Management AdviceManagement considerationsNephrops
are limited to a muddy habitat. This means that the distribution of suitable sediment defines the species distribution and the stocks are therefore assessed as eight separate functional units (Figure 22.214.171.124):
|Section||FU no.|| Name||ICES area|| Statistical rectangles|
|126.96.36.199*||5|| Botney Gut - Silver Pit||IVb,c|| 36-37 F1-F4; 35F2-F3|
|188.8.131.52||6|| Farn Deeps||IVb|| 38-40 E8-E9; 37E9|
|184.108.40.206||7|| Fladen Ground||IVa|| 44-49 E9-F1; 45-46E8|
|220.127.116.11||8|| Firth of Forth||IVb|| 40-41E7; 41E6|
|18.104.22.168||9|| Moray Firth||IVa|| 44-45 E6-E7; 44E8|
|22.214.171.124*||10|| Noup||IVa|| 47E6|
|126.96.36.199*||32|| Norwegian Deep||lVa|| 44-52 F2-F6; 43F5-F7|
|188.8.131.52*||33|| Off Horn Reef||lVb|| 39-41E4; 39-41E5|
|* The advice for these stocks was given in biannual advice in 2008, see ICES (2008a).|
An overview of advice for different functional units is presented in Table 184.108.40.206.
|Figure 220.127.116.11 Nephrops Functional Units in the North Sea and Skagerrak/Kattegat region (see Section 6.4.13).|
The overriding management consideration for these stocks is that management should be at the Functional Unit (FU) rather than the ICES Subarea level. Management at the Functional Unit level should provide the controls to ensure that catch opportunities and effort are compatible and in line with the scale of the resources in each of the stocks defined by the Functional Units. Current management of Nephrops
in Subarea IV (both in terms of TACs and effort) does not provide adequate safeguards to ensure that local effort is sufficiently limited to avoid depletion of resources in Functional Units. In the current situation vessels are free to move between grounds, allowing effort to develop on some grounds in a largely uncontrolled way and this has historically resulted in inappropriate harvest rates from some parts. This is a particular problem in the Farn Deeps where increased vessel activity from other parts of the UK occurred resulting in low stock levels.
There are no precautionary reference points defined for Nephrops
. This year ICES has provided advice on a range of catch options including a long term exploitation rate for Nephrops
based on a number of considerations:
- For many stocks ICES considers that exploitation rates between F0.1 and Fmax are an appropriate range of exploitation rates. For Nephrops both F0.1 and Fmax are often well defined.
- There are a substantial number of studies on exploitation of fin fish indicating that F0.1 is consistent with maximising sustainable long term yield, though this has not been studied directly for Nephrops and further work is required to ascertain whether fishing at F0.1 is consistent with the management objective of maximising yield of Nephrops. Fmax is often considered too high an exploitation rate, leading to increasing risk to SSB without any substantial increases in yield. In addition for stocks with high discard rates a reduction of harvest rates often reduces the rates of exploitation of smaller individuals allowing larger numbers for the fishery in subsequent years.
- A limited number of management simulations have been carried out (Dobby, 2007) on two of the FU in the NS showing that harvest ratios of close to F0.1 are appropriate for sustainable high long term yield with risks that ICES would consider precautionary. Ideally these simulations should be extended to other Nephrops FUs. The higher exploitation rates of the order of Fmax have not been explicitly tested but it’s unlikely that they will provide the same stability of yield and will have higher risks. These simulations did not explicitly deal with surveys that had different selection patterns from the fishery, there may be increases in risk associated with this, reinforcing the greater applicability of the lower exploitation rates.
- For a number of FUs in the North Sea (and Celtic Seas) the recent catches have been substantially higher than the rates implied by F0.1. In the case of Firth of Forth (FU8) these catches have been even higher than the rate implied by Fmax but this coincided with an apparent increase in the stock in the last few years.
In light of the above considerations, ICES advises that F should be in the range of F0.1
to maximize long term average yield without unduly risking SSB. ICES advice for individual functional units takes account of the current estimate of F and trend in SSB as follows:
|F relative to F0.1 and Fmax|
Stable or Increasing
|F > Fmax||Reduce F to Fmax||Reduce F to F0.1|
|Fmax > F > F0.1||Maintain current F||Reduce F to F0.1|
|F < F0.1||Increase F to F0.1||Maintain current F|
Where the new advice suggests a large change in landings, a constraint on the year to year change in catches as is typical of management plans and the Communication on Fishing Opportunities for 2010 [COM (2009) 224] might be considered.
Some combinations of F and trend in SSB are included for completeness, even though not all of these combinations have been applied to North Sea Nephrops
In general, catches of cod in the Nephrops
fisheries have been relatively low, particularly in recent years in inshore grounds of Subarea IV, but can vary amongst Functional Units. However, it is important that emerging year classes of cod should not be subject to high discard mortality. The capture of juvenile fish or other species such as haddock is also a problem in some of the Functional Units and discarding of these is a problem in some years. This problem is being addressed with the use of more selective gear and efforts are already being made in Scotland through the Conservation Credits scheme, requiring vessels targeting Nephrops
to use gear with larger square meshed panels (110 mm). Subject to evaluation of the effectiveness of these measures, further action may be required to reduce discards.
The advice is presented separately for each Functional Unit. In addition, there are increasing and significant landings from some isolated patches outside the Functional Units, most notably the Devil’s Hole area. Table 18.104.22.168 below shows that in 2008 overall landings in Subarea IV were around 22 100 tonnes, similar to landings in 2005. Landings from other rectangles have risen steadily and amounted to over 1 600 tonnes in 2008. To provide some guidance on appropriate future landings for these areas, the use of average landings of no more than 1500 tonnes (2006–2008) could be considered (Table 22.214.171.124). Impacts of fisheries on the ecosystem
Trawling for Nephrops
results in bycatch and discards of other species, including cod, haddock, and whiting. 80 mm is the predominant mesh size used in Nephrops
fisheries and the resulting proportion of fish discarded can be high. Initiatives are in place to reduce discarding (see Regulations and their effects below).
The high mud content and soft nature of Nephrops
grounds means that trawling readily marks the seabed, trawl marks remaining visible for some time. Burrowing fauna can be seen re-emerging from freshly trawled grounds, implying that there is some resilience to trawling.
Cod has been identified as a major predator of Nephrops
in some areas. The generally low level of the cod in the North Sea is likely to have resulted in reduced predation. Multi-species models applied in the past to the exploitation of Irish Sea stocks indicated that management strategies which lead to an increase in the cod stock are associated with a reduction in Nephrops
abundance. Therefore it may be expected that Nephrops
stocks in the North Sea may decrease when cod recovers.