Tools to examine population maturity and diet of Nephrops norvegicus on Irish fishing grounds

da Silva Santana, Cesar Augusto
The research presented within this thesis describes investigations on maturity and feeding ecology of Nephrops across some Irish and Scottish grounds in the Northeast Atlantic Ocean. This thesis aimed to address the gaps and ambiguities concerning the feeding ecology and the need for improvement in techniques related to the estimation of the size at maturity. The major objective of this study was to gain insights on the feeding ecology of Nephrops, as well as about the size at maturity of this species and, in particular, the effects of density-dependent processes on size at maturity and morphometry in Nephrops populations. These objectives were addressed using stable isotope analysis (SIA) in a Bayesian approach, multivariate morphometric techniques and a new methodology, developed in this study: theoretical size at the onset of maturity (TSOM), for estimation of the size at the onset of maturity of female Nephrops, which might be particularly useful in data poor situations or for large-scale studies including macro-ecological comparisons. Our estimates indicated that almost half the diet (47%) was made up of suspended particulate organic matter 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. Onset of maturity using TSOM varied from 18.4-33.7 mm carapace length for the Irish grounds. These estimates showed a significant negative linear relationship (R2 = 0.60) with population density at all grounds / years, and a significant positive linear relationship with average size in females (both mature and immature, R2 = 0.84). L50 was linked to the new TSOM metric by a significant positive linear relationship (R2 = 0.40). This set of linear relationships ultimately allowed TSOM and L50 to be estimated without a requirement for maturity stages to be distinguished. Furthermore, it was possible to discriminate samples morphologically across the Moray Firth, Firth of Clyde, Western Irish Sea and Aran Grounds in females and between the Western Irish Sea, Aran Grounds and Moray Firth in males. Therefore, this study showed, for the first time, morphometric variation in Nephrops populations among a variety of grounds in the northeast Atlantic.
NUI Galway
Publisher DOI
Attribution-NonCommercial-NoDerivs 3.0 Ireland