384 



Fishery Bulletin 90(2), 1992 



presence of at least three year-classes in the fall and 

 four in the spring. Smaller individuals (<19cmFL) 

 represented a larger proportion of the overall length 

 composition in spring and fall than in summer. 



River herring > 1 9 cm FL were more densely and ex- 

 tensively distributed in all seasons than smaller fish. 

 During summer and fall, most river herring <19cm 

 long were captured within the Bay of Fundy, although 

 a few occurred in the central Shelf region and in 

 Sydney Bight (fall only) (Fig. 9). The greatest overlap 

 in the spatial distribution of both size-groups was dur- 

 ing spring, especially in offshore regions (i.e., central 

 Scotian Shelf, southern Gulf of Maine). 



The mean fork length of trawl-caught (1979-84) river 

 herring varied significantly by season of capture and 

 depth (Table 2, Fig. 10). Interactions of season effect 

 within depth strata and depth effect within season were 

 all significant. Smaller fish were caught more frequent- 

 ly at depths <93m during spring and fall. Larger fish 

 occurred at all depths during summer (more numerous- 

 ly at < 93 m depth) and at depths ^93m during spring 

 and fall. In all three depth strata, river herring from 

 spring surveys averaged smaller in length than those 

 captured in summer and fall because of the greater oc- 

 currence of fish <19cm throughout the survey area 

 during spring (Fig. 9). 



Discussion 



Canadian groundfish survey data indicate persistent 

 patterns in the temporal and spatial distribution of 

 river herring off the Atlantic coast of Nova Scotia 

 which appear to be greatly influenced by oceanographic 

 features. In spring, river herring were most abundant 

 in the warmer, deeper waters of the Scotian Gulf, par- 

 ticularly along the edges of Emerald and Western 

 Banks and within the channel separating them, and in 

 regions of warm slope-water intrusion along the Sco- 

 tian Slope, the western and southern edges of Georges 

 Bank, and the eastern Gulf of Maine. In all seasons, 

 river herring occurred in the Bay of Fundy and off 

 southwestern Nova Scotia, regions characterized by 

 strong tidal mixing and upwelling, but were rarely 

 present on the eastern Scotian Shelf. 



Water temperature evidently influences temporal 

 and spatial patterns in river herring depth distribution. 

 In all seasons, most catches occurred within the 7-1 1°C 

 range but shifted from mid-depths offshore in spring 

 (101-183 m) to shallower, nearshore waters in summer 

 (46-82 m), and to deeper offshore waters in fall (119- 

 192 m). River herring were not present in colder 

 regions on the eastern and western Scotian Shelf. 

 Catches of river herring along the U.S. continental 

 shelf were most frequent at bottom temperatures of 

 4-7°C and depths <92m (Neves 1981). Spring catches 

 of river herring at bottom temperatures <5°C in the 

 Bay of Fundy indicate some flexibility in thermal selec- 

 tion, as might be expected of a migratory anadromous 

 fish. American shad, which are closely related to 

 alewives and blueback herring, can remain for ex- 

 tended periods in temperatures outside their usual 

 range (7-13°C) and migrate rapidly between areas with 

 different temperature regimes (Dadswell et al. 1987). 



Seasonal shifts in zooplankton abundance, which are 

 influenced by local oceanographic features, may also 

 influence river herring distribution patterns off Nova 

 Scotia. Both alewives and blueback herring are zoo- 



