Ling

Molva molva


Technical report
Published by

Marine and Freshwater Research Institute, Iceland

Published

7 June 2024

GENERAL INFORMATION

The common ling is one of the largest fish of the Gadiformes order reaching a maximum length of 200 cm, with a mean length of about 70-90 cm according to data from the annual Icelandic spring groundfish survey. It is a demersal fish that preys on fish and invertebrates and can be found at depths 10 and 1300 meters but is mainly caught at depths between 100 and 400 meters. It reaches sexual maturity at the age of 5-8 years and 60-80 cm total length. Ling spawns in May and June mostly along the edges of the south, southwest and west of the Icelandic continental shelf.

The fishery

Landing data available

In general sampling is considered good from commercial catches from the main gears (longlines and trawls). Sampling does seem to cover the spatial distribution of catches for longlines and trawls but less so for gillnets. Similarly, sampling does seem to follow the temporal distribution of catches (Figure 6).

Landings and discards

Data on landings by Icelandic vessels are collected by the Icelandic Directorate of Fisheries. Landings of Norwegian and Faroese vessels are registered through the Icelandic Coast Guard. Discarding is banned by law in the Icelandic demersal fishery. Based on limited data, discard rates in the Icelandic longline fishery for ling are estimated very low (<1% in either numbers or weight) (WGDEEP, ICES 2011:WD02). Measures in the management system such as converting quota share from one species to another are used by the fleet to a large extent and this is thought to discourage discarding in mixed fisheries. A description of the management system is given in the area overview (ICES 2019).

Data available

In general sampling is considered good from commercial catches from the main gears and seems to cover the spatial distribution of catches (Figure 6). Similarly, sampling does seem to follow the temporal distribution of catches (Figure 6).

Figure 6: Ling. Spatial distribution of length samples (black dots) from commercial catches in Icelandic waters (upper) and numbers of samples taken per month by project (bars) and proportion of landings per month (black line) (lower).

Length compositions

Most length measurements of ling are from longlines and bottom trawls (Table 2). The number of available length measurements increased in recent years in line with increased landings but in 2020 they were fewer (due to the covid pandemic). Length distributions from the Icelandic longline and trawling fleet are presented in Figure 7. Sampling from commercial catches of ling is considered good; both in terms of spatial and temporal distribution of samples (Figure 6).

Table 2: Ling. Number of available length measurements from Icelandic commercial catches.
Year Longlines Gillnets Demersal_seine Trawls Other Total
2000 1624 566 0 377 6 2573
2001 1661 493 0 37 0 2191
2002 1504 366 0 221 0 2091
2003 2405 300 0 137 143 2985
2004 2640 348 46 141 150 3175
2005 2323 31 101 349 180 2954
2006 3354 645 0 1157 405 5557
2007 3661 0 76 400 0 4137
2008 5847 357 15 819 150 7188
2009 9014 410 0 516 450 10390
2010 7322 57 0 1146 1200 9724
2011 7248 0 150 1234 750 9393
2012 11356 85 150 1411 1337 14339
2013 19405 267 122 993 1344 12131
2014 6448 1286 120 2089 2964 12907
2015 3315 1563 0 2615 3052 10545
2016 2483 2039 0 2460 1212 8194
2017 1637 485 0 1963 1226 5311
2018 1424 559 0 1603 712 4298
2019 3598 0 0 1830 819 6247
2020 1099 4 0 1718 498 3439
2021 1056 0 0 2028 466 3550
2022 563 370 0 1805 1534 4272
2023 1284 90 0 2423 0 3797

Figure 7: Ling. Length distribution from the Icelandic fleet from 2003-2023.

Age compositions

Aged data are available from 2000 onwards (Table 3). In previous years, most of the ling caught in the Icelandic spring survey were between age 5 and 8 but from longlines the age was between 6 and 9. The past several years have shown a much larger composition of older fish, common up to 12, from both sample sources (see Survey Data, next section).

