NEVES and DESPRES OCEANIC MIGRATION OF AMERICAN SHAD 



chronous species migration as suggested by 

 Leggett (1977). Coastal tagging studies during the 

 spring reveal an aggregation of many spawning 

 stocks that often detour into estuaries along the 

 coast (Sykes and Talbot 1958; Talbot and Sykes 

 1958; Chittenden 1974; Leggett 1977; White et 

 al.'"). However, the length oftime each population 

 has been inshore is unknown. Until stock iden- 

 tification at sea is feasible, the regional composi- 

 tion and extent of offshore mixing cannot be 

 documented. 



The location of winter collections (lat. 39°-41°N) 

 coincides with two previously published capture 

 records (Talbot and Sykes 1958; Walburg and 

 Nichols 1967), but the extent of overwintering in 

 deep water off the continental shelf is unknown. 

 Shad collections in the northern Gulf of Maine 

 during November and December were made at 

 depths >100 m and do not conform (based on pre- 

 vious studies) with the expected migration south 

 in late autumn. These and other shad captured in 

 deep water near Nova Scotia during March fVla- 

 dykov 1936) are outside the apparent wintering 

 area, south of Nantucket Shoals. The possibility 

 that some shad overwinter or become thermally 

 isolated in deepwater areas off Nova Scotia (Vla- 

 dykov 1936; Hodder 1966) needs further investi- 

 gation. 



Circulation patterns along the Atlantic coast do 

 not account for the seasonal distribution of shad 

 according to survey data or their coastal migration 

 routes based on tagging studies (Talbot and Sykes 

 1958; Leggett 1977). Bottom drift toward shore 

 and coastal drift south in the Middle Atlantic 

 Bight during winter (Bumpus 1973) would aid 

 migrants moving south, but seasonal shifts in di- 

 rectional flow along the east coast and their effect 

 on shad movements are liable to subjective in- 

 terpretation. Spawning populations moving north 

 and south concurrently could be helped or hin- 

 dered by circulation patterns in the mid-Atlantic 

 area. We believe that seasonal shifts in isotherms, 

 as influenced by circulation patterns, are of great- 

 er importance in defining the migratory route of 

 shad. 



tribution in the water column from three separate 

 sources: food habits, diel differences in catchabil- 

 ity, and effectiveness of various trawls in captur- 

 ing shad. Adult shad are zooplankton feeders and 

 consume primarily large copepods, mysids, and 

 euphausiids (Bigelow and Schroeder 1953; Hil- 

 debrand 1963; Leim and Scott 1966). The con- 

 sumption of food organisms such as mysids and 

 zoobenthos indicates that part of a shad's life is 

 spent near the ocean bottom (Leim 1924; Walburg 

 and Nichols 1967). In general, stomach analyses 

 reveal that shad feed at all depths but particularly 

 where concentrations of zooplankton occur. 



Trawling stations where shad were collected 

 during U.S. surveys ( 24 h/day) were partitioned by 

 capture time (Eastern Standard Time) into day 

 (0600-1800 h) and night ( 1800-0600 h). Chi-square 

 analysis on time of capture revealed that daytime 

 catches occurred significantly more often (P<0.01) 

 than night collections (Table 4). Of the night 

 catches, 25'^f occurred within 1 h of the daytime 

 interval. Shad were apparently closer to the bot- 

 tom during daylight hours and thus more suscep- 

 tible to bottom trawling gear. Further corrobo- 

 ration of this daytime occurrence nearer to the 

 bottom is evidenced by the frequency of shad 

 catches in foreign bottom trawls. During daylight 

 hours in March 1974-76, foreign research vessels 

 used herring trawls to sample 280 stations from 

 Long Island to Georges Bank and recorded shad at 

 71 (25'/( ) of these stations. Contemporary surveys 

 by the United States in the same area with the No. 

 41 Yankee trawl sampled 207 daytime stations 

 and collected shad at 22 ( ll'/f ) of them. Maximum 

 headrope distance off the bottom for the U.S. trawl 

 was 5 m. The larger foreign trawls had a higher 

 opening (6 m) which increased their effectiveness 

 on off-bottom species, although extra-trawl factors 

 such as vessel size, speed, and gear rigging cer- 

 tainly contributed to the greater overall fishing 

 power of these trawls (Grosslein 1969, 1971). 



We deduce from the above observations that 

 shad are vertical migrators like other schooling 

 planktivores such as herring, Cliipea harengus, 

 and mackerel. Scomber scombrus (Blaxter 1975; 



Vertical Distribution 



Presently there is little information on the 

 depths preferred by shad at sea. We inferred dis- 



'»White,R.L.,J. T.Lane, and P.E.Hamer. 1969. Popula- 

 tion and migration study of major anadromous fish, N.J, Div. 

 Fish Game Misc. Rep. No. 3M, 21 p. 



Table 4, — Chi-square test comparing the number of day and 

 night catches of shad during U.S. bottom trawl surveys. 1963-76, 

 Cape Hatteras, N.C. to Nova Scotia. 



Time 



Day (0600-1800 h) 

 Nighl (1800-0600 h) 

 Totals 



••p. 001 



Observed 



217 

 98 



Expected 



1575 

 157 5 



207 



