Estimating the lipid content of pelagic fish in the southern Benguela by visual assessment of their mesenteric fat

Abstract

A technique to estimate the lipid content of anchovy Engraulis encrasicolus and sardine Sardinops sagax using visual assessments of mesenteric fat is described. Anchovy are allocated to one of five and sardine to one of seven fat stages, depending upon the amount of fat associated with their intestine and stomach respectively. The accuracy of this technique was evaluated by relating body (less gonad) lipid content to fat stage and other morphometric parameters (fish length and mass) through general linear modelling; significant relationships for both species were derived. Fat stage by itself is a good predictor of anchovy body lipid content and accounts for 75% of the observed variability in body lipid content, whereas fat stage and fish mass together account for most of the variability (89%) in body lipid content of sardine. Application of the fat staging technique has revealed both spatial and temporal variability in pelagic fish condition. Spatially, the condition of anchovy recruits is higher off the South African west coast than the south coast, whereas the condition of anchovy adults increases in an eastward direction over the Agulhas Bank. Temporally, anchovy recruits exhibit marked interannual variability in their fat stage distributions, and sardine spawners show clear seasonal changes in their mean fat stage that are inversely related to their mean gonadosomatic index. This technique has the advantages of being quick and easy to apply, requires no specialised equipment and is cheap, and is therefore well-suited for use at sea. These factors, together with good reproducibility and accuracy, demonstrate its efficacy as a method for assessing the condition of pelagic fish.

African Journal of Marine Science 2005, 27(1): 45–53