Key signals
- Since catch advice was first issued for spotted wolffish, catches have consistently exceeded the recommended limits.
- The spotted wolffish stock is now at the precautionary threshold, and recruitment is at a historic low. If the stock falls below the threshold, management measures must be implemented to ensure its protection.
- The Marine and Freshwater Research Institute encourages the release of spotted wolffish after quotas have been filled. This is because studies have demonstrated high survival rates post-release.
General information
Spotted wolffish has been exploited for many years in Icelandic waters. It is an elongate fish characterised by dark spots on its skin and sharp, protruding teeth. In catches, the common length range of spotted wolffish is 60-90 cm, although the largest individual caught in Icelandic waters was 144 cm. Spotted wolffish is mainly found in the northern parts of the continental shelf of Iceland. They reside on sandy or muddy substrate at depths ranging from 100-400 m. In Icelandic waters, female spotted wolffish mature at an average length of 83 cm and age of 9 years. Prior to maturity, mean annual growth is approximately 6.5 cm.
The fishery
From 1982-1995, the catch of spotted wolffish was fairly stable, averaging just over 900 tonnes each year, the majority of which was caught by bottom trawlers. From 1995, catches gradually increased to a historical high of 3640 tonnes in 2006. This increase was mostly due to increased catch on longlines. Since 2006, the annual catch has declined between years and was 805 tonnes in 2024. Reduced catches in 2024 may be due to the fact that in December 2020 fishermen were granted a permission to release spotted wolffish, possibly leading to significant releases from 2022 onwards.
The main fishing grounds for spotted wolffish are located northwest of Iceland (Figure 1). Prior to 2000, most of the catches were taken in the NE area. However, the proportion of catches in the NW area gradually increased in 2000-2018, therefrom it has decreased a little. In 2024, catches in the NE and NW areas accounted for 84% of all catches (Figure 1 and Figure 2). Note that the majority of catches allocated to the ‘Other’ area in Figure 2 are taken from Dorbanki and an area to the east of Dorbanki.
Approximately 7% of the catch of spotted wolffish is caught at depths less than 100 m, and about 25-35% is caught from 100-200 m (Figure 3). From 2000-2004, approximately 50% of the spotted wolffish catch was caught at depths between 200 and 300 m, since then it has been 30-47%. The catch taken at more than 300 m has been relatively stable (20-25%) since 2000 (Figure 3).
Around 98% of landed spotted wolffish is caught on longline and in demersal trawl. From 2000-2003, demersal trawls caught more than 65-70% of the total catch. Following this period, the proportion of catch by longline increased, peaking at approximately 66% in 2013. Subsequently, the proportion of catch by longline has decreased steadily to 30% in 2024 (Figure 4).
| Year | Nr. Long Line | Nr. Bottom Trawl | Nr. Other | Long Line | Bottom Trawl | Other | Total catch |
|---|---|---|---|---|---|---|---|
| 2000 | 27 | 74 | 3 | 654 | 1185 | 12 | 1851 |
| 2001 | 32 | 66 | 6 | 654 | 1401 | 26 | 2081 |
| 2002 | 36 | 69 | 8 | 591 | 1488 | 30 | 2109 |
| 2003 | 52 | 69 | 6 | 757 | 1614 | 26 | 2397 |
| 2004 | 72 | 72 | 12 | 1412 | 1816 | 47 | 3275 |
| 2005 | 99 | 67 | 8 | 1573 | 1624 | 38 | 3235 |
| 2006 | 127 | 66 | 11 | 2052 | 1511 | 73 | 3636 |
| 2007 | 115 | 71 | 10 | 1391 | 1255 | 38 | 2685 |
| 2008 | 84 | 61 | 3 | 1073 | 990 | 24 | 2088 |
| 2009 | 84 | 62 | 9 | 1226 | 998 | 51 | 2275 |
| 2010 | 76 | 58 | 13 | 1045 | 786 | 71 | 1903 |
| 2011 | 79 | 57 | 10 | 934 | 642 | 38 | 1614 |
| 2012 | 79 | 61 | 13 | 1129 | 761 | 59 | 1950 |
| 2013 | 90 | 61 | 10 | 1575 | 788 | 39 | 2402 |
| 2014 | 85 | 55 | 9 | 1169 | 712 | 36 | 1917 |
