Variability of Nephrops norvegicus (L.) populations in Scottish waters in relation to the sediment characteristics of the seabed
Previous studies in localised areas have revealed spatial variability on Nephrops norvegicus population characteristics. In order to investigate this subject in a comprehensive way, a large amount of Nephrops data (catch rates, size compositions, female maturity condition, moult condition and burrow densities) and related sediment data, from different areas and grounds all around Scotland, were assembled and analysed. The available commercial fishery statistics on Nephrops were also used for this purpose. High variability in depth and sediment size composition was found in Scotland, between and within, Nephrops fishing areas. The East cost areas showed less variability than those from the West coast areas both in terms of depth and sediment types. The particle mean size of the sediments (in phi units) was found to be the most appropriate descriptor of sediment granulometry. Well defined relationships were found between the particle mean size and other sediment characteristics. Equations were provided to convert mean to median particle size and mean to percentage of silt and clay. Organic carbon was found to be linearly correlated to the particle mean size but different trend lines were found for the east and west coast. Contradictory trends in the relationships between the sediment size composition and depth suggested a major role of hydrographic and topographic factors in the distribution of the sediments. Large spatial variability in the biological characteristics of Nephrops living at different fishing grounds was found, between and within areas. Burrow density was directly associated with catch rates, and mean sizes were inversely correlated with catch rates. The female size statistics associated with the reproductive condition (e.g. the size at first maturity) were positively significant correlated with the female overall mean size. Populations with lower catch rates exhibited higher growth performance and size structures characterised by larger mean sizes. Females from populations with faster growth became sexually mature at a larger size than those with slower growth.