FISHERY BULLETIN: VOL. 85, NO. 2 



Table 9.— Instantaneous growth rates 

 (Ricker 1 979) of Decapterus punctatus 

 predicted from the von Bertalanffy 

 equation (fig. 11). Weights were con- 

 verted from lengths by using the regres- 

 sion: In Wt (g) = 2.96 In FL (mm) - 

 11.2. 



the specimens 121-143 mm FL and no individuals 

 >143 mm FL could be assigned an age. Replicate 

 counts of otolith increments from adults often dif- 

 fered by >10%, and were considered unreliable. In 

 addition, frequent growth interruptions occurred in 

 the outer portions of the otoliths of large specimens, 

 and the timing of formation of such marks was not 

 known. 



DISCUSSION 



Seasonal Distribution and 

 Relative Abundance 



Round scad apparently migrate shoreward across 

 the continental shelf as sea temperatures increase 

 in spring, then migrate into warm (>15°C) midshelf 

 (28-110 m) depths as inshore temperatures decline 

 in winter. Intermediate shelf depths in the South 

 Atlantic Bight are fairly warm year round, unlike 

 inshore waters which are seasonally cooled by cold 

 fronts (Atkinson et al. 1983) and outer shelf waters 

 which are intruded upon by cold-water upwellings 

 (Blanton et al. 1981). Magnuson et al. (1981) re- 

 ported that catches of round scad were proportional 

 to temperature, and laboratory studies (Wyllie et 

 al. 1976) have suggested a preferred temperature 

 of 27°C. Seasonal onshore and offshore movements 

 are made by round scad in the Gulf of Mexico (Klima 

 1971) and by other pelagic fishes in the South Atlan- 

 tic Bight (Wenner et al. 1979a, b, c, d, 1980) and 

 elsewhere (Allen and DeMartini 1983). 



Longshore migration does not appear to be a con- 

 sistent feature of the movements of round scad 

 along the southeastern Atlantic states. Differences 

 of catches (Table 4) made north and south of lat. 

 31°30'N are inconsistent, and limited meristic data 

 (see Results) provides no evidence of movements by 

 discrete stocks through the South Atlantic Bight. 



Although fish communities of live bottom habitats 

 may change (Chester et al. 1984), latitudinal differ- 

 ences in the distribution of demersal fishes in sandy, 

 open shelf habitats of the South Atlantic Bight are 

 not apparent (Wenner et al. 1979a). More substan- 

 tive information (tag-recapture studies, etc.) than 

 the limited data given above is needed to determine 

 accurately the movement patterns of round scad. 

 At the present time, available information suggests 

 that seasonal migration in round scad involves main- 

 ly onshore or offshore movement. 



The abundance of round scad in the South Atlan- 

 tic Bight is undoubtedly underestimated by catch 

 statistics (Table 2). First, benthic otter trawling is 

 generally inadequate for determining the abundance 

 of small, mobile pelagic species (Wenner et al. 

 1979a). Second, the attraction of round scad to 

 sponge-coral habitats in winter may have exag- 

 gerated the apparently large fluctuations in the 

 seasonal abundance of round scad in the South 

 Atlantic Bight. The sampling protocol assumes equal 

 probability of capture over the different habitat 

 types, but the probability of capturing round scad 

 over live-bottom habitats varies seasonally. An in- 

 creased proportion of live-bottom areas should be 

 sampled in winter to obtain more reliable estimates 

 of the abundance of round scad. The distribution, 

 extent, and adequacy of sampling of live-bottom 

 habitats are not well known (Wenner 1983). Thus, 

 sampling inadequacies and the seasonal attraction 

 of round scad to live-bottom habitat result in under- 

 estimation of abundance. 



Several factors may influence the attraction of 

 round scad to live-bottom habitat in winter. First, 

 round scad utilize live-bottom habitat in winter, 

 when invertebrate biomass has peaked and poten- 

 tial competitors and predators have decreased 

 (George and Staiger 1978^; Wenner et al. 1980, 

 1983, 1984; Sedberry and Van Dolah 1984). Second, 

 winter temperatures at live-bottom stations, though 

 not significantly different from temperatures at 

 sand-bottom stations, tend to be warmer, and scad 

 prefer warm waters. The greatest densities of round 

 scad occur in the midshelf where seasonal temper- 

 atures are generally warmest and the most highly 

 productive live-bottom areas are located (Miller and 

 Richards 1980; Sedberry and Van Dolah 1984). 

 Finally, the relief of live-bottom habitats (albeit low) 

 may serve to attract round scad; many coastal 

 pelagic fishes, including round scad, have an affin- 



^George, R. V., and J. C. Staiger. 1978. Epifaunal benthic 

 invertebrate and demersal fish populations in the Georgia Bight 

 continental shelf environment. South Atlantic Benchmark pro- 

 gram, Volume 3, Texas Instruments Inc. Draft Report, p. 211-254. 



262 