Table 3: Ling. Number of aged otoliths from the commercial catches.
Year Longlines Gillnets Demersal_seine Trawls Other Total
2000 650 200 0 150 0 1000
2001 550 193 0 37 0 780
2002 519 166 0 150 0 835
2003 900 100 0 150 50 1150
2004 750 100 46 100 50 996
2005 750 0 0 231 50 981
2006 1137 288 0 550 100 1975
2007 1300 0 50 100 0 1450
2008 1950 150 0 365 50 2465
2009 2550 150 0 400 150 3100
2010 2498 50 0 850 400 3398
2011 2546 0 50 700 250 3296
2012 3521 50 50 541 400 4562
2013 2590 100 50 350 450 3540
2014 665 225 20 399 514 1823
2015 595 300 0 483 520 1898
2016 440 345 0 460 220 1465
2017 310 85 0 370 225 990
2018 245 100 0 310 120 775
2019 385 0 0 340 140 865
2020 225 40 0 355 102 772
2021 180 0 0 398 100 678
2022 183 80 0 400 318 981
2023 320 20 0 564 0 904

Figure 8: Ling. Catch at age from the commercial fishery in Iceland waters. Bar size is indicative of the catch in numbers and bars are coloured by cohort.

Figure 9: Ling. Catch at age from the commercial fishery in Iceland waters. Biomass caught by year and age; bars are coloured by cohort.

Weight at age in catch

Mean weight at age in the catch is shown in Figure 10. Catch weights of the older year classes (8-12 years) have been increasing in recent years and have mostly been above average since 2018. The opposite is seen in catch weights of younger age classes, where the mean weight has been below the average for the past years.

Figure 10: Ling. Mean weight at age in the catch from the commercial fishery in Icelandic waters. Bars are coloured by cohort.

Figure 11: Ling. Catch weights by age from the commercial fishery in Icelandic waters.

Catch and effort

The CPUE estimates of ling from commercial fisheries in Icelandic waters have not been considered representative of stock abundance.

Survey data

The Icelandic spring groundfish survey, which has been conducted annually in March since 1985, covers the most important distribution area of the ling fishery. In addition, the autumn survey was commenced in 1996 and expanded in 2000, however a full autumn survey was not conducted in 2011 and therefore the results for 2011 are not presented.

Figure 12 shows distribution of ling in groundfish surveys in spring 2024 and autumn 2023. Figure 13 shows both a recruitment index and the trends in biomass from both surveys. Length distributions from the spring survey are shown in Figure 14 (abundance) and changes in spatial distribution in the spring survey are presented in (Figure 15).

Ling in both the spring and autumn surveys are mainly found in the deeper waters south and west off Iceland. Both the total biomass index and the index of the fishable biomass (>40 cm) gradually decreased in the spring survey until 1995 (Figure 13). In the years 1995-2003 these indices were half of the mean from 1985–1989. In 2003-2007, the recruitment indices increased and remained high until 2010. The index of the large ling (80 cm and larger) shows a similar trend as the total biomass index (Figure 13). The recruitment index of ling, defined here as ling smaller than 40 cm, showed a considerable increase in 2003-2007 and remained high until 2010. Then the juvenile index fell to a very low level in 2014 and remained low until 2021. Since then, the recruitment indices have been ascending, as well as the indices of larger fish as it reached the highest recorded value in the time series in 2024 (Figure 13).

Length distributions from the spring survey show a similar pattern as survey indices, with the 2012-2018 peak in abundance observed as high proportions of fish in the range of 60–100 cm, that has slowly decreased as they have reached sizes 80–120 cm (Figure 14). This pattern is likely to have caused the increase in ling sizes observed in the trawl samples (Figure 7).

Biomass indices in the autumn survey were low in 1996-2000 but have increased since then (Figure 13). There is consistency between the two surveys; the autumn survey biomass indices are however derived from substantially fewer ling caught. Also, there is an inconsistency in recruitment indices (<40 cm), where the autumn survey shows much lower recruitment, in absolute terms compared with the spring survey (Figure 13). This discrepancy is likely a result of much lower catchability of small ling (due to different gears) in the autumn survey, where ling less than 40 cm has rarely been caught.

