Whiting

Merlangius merlangus


Assessment report
Published by

Marine and Freshwater Research Institute, Iceland

Published

7 June 2024

General information

Whiting is a demersal gadoid species like cod and haddock, but smaller with a maximum length of about 80 cm, males and females being similar in size. In Icelandic waters, sexual maturity is reached at around 30 cm.

The Fishery

Whiting has been caught mainly as bycatch all around Iceland in recent years, though mostly around south and west of Iceland (Figure 1 and Figure 2). Annual catches have been between 500 and 1000 tonnes except for 2008-2012 when catches increased with a peaked in 2011 at 2602 tonnes (Figure 2). Increased catches in this period occurred almost exclusively in the southwest (Figure 2). In 2023, catches increased to similar levels as in 2013 (Figure 2, Figure 3, Figure 4 and Figure 6). Whiting is found at depths ranging from 10 to 300 m but is mostly caught between 100 and 250 m (Figure 3).

Whiting is mainly caught in demersal trawls but to some extent in Nephrops trawls, longline and demersal seine (Table 1, Figure 4). The number of boats reporting whiting catches increased with increased catches between 2007 and 2012 but has since then decreased (Figure 5).

Figure 1: Whiting. Spatial distribution of catches by all gears.

Figure 2: Whiting. Changes in spatial distribution of whiting catches as recorded in Icelandic logbooks.

Figure 3: Whiting. Depth distribution of whiting catches from bottom trawls, longlines, trawls and demersal seine from Icelandic logbooks

Figure 4: Whiting. Landings in tons and percent of total by gear and year

Figure 5: Whiting. Number of vessels (all gear types) accounting for 95% of the total catch annually since 1994. Left: Plotted against year. Right: Plotted against total catch. Data from the Directorate of Fisheries.

Table 1: Whiting. Number of Icelandic vessels landing whiting, and all landed catch divided by gear type.
Year Nr. Bottom Trawl Nr. Other Nr. Long Line Nr. Danish Seine Bottom Trawl Other Long Line Danish Seine Total catch
2000 86 103 170 25 1088 23 157 76 1344
2001 76 129 139 25 929 66 114 64 1173
2002 88 133 116 18 1068 48 90 93 1299
2003 72 111 133 27 733 37 153 102 1025
2004 68 110 142 25 704 26 224 84 1038
2005 77 107 171 34 518 11 205 63 797
2006 62 106 189 37 467 23 460 100 1050
2007 66 86 216 43 767 26 394 71 1258
2008 77 95 235 49 950 30 557 151 1688
2009 77 221 244 60 1439 45 520 303 2307
2010 63 276 212 50 2192 42 425 191 2850
2011 65 263 219 43 2486 24 345 109 2964
2012 65 263 223 45 956 19 320 174 1469
2013 53 234 218 41 659 7 255 62 983
2014 52 174 208 32 643 7 207 73 930
2015 44 160 186 30 675 4 116 65 860
2016 41 114 174 30 559 2 104 69 734
2017 34 64 147 24 401 1 108 43 553
2018 38 37 115 21 688 1 61 67 817
2019 47 64 109 26 605 1 93 61 760
2020 47 61 102 20 548 2 67 18 635
2021 48 103 92 25 820 3 58 71 952
2022 48 101 78 26 631 3 67 72 773
2023 49 159 66 30 946 12 112 64 1134

Length Distributions from commercial catches of whiting

Length measurements of whiting from commercial catches are scarce and missing for several years, but show substantial recruitment in 1980, 1996, 2007 and 2019 (Figure 7). Most whiting caught in the commercial fishery are 38-55 cm.

Figure 6: Whiting. Distribution of commercial catches 2023.

Figure 7: Whiting. Commercial length distribution from bottom trawls.

Survey Data

Annual Icelandic groundfish surveys have been conducted in March (IS-SMB) since 1985 and October (IS-SMH) since 1996. Both surveys cover the distribution area of whiting on Icelandic grounds. For monitoring, harvestable biomass and recruitment indices were estimated for both surveys (Figure 8). The harvestable biomass index is calculated as the biomass of individuals 40 cm and larger. The recruitment index is defined as whiting smaller than 20 cm.

