Habitat Bio
Depth Zone: Shelf (50 m - 200 m). Vertical Dist: Demersal/Benthic.
Nephrops is limited to a muddy habitat and requires sediment with a silt and clay content of between 10% and 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. Catches typically consist of a smaller proportion of females than males, due to the lower burrow emergence (resulting in lower catchability) of females during the egg bearing. It is likely that maximum stock size and fishery potential is constrained by the available space since the species competes for space and there are upper limits on density.
Environmental influence on the stock
Temperature and hydrographic factors are critical to recruitment success in
Nephrops, particularly during the larval phase. Different environmental factors such as sediment type and hydrography result in very different population structure, productivity, and vulnerability to fishing.
Effects of the fisheries on the ecosystem
There is a bycatch of other species in the
Nephrops fisheries in Division VIa. This bycatch reflects the species associated with muddy sediments. Estimates of discards of whiting and haddock are high in Division VIa. Bycatches of cod are low and the Scottish Conservation Credits Scheme is in place to minimize cod catches. The use of creels for
Nephrops fishing has increased in inshore areas in the Division VIa FUs. Discards and bycatch in the creel fisheries are considered to be low. The high mud content and soft nature of
Nephrops grounds means that trawling readily marks the seabed, with 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.
Geo Dist
Geo Dist: Shared between nations
Bio Assess
Sci Advice
A summary of the advice per area can be found in Table 5.4.33.1.
To protect the
Nephrops stocks in this management area, management should be implemented at the functional unit level.
There is no information available on the trends in the stock or exploitation status for the rectangles outside the FUs (‘other rectangles’) for which ICES provides advice. ICES advises that the catches in the other rectangles should not increase.
Results
 |
Figure 5.4.33.1 Nephrops Functional Units in Subarea IV and Division VIIa.  |
Assess Models
Scientific basis
The assessment and advice for
Nephrops stocks in Division VIa is primarily based on abundance estimates from underwater TV (UWTV) surveys together with fishery landings data and estimates of quantities of discards. Additional indicators of changes in stocks are derived from trends in length compositions and sex ratio in the catches.
Catch options tables for 2013 are derived for a range of F
MSY and other options by applying the appropriate harvest ratios to the population numbers estimate from the most recent UWTV survey. This assumes that population numbers remain stable in the interim year. Landings are derived from the resultant total catch numbers after multiplying by the recent average of proportion retained and the mean weight in the landings.
Uncertainties in assessment and forecast
There is a gap of 18 months between the survey and the start of the year for which the assessment is used to set management levels. It is assumed that the stock is in equilibrium during this period (i.e. recruitment and growth balance mortality) although this is rarely the case. The effect of this on the accuracy of the catch forecast has not been investigated. The calculations of MSY proxies are all based on yield-per-recruit analyses from a length-based age-structured population model. These analyses utilize average length–frequency data taken over a 3-year period and therefore assume that the stock is in equilibrium. However, it is unlikely that the
Nephrops stocks to which the approach has been applied are actually in equilibrium due to variable recruitment. MSY proxy estimates may vary in time due to changes in selection pattern.
Management
Management unit: Yes
Advice
Table 5.4.33.1 Nephrops in Subarea VIa. Summary of ICES advice by functional unit plus other rectangles.
| Year | Predicted landings corresponding to ICES advice | Agreed TAC 1) | ICES landings |
| | North Minch (FU11) | South Minch (FU12) | Firth of Clyde (FU13) | Sound of Jura (FU13) | Other rectangles | Total advice | | |
| 1992 | | | | | | ~11.4 | 12.0 | 10.8 |
| 1993 | | | | | | ~11.3 | 12.0 | 11.3 |
| 1994 | | | | | | 11.3 | 12.6 | 11.1 |
| 1995 | | | | | | 11.3 | 12.6 | 12.8 |
| 1996 | | | | | | 11.3 | 12.6 | 11.2 |
| 1997 | | | | | | 11.3 | 12.6 | 11.2 |
| 1998 | | | | | | 11.3 | 12.6 | 11.2 |
| 1999 | | | | | | 11.3 | 12.6 | 11.5 |
| 2000 | | | | | | 11.3 | 12.6 | 11.0 |
| 2001 | | | | | | 11.3 | 11.34 | 10.9 |
| 2002 | | | | | | 11.3 | 11.34 | 10.5 |
| 2003 | | | | | | 11.3 | 11.34 | 10.8 |
| 2004 | | | | | | 11.3 | 11.3 | 10.4 |
| 2005 | | | | | | 11.3 | 12.7 | 10.5 |
| 2006 | | | | | | -2) | 17.7 | 13.7 |
| 2007 | 3.2 | 7.2 | 3.765 | | 0.8 | 15.0 | 19.9 | 16.3 |
| 2008 | 3.2 | 7.2 | 3.765 | | 0.8 | 15.0 | 19.9 | 15.2 |
| 2009 | < 4.1 | < 5.0 | < 5.7 | | < 0.3 | 3) | 18.4 | 12.7 |
| 2010 | < 1.0 | < 4.1 | < 3.9 | | <0.25 | 3) | 16.1 | 12.2 |
| 2011 | < 3.1 | < 4.0 | < 4.1 | < 0.5 | < 0.25 | 3) | 13.7 | 12.8 |
| 2012 | < 3.2 | < 5.5 | < 4.2 | < 0.9 | 4) | 3) | 14.1 | |
| 2013 | < 4.2 | < 5.8 | < 5.6 | < 0.8 | 4) | 3) | | |
Weights in thousand tonnes.
1) Subarea VI and EC waters of Division Vb.
2) Effort should not be increased.
3) ICES advises that stocks should be managed by functional unit.
4) ICES advises that the catches in the other rectangles should not increase.
Sources
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:
ICES. Nephrops in Division VIa. Report of the ICES Advisory Committee, 2012. ICES Advice, June 2012.
http://www.ices.dk/committe/acom/comwork/report/2012/2012/Neph-Via.pdf
to turn layers on and off 