McBride and McKown: Consequences of dispersal to temperate estuaries for Caranx hippos 



533 



dinal range, sudden and irregular freezes are more likely 

 agents for actual hypothermal mortality, regardless of lati- 

 tude I Storey, 1937 ). There are, for example, several instances 

 of hypothermal mortality of C. hippos on Florida's Atlantic 

 and Gulf coasts (Storey and Gudger, 1936; Miller, 1940; Gal- 

 loway, 1941; Gilmore et al., 1978; Snelson and Bradley, 1978; 

 Provancha et al., 1986). If individuals dispersed to temper- 

 ate estuaries react in a similar manner to those dispersed 

 to subtropical estuaries and emigrate from estuaries during 

 autumn, then they may successfully migrate south to suit- 

 able overwintering habitats. We observed similar sizes of 

 YOY C. hippos disappearing from estuaries and appearing 

 on the continental shelf during September and October, 

 which is strong circumstantial evidence of a migration pat- 

 tern before hypothermal temperatures are reached. 



During the summer, coastal water temperatures at tem- 

 perate and subtropical latitudes are similar (e.g. Mountain 

 and Holzwarth, 1989) and produce a favorable growth cli- 

 mate for C. hippos across a wide latitudinal range. Monthly 

 sizes of YOY C. hippos were similar across latitudes, and the 

 apparent growth rates were high in relation to other YOY 

 estuarine fish species in temperate waters (Rountree and 

 Able, 1992). Our comparative data suggested no size disad- 

 vantage for individuals dispersed to temperate estuaries. 



Overwintering mortality is size-dependent for some species 

 that migrate offshore (Conover, 1990), so that if C. hippos 

 in temperate waters were smaller by autumn than those in 

 subtropical estuaries, then their overwinter survival could 

 be lower. Nevertheless, individuals in temperate waters may 

 be at a disadvantage to equal-size individuals in subtropi- 

 cal waters because the latter do not need to migrate as far 

 to reach overwintering habitats. Our preliminary growth 

 model was confounded by a prolonged (about 3 months) 

 presence of small fish, and presumably there was consid- 

 erable age variation within our late-summer samples. The 

 continued presence of small fish into September confounded 

 analysis of length frequencies and created the appearance 

 of sudden shifts in average fish size in late summer. The 

 discrepancy between predicted size in temperate estuaries 

 during autumn and the modal size observed on the shelf (e.g. 

 14 vs. 17 cm FL) could be the result of smaller fish having 

 a higher mortality rate during this habitat shift. Further 

 work with growth modeling will improve with age-specific 

 data. The growth and size data of our study, however, sug- 

 gest that C. hippos emigrated from estuaries largely as YOY 

 and at the same time and sizes across latitudes. These data 

 suggest that the same behavioral cue to migrate operated on 

 individuals dispersed to both biogeographic regions. 



