Mackerel icefish size and age differences and long-term change at South Georgia and Shag Rocks
This study analysed the total length (LT)-frequency distribution of mackerel icefish Champsocephalus gunnari at South Georgia and Shag Rocks from nine bottom trawl surveys at South Georgia and eight at Shag Rocks between 1987 and 2002. The estimated mean LT of age-classes 1+, 2+, 3+ and 4+ years during January were, respectively, 14·7, 23·5, 29·8 and 35·1 cm at South Georgia. Age-classes 1+, 2+ and 3+ years were 18·3, 26·2 and 33·8 cm at Shag Rocks. The derived Bertalanffy growth parameters for South Georgia were: L∞ = 51·7 cm, k = 0·27 and t0 = −0·26. The mean LT of each age-class of C. gunnari at Shag Rocks was significantly larger than at South Georgia, equivalent to c. 5 months growth, although the annual growth in LT was similar. This is further evidence that C. gunnari hatched earlier at Shag Rocks. At South Georgia, the mean LT of age-classes 1+ and 3+ years were correlated, and significantly decreased between 1987 and 2002, and were smaller following warmer summers. This decrease in the size of C. gunnari may be the result of reduced food availability linked to climate warming.