Stone and Jessop. Seasonal distribution of Alosa pseudoharengus and A aestivalis 



377 



58 °N 



Figure 1 



Topographical map of Cana- 

 dian groundfish survey areas 

 (1970-89) and geographic fea- 

 tures mentioned in text. Spring 

 (1979-85), summer (1970-89), 

 and fall (1978-84) survey cov- 

 erage of the Scotian Shelf and 

 Bay of Fundy (- - -) is shaded 

 light-gray. Spring (1986-89) 

 survey coverage of the eastern 

 shelf and Georges Bank (...) 

 is shaded dark-gray. Offshore 

 banks are delineated by the 

 100 m depth contour; basins 

 and the outer edge of the con- 

 tinental shelf are delineated by 

 the 200 m depth contour. 



catches in offshore bottom-trawl fisheries from 1984 

 to 1989 has averaged l,400t, which is <17% of the 

 freshwater exploitation (Statistics Branch, Dep. Fish. 

 Oceans, P.O. Box 550, Halifax, Nova Scotia B3J 2S7), 

 only a limited amount of information on marine dis- 

 tribution is available from commercial fishing opera- 

 tions. More comprehensive information comes from the 

 bycatch of the annual bottom trawl surveys of the Sco- 

 tian Shelf-Bay of Fundy region conducted by the Cana- 

 dian Department of Fisheries and Oceans (DFO) to 

 monitor temporal changes in the abundance of commer- 

 cially exploited groundfish species and associated 

 environmental conditions (Halliday and Koeller 1981). 

 Additional information on river herring distribution 

 was obtained from 10 years of bycatch data from 

 foreign and domestic fishing operations compiled by 

 the DFO International Observer Program. 



Description of survey srea 



The survey area, which includes the Scotian Shelf, 

 eastern Gulf of Maine, Bay of Fundy, and, recently, 

 Georges Bank, is topographically and hydrographical- 

 ly complex (Fig. 1). The oceanographic and biological 

 characteristics of this area influence the distribution 

 of other species (Sinclair 1988) and are assumed to do 

 so for river herring. 



The Scotian Shelf is characterized by deep central 

 basins (>200m) and shallow offshore banks (<50m). 

 Over the deeper parts of the shelf, three distinct ver- 

 tical layers occur: a surface mixed layer with seasonal 

 temperature changes, an intermediate layer with tem- 

 peratures <5°C regardless of season, and a warm 

 bottom layer derived from cross-shelf intrusions of 

 slope water (Hatchey 1942, Smith et al. 1978). The 

 coldest water at any depth occurs in the northeastern 

 Shelf and the warmest in the bottom waters of the cen- 

 tral Shelf (also termed the Scotian Gulf) where intru- 

 sion of warm slope water to adjacent Emerald and 

 LaHave Basins is a persistent feature (McLellan 1954). 

 Nutrient-rich upwellings along the shelf-slope interface 

 sustain high levels of biological productivity (Fournier 

 et al. 1977). Consequently, pelagic fish production on 

 the shelf-slope is much higher than on the shelf (Mills 

 and Fournier 1979). 



The Bay of Fundy and eastern Gulf of Maine regions 

 are vertically well mixed due to the action of strong 

 tidal currents (Greenburg 1984). Strong, persistent 

 summertime fronts in sea-surface temperature occur 

 near the mouth of the Bay and off southwestern Nova 

 Scotia (Loder and Greenburg 1985). The upwelling of 

 nutrient-rich deep water from the Gulf of Maine and 

 Scotian Shelf supports high biological productivity 

 during spring and summer (Fournier et al. 1984). Sec- 

 ondary production in the outer Bay of Fundy occurs 