Changes in spatial distribution as observed in surveys: According to the spring survey most of the increase during the 2012–2018 peak in ling abundance was in the western area, but an increase was seen in most areas. However, most of the index in terms of biomass comes from the southwestern area or around 40% compared to around 30% between 2003 and 2011. Since 2016, the amounts of biomass in the west and southwest have, however, reduced while the proportions in the southeast have increased, leading to a greater contribution of ling from the northwest and southeast to the total index. A similar pattern is observed in the autumn survey.

Figure 12: Ling. Location and abundance of ling in the spring survey (SMB) in 2024 and the autumn survey (SMH) in 2023.

Figure 13: Ling. Total biomass indices, biomass indices >40 cm, biomass indices >80 cm, and abundance indices <40 cm. The lines with shaded area show the spring survey index from 1985 and the points with the vertical lines show the autumn survey from 1996. The shaded areas and vertical lines indicate +/- standard error.

Figure 14: Ling. Survey biomass indices in the spring survey by year from different parts of the continental shelf (upper figure) and as proportions of the total (lower figure)

Figure 15: Ling. Length distribution (grey area) from the spring survey. Black lines are the average mean of the period.

Figure 16: Ling. Age disaggregated indices in the autumn survey (left), gillnet survey (middle) and the spring survey (right). Fill colours indicate cohorts. Note different scales on y-axes.

Stock weight at age

Mean weight at age in the survey is shown in Figure 17. Stock weights are obtained from the groundfish survey in March and are also used as mean weight at age in the spawning stock.

Figure 17: Ling. Stock weights from the spring survey in Icelandic waters. Bars are coloured by cohort.

Stock maturity

Ling in Icelandic waters are mature at the age of 5-8 years and 60-80 cm total length. Maturity at age data is taken from the spring groundfish survey in March and prior to 1985 the proportion mature is assumed fixed at 1985 levels. Maturity-at-age five, six and seven has been decreasing for the past few years (Figure 18 and Figure 19), and in 2023, the mean length at maturity was around 75 cm (Figure 20).

Figure 18: Ling. Maturity at age in the survey. Bars are coloured by cohort. The values are used to calculate the spawning stock.

Figure 19: Ling. Proportion mature at age from the spring survey.

Figure 20: Ling. Proportion mature at length from the spring survey. The black line is year 2024.

Analytical assessment using SAM

In 2022, Ling in 5.a was reassessed as the previously benchmarked Gadget model had begun to show great instability in retrospective patterns in recent years. As a part of a Harvest Control Evaluation requested by Iceland (WKICEMSE, ICES 2022a), the stock was benchmarked (WKICEMSE, ICES 2022c) which resulted in changes in the assessment method and updated reference points. Model setup and settings are described in the Stock Annex (ICES 2022b).

Diagnostics

Model fit

Figure 21 shows the overall fit to the survey indices described in the stock annex. In general, the model appears to follow the stock trends historically. Furthermore, the terminal estimate is not seen to deviate substantially from the observed value for most length groups, with model overestimating the abundance in the two largest length group. Summed up over survey biomass the model overestimates the biomass in the terminal years. The overview of model parameter estimates are shown in Figure 26.

Figure 21: Ling. Model fit to indices from the spring survey, autumn survey and gillnet survey. Black dots are observed values and the black line is the model fit.

Results

Population dynamics of the ling estimated in this model show a clear trend of a high recruitment period from 2004-2010, corresponding with increased spawning stock biomass (SSB) and catches during the 2010-2019 period. Fishing mortality remained rather steady until 2015 but has declined since then (Figure 22).

Figure 22: Ling. Model results of population dynamics overview: estimated catch, average fishing mortality over ages 8-11 (Fbar), recruitment (age 2), and spawning stock biomass (SSB). Catch and fbar values in 2024 are projections.

Retrospective analysis

Analytical retrospective analysis shows a revision of spawning stock biomass over the 5-year peel (Figure 23). Estimates of F and recruitment are decently stable except for the apparent peak in 2017-2018. As explained in reference to the survey indices, this is likely the influence of highly variable survey indices that, for the smallest sizes in the most recent years, have no repeated observations at larger sizes with which this influence can be tempered. Therefore, it is expected that these recruitment peaks may simply be the result of uncertainty in survey indices and are likely to disappear in the coming assessment years.