| 2015 | 80 | 53 | 8 | 1010 | 683 | 24 | 1718 |
| 2016 | 70 | 51 | 4 | 1032 | 676 | 18 | 1726 |
| 2017 | 60 | 54 | 5 | 820 | 589 | 18 | 1427 |
| 2018 | 63 | 50 | 2 | 722 | 625 | 10 | 1357 |
| 2019 | 48 | 50 | 6 | 736 | 640 | 20 | 1396 |
| 2020 | 57 | 52 | 4 | 664 | 630 | 14 | 1308 |
| 2021 | 46 | 49 | 4 | 634 | 559 | 13 | 1205 |
| 2022 | 40 | 43 | 2 | 334 | 425 | 6 | 765 |
| 2023 | 34 | 39 | 2 | 244 | 374 | 6 | 624 |
| 2024 | 30 | 46 | 5 | 221 | 532 | 14 | 768 |
In 2000-2006, the number of longliners reporting catches of ≥1000 kg/year of spotted wolffish increased from 27 to 127 (Table 1). At the same time catches on longline increased from 654 to 2052 tonnes. Since then, the number of longliners has decreased and in 2008-2023 they were on the average 67 each year. In 2024 they were only 30 and the catch was similar as it was in 1997. The number of trawlers that caught > 1000 kg of spotted wolffish has gradually declined from 74 in 2000 to 46 in 2024. Catches in demersal trawl since 2008 have dropped to about 50% of the catches from 2000-2007, and from 2021–2024 the catch was similar to the levels in 1994–1995 (Table 1). In 1996, longliners caught greater than 400 tonnes of spotted wolffish. This signified the increased targeting of spotted wolffish by longliners because prior to 1996 the annual catch was usually less than 100 tonnes. This increased effort could be related to the fact that in the fishing year 1996/97, the closely related species, Atlantic wolffish, was included in ITQ system for the first time.
The number of vessels accounting for 95% of the annual catch of spotted wolffish ranges from 75-150 (Figure 5). Despite an increase in catches from 1996-2003, the number of vessels remained relatively stable during this period. However, from 2004-2006, the number of vessels increased when annual catches exceeded 3000 tonnes. Since 2007, a drop in the number of vessels accounting for 95% of the catches has coincided with catch reductions (Figure 5).
Catch per Unit Effort (CPUE) and Effort.
When catch per unit effort (CPUE) is estimated, changes in fleet composition, technical improvements, and differences in gear setup have not been considered. Therefore, CPUE estimates of spotted wolffish in Icelandic waters are not considered to be representative of stock abundance.
CPUE of longline (kg/1000 hooks) and demersal trawl (kg/tow-hour) were calculated as the total weight in sets or tows. In both cases, data was either constrained so that spotted wolffish accounted for more than 10% of the catch, or that spotted wolffish was part of the catch according to logbook records. Here, CPUE is calculated for each fishing trip and the median CPUE of all fishing trips in each year is presented (Figure 6).
Estimates of CPUE from demersal trawl were catch was ≥10% of the total catch was lowest in 2001 (83 kg/h) Since then it generally increased and was highest in 2014 (134 kg/h), since 2019 it has been decreasing (Figure 6). Where the catch was >0, CPUE increased from 2000 to its highest level in 2004 (74 kg/h). Therefrom it decreased to 2007 (36 kg/h) and has since oscillated between 32-56 kg/h with no clear trend, and has been declining since.
The estimated longline CPUE where catch was ≥10% oscillated between 16-57 kg/ 1000 hooks in 2000-2024, with no clear trend (Figure 6). Where the catch was >0, CPUE decreased gradually from 2000 (10 kg/1000 hooks) to 2018 (3 kg/1000 hooks), and has remained in the range of 3-5 kg/1000 hooks since.
Sampling and Age distribution of landed spotted wolffish
Between 300 and 1900 otoliths have been sampled annually for age reading by the MFRI. The number of samples collected from longline and demersal trawl ranged from 1-48 and 5-32 respectively (Table 2, Figure 7). Samples were not taken from other gear types because they represent a small proportion (~2%) of the total catch.
In samples from commercial catches in 2015 about 400 specimens were aged. The estimated age range was 5-16 years, whilst ages 8 and 9 were most common, accounting for approximately 40% of the readings. Age has not been determined for spotted wolffish landed since 2016, but recently age determination began for spotted wolffish from the spring groundfish survey.
| Year | BMT, samples | BMT, lengths | BMT, otoliths | LLN, samples | LLN, lengths | LLN, otoliths |
|---|---|---|---|---|---|---|
| 2007 | 7 | 667 | 230 | 7 | 983 | 334 |
| 2008 | 9 | 838 | 352 | 9 | 880 | 391 |
| 2009 | 7 | 861 | 350 | 6 | 615 | 200 |
| 2010 | 10 | 964 | 355 | 7 | 794 | 295 |
| 2011 | 7 | 527 | 246 | 8 | 766 | 329 |
| 2012 | 11 | 1092 | 451 | 9 | 1097 | 432 |
| 2013 | 5 | 547 | 200 | 17 | 2175 | 788 |
| 2014 | 32 | 2982 | 800 | 48 | 4916 | 1101 |
| 2015 | 22 | 2113 | 500 | 21 | 2073 | 475 |
| 2016 | 12 | 1266 | 300 | 15 | 1825 | 350 |
| 2017 | 9 | 954 | 225 | 8 | 1000 | 200 |
| 2018 | 9 | 964 | 225 | 8 | 869 | 200 |
| 2019 | 12 | 1017 | 299 | 19 | 1389 | 300 |
| 2020 | 13 | 947 | 300 | 4 | 375 | 100 |
| 2021 | 11 | 777 | 270 | 18 | 1061 | 336 |
| 2022 | 10 | 684 | 203 | 3 | 167 | 60 |
| 2023 | 7 | 684 | 120 | 1 | 8 | 8 |
| 2024 | 12 | 951 | 240 | 2 | 156 | 40 |
Length distribution of landed spotted wolffish
Mean length of spotted wolffish sampled from commercial catches has generally been increasing from 2008 (72 cm) to 2019 (78 cm), since then it has decreased to 75–77 cm from 2020–2024 (Figure 9). Note that in 2022, some of the samples collected were from catch in Dohrnbank, an area that has not been sampled before. The spotted wolffish in these samples were unusually small, which partly explains the low mean length from samples in 2022.
Survey data
The Icelandic spring groundfish survey (hereafter 'spring survey', IS-SMB), which has been conducted annually in March since 1985, covers the most important areas in terms of the distribution of the spotted wolffish fishery. In addition, the Icelandic autumn groundfish survey (hereafter 'autumn survey', IS-SMH) was commenced in 1996 and expanded in 2000. However, a full autumn survey was not conducted in 2011 due to a labour dispute. For spotted wolffish, the spring survey is considered to measure changes in abundance/biomass better than the autumn survey, although from 1996-2003 the Iceland-Faroe ridge was not sampled in the spring survey.
Indices of total biomass and harvestable biomass have been decreasing since 1996 and were at a historic low in 2020. An upward revision was observed in 2021 and the indices have been at a similar level since then (Figure 10).
The recruitment index was high in the years 1992-2000, after which it decreased gradually to a historic low in 2012. Thereafter, the recruitment index from the spring survey has been low; a small increase was observed from 2021-2024, but the index decreased (by ~60%) to a historic low in 2025 (Figure 10).
Since 2012, spotted wolffish has mostly been caught in the slope areas northwest and north of Iceland in the spring survey (Figure 11 and Figure 12). Biomass indices from the NW area have been relatively stable throughout the survey period. Greater changes have taken place in the NE area, where biomass increased in 1985-1996 but has decreased significantly since then (Figure 12). In 1996-2003 the Iceland-Faroe ridge was not sampled in the spring survey, which seems to have resulted a 15-20% underestimation of the total biomass index for spotted wolffish.
Spotted wolffish spawn in late summer or autumn and the distribution of the catch is similar in the autumn and spring surveys, suggesting a spatial proximity between spawning and feeding grounds. (Figure 11 and Figure 12).
However, in 1996-2003, a lower proportion of the autumn survey biomass was measured in the NE area compared to the spring survey. In the 2024 autumn survey, the majority of spotted wolffish were caught at the slope areas northwest of Iceland; however, the biomass there has been decreasing since 2006. In accordance with the spring survey, the biomass index in the NE area has been decreasing since 1996, however, from 2022–2024, both the numbers and proportions have shown small increases over the respective levels from 2012–2022 (Figure 11 and Figure 12). The majority of the catches allocated to ‘Other’ in Figure 12 were taken just beyond the southern edge of the Northwest region.
The mean length in spring survey decreased from 1986 (62.9 cm) to 1995 (52.1 cm) due to increased abundance of fish smaller than 60 cm (Figure 13). Thereafter, the mean length increased to 69.4 cm in 2019, due to lower abundance of fish smaller than 60 cm. The mean length has decreased since. The number of spotted wolffish caught in the spring survey increased from 1273 fish in 1990 to 2744 fish in 1997. Since then, the number has been declining and reached the lowest level of 353 fish in 2020. Since then, the number of observed fish increased to between 450 and 550 individuals.
The mean length of spotted wolffish in the autumn survey has increased from 1996 (58.8 cm) to 2018 (70.8 cm). This is in accordance with the spring survey and the reason is decreased abundance of fish smaller than 60 cm (Figure 14). The average number of spotted wolffish caught in the autumn survey was 250 fish in the years 1996-2006. Since then, the number has been decreasing and was on average 90 fish in the years 2010-2017. Since 2018, the number of fish caught has ranged 53–80 with an average of 64.
In 2020, age determination for spotted wolffish from IS-SMB was initialised. Currently, individuals from 2014-2024 have been aged, a total of 2034 fishes. The age of spotted wolffish ranged from 1-16 years, with approximately 70% of the fish between 7 and 12 years (the weighted mean was 8.9 years old). The main purpose of the age determination is to provide data for stock assessment with the Gadget model.
Stock assessment
The application of rfb-rule
- r is calculated as the average of last two years values, divided by average of three preceding years values which results in r = 1.033 (Figure 15, Table 3).
Figure 15. Spotted wolffish. Total biomass index. The red lines show the average of last two years values and the three preceding years used to calculate r. The dashed horizontal line shows Itrigger. The black point shows Iloss.
- f is the length-based fishing pressure proxy. The mean length from catches in 2024 that are above Lc was 80 cm and the target reference length was 78 cm (Figure 16 and Figure 17). This resulted in a value of f\(^{-1}\) = 1.029, and f = 0.972
b is the biomass safeguard and is used to reduce catch advice when index falls below the threshold Itrigger. The lowest observed index Iloss for spotted wolffish is 2402 and was recorded in the year 2020, therefore, Itrigger = 1.4 * 2402 (Figure 15). The biomass index in 2025 was 3575, which is above Itrigger leading to b = 1.
m is the tuning parameter and for slow growing species (with von Bertanlaffy K < 0.2), m equals to 0.95.
Management
The Ministry of Food, Agriculture and Fisheries is responsible for the management of Icelandic fisheries and the implementation of legislation. Spotted wolffish was included in the ITQ system in 2018. The MFRI advised catch based on Fproxy for the first time in the fishing year 2012/2013. For the first four fishing years, the advice was based on the average catch in the years 1985-1997 which was approximately 900 tonnes, and the stock size was rather stable during these years. When advising catch for the fishing year 2016/2017, it was decided to use 70% of the average Fproxy from the years 2001-2015 as the target Fproxy, but the biomass index from the spring survey decreased about 20% at this time. For the fishing year 2019/2020 it was decided, due to low spawning stock biomass and poor recruitment, to set target Fproxy on a more precautionary basis than the years before. The mean Fproxy for the years 1985-1998 was chosen, because fishing pressure during this period did not have any observed detrimental effects on the stock biomass (Figure 15). Catches of spotted wolffish in the fishing years 2012/2013-2015/2016 were around 100% higher than recommended by the MRI and around 40% higher in each of the next three fishing years and in last fishing year it was about 150% higher (Table 4).
The spotted wolffish stock is now at a historical low level. In 2020, stock indices were lower than any points observed in the IS-SMB time series. The size of the spawning stock is likely to be below any candidate value of Blim. Management of spotted wolffish fishing effort is difficult as it is mainly taken as bycatch. Therefore, the advised catch levels are expected to decline in coming years or potentially an advised landing ban of spotted wolffish. Another possibility is that fishermen release spotted wolffish beyond quota. In June 2020, the MFRI proposed that fishermen were granted license to release spotted wolffish. This licence was granted by the Ministry of Industries and Innovation on 14 December 2020.
To reverse the downward trend of the stock, the MFRI recommended fishermen to release spotted wolffish that were caught beyond quota. Research has shown that the survival rate is high for post-catch released spotted wolffish. The reduction in catches in 2022 suggests that fishermen were releasing spotted wolffish; however, there are limited registrations of releases in the logbooks. A potential explanation for this is that only the amount could be recorded, but this limitation has been fixed. Registering the catch-release is important because the fraction of released fish that survive (research suggests 90%) can be estimated from these records.
Fishing year | Recommended TAC | National TAC | Total catch |
|---|---|---|---|
2012/2013 | 900 | 2 042 | |
2013/2014 | 900 | 2 250 | |
2014/2015 | 900 | 1 655 | |
2015/2016 | 900 | 1 913 | |
2016/2017 | 1 128 | 1 587 | |
2017/2018 | 1 080 | 1 553 | |
2018/2019 | 1 001 | 1 001 | 1 425 |
2019/2020 | 375 | 375 | 1 310 |
2020/2021 | 314 | 314 | 1 295 |
2021/2022 | 377 | 377 | 904 |
2022/2023 | 334 | 334 | 661 |
2023/2024 | 296 | 296 | 745 |
2024/2025 | 296 | 296 | |
2025/2026 | 299 | ||
2026/2027 | 299 |
Survival of released spotted wolffish after catch
Fish farming of spotted wolffish was carried out in Iceland from 2001-2006. Adult fish used to produce larvae were collected in conventional bottom trawl fishing and kept onboard in a container with flowing seawater. Observed mortality was negligible both on the fishing vessel and at the farming station where the fish were moved after landing. Observed mortality was also negligible for spotted wolffish sampled by handline for the fish farming.
From 2015-2017, a total of 102 spotted wolffish were tagged with conventional tags in IS-SMB and IS-SMH, and 43 individuals were tagged with DST tags. Recaptures rates were good, indicating that spotted wolffish can tolerate release after catch in bottom trawl. In the tagging experiments, it was a surprise that some of the spotted wolffish survived up to one hour on the fishing ramp. Most of these fish appeared lifeless when put into a container with flowing seawater, but after 5-10 minutes they began to swim actively in the container. In 2021 and 2022, 150 and 240 spotted wolffish were tagged with T-bar anchor tags in IS-SMB respectively. This was carried out to investigate the survival of released fish post-capture in bottom trawl.
Grant and Hiscock (2014) showed that 90% of Atlantic wolffish survived when released two hours after catch in bottom trawl, i.e., the fish could survive two hours on the fishing ramp or the conveyor belt after catch. The authors of this research assumed a similar conclusion could be applied to the related spotted wolffish. In 2020, MFRI carried out preliminary research on how long spotted wolffish can survive on the fishing ramp and conveyor belt in IS-SMH. The results indicate that spotted wolffish can survive up to two hours after catch on the fishing ramp and conveyor belt. Research have shown that spotted wolffish and Atlantic wolffish can tolerate sea with low oxygen content (Foss, Evensen, and Øiestad (2002)), which supports the result of the two studies.
In 2020, when MFRI recommended that fishermen can release spotted wolffish beyond quota, it was known that survival was high when released after capture in bottom trawl. However, no studies or observation had been carried out on the survival of released spotted wolffish after longline catch. Therefore, the MFRI began to investigate this in autumn 2020. Preliminary results suggest that survival of spotted wolffish is high after being released after capture in longline, it is scheduled to continue this research this year.