Both the total biomass index and harvestable biomass index in IS-SMB increased from 2003 to a maximum in 2005 but decreased to a low level in 2015 (Figure 7). Since then, both indices have increased, and harvestable biomass is approaching the highest value in the timeseries (1990). The biomass indices from IS-SMH are much more variable but show similar trends as IS-SMB. Recruitment indices show similar trends in both surveys (Figure 7). Strong recruitment was observed in 2003, 2007 and 2019-2021 in IS-SMH and in 2004, 2008 and 2021-2022 in IS-SMB. These peaks can be observed in the length distributions (Figure 7 and Figure 8), and in the harvestable biomass indices 2-3 years later.

Spatial distribution of whiting from the spring survey is similar to what is observed in commercial catches, that is, mostly in the south of Iceland (Figure 1, Figure 2, Figure 9 and Figure 10). The autumn survey however shows the highest indices in a larger area, southeast, southwest, and west (Figure 11 and Figure 12). The recent increase in the biomass indices has mostly taken place in the southeast and southwest areas (Figure 10 and Figure 12).

Figure 8: Whiting. Total biomass indices (upper left), harvestable biomass indices (<39 cm, upper right), and juvenile abundance indices (<21 cm, lower) from IS-SMB (blue) since 1985 and IS-SMH (red) since 1996, along with 95% CI.

Figure 9: Whiting. Length disaggregated abundance indices from the March survey 1985 and onwards.

Figure 10: Whiting. Length disaggregated abundance indices from the October survey 1996 onwards (execept for 2011).

Figure 11: Whiting. Spatial distribution from IS-SMB in the current year.

Figure 12: Whiting. Spatial distribution of biomass index from IS-SMB since 1985.

Figure 13: Whiting. Spatial distribution of catches from IS-SMH last year.

Figure 14: Whiting. Spatial distribution of biomass index from IS-SMH since 1996, excluding 2011.

Stock assessment

The assessment is for this stock based on ICES constant harvest rate \((chr)\)-rule for data limited stocks, where a constant harvest rate (Fproxy,MSY), that can be considered a proxy for harvest rate at maximum sustainable yield, is applied to the spawning stock biomass index (ICES 2021). The \(\text{chr}\)-rule has the following form:

\[A_{y + 1} = I_{y}\ F_{\text{proxy},\text{MSY}}\ b\ m\]

where \(A_{y + 1}\) is the advised catch, \(I_{y - 1}\) is the last year’s index, \(F_{proxy,MSY}\) is the fishing pressure proxy at MSY, \(b\) is a biomass safeguard (reducing the catch when biomass index drops below a trigger value) and m is a multiplier that is applied to maintain the probability of the biomass declining below \(B_{\lim}\) under 5%.

\(F_{\text{proxy},\text{MSY}}\) is the ratio of the catch and index for the set of historical years (U) where \(f\) > 1:

\[F_{\text{proxy},\text{MSY}} = \frac{1}{u}\begin{matrix} \sum_{y \in U}^{}\frac{C_{y}}{I_{y}} \\ \end{matrix}\]

\(f\) is the length-ratio component where:

\[f = \frac{{\overline{L}}_{y - 1}}{L_{F = M}}\]

where \(\overline{L}\) is is the mean catch length above \(L_{F = M}\).

\(L_{F = M}\) is calculated as:

\[L_{F = M} = 0.75L_{c} + 0.25L_{\infty}\]

where \(L_{c}\) is the length where frequency is half that of the modal value (Figure 14), and L∞ is von Bertalanffy L∞.

\(b\) is the biomass safeguard and is used to reduce catch advice when index falls below trigger,

\[b = \min\left( 1,I_{y} - 1/I_{\text{trigger}} \right)\]

where \(I_{\text{trigger}}\) = \(i_{\text{loss}} \bullet 1.4\)

\(m\) is a multiplier based on stock growth. \(K\) for whiting could not be estimated reliably since the most recent data is from 1973 and hence, K from other areas was used instead. K for whiting is 0.38 and therefore \(m\) is 0.5 (0.32 <K < 0.45 yr-1).

Figure 15: Whiting. Length frequency distribution from catches. Red line is the length at first capture.