Mohn’s ρ was estimated to be -0.0567807 for SSB, 0.091087 for F, and 0.2213527 for recruitment. Neither observation nor process residuals show obvious trends (Figure 24 and Figure 25).

Figure 23: Ling. Retrospective plots illustrating stability in model estimates over a 5-year “peel” in data. Results of spawning stock biomass, fishing mortality F, and recruitment (age 2) are shown.

Figure 24: Ling. Observation error residuals of the SAM model.

Figure 25: Ling. Process error residuals of the SAM model.

Figure 26: Ling. Illustration of estimated model parameters.

Reference points

As part of the WKICEMP 2022 HCR evaluations (ICES 2022c), the following reference points were defined.

Table 4: Ling. Reference points adopted from ICES WKICEMP 2022 (ICES 2022).
Framework Reference_point Value Technical_basis
MSY Approach MSY Btrigger 11100 Bpa
FMSY 0.3 F that produces MSY in the long term
Precautionary Approach Blim 9000 Bloss (SSB in 1993)
Bpa 11100 Blim x e1.645 * σB
Flim 0.95 Fishing mortality that in stochastic equilibrium will result in median SSB at Blim.
Fpa 0.62 Maximum F at which the probability of SSB falling below Blim is <5%
Management plan MGT Btrigger 11100 According to the harvest control rule
FMGT 0.3 According to the harvest control rule

The harvest control rule (HCR) for the Icelandic Ling fishery, which sets a TAC for the fishing year y/y+1 (September 1 of year y to August 31 of year y+1) based on a fishing mortality FMGT of 0.30 applied to ages 8 to 11 modified by the ratio SSB\(_{y}\)/MGT B\(_{\text{trigger}}\) when SSB\(_{y}\) < MGT B\(_{\text{trigger}}\), maintains a high yield while being precautionary as it results in lower than 5% probability of SSB < B\(_{\lim}\) in the medium and long term. WKICEMSE 2022 concluded that the HCR was precautionary and in conformity with the ICES MSY approach (ICES 2022c).

Management

The Icelandic Ministry of Food, Agriculture and Fisheries is responsible for management of the Icelandic fisheries and implementation of legislation. The Ministry issues regulations for commercial fishing for each fishing year (1 September–31 August), including an allocation of the TAC for each stock subject to such limitations. Ling in 5.a has been managed by TAC since the 2001/2002 fishing year.

Landings have exceeded both the advice given by MFRI and the set TAC from 2002/2003 to 2013/2014 but amounted to less than two thirds in 2015/2016 (Table 5). Overshoot in landings in relation to advice/TAC has been decreasing steadily since the 2009/2010 fishing year, with an overshoot of 53% to 35% in 2010/2011, 24% in 2011/2012 and 4% in 2012/2013. The reasons for the implementation errors are transfers of quota share between fishing years, conversion of TAC from one species to another (Figure 18) and catches by Norway and the Faroe Islands by bilateral agreement. The level of those catches is known in advance but has until recently not been taken into consideration by the Ministry when allocating TAC to Icelandic vessels. There is no minimum landing size for ling.

There are agreements between Iceland, Norway and the Faroe Islands relating to a fishery of vessels in restricted areas within the Icelandic EEZ. Faroese vessels are allowed to fish 5600 t of demersal fish species in Icelandic waters which includes maximum 1200 tonnes of cod and 40 t of Atlantic halibut. The rest of the Faroese demersal fishery in Icelandic waters is mainly directed at tusk, ling, and blue ling.

