Relevance of Some Environmental Parameters to Distribution Patterns of Groundfish and Implications for Reasonable Survey Design: Case Study Atlantic Cod off Greenland

Since the taking up of regular investigations in fisheries biology in the 1920s, much attention has been directed at the occurrence of Atlantic cod (Gadus morhua) in relation to environmental conditions. The majority of contributions deal with climatic effects on migration or recruitment success in terms of large geographic or temporal scales (Jensen, 1939; Hansen, 1949; Bratberg and Hylen, 1964; Meyer, 1964; Stein and Messtorff, 1990). In contrast, the present analysis is focused on possible impacts of environmental parameters in terms of near bottom temperature, depth and position on the spatial distribution patterns of Atlantic cod off West and East Greenland, based on groundfish survey data covering a relatively short period of 14 years from 1982 until 1995 and in a limited survey area. The results are discussed in order to assess their relevance in relation to appropriate survey designs. Material and Methods Since 1982, the demersal fish assemblage off West and East Greenland (NAFO Division 1 and ICES Subarea XIV) has been monitored annually by German groundfish surveys. The survey strategy was designed for cod as the target species. The survey area was the continental slope off West and East Greenland south of 67°N from the 3-mile offshore line down to the 400 m isobath. Positions of hauls were randomly selected within trawlable areas and are illustrated in Fig. 1. During 1982-95, 2 245 successful sets were carried out, the numbers of valid sets by year are listed in Table 1. In 1984, 1992, 1994 and 1995, the survey area was not covered adequately due to technical problems. The catches of Atlantic cod (Gadus morhua L.) in numbers were raised to abundance per nautical square mile. Gear parameters used for converting and a detailed description of the survey method are given by Rätz (1996). As a standard procedure, near bottom temperatures were measured directly before or after trawling in the vicinity of the swept area by a CTD-sonde with a precision of a hundredth °C. Table 1 lists the available numbers of temperature values by year. During the 14-year time series, a total of 1 847 measurements were conducted (82% of the hauls). Numerical values of catch positions (longitude and latitude) were converted to decimals and the mean depth was determined from the average of the minimum and maximum recorded depths during an individual haul. In order to avoid pronounced year effects, individual cod abundance estimates in numbers per nautical square mile derived from trawl catches were standardized by division using the stratified means. Temperature measurements were standardized by division using the weighted means (by survey area) for the same reason. Table 1 lists the means on which standardizations are based.