Importance of suspended particulate organic matter in the diet of Nephrops norvegicus (Linnaeus, 1758)

Abstract

the extent to which commercially important Nephrops norvegicus lobsters feed on particulates in the wild is unknown, even though this could be an important way for burrow-dwelling females to avoid starvation during the long breeding season. this was investigated using δ 13 c and δ 15 n isotopic signatures in tissues with long and short turnover rates to provide diet discrimination and compare this between males and females. Secondary objectives examined size-related differences and calculated the trophic position based on the new results. Almost half the diet (47%) was made up of suspended particulate organic matter (poM susp) alone. Fish was another important item in the diet, with plankton and invertebrate sources coming much lower down in dietary importance. Significantly more suspension feeding was observed in small or medium sized individuals than large ones in both sexes. However, there were no sex-related patterns, despite females being restricted to burrows for part of the analysis period. female diet was almost identical to males and poM susp comprised a large component of the diet in both sexes. The trophic position was estimated at 2.94 ± 0.16 (mean ± SD), which was at the lower end of the range reported in previous studies (2.60 to 4.32). Dublin Bay prawn, Nephrops norvegicus, is a decapod crustacean and an important economic resource in Europe: global production of this fishery was 59,033 tons in 2016 of which the United Kingdom and Republic of Ireland were the main producers, capturing up to 32,708 and 10,379 tons per annum respectively during 2012-2016 1. Nephrops populations are distributed on semi-isolated mud patches which are assessed by ICES as separate Functional Units (FU), however this resource is not 'managed' via fishing quotas, and some FUs periodically display signs of over-exploitation 2,3. The Marine Strategy Framework Directive and reformed Common Fisheries Policy (CFP) require ecosystem-based fisheries management which observes interactions among all components of the ecosystem, including trophic interactions 4-6. Although not currently managed under the CFP, key gaps and ambiguities exist in knowledge of Nephrops' diet and feeding ecology which should be addressed, given their economic importance and occasional over-exploitation. Nephrops individuals are known to be opportunistic predators and scavengers, which seem to have a diet driven by prey abundance rather than prey preference 7-10. Diet from stomach contents analyses seems to be similar across a wide geographical range in the northeastern Atlantic and Mediterranean, composed mainly of crustaceans, polychaetes, molluscs and echinoderms 8,9. A considerable contribution to the diet is also made by fish in southern Atlantic and Mediterranean samples 11,12. However, some mystery surrounds the extent of feeding on particulates in Nephrops. In the absence of alternative food sources, such as in the aquarium, Nephrops was demonstrated to feed on planktonic items larger than 300-500 µm, which were later recovered from the stomach and...

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