FISHERY BULLETIN VOL 77. NO 1 



Isakov"; Rikhter'^), following the diel movements 

 of zooplankton in the water column. This reliance 

 on zooplankton for food may be an additional fac- 

 tor influencing shad distribution during the year. 

 Zooplankton distribution in the Gulf of Maine dur- 

 ing summer and autumn is closely tied to local and 

 regional hydrography (Redfield 1941; Sherman 

 1966; Cohen'-''); concentrations generally occur 

 along areas of current convergence and divergence 

 (Zinkevich 1967) and at depths <100 m (Bigelow 

 1926; Whiteley 1948). During winter, the waters 

 around Georges Bank are nearly devoid of zoo- 

 plankton, whereas sizeable neritic populations 

 occur from Nantucket Shoals to southern Long 

 Island (Clarke 1940; Grice and Hart 1962; Zin- 

 kevich 1967). Sette (1950) concluded that water 

 temperature had a limiting rather than causal 

 influence on the seasonal movements of mackerel, 

 and Redfield (1941) noted a parallelism between 

 mackerel distribution and areas of zooplankton 

 abundance. Similarly, Zinkevich (1967) related 

 herring movements to water temperature and 

 seasonal shifts in zooplankton concentrations. 

 Catches of shad during bottom trawl surveys 

 along Georges Bank, Gulfof Maine perimeter, and 

 south of Nantucket Shoals may therefore be re- 

 lated to zooplankton abundance in these areas, but 

 direct evidence is lacking. 



Coastal Migration 



Tagging studies and the location of NMFS and 

 ICNAF catches during the spring indicate that 

 most shad populations move toward the mid- 

 Atlantic coast from offshore waters, between lat. 

 36° and 40°N in the winter and early spring. The 

 time an4 location of tag returns by the mid- 

 Atlantic shad fishery demonstrate that shad from 

 most populations occur in this region during the 

 spring (Talbot 1954; Talbot and Sykes 1958; 

 Leggett 1977; White et al. see footnote 10). Shad 

 tagged near southern Long Island in early spring 

 were recaptured on spawning runs as far south as 

 North Carolina (Talbot and Sykes 1958). Tagging 



"Isakov, V. I, 1976. The peculiarities of diurnal vertical 

 migrations of mackerel in the northwestern Atlantic. Int. 

 Comm. Northwest Atl. Fish. Annu. Meet. 1976, Res. Doc. No. 

 lll.Senal No. 3934. 3 p. 



'^Rikhter, V. A. 1976. Proposal on trawUng surveys for 

 estimation of pelagic fish stocks in ICNAF Subarea 5 and Statis- 

 tical Area 6. Int. Comm, Northwest Atl. Fish, Annu. Meet, 

 1976, Res, Doc, No, 116. Serial No, 3939, 3 p, 



^•*Cohen, E, B, 1975, An overview of the plankton com- 

 munitiesof the Gulf of Maine, Int, Comm, Northwest Atl, Fish, 

 Annu. Meet, 1975, Res. Doc, No, 106, Serial No, 3599, 16 p. 



of shad in North Atlantic rivers during the spawn- 

 ing period produced recaptures as far south as the 

 North Carolina coast in subsequent years (Talbot 

 1954; Vladykov 1956; Talbot and Sykes 1958; 

 Leggett 1977). These tag returns provide an ap- 

 proximate geographical range of entry into coastal 

 waters by returning oceanic migrants (lat. 36°- 

 40°N). 



Assuming that the 3°and 15"C isotherms define 

 the northern and southern limits respectively of 

 shad movements at sea, prespawning adults re- 

 turning to coastal waters from the ocean would 

 face a thermal barrier south of Cape Hatteras. 

 Offshore bottom temperatures along the South At- 

 lantic coast remain above 17.5°C during the year, 

 whereas bottom temperatures on the continental 

 shelf north of Cape Hatteras and inshore tempera- 

 tures for the South Atlantic coast drop below 15°C 

 by December ( Figure 9). The proximity of the Gulf 

 Stream to North Carolina creates a narrow coastal 

 corridor at Cape Hatteras, providing the only mi- 

 gratory route to southern rivers if shad returning 

 to these home rivers are to remain within their 

 marine temperature regime. Migration toward 

 shore north of Cape Hatteras and then south along 

 the coast appear to be essential prerequisites for 

 successful homing to South Atlantic rivers. In con- 

 trast, shad returning to North Atlantic rivers dur- 

 ing the spring are not obliged to follow a coastal 

 route because offshore temperatures in the Middle 

 Atlantic Bight are well within the shad's range of 

 oceanic occurrence (Figure 9). However, tag re- 

 turns from adults tagged on spawning runs into 

 North Atlantic rivers indicate that many (most?) 

 adults do enter coastal waters in the lower mid- 

 Atlantic region and migi'ate north along the coast 

 to reach home rivers as repeat spawners the fol- 

 lowingspring(Talbot 1954; Leggett 1977). Results 

 of Atlantic coast tagging are consistent with our 

 upper temperature limit ( 15°C) for shad migration 

 at sea; all prespawning, oceanic migrants enter 

 inshore waters as far south as North Carolina. The 

 significance of the Cape Hatteras region to other 

 aspects of northern versus southern shad biology 

 was discussed by White and Chittenden ( 1977). 



Based on our proposed migratory route, large 

 shad catches in ICNAF Division 6B during the 

 spring would consist of shad entering home rivers 

 and populations moving toward and along the 

 coast. Catches in Chesapeake Bay and the sounds 

 of North Carolina from late November to early 

 December (Hildebrand and Schroeder 1928; Tal- 

 bot and Sykes 1958; Walburg and Nichols 1967) 



208 



