NEVES: OFFSHORE DISTRIBUTION OF ALEWIFE AND BLUEBACK HERRING 



recorded low volumes of zooplankton along the 

 northern coast of Maine. Confirmation of river 

 herring abundance in this region requires more 

 extensive investigation. 



The apparent difference in preferred depth dis- 

 tribution between the alewife and blueback her- 

 ring (Table 3) may be related to the diagnostic 

 character differences between these two species. 

 The alewife has a slightly larger eye, an adapta- 

 tion usually associated with an existence at 

 greater depths (Marshall 1966). In addition, green 

 wavelengths (ca. 500 m/x) penetrate waters of the 

 continental shelf most effectively (Wald et al. 

 1957). Could the color of the dorsum, green in the 

 alewife and blue in the blueback, be a counter- 

 shading mechanism for reduced predation within 

 the depth ranges most frequently occupied by each 

 species? No direct evidence is available to support 

 either the eye size or dorsal coloration postulates; 

 however, the vertical segregation evidenced at sea 

 has also been reported for juveniles in rivers 

 (Loesch and Kriete^°). Juvenile blueback herring 

 occur in the upper levels of the river water column, 

 and juvenile alewives frequent midwater depths. 



Diel differences in catchability of river herring 

 were examined by partitioning capture time 

 (eastern standard time) during survey cruises (24 

 h/d) into day (0600-1800 h) and night (1800-0600 

 h). Chi-square analysis with correction for con- 

 tinuity on time of capture revealed that catches 

 were made significantly more often (P <0.01) dur- 

 ing the day than at night (Table 5). Alewives and 

 blueback herring were apparently closer to the 

 bottom during daylight, and thus more susceptible 

 to bottom trawling gear. This diel difference in 

 depth distribution has also been reported for 

 juvenile river herring in estuaries ( Warriner et al. 

 1969; Loesch et al.^^) and adult alewives in the 

 Great Lakes (Janssen and Brandt 1980). 



I deduce from the above observations that river 

 herring are vertical migrators like other schooling 

 clupeids such as American shad and sea herring, 

 Clupea harengus (Blaxter 1975; Neves and De- 

 spres 1979), which follow the diel movements of 

 zooplankton in the water column. This reliance on 

 zooplankton for food may partly account for the 



Table 5. — Chi-square test comparing the number of day and 

 night catches of alewives (A) and blueback herring (B) during 

 U.S. bottom trawl surveys, 1963-78, Cape Hatteras, N.C., to 

 Nova Scotia. 



'"Loesch, J. G., and W. H. Kriete, Jr. 1976. Biology and 

 management of river herring and shad. Completion Rep. 

 Anadromous Fish Proj. 1974-1976, Va. AFC 8-1 to 8-3, 226 p. 



"Loesch, J. G., W. H. Kriete, Jr., H. B. Johnson, B. F Holland, 

 and M. W. Street. 1977. Biology and management of mid- 

 Atlantic anadromous fishes under extended jurisdic- 

 tion. Annu. Rep. Anadromous Fish Proj. 1977, N.C.-Va. AFCS 

 9-1, 183 p. 



"P<0.01. 



disjunct distribution of river herring in offshore 

 waters during most seasons. Zooplankton dis- 

 tribution in the Gulf of Maine during summer and 

 autumn is closely related to local and regional 

 hydrography (Redfield 1941; Sherman 1970; Cohen 

 footnote 9), and concentrations generally are 

 along areas of current convergence and divergence 

 (Zinkevich 1967). The waters around Georges 

 Bank during the winter 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 the 

 planktivorous Atlantic mackerel. Scomber scom- 

 brus. Similarly, Neves and Despres (1979) related 

 American shad distribution to bottom tempera- 

 tures and possibly seasonal shifts in zooplankton 

 concentrations. Catches of river herring in specific 

 areas along Georges Bank, the perimeter of the 

 Gulf of Maine, and south of Nantucket Shoals may 

 therefore be related to zooplankton abundance in 

 these regions, although direct evidence is lacking. 

 Critical data on the oceanic life history of most 

 anadromous fishes are lacking, and my synthesis 

 of NMFS survey data and previous studies on the 

 alewife and blueback herring should be consid- 

 ered tentative. Unanswered questions such as 

 stock identification and mixing, and time and di- 

 rection of migrations at sea during the year must 

 await prescribed oceanic research. 



ACKNOWLEDGMENTS 



I am indebted to the Resource Surveys Investi- 

 gation section and other staff members at the 

 Northeast Fisheries Center, NMFS, NCAA, Woods 

 Hole, for their cooperation in this study. Paul 

 Eschmeyer, Garland Pardue, Louis Helfrich, and 

 Ralph Mayo kindly reviewed the manuscript. Spe- 

 cial thanks go to Linda Depres for supplying the 

 1978 data, providing fish specimens for stomach 



483 



