Research Document 1998/13

Spatial analysis of northern Atlantic cod distributions with respect to bottom temperature and estimation of biomass using potential mapping in SPANS

By D.W. Kulka

Abstract

The spatial analysis technique (referred to as SPANdex) used in this study to estimate biomass converts point estimates, in this case individual fall survey set catch rates, into continuous surfaces (density subareas) that perform as survey strata. The strata vary over time taking into account stock distributional shifts. This and the increased number of sets per stratum result in lower within strata variability. Analysis of spatial patterns of distribution and density of cod using the potential mapping technique shows that cod in NAFO Divs. 2J, 3K and 3L has undergone several major changes during the 1980's and 1990's. During the fall in the early 1980's, the cod were found to be aggregated into four areas: North, along the Labrador coast from about Lat. 51° to the shelf edge north of Lat. 54° 30'; Middle, along the shelf edge between Lat. 51° 30' and Lat. 53°; South, along the northeast slope of the Grand Bank to Lat. 50° 30' and; Grand Bank, on the top of the bank at the border of NAFO Div. 3L and 3NO. Among years, how far along the migration of the cod from the inshore to the shelf edge was during the survey affected how far inshore these concentrations extended. After 1985, this pattern started to change as the North concentration started to diminish. It was the largest concentration from 1983 to 1988 and was the first to disappear from the Labrador Shelf during the collapse of the stock. At the same time, the Middle and South (Labrador Shelf) concentrations first increased in terms of extent of high density areas and biomass, then rapidly declined until 1993 where only a small remnant of the South concentration remained. During 1989 to 1991, the remaining cod hyper-aggregated in the South area. In the area along the Labrador coast where the bottom temperature was more variable among years, the cod distribution appeared to vary somewhat independently with respect to temperature. There was no downturn in the fall bottom temperatures during the period of decline. Bottom temperature appeared not to change the behaviour of the migrating cod or directly drive the decline of the stock. On the other hand, the pattern of disappearance of the concentrations (north to south) with respect to the pattern of fishing offshore (not north to south) suggests that fishing pressure was also not the primary cause of the decline

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