Vertical migrations of mackerel icefish (Champsocephalus gunnari) on the South Georgia shelf
High values of coefficient of variation resulting from the calculation of mackerel icefish biomass (Champsocephalus gunnari) from trawl survey data on the South Georgia shelf (Parkes et al., 1989) challenge the rationale for round-the-clock trawling and call for clarification of the techniques used. The objective of the present study is to analyse data on daily vertical migrations of mackerel icefish. The study used data from research vessels which had been fishing for commercial concentrations of C. gunnari in years of high abundance of the species (1982/83, 1983/84 and 1986/87). Catch-per-hour of fishing was analysed by the time of day. Catches made by both bottom trawls and midwater trawls were taken into account. Information from the round-the-clock hydroacoustic observations on vertical migrations of C. gunnari as well as from observations on fish feeding is also considered in the paper. A distinct pattern of daily vertical migrations of C. gunnari was observed on the South Georgia shelf. During the day, fish concentrate near the bottom in concentrations 5 to 7 m thick. At night, fish disperse and migrate to the upper layers 50 to 60 m beneath the surface. The main peak of fish feeding activity was also observed at night. Fishing during this period with bottom trawls had yielded almost no catches at all. The information obtained on vertical migrations of C. gunnari may be used for clarification of survey techniques used in stock assessment. For a number of years scientists of AtlantNIRO have been conducting trawl surveys for C. gunnari during daylight hours only.