Decadal changes in the Scotian shelf large marine ecosystem

The eastern and western portions of the Scotian Shelf were treated separately based on differences in physical oceanographic characteristics and differences in fish and other faunal compositions. The central and western shelf are warmer, experienced a cold period during the 1960s, and then warmed to. above average until 1998 when an intrusion of cold Labrador Shelf water caused significant cooling. The eastern shelf cooled from about 1983 to the early 1990s and bottom temperature has remained below average since then. This cold period is associated with increased abundance of cold water fish (capelin, turbot) and invertebrates (snow crab, shrimp) usually more prevalent in colder Gulf of St. Lawrence and Newfoundland waters. Colder temperatures are also implicated in reductions in growth rates of demersal fishes. Phytoplankton greenness and total copepod indices are higher and show peaks earlier in the year during the early 1990s than during the 1960s. An increase in the abundance of arctic copepod species also accompanied the cooling of the eastern shelf. Trawlable demersal biomass has declined on both portions of the shelf but most rapidly on the eastern shelf since the early 1980s. However, total finfish biomass (corrected for relative catchabilities) shows a variable increase since the mid-1980s. Pelagic fish biomass has increased, and the ratio of demersal to pelagic biomass has decreased, especially in the east. Average size of demersal fishes decreased by 60 to 70% since 1970 in both systems, and both have shown significant changes in the integrated community size frequency (ICSF) with a reduction in numbers of large fish. Fishing effort, which increased rapidly following the 1977 establishment of Canada's 200-mile exclusive economic zone, was negatively correlated with community size structure. A reduction in effort in the early 1990s is associated with an apparent reversal in the trend of decreasing ICSF The decrease in ICSF was more rapid in the colder eastern shelf and was considered to be related to differences in growth rates between the two systems. Grey seal abundance has increased exponentially since the 1960s, especially on the eastern shelf, however, the impact of this increase on levels of predation remains uncertain due to the paucity of comparative diet information prior to the mid-1980s.