Nutritional status and trophic dynamics of the Norway lobster Nephrops norvegicus (L.)

Nephrops norvegicus is a mid-sized benthic decapod lobster found at depths of between 30-300 metres along the eastern Atlantic coast from Norway to Morocco, and within the Mediterranean sea. They inhabit marine muddy sediments in which they excavate burrow systems and from which they make short excursions to feed by predation and scavenging. The females of this species are known to reside within their burrows over the winter period while brooding their eggs, although their feeding behaviour over this period has not been investigated. Two aspects of the nutrition of N. norvegicus, namely nutritional status (the condition of an animal, which is maintained through a balance between energy intake and energy expenditure) and trophic dynamics (the flow of energy from exogenous inputs throughout food webs in an ecosystem) have been studied, to answer questions relating to the influence of season, sex and site on nutrition in this species. Specific questions that have been addressed include: with regard to season, is there a reduction in the nutritional status of N. norvegicus over the winter due to a decrease in primary production? With regard to sex, do females have a reduced nutritional status to males when they emerge from their winter brooding period, indicating that females go through a period of fasting over the winter? Also do females employ a filter feeding strategy over the winter? With regard to site, do N. norvegicus at two different sites on the west coast of Scotland (i.e. the Clyde Sea Area [CSA] and the North Minch [NM]) occupy different trophic levels and have different fatty acid signatures, which would indicate that their diets are made up of different prey species? A range of biochemical markers was tested to describe how N. norvegicus reacts to a period of forced starvation, and thus to determine their nutritional status. There was clear metabolic depression, indicated by an increase in the copper concentration of the hepatopancreas in both males and females. In contrast to the findings of previous studies, lipids in the hepatopancreas did decrease with starvation, however was mitigated by the metabolic depression and only became significantly lower than fed individuals after 12 weeks in males and 20 weeks in females. Lipids and water combined made up 80% of the mass of the hepatopancreas, and when the lipids decreased the proportional mass was replaced by water. However, as indicated by the hepatosomatic index (HSI), the total mass of the hepatopancreas decreased over the period of starvation. A temperature effect was also seen which influenced the amount of reserves required by the animal under these conditions. Using the results from the starvation trials, a predictive tool was developed for determining the nutritional status in N. norvegicus from the wild, and for calculating threshold values which indicate whether animals were in a starved state. These measurements and threshold values were applied to field caught animals, assessing the factors of season, sex and site. The seasonal analysis showed that there was low nutritional status in N. norvegicus during the winter of 2008-2009, whereas the nutritional status of winter 2009-2010 was not as low. The site analysis showed that N. norvegicus in the CSA, where burrow density is 0.85 m-2, were found to have a lower nutritional status than N. norvegicus in the NM, where burrow density was only 0.55 m-2. These results are consistent with the notion that in high density areas N. norvegicus has a lower nutritional status than in low density areas. Females were found to have a larger lipid store within the hepatopancreas than males, presumably to sustain reproduction. Females were also found to be metabolically depressed over the winter and spring months of early 2009. However they were not in an advanced state of starvation over this period. It is therefore concluded that over the winter months females reduce feeding, perhaps in a response to reduced mobility brought about by the brooding stage of reproduction, but do not follow different feeding strategies to males over this time. The differences of trophic level between N. norvegicus in the CSA and the NM were assessed through the use of stable isotope analysis. It was found that although the animals in the CSA had higher 15N values than the animals in the NM, the animals in the NM were actually feeding at 0.6 trophic level units higher than those in the CSA. The high 15N values in the CSA were ascribed to organic enrichment of nitrates in that area. There was no significant difference between males and females at the end of the winter period, indicating that when females are brooding their eggs they do not use different feeding strategies (e.g. filter feeding) as an alternative means of nutrition. The differences between the Fatty Acid (FA) signatures of N. norvegicus in the CSA and the NM were determined by fatty acid analysis. A clear difference was demonstrated between sites, indicative of differences in the dietary composition of the two populations, and suggestive of a greater input from pelagic food sources (either zooplankton or fish) in the NM. However, no significant difference was found in FA signatures between male and female N. norvegicus at either site or at any time of the year. Thus, consistent with the conclusions from the other monitoring methods, the FA signatures indicate that females and males had similar feeding patterns at all times of year, with regard to the type of food consumed. As well as addressing issues directly concerned with the nutrition of N. norvegicus in relation to season, sex and site, the results of these studies have a number of wider applications, including the development of optimal feeding strategies for holding lobsters under impoundment conditions for long periods, and for identifying the procedures required to assess more directly the dietary composition of N. norvegicus populations, and especially the contribution of fish to their diet.