Working Group on the Assessment of Dermersal Stocks in the North Sea and Skagerrak (WGNSSK)

Anja Helene Alvested, Jurgen Batsleer, Alan Baudron, Ewen D. Bell, Chun Chen, Harriet Cole, José De Oliveira, Jordan P. Feekings, Raphaël Girardin, Ghassen Halouani, Holger Haslob, Alexander Kempf, Alexandros Kokkalis, Tiago Malta, Carlos Mesquita, Tanja Miethe, Iago Mosqueira, Nikolai Nawri, Coby Needle, Anders NielsenJ. Rasmus Nielsen, Erik Olsen, Alessandro Orio, Yves Reecht, Jon Egil Skjæraasen, Andreas Sundelöf, Jon Svendsen, Guldborg Søvik, Marc Taylor, Mats Ulmestrand, Wouter van Broekhoven, Lies Vansteenbrugge, Nicola Walker, Fabian Zimmermann

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Abstract

The main terms of reference for the The ICES Working Group for the Assessment of Demersal Stocks in the North Sea and Skagerrak (WGNSSK) were: to update, quality check and report relevant data for the working group, to update and audit the assessment and forecasts of the stocks, to produce a first draft of the advice on the fish stocks and to prepare planning for benchmarks in future years. Ecosystem changes have been analytically considered in the assessments for cod, haddock and whiting in the form of varying natural mortalities estimated by the ICES Working Group on Multi Species Assessment Methods (WGSAM).

Benchmarks and Inter-benchmarks in 2019/2020
Full benchmarks were conducted during 2020 for WGNSSK stocks. These were on 3.a turbot, 3.a whiting, sole in 7.d and sole in 4. However, there were no inter-benchmark protocol (IBP) meetings during 2020.

State of the Stocks
The main impression in recent years is that fishing pressure has been reduced substantially for many North Sea stocks of roundfish and flatfish compared to the beginning of the century. All fish stocks with agreed reference points (Category 1 stocks) are above Blim, apart from cod in 4, 7.d and 20, and only the SSBs of cod in 4, 7.d and 20 and sole in 4 are below MSY Btrigger at the beginning of 2020. Several North Sea stocks are exploited at or below FMSY levels (haddock in 4, 6.a, plaice in 4 and 20 and sole in 7.d); however, several others are being fished above FMSY (cod in 4, 7.d and 20, saithe in 3.a, 4 and 6, whiting in 4 and 7.d, sole in 4, plaice in 7.d, turbot in 4 and witch in 3.a, 4 and 7.d). An important feature is that recruitment still remains poor compared to historic average levels for most gadoids, although there are signs of a strong recruitment for
haddock and whiting in 2019. Recruitment in 2019 continues on a high level also for flatfish stocks of plaice, sole and turbot.

All Nephrops stocks with agreed biomass reference points (Category 1 stocks, excluding nep.fu.3-4) are currently above MSY Btrigger, and all Nephrops stocks with defined FMSY (Category 1 stocks, including) are being fished above FMSY in 2019, apart from Nephrops in FU 7 (nep.fu.7) and FU 3-4 (nep.fu.3-4) which are fished sustainably.

WGNSSK is also responsible for the assessment of several data-limited species (Category 3+stocks) that are mainly by catch in demersal fisheries (brill in 3.a, 4 and 7.d-e, lemon sole in 3.a, 4 and 7.d, dab in 3.a and 4, flounder in 3.a and 4, sol in 7.d turbot in 3.a, whiting in 3a), along with grey gurnard in 3.a, 4 and 7d and striped red mullet in 3.a, 4 and 7.d. Biennial precautionary approach (PA) advice was provided in 2015 for the first time, and again in 2017 and 2019; for 2020, biennial advice was either PA, where catch advice was still needed, or simply reporting stock status where no catch advice was needed. Reopening of advice was triggered for several Category 1 stocks in the autumn, following the availability of Q3 survey results in 2019, namely cod in 4, 7.d and 20, haddock in 4, 6.a and 20, plaice in 4 and 20, sole in 4, and Nephrops in FU 6, 7 and 8 (Annex 7).

