Fishery p r o d u c t i o n and food webs have been studied on the Scotian Shelf and upper c o n t i n e n t a l slope along a transect running 270 km SE of Halifax, Canada. The area (ICNAF D i v i s i o n 4W) supports a fishery of roughly O.15 x 1 0 6 m e t r i c tons. O v e r a l l primary p r o d u c t i o n of the shelf waters is 102 g C m -2 year -I and of the Slope waters about 128 g C m -2 year-1. Demersal fish p r o d u c t i o n (average 4.1 kcal m-2 year-l) is highest over the shelf and declines in an o f f s h o r e direction, while pelagic fish p r o d u c t i o n (average 16 kcal m-2 year-l) is highest over the slope and declines in an inshore direction. H y p o t h e t i c a l food webs of these two i n t e r g r a d i n g e c o s y s t e m s have been constructed, based on data for primary production, fish catches, and the biomasses of z o o p l a n k t o n and macrobenthos. These lead us to suggest that there are basic d i f f e r e n c e s in food chains and e f f i c i e n c i e s between the two e c o s y s t e m s that account for their d i f f e r e n c e s in production. A l t h o u g h p r i m a r y p r o d u c t i o n is 17% higher on an average on the Nova Scotian transect than in the North Sea, the apparent z o o p l a n k t o n and m a c r o b e n t h o s p r o d u c t i o n is 31% lower and m a c r o b e n t h o s p r o d u c t i o n may also be lower. Overall fish catch from the Scotian Shelf and slope is about 47% lower per unit area than the catch in the North Sea, despite the fact that the demersal catches are identical. This is a c c o u n t e d for by a much lower overall pelagic catch from the Nova Scotian area, c e n t e r e d in a region that is small c o m p a r e d to the total area. Fish p r o d u c t i o n in different regions cannot be p r e d i c t e d m e r e l y on the basis of d i f f e r e n c e s in level of p r i m a r y production, but must take into account d i f f e r e n c e s in the structure of the ecosystems.