Application of the triacylglycerol/sterol condition index to the interpretation of larval lobster Homarus americanus distribution in close proximity to Georges Bank, Gulf of Maine

Abstract

The triacylglycerol/sterol condition index was applied to larval lobster populations in the vicinity of Georges Bank in the Gulf of Maine. This index is related to larval size by an increasing power function which explains around 40% of the variation. The poor fit can be explained by the uneven increase in triacylglycerol levels during development within each moult stage. Increased pigmentation is not related to larval condition, as measured by lipid storage, and masks the increased yellowish hue of lipids as development proceeds. The larval triacylglycerol/sterol index appears to undergo a diurnal cycle in Stage III and IV lobsters, with lowest values at midday and highest values after dark. This pattern cannot be explained by nocturnal feeding, which leaves the possibilities that satiated larvae descend below the surface metre during daylight and are therefore underrepresented in our collections by being more vertically diffuse, and/or that healthy well-fed larvae would be more likely to detect the trawl and escape during daylight. Few Stage I and II lobster larvae were found in the vicinity of Georges Bank with a condition index less than 0.1, which is the level laboratory studies indicate approaches the 'point-of-no-return'. The condition of all developmental stages was found to be better in individuals located off Georges Bank. This is not ecologically significant in the case of the first 2 stages because such a small proportion of the population was actually located off the bank. It is not resolved how the third and fourth stages arrive off Georges Bank, since shoal water hatching is the norm, but their lipid reserves are significantly greater than identical developmental stages located on the bank. Finally, the density of Stage IV larvae in the adjacent surface waters over the Gulf of Maine is twice that found over Georges Bank. This suggests that the lobster has evolved a life cycle in offshore waters in which larvae are hatched in shoal waters over the banks but the last 2 planktonic/pelagic stages either seek or are transported to, and by Stage IV thrive in, the warmer stratified layer over the deeper waters of the Gulf of Maine.