Temporal trend monitoring: Contaminant levels in tissues of Atlantic cod.

Abstract

The study of trends in trace contaminant concentrations in marine species has been of interest to the International Council for the Exploration of the Sea (ICES) since the early 1970s. The investigation of temporal trends in contaminant levels in fish, both as monitors of their environment and from a human health concern, is a topic under consideration by the ICES Working Group on the Statistical Aspects of Trend Monitoring (WGSATM) as a part of the Cooperative ICES Monitoring Studies Programme (CMP). Many problems were encountered during the first attempts by WGSATM to analyse the CMP data. In addition to the presence of outlying observations and significant differences between years in their coefficients of regression of (log) contaminant level(s) on (log) biological variable(s), many inconsistencies were found in the annual sampling structures (Anon., 1987). These often reflected the inability to replicate the size (length, age) structure characterizing earlier samples and the failure to obtain a sufficiently wide range in the covariables selected for study. These inconsistencies affected the annual regression lines, yielding, in many instances, insignificant regressions. Further discussion within WGSATM identified many sampling and handling differences which probably arose from the too general nature of the past guidelines for trend monitoring. The Working Group emphasized the importance of essentially identical structures within all samples comprising a temporal trend study and emphasized that trend monitoring can only be done effectively if all steps of the study are thoroughly described and properly followed by all participants. Therefore, the WGSATM recommended that a series ofleaflets be prepared giving specific details of sampling, handling, and analysis for each species being studied. The first leaflet (Vthe et al., 1991) is a general introduction, describing the problems and requirements when monitoring temporal trends in contaminant levels in the marine environment, particularly biota. This document describes the Canadian techniques used to study temporal trends in contaminant levels, both tissue concentrations and burdens (tissue concentration times the total weight of that tissue in the animal), in individual Atlantic cod (Gadus morhua) muscle and liver tissues over the period 1977-1985. The decision to study Atlantic cod was based, inter alia, upon the importance of this species to Canada and the North Atlantic community, its widespread distribution within the ICES area, and the identification of a relatively discrete and stable cod stock in the southern Gulf of St. Lawrence. The procedural descriptions which follow are given only in outline form, along with the bases upon which such procedures were developed. Full details, including the statistical analyses carried out on the data, are given in the following references: Scott et al. (1978, 1981, 1983), Misra et al. (1988, 1989a,b,c), and Misra and Vthe (1985, 1986, 1987a,b). The chemical analytical procedures for the measurement of trace contaminants are not considered here, but are included in the above-mentioned references.