Table 5: Ling. Recommended TAC, national TAC, and catches (tonnes).
Fishing year Recommended catch National TAC Catches Iceland Catch other nations Total catch
2010/2011 7500 7500 9327
9327
2011/2012 8800 9000 10072 0 10072
2012/2013 12000 12000 11125 44 11140
2013/2014 14000 14000 11794 1763 12982
2014/2015 14300 14300 11684 1974 13658
2015/2016 16200 16200 9773 1456 11229
2016/2017 9343 9343 7291 1135 8426
2017/2018 8598 8598 7017 1309 8326
2018/2019 6255 6255 6927 1101 8028
2019/2020 6599 6599 5972 1183 7155
2020/2021 5700 5700 6201 1012 7213
2021/2022 4735 4735 5814 885 6699
2022/2023 6098 6098 7511 926 8437
2023/2024 6566 6566


2024/2025 6479



Figure 27: Ling. Net transfer of quota to and from ling in the Icelandic ITQ system by fishing year. Between species (upper): Positive values indicate a transfer of other species to ling, but negative values indicate a transfer of ling quota to other species. Between years (lower): Transfer of quota from given quota year to the next quota year.

Management considerations

All the signs from commercial catch data and surveys indicate that ling is at present in a good state. This is confirmed in the SAM assessment. However, the drop in recruitment since 2010 will probably result in a rapid decrease in sustainable catches in the near future.

Currently the longline and trawl fishery represent 95% of the total fishery, while the remainder is assigned to gillnets. Should those proportions change dramatically, so will the exploratory patterns as the selectivity of the gillnet fleet is substantially different from other fleets.

Table 6: Ling. Catches in Icelandic waters by country (Source STATLANT).
Year Faroe_Islands Germany Iceland Norway UK
2002 1631 0 2843 45 0
2003 570 2 3585 108 5
2004 739 1 3727 139 0
2005 682 3 4313 180 20
2006 962 1 6283 158 0
2007 807 0 6599 185 0
2008 1366 0 7738 179 0
2009 1157 0 9616 172 0
2010 1095 1 9868 168 0
2011 588 0 8789 249 0
2012 875 0 10695 248 0
2013 1030 0 10213

2014 1604 0 12483 158 0
2015 1132 0 11653 250 0
2016 952 0 8702 230 0
2017 730 0 7792 244 0
2018 993 0 6866 203 0
2019 1023 0 7061 184 0
2020 971 0 5853 237 0
2021 832 0 6205 91 0
2022 706 0 6818 132 0
2023 825 0 7531 178 0

::: {#tbl-table7 .cell tbl-cap=’ Ling. Assessment summary by calendar year. Catches are ICES estimates.’} ::: {.cell-output-display}