The summary of stock status is as follows:
1) Nephrops:
Category 1:
a) FU 3-4 (nep.fu.3-4): The stock size is considered to be stable. The estimated harvest rate for this stock is currently below FMSY. No reference points for stock size have been
defined for this stock.
b) FU 6 (nep.fu.6): The stock abundance has increased since 2015, and currently it is above MSY Btrigger. The harvest rate has shown an increase since 2018, and is above FMSY in 2019.
c) FU 7 (nep.fu.7): The stock size has been above MSY Btrigger for most of the time-series. The harvest rate has increased in 2019 but remains below FMSY.
d) FU 8 (nep.fu.8): The stock size has been above MSY Btrigger for the entire time-series. The harvest rate is varying, increased in 2019 and is now above FMSY.
e) FU 9 (nep.fu.9): The stock has been above MSY Btrigger for the entire time-series. The harvest rate has fluctuated around FMSY in recent years and is now above FMSY.
Category 4:
f) FU 32 (nep.fu.32): The available data is non-conclusive with regard to stock status, in recent years landings have relatively low.
g) FU 33 (nep.fu.33): The state of this stock is unknown. Landings have been relatively stable since 2004, fluctuating without trend at around 1000 tonnes. The mean density
of Norway lobster decreased 2017 to 2019. Advice was provided for this stock in 2019 (although it was not scheduled) because of the availability of data from a UWTV survey conducted in 2018.
h) FU 34 (nep.fu.34): The current state of the stock is unknown.
i) FU 5 (nep.fu.5): The status of this stock is uncertain. Assuming the density has been constant since 2012, the harvest rate in 2018 and 2019, corresponding to the total landings, has decreased and now below the MSY proxy reference point.
j) FU 10 (nep.fu.10): The current state of the stock is unknown.
Category 5:
k) Out of FU (nep.27.4outFU): The current state of the stock is unknown. 
No new advice was provided in spring 2020 for Nephrops stocks but advice was delayed until autumn 2020:
2) Cod (cod.27.47d20): Fishing pressure has increased since 2016, and is above Flim in 2019. Spawning-stock biomass has decreased since 2015 and is now below Blim. Recruitment since 1998 remains poor. Currently, fishing pressure on the stock is above FMSY, Fpa and Flim; the spawning-stock size is below MSY Btrigger, Bpa and Blim.
3) Haddock (had.27.46a20): Fishing pressure has declined since the beginning of the 2000s, but it has been above FMSY for most of the time-series. Only in 2019, fishing pressure is at FMSY. Spawning-stock biomass has been above MSY Btrigger in most of the years since 2002. Recruitment since 2000 has been low with occasional larger year classes. The 2019 yearclass is estimated to be one of the largest since 2000. Currently, fishing pressure on the stock is at FMSY, but below Fpa and Flim, and spawning stock size is above MSY Btrigger, Bpa and Blim.
4) Whiting (whg.27.47d): Spawning-stock biomass has fluctuated around MSY Btrigger since the mid-1980s and is just above it in 2020. Fishing pressure has been above FMSY throughout the time-series, apart from 2005. Recruitment (R) has been fluctuating without trend, but the 2019 year-class is estimated to be the largest since 2002. Currently, fishing pressure on the stock is above FMSY, but below Fpa and Flim; spawning-stock size is above MSY Btrigger, Bpa and Blim.
5) Saithe (pok.27.3a46): Spawning-stock biomass has fluctuated without trend and has been
above MSY Btrigger since 1996. Fishing pressure has decreased and stabilized at or above FMSY since 2014. Recruitment has shown an overall decreasing trend over time with lowest levels in the past 10 years. Currently, fishing pressure on the stock is above FMSY and Fpa, but below Flim; spawning-stock size is above MSY Btrigger, Bpa and Blim.
6) Plaice (ple.27.420): The spawning-stock biomass is well above MSY Btrigger and has markedly increased since 2008, following a substantial reduction in fishing pressure since 1999. Recruitment in 2019 is estimated to be the second highest in the time-series. Since 2009, fishing pressure has been estimated below FMSY. Currently, fishing pressure on the stock is below FMSY, Fpa and Flim, and spawning-stock size is above MSY Btrigger, Bpa and Blim.
7) Sole (sol.27.4): The spawning-stock biomass has fluctuated around Blim since 2003, and has been estimated to be below MSY Btrigger since 1999. Fishing pressure has declined since 1999 and is above FMSY in 2019. Recruitment in 2019 is estimated to be the highest since the start of the time series in 1957. Currently, fishing pressure on the stock is above FMSY, but below Fpa and Flim, and spawning-stock size is above MSY Btrigger, Bpa and Blim.
8) Plaice (ple.27.7d): The spawning-stock biomass has increased rapidly from 2010 following a period of high recruitment between 2009 and 2015, and is now still well above the MSY Btrigger, despite a decline since 2016. Fishing pressure has declined since the early 2000s, with an increase in the recent years to slightly above FMSY. Recruitment in 2019 is currently estimated to be highest in the time series since 1980. Currently, fishing pressure on the stock is above FMSY, but below Fpa and Flim, and spawning stock size is above MSY Btrigger, Bpa and Blim.
