WHITE and CHITTENDEN: AGE DETERMINATION OF ATLANTIC CROAKER 



recovering gonads in late November and 

 thereafter. 



2) Reach maturity when greater than 200 mm 

 long as they approach at least age II (Welsh 

 and Breder 1924; Wallace 1940; Haven 

 1954). 



3) Have a maximum size of about 500 mm 

 (Hildebrand and Schroeder 1928; Gunter 

 1950) and large average size so that they 

 have supported important commercial food 

 fisheries (Gunter 1950; Haven 1957; Joseph 

 1972). 



Maturity is reached about 1 yr later in cold- 

 temperate waters and typical sizes are much 

 larger, although growth rates appear similar. 

 Therefore, the typical maximum age is probably 

 about 2-4 yr north of Cape Hatteras. If so, the 

 total annual mortality rate must be lower north 

 of Cape Hatteras. Assuming negative exponential 

 survivorship, the theoretical approximate total 

 annual mortality rates would be 90, 78, and 68% 

 for life spans of 2, 3, and 4 yr, respectively. 



The existence of an abrupt change at Cape 

 Hatteras in the life histories and population 

 dynamics of species whose ranges traverse this 

 area has apparently not been recognized, par- 

 ticularly as a possible general phenomenon; 

 although Cape Hatteras has long been recognized 

 as a significant zoogeographic boundary [see 

 Briggs' (1974) review]. Gunter (1950) noted dif- 

 ferences in the sizes and some aspects of the life 

 histories of certain fishes of the Gulf of Mexico and 

 mid-Atlantic coast of the United States. However, 

 he gave no consideration to the possibility that an 

 abrupt change might occur near Cape Hatteras. 

 Although the Cape Hatteras connection has not 

 been recognized, the pelagic, anadromous 

 American shad, Alosa sapidissima, also shows 

 changes in life history there that are similar to 

 those herein documented for croaker. Runs of shad 

 native to streams north of Cape Hatteras consist 

 primarily of somewhat older fish (ages IV- VII and 

 older) and include many repeater spawners in 

 contrast to the younger fish (ages IV- VI) and the 

 complete or virtual absence of repeat spawners 

 south of Cape Hatteras (for pertinent literature 

 see Walburg and Nichols 1967; Chittenden 1975). 

 La Pointe (1958) reported similar growth rates in 

 shad native to streams throughout their range. 

 Therefore, the geographic differences in age 

 compositions should result in differences in life 

 spans, ages at maturity, maximum ages, 



maximum and average sizes, and mortality rates 

 as in croaker. 



The life histories and population dynamics of 

 two species with different life styles but primarily 

 coastal habit have been shown to change abruptly 

 at Cape Hatteras. This may represent a general 

 phenomenon as Gunter (1950) apparently ob- 

 served. However, similar comparisons are 

 necessary in other species, especially noncoastal 

 forms, to see how far the inference extends. 



The reason for the geographical differences in 

 population dynamics is not clear. However, shad 

 exhibit great somatic weight loss (about 25-55% 

 depending upon sex and size) associated with 

 migration and spawning (Leggett 1972; Chitten- 

 den 1976). Leggett (1972) suggested that the low 

 frequency of repeat spawning shad in southern 

 streams might be due to increased use of body 

 reserves during spawning migrations that occur 

 at higher average temperatures. Croaker also 

 show somatic weight loss associated with mat- 

 uration and spawning, although we did not ob- 

 serve weight loss comparable to that in shad. 

 However, we had no data for the post-peak 

 spawning period December-February when 

 weight loss may have been greater. It is pertinent 

 here that Chittenden has observed many 

 emaciated spot, Leiostomus xanthurus, in the Gulf 

 of Mexico during January, which is about when 

 this species spawns. The observed differences in 

 population dynamics north and south of Cape 

 Hatteras may be largely the result of different 

 temperature regimes that affect age at mat- 

 uration, spawning-associated somatic weight loss, 

 and the magnitude of a subsequent post-spawning 

 mortality. 



ACKNOWLEDGMENTS 



For assistance with field collections we are 

 indebted to R. Clindaniel, C. H. Stephens, G. 

 Graham, J. Surovik, M. Carlisle, and to Captains 

 R. Foreman, R. Foreman, Jr., J. Torres, H. For- 

 rester, and M. Forrester. C. E. Bryan and W. Cody 

 of the Texas Parks and Wildlife Department made 

 collections offish from the Gulf in November. S. M. 

 Lidell directed us to large croakers near the reef. J. 

 Merriner and J. Musick of the Virginia Institute of 

 Marine Science loaned scales from Chesapeake 

 Bay. J. McEachran, W. Neill, R. Noble, L. Ringer, 

 R. Stickney, K. Strawn, and M. VanDenAvyle of 

 Texas A&M University reviewed the manuscript 

 and L. Ringer programmed certain statistical 



121 