Year Recruitment Rec_upper Rec_lower SSB SSB_upper SSB_lower F_age2 F_upper F_lower Catch
1979 2607 3702 1836 17529 22480 13668 0.45 0.70 0.29 5284
1980 2830 3746 2139 16508 21083 12926 0.50 0.81 0.31 4601
1981 3175 4074 2475 15038 19153 11807 0.55 0.88 0.34 4577
1982 3536 4491 2784 13679 17208 10874 0.66 0.99 0.45 4868
1983 3793 4800 2998 12175 15085 9826 0.75 1.03 0.55 4907
1984 3718 4697 2943 10566 13006 8583 0.65 0.84 0.50 3931
1985 3432 4329 2720 10350 12499 8571 0.54 0.70 0.42 3525
1986 3508 4433 2776 10726 12644 9099 0.50 0.63 0.40 3710
1987 3769 4763 2983 11966 13873 10321 0.59 0.73 0.48 4928
1988 3635 4572 2891 12262 14056 10696 0.65 0.80 0.54 5764
1989 3279 4085 2632 11695 13354 10242 0.64 0.78 0.53 5600
1990 2867 3547 2318 11562 13218 10114 0.65 0.78 0.55 5575
1991 2619 3244 2115 10063 11465 8832 0.69 0.83 0.58 5696
1992 2705 3351 2183 9599 10703 8610 0.66 0.79 0.55 5072
1993 2918 3610 2359 9187 10062 8388 0.56 0.67 0.47 4121
1994 2696 3352 2169 11442 12414 10546 0.51 0.59 0.44 3701
1995 2526 3145 2029 11921 12908 11009 0.56 0.65 0.48 3964
1996 2518 3130 2025 11727 12690 10838 0.58 0.67 0.51 4036
1997 2628 3259 2118 10548 11442 9724 0.57 0.66 0.49 3953
1998 3069 3801 2478 10602 11534 9745 0.57 0.66 0.50 4080
1999 3794 4691 3068 10714 11650 9854 0.64 0.74 0.56 4336
2000 4500 5559 3643 10727 11683 9849 0.46 0.53 0.40 3185
2001 4817 5970 3886 11711 12733 10771 0.45 0.52 0.39 3366
2002 5966 7349 4843 13034 14161 11997 0.51 0.59 0.44 4133
2003 6997 8653 5658 15094 16400 13892 0.47 0.54 0.41 4159
2004 8179 10209 6552 17431 18889 16086 0.48 0.56 0.42 4590
2005 9047 11291 7248 20220 21876 18690 0.45 0.52 0.39 5023
2006 10378 12982 8296 23013 24846 21315 0.54 0.62 0.47 6979
2007 10484 13162 8351 27369 29573 25329 0.49 0.56 0.43 7221
2008 11123 13755 8995 29851 32345 27550 0.52 0.60 0.46 8891
2009 10838 13505 8698 32994 35758 30443 0.55 0.63 0.48 10316
2010 7193 8895 5816 31812 34563 29280 0.54 0.62 0.47 10619
2011 5015 6235 4034 25429 27754 23299 0.46 0.53 0.40 9768
2012 3772 4746 2998 31969 34901 29283 0.56 0.65 0.48 11557
2013 4398 5590 3460 32193 35298 29362 0.44 0.52 0.38 12157
2014 4208 5354 3308 38072 41892 34601 0.50 0.58 0.43 14012
2015 4285 5471 3356 36308 40209 32785 0.52 0.60 0.44 13205
2016 5363 6944 4142 40286 45051 36025 0.40 0.48 0.34 10023
2017 4216 5448 3263 35840 40372 31817 0.40 0.47 0.33 9219
2018 3141 4121 2394 37759 42719 33374 0.35 0.43 0.30 9685
2019 2760 3699 2059 31796 36304 27847 0.37 0.44 0.31 9080
2020 2243 3151 1597 32065 37223 27622 0.32 0.39 0.26 7118
2021 2818 4200 1891 29799 35220 25213 0.29 0.36 0.23 7066
2022 2524 4135 1541 32695 39642 26965 0.30 0.38 0.23 8328
2023 2869 5241 1570 31333 39519 24842 0.34 0.45 0.25 8382
2024 3054 6171 1511 29017 38985 21598 0.33 0.52 0.22 7264

::: :::

References

ICES. 2011. “Report of the Working Group on the Biology and Assessment of Deep-Sea Fisheries Resources (WGDEEP), 2 March–8 March, 2011, Copenhagen, Denmark. ICES Cm 2011/Acom:17.” International Council for the Exploration of the Seas; ICES publishing.

2012. “Report of the Working Group on the Biology and Assessment of Deep-Sea Fisheries Resources (WGDEEP), 28 March–5 April, 2012, Copenhagen, Denmark. ICES Cm 2012/Acom:17.” International Council for the Exploration of the Seas; ICES publishing.

2017. “Report of the Workshop on Evaluation of the Adopted Harvest Control Rules for Icelandic Summer Spawning Herring, Ling and Tusk (WKICEMSE), 21–25 April 2017, Copenhagen, Denmark. ICES CM 2017/ACOM:45.” International Council for the Exploration of the Seas; ICES publishing.

2022a. “11.2 Icelandic Waters ecoregion – Fisheries overview.” International Council for the Exploration of the Seas; ICES publishing. https://doi.org/10.17895/ices.advice.21487635.v1

2022b. Iceland request for evaluation of a harvest control rule for tusk in Icelandic waters. In Report of the ICES Advisory Committee, 2022. ICES Advice 2022, sr.2022.6d, https://doi.org/10.17895/ices.advice.19625823

2022c. “Stock Annex: Ling (Molva molva) in Division 5.a (Icelandic grounds).” International Council for the Exploration of the Seas; ICES publishing. Unpublished

2022d. Workshop on the evaluation of assessments and management plans for ling, tusk, plaice and Atlantic wolffish in Icelandic waters (WKICEMP). ICES Scientific Reports. Report. https://doi.org/10.17895/ices.pub.19663971.v1