Nephrops is limited to muddy habitat, and requires sediment with a silt and clay content of between 10–100% to excavate its burrows. This means that the distribution of suitable sediment defines the species distribution. Adult Nephrops only undertake very small scale movements (a few 100 m) but larval transfer may occur between separate mud patches in some areas. This makes some stocks, particularly those with lower average density, vulnerable to localized depletion. Catch rates and composition vary daily and seasonally between different areas and sexes due to different emergence patterns and underlying population densities. After the onset of maturity male Nephrops grow faster and attain a large size than female. Density limits growth and grounds with high Nephrops density (>0.7/m²) have smaller average size that those with low density.

Environmental influence on the stock

Temperature and hydrographic factors, particularly during the larval phase are critical to recruitment success in Nephrops. Some stocks in Division VII such as FU15 have well known and understood larval retention mechanisms (i.e. Western Irish Sea Gyre). Other stocks such as the Porcupine Bank have less well understood larval retention mechanisms. This results in very different population structure, productivity and vulnerability to fishing. Increasing water temperature leading to shorter larval development times is thought to improve recruitment in areas such as the Irish Sea. Increased storminess related to the NAO has also been linked to reduced recruitment and low catch rates several years later on the Porcupine Bank.

Effects of the fisheries on the ecosystem

Trawling for Nephrops results in bycatch and discards of other commercial species, including cod, haddock, whiting, hake, monkfish, and megrim. Given that 80 mm is the predominant mesh size used in Nephrops fisheries the resulting discard rates of small Nephrops and fish can be high.

The high mud content and soft nature of Nephrops grounds means that trawling readily marks the seabed, trawl marks remaining visible for some time. Despite the high intensity of fishing (some areas are impacted >7 times/year) burrowing fauna can be seen re-emerging from freshly trawled grounds, implying that there is some resilience to trawling.

Ecoregion: Celtic Sea.

The above excerpts are from the first two pages of the ICES advice, the supporting information to this advice can be read in full at the following reference: