Key signals
Following a peak in landings from 2008-2013, landings since 2017 have been at historically low levels.
Biomass indices from the Icelandic Autumn Survey declined from 2009 to 2022, and have increased in the past three years.
From 2010 to 2020 recruitment levels were minimal or low. These have since increased to levels seen in early 2000s.
Spawning stock biomass (SSB) has increased since 2020 and is currently at a similar level to 2010.
Biomass levels are expected to increase under current conditions due to improved recruitment.
General information
Blue ling (Molva dypterygia) in ICES Division 5.a and Subarea 14 are assessed as one stock. Blue ling is a deep-water gadoid species primarily distributed along the continental shelves and slopes of the Northeast Atlantic, particularly around Iceland, the Faroe Islands, Norway, the British Isles, and off East Greenland. In Icelandic waters, it is commonly found in deeper regions over soft seabeds in the southeast, south, and west of Iceland, at depths greater than 500 meters. The majority of the population is concentrated between 300 and 800 meters, and it is rarely found at depths shallower than 200 meters. During the winter months, blue ling tends to stay at deeper depths, while in the spring and summer, it ascends to shallower areas.
The blue ling is smaller than the common ling (Molva molva), reaching an average length of approximately 80 cm and a maximum length of around 150 cm, according to the Icelandic autumn groundfish survey. Blue ling is a slow-growing species, living up to 20 years or more. Sexual maturity is typically reached when individuals are 75-90 cm. Sexual dimorphism is notable with females growing faster and reaching maturity at larger sizes and older ages than males. Spawning occurs in the spring at depths of 500-1000 meters or deeper, in waters with temperatures ranging from 5-8°C, outside the continental shelf off the southern coast of Iceland, south and west of the Faroe Islands, west of Norway, and west of Scotland to Northern Ireland. Off Iceland, blue ling spawns from February to April, peaking in March in waters with temperatures of 5-6°C. Known spawning grounds include depths of 600-1000 meters south of the Westman Islands and along the fishing limits of the Reykjanes Ridge (Franshóll). The eggs are pelagic, measuring 1.3-1.5 mm in diameter, and hatch in the upper layers of the ocean. The larvae drift long distances west and north, traveling far into the open sea toward Greenland, where they grow alongside the larvae of cod, herring, capelin, silver hake, and common skates. The juvenile fish settle on the seabed when they reach 8-10 cm in length, typically between August and October (Jónsson and Pálsson (2013))
Fishery
Landings trends
Since 1960, landings of blue ling in Icelandic water have shown three distinctive peaks (Figure 1). In the early 1980s, catches were at a historical maximum, driven primarily by bottom trawl fisheries targeting spawning aggregations near the Westman Islands (Figure 1, Figure 2). In 1993, the Icelandic bottom trawl fleet fished on aggregations of spawning blue ling in a small area on the Reykjanes Ridge at the border between Division 5.a and Subarea 14 (Figure 1, Figure 2). The third peak began in 2008 when the longline fleet started targeting blue ling. Landings peaked in 2010 and subsequently declined to a historic low in 2020. In between these peaks, catches have generally been below 3000 t and blue ling has largely been bycatch in redfish, greater silver smelt, and Greenland halibut bottom trawl fisheries, and tusk longline fisheries.
Before 2008, the majority of the catches of blue ling in 5.a were caught by trawlers (Figure 2) as bycatch. From 2008-2013 there was a substantial change in the fishery as longliners started targeting blue ling and their proportion of catches increased from 7–20% in 2001–2007 to around 70% in 2011 (Figure 2). The trend has reversed and from 2016–2025 the proportion of longline catches decreased to 10–30%.
Historically, landings in Subarea 14 have been small compared to Division 5.a (Figure 1). Total international landings from Subarea 14 have been highly variable over the years, ranging from a few tonnes in some years to around 3700 t in 1993 and 950 t in 2003. Most of the landings in 2003 were taken by Spanish trawlers (390 t). Since then, no further information is available on this fishery. The high landing values in Subarea 14 are very occasional, and in most years, total international landings have been between 50 and 200 t. Preliminary landings in 2025 were 17 t.
Preliminary total landings in 2025 were 640 tonnes of which the Icelandic fleet caught 624 tonnes.
Spatial and temporal patterns
The main bottom trawl fishing grounds are located northwest of Iceland close to the shelf break, and southwest of Iceland at the base of Reykjanes Ridge (Figure 4). Fisheries also operate along the continental shelf south and southeast of Iceland, and directly west of Iceland (Figure 4, Figure 6). Most of the catches by trawlers are taken in waters between 200 m and 700 m (Figure 3). When targeting blue ling, longliners typically operated in waters from 200 m to 500 m (Figure 3), and catches were distributed in deeper waters west, southwest and south of Iceland (Figure 4, Figure 6).
| Year | Nr. Bottom Trawl | Bottom Trawl | Nr. Long Line | Long Line | Nr. Other | Other | Total catch |
|---|---|---|---|---|---|---|---|
| 2000 | 108 | 801 | 44 | 808 | 36 | 25 | 1 634 |
| 2001 | 110 | 597 | 39 | 131 | 48 | 34 | 762 |
| 2002 | 105 | 986 | 41 | 256 | 25 | 23 | 1 264 |
| 2003 | 105 | 883 | 47 | 197 | 33 | 17 | 1 098 |
| 2004 | 112 | 894 | 53 | 145 | 45 | 44 | 1 083 |
| 2005 | 106 | 1 261 | 60 | 108 | 51 | 126 | 1 496 |
| 2006 | 105 | 1 477 | 69 | 151 | 47 | 107 | 1 734 |
| 2007 | 97 | 1 544 | 90 | 374 | 64 | 76 | 1 995 |
| 2008 | 95 | 2 111 | 92 | 1 454 | 61 | 88 | 3 653 |
| 2009 | 89 | 2 242 | 87 | 1 677 | 69 | 211 | 4 129 |
| 2010 | 85 | 2 202 | 96 | 3 978 | 73 | 198 | 6 378 |
| 2011 | 81 | 1 630 | 97 | 4 140 | 58 | 134 | 5 904 |
| 2012 | 79 | 1 449 | 78 | 2 425 | 52 | 332 | 4 207 |
| 2013 | 75 | 1 300 | 72 | 1 420 | 45 | 48 | 2 769 |
| 2014 | 72 | 923 | 74 | 628 | 36 | 43 | 1 594 |
| 2015 | 67 | 821 | 78 | 868 | 36 | 22 | 1 712 |
| 2016 | 66 | 701 | 54 | 213 | 31 | 10 | 925 |
| 2017 | 57 | 436 | 53 | 169 | 26 | 14 | 619 |
| 2018 | 65 | 363 | 60 | 132 | 20 | 7 | 502 |
| 2019 | 58 | 238 | 54 | 161 | 27 | 16 | 415 |
| 2020 | 58 | 264 | 47 | 71 | 21 | 9 | 344 |
| 2021 | 59 | 286 | 41 | 33 | 20 | 4 | 323 |
| 2022 | 55 | 338 | 37 | 86 | 20 | 2 | 427 |
| 2023 | 52 | 304 | 36 | 96 | 20 | 4 | 404 |
| 2024 | 55 | 427 | 28 | 42 | 16 | 3 | 473 |
| 2025 | 52 | 555 | 25 | 43 | 20 | 24 | 622 |
Catch per unit of effort from commercial fisheries
Effort and nominal catch per unit effort (CPUE) data from the Icelandic trawl and longline fleet are given in Figure 8. Due to changes in the fishery (expansion into new areas, fleet behaviour, etc.) and technical innovations, CPUE is not considered a reliable index of biomass abundance of blue ling and therefore no attempt has been made to standardize the series. However, looking at fluctuations in CPUE and effort may be informative regarding the development of the fishery. CPUE from longlines was high from 2008-2013 but has decreased markedly since. CPUE from trawls has been gradually decreasing over time. Effort from bottom trawls peaked in 2009 and has since declined to the same level observed in the early 2000s. Effort from longlines peaked in 2011 but has remained relatively stable since.
Landings and discards
Landings data before 1982 (and foreign landings until 2005) are sourced from the ICES STATLANT database. Icelandic landings from 1982–1993 were collected by the Fisheries Association of Iceland, and from 1994 onward by the Directorate of Fisheries. Foreign landings were recorded by the Coast Guard before 2014 and by the Directorate of Fisheries thereafter.
Discard rates of blue ling are unknown. Discarding is prohibited by law. Management measures, such as inter-species quota transfers and landing flexibility, discourage discarding. To help prevent high grading and quota mismatches, fisheries are permitted to land catches that are exempt from quota deductions, provided that the proceeds from their sale are transferred to a government-managed fund. More information is available at https://www.responsiblefisheries.is/seafood-industry/fisheries-management.
Data and sampling
Commercial data
Fishing areas and sampling locations are shown in Figure 9 and Figure 10. In 2025, 7 samples were collected from bottom trawls and 0 from longlines (Figure 10, Table 2). As of January 2026, the sampling frequencies for blue ling from longline were increased to improve coverage.
Length compositions
Length distributions from the Icelandic trawl and longline catches for the period 2007–2025 are shown in Figure 11 and Figure 12 respectively. From 2003-2010, mean length in catches ranged 82-85 cm and 85-93 cm for bottom trawl and longline respectively. From 2011-2019, an increase in mean length from 86-102 cm and 93-109 cm in bottom trawl and longline respectively was seen due to poor recruitment. Evidence of recruitment was seen in the catches in 2019 for bottom trawl and 2020 for longline. The mean length has since fluctuated between 83 and 94 cm in bottom trawl, and 88 and 99 cm in longline.
| Year |
Bottom Trawl
|
Long Line
|
||
|---|---|---|---|---|
| Num. samples | Num. lengths | Num. samples | Num. lengths | |
| 2000 | 3 | 420 | 8 | 931 |
| 2001 | 6 | 631 | 1 | 39 |
| 2002 | 10 | 1 427 | 4 | 399 |
| 2003 | 6 | 790 | 3 | 295 |
| 2004 | 6 | 851 | 1 | 150 |
| 2005 | 12 | 1 164 | 1 | 94 |
| 2006 | 9 | 824 | 0 | 0 |
| 2007 | 12 | 1 461 | 2 | 238 |
| 2008 | 13 | 1 685 | 14 | 1 960 |
| 2009 | 23 | 2 894 | 15 | 1 940 |
| 2010 | 29 | 3 161 | 38 | 5 191 |
| 2011 | 12 | 1 364 | 44 | 6 513 |
| 2012 | 11 | 1 135 | 27 | 3 829 |
| 2013 | 6 | 757 | 15 | 1 564 |
| 2014 | 5 | 411 | 11 | 1 222 |
| 2015 | 4 | 394 | 0 | 0 |
| 2016 | 3 | 309 | 0 | 0 |
| 2018 | 2 | 240 | 1 | 120 |
| 2019 | 1 | 114 | 1 | 120 |
| 2020 | 2 | 126 | 1 | 120 |
| 2021 | 7 | 353 | 0 | 0 |
| 2022 | 6 | 282 | 3 | 253 |
| 2023 | 5 | 406 | 1 | 69 |
| 2024 | 5 | 527 | 0 | 0 |
| 2025 | 7 | 702 | 0 | 0 |
Age compositions
There is no reliable data on the age composition of blue ling in Icelandic waters.
Weight at age in the catch
There is no reliable weight-at-age data for blue ling in Icelandic waters.
Survey data
In Division 5.a, the Icelandic spring groundfish survey (IS-SMB) covers only the shallower part of the depth distributional range of blue ling and shows high interannual variance (Figure 13). It is thus unknown to what extent the spring indices reflect actual changes in total blue ling biomass, given that it does not cover the depths where largest abundance of blue ling occurs. The shorter Icelandic autumn groundfish survey (IS-SMH) covers deeper regions and is therefore more likely to reflect the true biomass dynamics (Figure 13). Due to industrial action, only part of IS-SMH was conducted in 2011.
In Subarea 14, data is available from two surveys in east Greenlandic waters: (1) the Greenland halibut survey, and (2) the Shrimp survey. The Greenland halibut survey has run from 1998 to present. The survey covers a depth range of 400-1500 m and was not conducted from 2017-2021. The shrimp survey is a stratified-random trawl survey which was initiated in 2008 and has a depth range of 0-600 m. The survey was not conducted in 2017-2019 and 2021.
Length-disaggregated survey indices from IS-SMH and the Greenlandic surveys are shown in Figure 14. From 2000, all biomass indices rose to peaks in 2009, then gradually declined to 2002-2005 levels between 2018 and 2022. The past three years saw an increase in biomass levels. A large increase of more than 200% in the recruitment index was observed in 2008 but in 2010 it decreased to its lowest observed value and remained low until 2017. From 2017-2020, small fluctuations in recruitment were seen, and from 2022 to present, recruitment has been at similar levels to 2000-2005 (Figure 14).
Length distributions from the Autumn survey are shown in Figure 16. From 2000-2009, mean length ranged from 68-77 cm and increased to 94 cm in 2016 owing to the very low recruitment. Since 2017, the mean length has been decreasing and was between 79 and 82 cm in the past three years.
In Icelandic waters, sexual maturity in blue ling occurs at lengths of 75-90 cm, with males maturing earlier than females and typically at a smaller size (Figure 17). Estimates of the length at which 50% of the stock are mature (L50) are 72 cm and 83 cm for males and females respectively, and 76 cm when pooled across sexes (Figure 17).
Stock weight at age
No data is available due to difficulties in ageing blue ling.
Stock maturity at age
No data is available due to difficulties in ageing blue ling.
Stock assessment
Model and data inputs
The stock was benchmarked in 2025 (ICES (2025)) where the basis for advice was changed and reference points updated. The stock is now a Category 1 stock, and the approved assessment is an age-length based assessment model (GADGET). A brief overview of the model settings and input data are given below, a full description of the assessment model can be found in the stock annex.
The model runs from 1975 to present. Each year is divided into quarterly time-steps.
Two sub-stocks are modelled:
- An immature stock that has an age range of 2-15 years and a length range from 20-112 cm. Each length bin is 4 cm.
- A mature stock that has an age range of 5-20 years and a length range of 36-140 cm. The oldest age is treated as a plus group (20 years and older). Each length bin is 4 cm.
Movement from the immature stock to mature stock occurs via:
- Maturation (using a length-based ogive).
- Ageing (15 year old immature fish automatically move to the mature stock at the end of the year).
Recruitment to the immature stock occurs in the first step at age 2.
Fish are removed from the stock via natural mortality (M = 0.15 for all ages of both stocks) and commercial exploitation.
Three commercial fleets and one survey fleet:
- Bottom trawl (Iceland and Greenlandic waters combined)
- Longline
- Foreign vessels
- Autumn survey (Icelandic autumn groundfish survey, 2001-present)
All fleets have a logistic selection curve. The foreign fleet shares the longline fleet’s selection pattern.
Spawning stock biomass is defined as the female portion of the mature stock to account for differences in growth between sexes. The proportion of females is calculated by applying a length-based sex ratio to the numbers-at-length output from the model.
The model uses multiple disparate datasets. The input data include:
Length disaggregated survey indices from the Icelandic Autumn goundfish Survey (IAGS), the Greenland halibut survey from east Greenland (GHLE), and the shrimp survey (SFE) from east Greenland. Survey indices from the three surveys were standardised using the Cochran method (Cochran (1977)) and combined resulting in a single combined survey index that covers Icelandic (27.5a) and east Greenlandic (27.14) waters. For the assessment model, the combined survey index was dis-aggregated into the following length bins: 20-44 cm, 44-56 cm, 56-68 cm, 68-80 cm, 80-92 cm, 92-104 cm, 104-140 cm.
Length distributions from the Icelandic commercial bottom trawl fleet.
Length distributions from the Icelandic commercial longline fleet.
Quarterly landings from Iceland and Greenland.
Maturation data from the Autumn survey (IAGS).
An overview of the input data and their annual availability is shown in Figure 18.
Diagnostics and fit
Figure 19 shows the overall fit to the length-disaggregated survey indices. In general, the model captures the observed trends over time for each length interval. For the smaller fish (20-56 cm), the recruitment spikes in 2016 and 2022 enter the stock a year early which is a result of fixing the growth rates in the model. The model underestimates the recruitment peaks in 2008 and 2009. Removing the penalty on recruitment variation (which acts to smooth recruitment) does not alleviate this misfit. The most likely reason that the model does not pick up these peaks is that the model cannot reconcile these peaks with what is seen in the length distribution data. Blue ling enter the fishery between 50-60 cm (Figure 21, Figure 22), and the model fits the indices from 56 cm onwards well, suggesting that biomass of the exploitable portion of the stock is well estimated. The deviation between the terminal estimates and the observed values are small for all length groups except 104-140 cm. For this group, the model estimates remain stable from 2017-2025 rather than tracking the observed downward trend.
The model’s fit to the Autumn survey composition data is shown in Figure 20. Overall, the fits to the observed length distributions from the Autumn survey are good. The model captures the age truncation seen in the observed survey data with the median length increasing from ~75 cm in 2010 to ~95 cm in 2016 due to poor recruitment (Figure 20), although during this period, the model is slightly overestimating the proportion of smaller fish and misses a large peak in 2013. A recruitment spike in 2017 led to a bimodal length distribution from 2017 to 2022. The bimodal distribution is visible in the observed survey data, though not as clearly defined as in the model. Blue ling, like many deep-water species, has a slow growth rate, and tracking cohorts based on length distributions (or age distributions) can be difficult. This is visible, for instance, in the observed peaks in 2018 and 2019, which are highly unlikely to be a single cohort. The model’s misfits are largely concerned with over- or underestimating the proportion of small fish, which is likely because there is little data for small, young fish, so estimations of growth are highly uncertain.
The model fits the maturity data well highlighting that the \(l_{50}\) and the \(\lambda\) parameters are well estimated (Figure 24). In 2012 and 2013, when the fit does not look as strong, less than 10 small fish (< 60 cm) in total were caught so the corresponding observed values are highly uncertain.
The fits to the commercial length distributions are shown in figures Figure 21, Figure 22, and Figure 23. See Residuals section below for discussion.
Length-based residuals are shown for each catch component in Figure 25. Residual patterns are visible for each fleet. The Autumn survey and bottom trawl fleet show similar patterns that reveal slight over-estimations of the proportion of smaller fish (< 75 cm) and underestimations of fish > 75 cm, whereas the opposite pattern is seen for the longline fleet (Figure 25). Dome-shaped selectivities were trialed for both commercial fleets during ICES (2025); however, this did not alleviate the residual patterns. Therefore this is potentially an indication of a shift in time in selection and/or growth. It should also be noted that commercial sampling, particularly in later years, has been fairly low, thus there is also considerable uncertainty in the observed data in later years.
Results
The model results are presented in Figure 26. Spawning stock biomass is above Blim and Bpa and has been gadually increasing since 2019, a trend also seen in total biomass, and owing primarily to a large recruitment spike in 2017. From 2012-2018, recruitment was minimal; however, whilst the values from 2020 to 2023 remain low and below the long-term average, the values for 2019 and 2024 increased levels seen from 2001-2010. Fishing mortality is below Fmsy and Fpa and has gradually declined since 2011 to a historic low in 2025.
The estimated selection patterns are as expected Figure 27. The Autumn survey selects a greater proportion of smaller fish than the commercial fleets, and the bottom trawl fishery selects smaller fish than the longline fishery.
Results of the retrospective analysis are seen in Figure 28. All Mohn’s Rho values are within acceptable limits and all peels are within the confidence intervals.
Short-term projections
Short-term forecasts for blue ling are done in Gadget using the settings described below.
- F and M before spawning: NA
- Weight-at-age in the stock: GADGET uses a weight–length relationship and von Bertalanffy growth (no weights-at-age are supplied to GADGET)
- Weight-at-age in the catch: GADGET uses a weight–length relationship and von Bertalanffy growth (no weights-at-age are supplied to GADGET)
- Maturity: A length-based ogive estimated within Gadget
- Exploitation pattern:
- Landings: logistic selection-at-length by fleet, with parameters estimated within GADGET. Catch proportions by fleet are assumed fixed based on last seven years.
- Intermediate year assumptions: Catch is set equal to the TAC during the fishing season and projections for the fol- lowing year run at a selected harvest rate that corresponds with the selected fishing mortality. If the TAC is exceeded prior to the intermediate year, catches from the corresponding quarter in the previous year are assumed.