Comments on the assessment and Advice
Following ICES guidelines for data-limited stocks (ICES (2025)), the ICES rfb-rule was utilized for the assessment of spotted wolffish in 2025. The method was first applied in 2023 and incorporates life-history traits, exploitation characteristics, and other relevant parameters for data-limited stocks into an empirical assessment rule (ICES (2025)). The rfb-rule has the following form:
\[A_{y + 1} = A_{y}rf^{-1}bm\]
where Ay+1 is the advised catch, Ay is the current advice (issued in the previous year), The parameter r corresponds to the rate of change in the biomass index (the ICES “2 over 3” rule), f is a length-based proxy for the exploitation level (an MSY reference length divided by the mean catch length), and b is a biomass safeguard (reducing the catch when the biomass index drops below a trigger value).
The parameter r is calculated by dividing the mean of the last two survey index (I) values by the mean of the three preceding survey index values:
\[r = \frac{\sum_{i = y - 2}^{y - 1}\left( I_{i}\text{/}2 \right)}{\sum_{i = y - 5}^{y - 3}\left( I_{i}\text{/}3 \right)}\]
The length-based fishing pressure proxy f is calculated as follows:
\[f = \frac{L_{F = M}}{{\overline{L}}_{y - 1}}\]
where \({\overline{L}}_{y - 1}\) is the mean length in the observed catch that is above the length at first capture (Lc). The length at first capture is defined as the length at 50% of modal abundance (Figure 16). The target reference length (\(L_{F = M}\)) is the expected mean length when fishing at MSY and is calculated via:
\[L_{F = M} = 0.75L_{c} + 0.25L_{\infty}\]
where Lc is the length at first capture (see above) and L∞ is von Bertalanffy growth parameter. This assumes that the ratio \(M/K\) is equal to 1.5.
The biomass safeguard b is used to reduce catch advice when the index falls below a threshold denoted by Itrigger:
\[b = \min\left\{ 1,\frac{I_{y}}{I_{trigger}} \right\}\,\]
where Itrigger = 1.4Iloss, and Iloss is the lowest observed index value (see Figure 15). Note that b cannot exceed a value of 1.
The multiplier m is based on stock growth. The von Bertalanffy growth parameter K for spotted wolffish is less than 0.2, therefore m = 0.95 (ICES (2025)).
Analysis on the assessment and advice
The assessment is based on the rfb-rule for ICES category 3 data-limited stocks. The Icelandic spring trawl survey (IS-SMB) was used as the index for the stock development. The advice follows the equation outlined above (Ay+1 = Ay rf\(^{-1}\)bm), and with parameter values inserted, Ay+1 = 296 * 1.033 * 1.029 * 1 * 0.95. The advice for the fishing years 2025/2026 and 2026/2027 is 299 t, which is a 1% addition from the previous year (296 t). The main difference from the previous assessment is that the small positive trend in the biomass index means that most recent index value is now above the reference point Itrigger so the biomass safeguard has no effect on the advice.
Ay: Catch advice for 2024/2025
296
Stock biomass trend
Index A (2024-2025)
3 426
Index B (2021-2023)
3 315
r: Stock biomass trend (index ratio A/B)
1.033
Fishing pressure proxy
Mean catch length (Lmean = L2024)
80
MSY proxy length (LF=M)
78
f: Fishing pressure proxy (LF=M/Lmean)
0.972
Biomass safeguard
Last index value (I2025)
3 575
Index trigger value (Itrigger=Iloss*1.4)
3 363
b: Index relative to trigger value, min{I2025/Itrigger, 1}
1
Precautionary multiplier to maintain biomass above Blim with 95% probability
m: Multiplier (generic multiplier based on life history)
0.95
Advice calculation1)
299
Stability clause (+20% / -30% compared to Ay, only applied if b>1)2)
0
Catch advice for 2025/2026 and 2026/2027
299
% advice change3)
1
1) Ay × r × 1/f × b × m
2) min{max(0.7Ay, Ay+1), 1.2Ay}
3) 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.