Spatial and temporal variability in length, weight and lipid content at age of the lesser sandeel Ammodytes marinus Raitt in the northern North Sea and adjacent waters
In order to investigate the spatial variability in length and weight at age seen in Ammodytes marinus during the period 1974-1977 (Anon. 1978), samples of A. marinus were collected during 1991 from Shetland, Fair Isle, the Fisher Bank, the Skagerrak and Rona. The samples were collected from the five sites during July when the peak in summer growth rate would have passed and any spatial differences resulting would be most marked. Greatest differences in length, weight and total lipid content were found to exist between fish from Shetland and the Fisher Bank. At the Fisher Bank fish were significantly longer, heavier and contained more lipid than fish of the same age at Shetland. Principal Component Analysis (PCA) was carried out on the 0 group and pooled 1 and 2+ group data. 82.5% (0-group) and 72.0% (1 and 2+ group) of the variance within the data was explained by the first two principal components. Plots of the scores of the first two principal components from the respective PCA's supported the spatial differences revealed in the analysis of the length, weight and lipid data. Sea temperatures and zooplankton biomass were suggested as factors influencing the spatial differences in size at age. Samples of A. marinus were also collected from Shetland and the Fisher Bank throughout the summer of 1991 to investigate differences in growth rate and lipid accumulation over a protracted period. Temporal differences in size at age could therefore be investigated. Over the four month study period fish of all age classes were found to be longer, heavier and containing more lipid at the Fisher Bank that at Shetland supporting the earlier analysis.