- Stock–recruitment model used: Fixed hockey-stick recruitment function, with geometric mean recruitment of the past three years set as the asymptote.
- Catch scenarios: F=Fmsy, F=0 and F=Fsq
The results of the prognosis are shown in Table 3.
Year | Rec. at age 2 ('000s) | Catch (t) | SSB (t) | F10-20 | Approach |
|---|---|---|---|---|---|
2026/2027 | 3,288 | 1,898 | 13,396 | 0.092 | Fmsy |
2027/2028 | 3,288 | 1,933 | 14,878 | 0.091 | |
2028/2029 | 3,288 | 1,933 | 15,377 | 0.086 | |
2026/2027 | 3,288 | 0 | 13,396 | 0.000 | Zero catch |
2027/2028 | 3,288 | 0 | 15,218 | 0.000 | |
2028/2029 | 3,288 | 0 | 17,165 | 0.000 | |
2026/2027 | 3,288 | 774 | 13,396 | 0.036 | SQ |
2027/2028 | 3,288 | 827 | 15,082 | 0.036 | |
2028/2029 | 3,288 | 866 | 16,430 | 0.034 |
Management
History
Before the 2013/2014 fishing year the Icelandic fishery was not regulated by a national TAC or ITQs. The only restrictions on the Icelandic fleet regarding the blue ling fishery were the introduction of closed areas in 2003 to protect known spawning locations of blue ling, which are in effect. As of the 2013/2014 fishing year, blue ling is regulated by the ITQ system (regulation 662/2013) used for many other Icelandic stocks such as cod, haddock, tusk and ling.
Until 2026, blue ling in Division 5.a and Subarea 14 was a category 3 stock. From the 2013 until 2019, the ICES Fproxy rule was used as an advice basis. This was changed to a data-limited rule based on survey trends (“2-over-3 rule”) in 2019 because the low recruitment estimates at the time were considerably lower than those observed in the period used for the calculation of the Fproxy. In 2022, following improvements to the advice rules for category 3 stocks (ICES (2021)), the ICES rfb rule was employed. In 2025, the stock was benchmarked and moved to category 1 (ICES (2025)), whereby advice was based on the MSY approach and delivered by an age- and length-based model.
Since 2021/2022, other species have been transferred to blue ling for the first time since it was regulated into the ITQ system (Figure 30).
Reference points
Reference points and their bases area are shown in Table 4. The reference points Fmsy and Fpa were re-evaluated in 2026 (see ICES (2026) for details).
Approach | Reference point | Value | Basis |
|---|---|---|---|
MSY approach | FMSY | 0.09 | Fpa |
MSY Btrigger | 7 094 | Bpa | |
Precautionary approach | Blim | 5 105 | Mean SSB (2003–2005) |
Bpa | 7 094 | Blim × e1.645σ, σ = 0.2. | |
Fpa | 0.09 | Maximum F at which the probability of SSB falling below Blim is < 5% | |
Flim | 0.19 | Fishing mortality that in stochastic equilibrium will result in median SSB at Blim |
Management considerations
As for many other deep-sea species, blue ling is susceptible to depletion of spawning aggregations (Large et al. (2010)). Closed areas to protect known spawning aggregations were introduced in 2003 in Icelandic waters. Maintaining the current closed areas to protect spawning aggregations is advised.
References
Cochran, William Gemmell. 1977. Sampling Techniques. John wiley & sons.
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.
ICES. 2025. Benchmark Workshop on Selected Deep-Sea Fisheries Stocks (WKBDEEP). International Council for the Exploration of the Seas; ICES publishing. https://doi.org/10.17895/ices.pub.28882295.
ICES. 2026. Northwestern Working Group (NWWG). International Council for the Exploration of the Seas; ICES publishing.
Jónsson, G., and J. Pálsson. 2013. Íslenskir Fiskar. Mál og menning.
Large, Philip A, Guzman Diez, James Drewery, et al. 2010. “Spatial and Temporal Distribution of Spawning Aggregations of Blue Ling (Molva Dypterygia) West and Northwest of the British Isles.” ICES Journal of Marine Science 67 (3): 494–501.