Atlantic genetic stock, recruit- 

 ment variations between spring and summer cohorts 

 may be significant, year to year, and consideration 

 should be given to monitoring the annual contribu- 

 tion of the spring-spawned and summer-spawned 

 cohorts as the stock may be healthier when the 

 spring-spawned predominate for several years run- 

 ning (Chiarella and Conover, 1990). The morphomet- 

 ric separation by year class provides evidence that 

 the environment likely affects bluefish size and 

 shape; this may prove to be a useful tool in separat- 

 ing yearling bluefish by the geographic area in which 

 they spent their first year of development. 



Conclusions 



Previous larval and tagging studies neither support 

 nor refute the existence of one or more MAB blue- 

 fish stocks, one that spawns just south of Hatteras, 

 North Carolina, in the spring and the other off New 

 England in the summer. The mtDNA analysis by 

 Graves et al. ( 1993) suggests that there is one mid- 

 Atlantic stock which from our results produces sev- 

 eral environmentally induced morphotypes. 



