Misinterpretations of elevated catch-per-unit-effort (CPUE) in the northern cod (Gadus morhua) fishery contributed to overestimations of stock size, inflated quotas, and unsustainable fishing mortality in the 1980s and early 1990s. We hypothesize that concentration of the fish and fishery led to extreme hyperstability in the CPUE-abundance relationship. In the late 1980s, migrant cod began to concentrate within the Bonavista corridor, their most southerly cross-shelf migration route. By the spring of 1990, approximately 450 000 t was concentrated within 7000 km2 at densities quadruple those of the 1980s. Densities remained high through 1992, while abundance declined fivefold. During this period, cod hyperaggregated (local densities increased with decreasing biomass) in the Bonavista corridor and CPUE increased. To the north, no hyperaggregation occurred, and densities and CPUE declined with biomass. In the Bonavista corridor from 1990 to 1993, CPUE was hyperstable with local cod density. Areas of high cod densities (>0.1 fish·m-2) shrunk as regional estimates of cod biomass declined. The spatial extent of the fishery contracted proportional to the shrinkage in area occupied by the fish. Hence, CPUE was related to abundance at the local scales of a fishing set (local acoustic density) but not to abundance at regional or stock scales.

Research on northern cod (Gadus morhua) from 1983 to 1994 indicated that a southward shift in distribution in the early 1990s was real and not an artifact of sequentially fishing down local populations. In the early 1990s, seasonal fishery and survey data showed distribution changes where there was no fishery, and large tonnage and densities (450 000 t, densities fourfold higher than 1980s levels) appeared in the south concurrent with declines in the north. All fishery, acoustic, and trawl survey indices increased in the south, while the stock declined. Southern-caught cod in the early 1990s exhibited northern characteristics: (i) antifreeze production capacities above historical norms and equivalent to those of northern fish, (ii) vertebral counts above historic norms and equalling northern counts, and (iii) declines in size-at-age to levels associated with northern fish. The cause of the shift is thought to be a combination of abiotic (climate) and biotic (capelin (Mallotus villosus)) environmental changes and cumulative long-term fisheries effects on cod behavior. The shifted distributions increased vulnerability to Canadian and foreign fisheries and led to a rapid decline in abundance, both before and after the moratorium on fishing in Canadian waters in 1992. Rebuilding will occur in three steps: environmental restoration, recolonization by adults, and enhanced recruitment across the shelf.

Biological changes in the ecosystem of the Northeast Newfoundland Shelf during the late-1980s and 1990s included a collapse in the biomass of cod (Gadus morhua), a substantial increase in the biomass of northern shrimp (Pandalus borealis), and an expansion in the area fished for shrimp. The timing and magnitude of changes in cod biomass and the quantity of shrimp consumed by cod were explored to determine if they were consistent with the hypothesis that the increase in shrimp biomass was a consequence of a reduction in predation pressure from cod. Results are equivocal because the timing of both the increase in the shrimp stock and the decline in the cod stock remain unclear and there is considerable uncertainty in the estimates of consumption of shrimp by cod. Nevertheless, it appears that an initial increase in shrimp biomass must have occurred during the early to mid-1980s and was not related to changes in the cod, whereas a larger increase in shrimp biomass in the 1990s was related at least in part to the collapse of the cod.

Blue hake (Antimora rostrata) are continuously distributed in slope waters off North America and into the Northeast Atlantic. This study focuses on their distribution from the Scotian Shelf in the south to the continental slope just south of Davis Strait. Although most survey effort took place at less than 500 m 91% of survey sets containing blue hake occurred at greater than 500 m. They were found as shallow as 200 m, but percent occurrence at less than 500 m (0.25%) was much lower than at depths exceeding 500 m (reaching 70% at >1400 m). Catch rate increased steadily to the maximum depths fished with commercial gear and peaked at about 1 400 m in survey trawls, although depths greater than 1 200 m were poorly sampled. Deep (1 3712 286 m) longline sets from the 1960s indicated that blue hake were relatively common beyond the standard depths of either scientific surveys or commercial fishing. The deepest capture was from the deepest set fished at 2 286 m. Catch rate increased faster with depth in the southern part of our study area. Sets with blue hake spanned a range of bottom temperatures between 0.9 and 8.7°C, but 97% of the survey sets with blue hake were associated with bottom temperatures greater than 3°C and less than 4.5 o C. Fish size (total length or TL) ranged between 5 and 65 cm. Only 29 of 74 569 fish measured from otter trawls were less than 15 cm, but in longline catches, 37 of 867 fish averaging 10 cm TL were taken, amongst the smallest recorded worldwide. Previous studies in US waters found no evidence of spawning and scant evidence of mature individuals and it was thought that spawning might take place to the north in Canadian waters; however, no mature fish, eggs or larvae have been found. The key meristic character used to distinguish North Pacific blue hake from those in other parts of the world was the count of gill filaments. We found considerable overlap in the range of number of gill filaments from our study area compared with those in the North Pacific.

No abstract