Analysis on the assessment and advice

The assessment is based on the chr-rule for ICES category 3 data-limited stocks and is applied for the fishing year (2024/2025). The Icelandic spring trawl survey (IS-SMB) for individuals ≥40 cm was used as the spawning stock biomass index. The advice is according to Ay+1 = Iy-1 \(F_{proxy,MSY}\) b m or 25762 t * 0.161 * 1 * 0.5 which result in advice that differs more than +20% from last year’s advice and hence, the stability clause is applied. The advice for 2024/2025 is therefore set at 1571 t (20% increase from last year’s advice) (Table 2). In 2019-2021, the advice was based on the ICES framework for data limited stocks (Category 3.2) where the ratio of the mean of the last two survey indices (Index A) to the mean of the three preceding values (Index B) is multiplied by the last year’s advice. This method is no longer considered precautionary and hence, the new rule.

Table 2: Whiting. Advice calculations.

Ay: Catch advice for 2023/2024

1 309

Biomass index

Latest index value (I2024)

25 762

Fishing pressure proxy**

FMSY proxy: MSY proxy harvest rate (average of the ratio of catch to biomass index for the years for which f>1, where f = Lmean/ LF = γM,K = ϴM )

0.161

Biomass safeguard

Index trigger value (Itrigger=Iloss*1.4)

2 152

b: Index relative to trigger value, min{I2024/Itrigger, 1}

1

Precautionary multiplier to maintain biomass above Blim with 95% probability

m: Multiplier (generic multiplier based on life history)

0.5

Advice calculation1)

2 080

Uncertainty cap (+20% / -30% compared to Ay, only applied if b>1)

1

Catch advice for 2024/2025

1 571

% advice change2)

20

1) A~y+1~ = I~y~ × F~MSY proxy~ × b × m, limited by stability clause

2) The figures in the table are rounded. Calculations were done with unrounded inputs, and compared values may not match exactly when calculated using the rounded figures in the table.

Application of THE CHR-rule

Figure 16: Whiting. IS-SMB biomass index since 1985. The broken line shows Itrigger which is the lowest index value in the time series multiplied by 1.4.

  • f is the length-ratio component. The annual mean length from catches and the target reference length (Lc, the length where frequency is half that of the modal value * 0.75 + L∞ * 0.25) is 48. The points along the line in Figure 17 indicate years when f>1.

Figure 17: Whiting. Annual fishing pressure proxy (Fproxy) for years for which suffucient data was available.

  • b is the biomass safeguard and is used to reduce catch advice when index falls below trigger. Iloss for whiting is 1537 and was based on the lowest biomass index. Itrigger is Iloss *1.4 or 2152 (Figure 16). The biomass index this year is 25 762, which is above Itrigger and hence, b is 1.

  • m is, as previously explained, the tuning parameter and for slow growing species (with von Bertalanffy 0.32 <K < 0.45 yr-1), m equals 0.5.

Management

Whiting has not been subject to management such as TAC limitations, but advice has been given by the Marine and Freshwater Research Institute since 2019 (Table 3).

Table 3: Whiting. Recommended TAC, national TAC, and catches (tonnes).
Fishing year Advice National TAC Catch
2001/02 - - 1 192
2002/03 - - 1 309
2003/04 - - 1 001
2004/05 - - 964
2005/06 - - 895
2006/07 - - 1 030
2007/08 - - 1 812
2008/09 - - 1 984
2009/10 - - 2 835
2010/11 - - 3 249
2011/12 - - 1 601
2012/13 - - 1 060
2013/14 - - 1 034
2014/15 - - 877
2015/16 - - 690
2016/17 - - 642
2017/18 - - 844
2018/19 - - 780
2019/20 836 - 607
2020/21 1 003 - 844
2021/22 1 137 - 826
2022/23 1091 - 1252
2023/24 1309 - -
2024/25 1571 - -

References

ICES. 2021. Tenth Workshop on the Development of Quantitative Assessment Methodologies based on LIFE-history traits, exploitation characteristics, and other relevant parameters for data-limited stocks (WKLIFE X). ICES Scientific Reports. Report. https://doi.org/10.17895/ices.pub.5985