Austin et al : Morphometnc separation of annual cohorts within mid-Atlantic Pomatomus saltalnx 417 



Discussion 



Several authors (Nyman and Conover, 1988; McBride, 

 1989; Simpson et al.. 1990) working in the Long Is- 

 land region and analyzing length-frequency data, 

 reported exclusively spring-spawned young-of-the- 

 year during 1985-86. Both spring-spawned and some 

 late arriving summer-spawned young-of-the-year 

 were collected there in 1987, and then a shift occurred 

 to predominantly summer-spawned fish during the 

 summer-fall of 1988. 



Because ratios continued to be similar for the year- 

 ling fish in our collections and because length fre- 

 quencies were also similar, our 1987 samples from 

 Hatteras and the Chesapeake Bay were apparently 

 from the predominantly spring-spawned 1986 year 

 class. Our 1988 samples, the 1987 year class, were 

 also predominantly spring spawned but were classi- 

 fied morphologically separate from the 1986 year 

 class. 



Our 1989 samples (the 1988 year class) from 

 Hatteras, Chesapeake Bay, and Long Island showed 

 a split classification, some the same as our 1988 



samples (the 1987 year class) and the rest, separate 

 from both 1987 and 1988 year classes. The 1990 

 samples, from a dominant 1989 year class, collected 

 in the Chesapeake Bay, overlapped slightly between 

 the 1987 and 1988 year classes (Fig. 1). 



If the shift from spring-spawned to summer- 

 spawned young-of-the-year noted by the above au- 

 thors for Long Island holds for Chesapeake-Hatteras 

 yearling fish, then the morphologically distinct 1986 

 year class from our 1987 collections was composed 

 of spring-spawned fish. The 1988-90 collections of 

 the 1987-89 year classes, may be a mix of spring- 

 and summer-spawned fish but show no separation. 

 As stated earlier, however, our length frequencies of 

 yearling fish in all years ( 1987-90) suggest that our 

 samples were all spring-spawned fish (Table 2). An 

 alternate explanation is that the morphologically 

 distinct yearling fish, representing separate year 

 classes or annual cohorts, and spawned in differing 

 environments each year are demonstrating environ- 

 mental or phenotypic plasticity. That is, that their 

 morphological characters are environmentally deter- 

 mined and are different each year. 