9) Turbot (tur.27.4): Recruitment is variable without a trend. In 2019 recruitment is estimated to be above average of the time series. Fishing pressure has decreased since the mid-1990s, and has been just below FMSY since 2012. The spawning-stock biomass has increased since 2005 and has been above MSY Btrigger since 2013. This stock was upgraded to Category 1 from Category 3 following an inter-benchmark during 2018. Currently, fishing pressure on the stock is above FMSY, but below Fpa and Flim; spawning stock size is above MSY Btrigger, Bpa and Blim.
10) Witch (wit.27.3a47d): Fishing pressure has been above FMSY since the beginning of the time-series. Spawning-stock biomass that was below Blim around 2010, has increased since then and is now above MSY Btrigger. Recruitment has declined since 2010 and is currently at a low level. This stock was upgraded to Category 1 from Category 3 following a benchmark during 2018. Currently, fishing pressure on the stock is above FMSY and at Fpa, but below Flim, and spawning stock size is above MSY Btrigger, Bpa and Blim.
11) Norway pout (nop.27.3a4): The stock size is highly variable from year to year, due to recruitment variability and a short life span. Spawning-stock biomass is estimated to have been fluctuating above Bpa for most of the time-series. Fishing pressure declined between 1985 and 1995 and has been fluctuating at a lower level since 1995. Recruitment in 2018 and 2019 was above the long-term average. Currently, spawning stock size is above Bpa and Blim; no reference points for fishing pressure or for MSY Btrigger have been defined for this stock.
12) Category 3–6 finfish stocks: In 2020, new advice has been produced for bll.27.3a47de, lem.27.3a47d, tur.27.3a, sol.7d (all Category 3 stocks) and whg.27.3a (Category 5). Advice was not provided for gug.27.3a47d, dab.27.3a4, fle.27.3a4, mur.27.3a47d (Category 3) and pol.27.3a4 (Category 5).
a) Brill (bll.27.3a47de): The biomass index has been gradually increasing over the timeseries until 2015, and has then decreased. Currently, fishing pressure on the stock is below FMSY proxy and spawning stock size is above MSY Btrigger proxy.
b) Grey gurnard (gug.27.3a47d): The time-series of mature biomass index of grey gurnard from the International Bottom Trawl Survey quarter 1 (IBTS-Q1) shows a strong increase from the beginning of 1990s and has since fluctuated at a high level. Fishing pressure is estimated to be below the FMSY proxy. No reference points for stock size have been defined for this stock.
c) Lemon sole (lem.27.3a47d): Total mortality has fluctuated without trend. Spawningstock biomass increased from 2007 to 2012, and has remained stable since, albeit with a small decline in 2018. Recruitment has shown a mostly downwards trend since a peak in 2011, but relatively high recruitment is estimated for 2019. Currently, fishing pressure on the stock is below FMSY proxy. No reference points for stock size have been defined for this stock.
d) Turbot (tur.27.3a): Catches peaked in the late 1970s and early 1990s and have been more stable in recent years. Relative exploitable biomass (B/Bmsy) declined towards
2000 without a trend in later years. Relative fishing pressure (F/Fmsy) peaked in the late 1970s and early 1990s without a trend in more recent years. Currently, fishing pressure on the stock is above FMSY proxy and spawning stock size is above MSY Btrigger proxy.
e) Whiting (whg.27.3a): Catches have been relatively low in recent years after a substantial industrial fishery ceased in the mid-1990s. The stock size indicator has been fluctuating and is now around the long-term mean. ICES cannot assess the stock and exploitation status relative to MSY and precautionary approach reference points because the reference points are undefined.f) Sole (sol.27.7d): This stock was downgraded from Category 1 to Category 3 following the Interbenchmark in 2019 and Benchmark in 2020. The XSA assessment is indicative
of trends only. The spawning-stock biomass (SSB) has been fluctuating without trend and has been above MSY Btrigger since 2010. Fishing pressure (F) has shown a decreasing trend since 2009 and has been below FMSY proxy since 2016. Recruitment has been fluctuating without trend. In 2019, the recruitment is estimated to be the highest of the time series.

Summary of retrospective analysis (WKFORBIAS decision tree)
To quantify retrospective patterns in the assessments of category 1 stocks, estimates of five year retrospective peels are produced for fishing pressure, SSB and recruitment and plotted with confidence bounds of the current assessment. The retrospective statistics (Mohn’s rho) are reported as a measure of quality. Following the decision tree formulated by WKFORBIAS (ICES 2020) to ensure more consistency in how advice is provided. Only sole in 4 and cod 4 7d and 20 showed significant retrospective patterns in SSB (Mohn’s rho above 0.2). For cod advice is given as usual since only 2 out of recent 5 peels and only 1 out of recent 3 peels fall outside the confidence bounds. A benchmark is planned in 2021 to address the retrospective pattern for cod further. For sole most of the retrospective peels fall outside the confidence bounds. The stock has recently undergone a benchmark and the retrospective pattern could not be solved yet. However, the target F (FMSY) in the forecast for 2021 is well below the F05 estimated using EqSim, and advice is given as usual this year.
Original languageEnglish
Place of PublicationCopenhagen, Denmark
PublisherICES
Number of pages1,140
DOIs
Publication statusPublished - 2020
SeriesICES Scientific Report
Number61
Volume2
ISSN2618-1371

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