FISHERY BULLETIN: VOL. 83, NO. 4 



Table 2.— Mean back-calculated standard length of 10 spot from 

 each of two collections of the same cohort (average spawning 

 date 22 December 1979) taken about 2 mo apart in 1980. 



^Larval mean age, 23 d; mean size, 5.7 mm 

 ^Larval mean age, 83 d; mean size, 15.0 mm 



DISCUSSION 



Although spot is a winter spawner, it spawns in 

 relatively warm water. Very young larval spot (<15 

 d) occurred only in water above 19.3°C, an observa- 

 tion corroborated by experimental evidence in- 

 dicating that spot spawn only between 17.5° and 

 25°C (Hettler and Powell 1981). In late fall and early 

 winter off North Carolina, such warm temperatures 

 are found only on the outer continental shelf near 

 the Gulf Stream. Newly ripe adults probably 

 emigrate in the fall of the year from the cooling 

 waters of bays and sounds in Virginia (Hildebrand 

 and Schroeder 1928), North Carolina (Roelofs 1951), 

 and South Carolina (Dawson 1958) to spawn in such 

 warm waters. Hildebrand and Schroeder (1928) and 

 Dawson (1958) also suggested that spot spawn along 

 the outer continental shelf. 



Warm coastal waters in the fall and the influence 

 of warm Gulf Stream waters later in the season may 

 provide a suitable spawning temperature regime 

 over a long period. The extended (4.5 mo) spawning 

 season of spot is typical of the general pattern for 

 Atlantic coast sciaenids (Powles 1981). The spawn- 

 ing season of spot in North Carolina in 1979-80 was 

 similar to that found by Hildebrand and Cable (1930) 

 in North Carohna and by Dawson (1958) in South 

 Carolina. Because most of the larvae caught off 

 North Carolina were spawned in December and 

 January, we conclude that these are the months of 

 peak spawning. This conclusion is supported by the 

 observation that peak estuarine immigration occurs 

 in February and March (unpublished data from R. 

 M. Lewis, Beaufort Laboratory; Thayer et al. 1974) 

 for fish we estimate to have been about 2 mo-old. 

 Hildebrand and Cable (1930) and Lewis and Judy 

 (1983) also inferred, from length-frequency informa- 

 tion, that peak spawning occurs in December and 

 January. 



The trend of decreasing larval age and size (Figs. 



2, 3) with distance from shore supports the idea that 

 spot spend virtually their entire larval period in the 

 ocean. Berrien et al. (1978) and Lewis and Judy 

 (1983) also noted an inverse trend of size with 

 distance from shore in the same area to 79 km off- 

 shore A similar trend may exist in the Gulf of Mex- 

 ico where Fruge (1977) found small larval spot to 

 be most abundant 60-80 km off the Louisiana coast. 

 By the time larvae have been transported to shore 

 and enter estuarine nursery areas, they have reach- 

 ed the late larval or early juvenile stage. 



Although the mechanism is unclear by which lar- 

 val spot from 74 to 93 km offshore arrive at the 

 estuary in about 60 d, their initial onshore movement 

 is probably a passive transport by water currents in 

 Onslow Bay. A consistent counterclockwise eddy 

 (Stefansson et al. 1971) and a strong indication of 

 bottom drift in a northerly direction on the outer 

 and mid-continental shelf and directly to the coast 

 inshore during January-April (Bumpus 1973) could 

 aid in the transport of larvae. Nelson et al. (1977) 

 considered that zonal Ekman transport was a signifi- 

 cant mechanism for movement of larval Atlantic 

 menhaden, Brevoortia tyrannus, from offshore 

 spawning grounds to inshore nursery grounds in the 

 same study area at about the same season of year. 

 Data from recent years, however, does not lend sup- 

 port for this hypothesis (Schaaf^). A recent analysis 

 by Yoder (1983) suggested that mean Ekman 

 transport does not favor onshore flow in surface 

 waters during winter off the southeastern United 

 States. Rather, cross-shelf transport of larval fishes 

 may depend on highly variable, short-term meteo- 

 rological events which reverse the mean surface flow. 

 Variations in transport rates of larvae in the ocean 

 as well as spawning at variable distances from shore 

 may be responsible for the seasonal differences in 

 age and length at immigration (Fig. 5). In addition, 

 factors affecting growth, such as temperature and 

 the distribution of food organisms, interact with the 

 physical factors of transport to produce the temporal 

 pattern of age and length observed in a given 

 year. 



Young spot undergo several environmentally 

 related changes in growth during their larval and 

 juvenile stages. Growth in length of larval spot is 

 rapid (initially approaching 7%/d) and coincides with 

 the winter peak of plankton productivity in the 

 relatively warm water of the outer continental shelf 

 (Tbrner et al. 1979; TUrner 1981; Yoder et al. 1981; 



^W. E. Schaaf, Southeast Fisheries Center Beaufort Laboratory, 

 National Marine Fisheries Service, NOAA, Beaufort, NC 

 28516-9722, pers. commun. January 1984. 



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