| Year |
Bottom Trawl
|
Demersal Seine
|
||
|---|---|---|---|---|
| Num. samples | Num. lengths | Num. samples | Num. lengths | |
| 2000 | 2 | 112 | 30 | 3 322 |
| 2001 | 0 | 0 | 50 | 8 061 |
| 2002 | 1 | 1 008 | 38 | 5 123 |
| 2003 | 0 | 0 | 46 | 7 075 |
| 2004 | 0 | 0 | 56 | 9 145 |
| 2005 | 1 | 152 | 52 | 7 799 |
| 2006 | 3 | 132 | 48 | 6 904 |
| 2007 | 0 | 0 | 45 | 6 591 |
| 2008 | 0 | 0 | 43 | 5 826 |
| 2009 | 0 | 0 | 29 | 4 810 |
| 2010 | 0 | 0 | 23 | 3 286 |
| 2011 | 0 | 0 | 25 | 4 010 |
| 2012 | 0 | 0 | 25 | 3 046 |
| 2013 | 0 | 0 | 16 | 2 228 |
| 2014 | 0 | 0 | 23 | 3 286 |
| 2015 | 0 | 0 | 22 | 2 901 |
| 2016 | 0 | 0 | 14 | 2 033 |
| 2017 | 0 | 0 | 13 | 1 217 |
| 2018 | 0 | 0 | 9 | 1 258 |
| 2019 | 0 | 0 | 9 | 889 |
| 2020 | 2 | 62 | 3 | 445 |
| 2021 | 0 | 0 | 8 | 762 |
| 2022 | 2 | 358 | 20 | 2 575 |
| 2023 | 1 | 16 | 15 | 1 758 |
| 2024 | 3 | 491 | 24 | 3 011 |
Key signals
Survey biomass were high until 2004, then declined rapidly and have remained low since.
Recruitment has been poor since 2015.
In both surveys and catches, the peak of the length distribution has gradually shifted to the right — toward larger fish - reflecting a prolonged period of poor recruitment.
Fishing mortality (F) was above FMSY until 2009, and has remained near the threshold since.
The dab stock in 5a is in severe decline — or has already collapsed.
General information
Dab is commonly found in shallow waters all around Iceland, with the highest abundance off the southwest coast. This demersal species inhabits sandy or muddy substrates and occurs at depths ranging from the intertidal zone to 150 meters, though it is most frequently found between 20 and 40 meters.
Females grow larger than males; only a small proportion of males exceed 30 cm in length, while a similar proportion of females grow beyond 35 cm. Size at sexual maturity also differs markedly between the sexes: approximately 50% of males are sexually mature at a length of 12 cm, whereas females typically reach the same maturity level at around 22 cm.
The fishery
Dab fishing grounds in 2005- 2024, as reported by mandatory logbooks, are shown on Figure 1 and Figure 2. Main fishing grounds for dab are in the west and southwest of Iceland, with smaller fishing grounds in the southeast and several fjords in the north (Figure 1 and Figure 2). Before 2005, around 20-30% of the catch came from the southeast area, compared to 0-10% after 2005, suggesting a shift in the fishing distribution or distribution of the stock.
Dab is caught in relatively shallow water, with most of the catch (60-80%) taken between 21-80 m depth (Figure 3).
Dab is mainly caught in demersal seine or around 95% of the total catch. This proportion has been stable through the years, however for last four years proportion of landed catch by bottom trawlers increased slightly (Figure 4). Since 2000, the number of seiners and trawlers reporting annual catches of dab in total have decreased (Table 1; Figure 5).
| Year | Nr. Demersal Seine | Nr. Bottom Trawl | Nr. Other | Demersal Seine | Bottom Trawl | Other | Total Catch |
|---|---|---|---|---|---|---|---|
| 2000 | 88 | 48 | 173 | 2 939 | 59 | 26 | 3 024 |
| 2001 | 83 | 49 | 179 | 4 327 | 32 | 22 | 4 381 |
| 2002 | 86 | 44 | 132 | 4 329 | 28 | 10 | 4 366 |
| 2003 | 88 | 34 | 135 | 4 162 | 22 | 29 | 4 213 |
| 2004 | 86 | 36 | 139 | 2 896 | 33 | 24 | 2 953 |
| 2005 | 72 | 26 | 114 | 2 079 | 20 | 17 | 2 116 |
| 2006 | 63 | 29 | 98 | 1 055 | 15 | 10 | 1 081 |
| 2007 | 57 | 27 | 111 | 780 | 22 | 12 | 814 |
| 2008 | 54 | 41 | 95 | 762 | 24 | 11 | 798 |
| 2009 | 58 | 37 | 99 | 846 | 28 | 14 | 888 |
| 2010 | 47 | 26 | 81 | 578 | 25 | 9 | 612 |
| 2011 | 49 | 27 | 85 | 866 | 27 | 9 | 903 |
| 2012 | 44 | 25 | 82 | 841 | 10 | 9 | 860 |
| 2013 | 42 | 19 | 70 | 705 | 11 | 7 | 723 |
| 2014 | 32 | 16 | 58 | 491 | 7 | 8 | 505 |
| 2015 | 30 | 16 | 65 | 472 | 18 | 10 | 500 |
| 2016 | 27 | 15 | 42 | 333 | 3 | 5 | 341 |
| 2017 | 28 | 13 | 36 | 226 | 2 | 3 | 231 |
| 2018 | 34 | 12 | 30 | 428 | 10 | 7 | 445 |
| 2019 | 36 | 17 | 36 | 487 | 8 | 7 | 502 |
| 2020 | 35 | 13 | 34 | 388 | 15 | 10 | 412 |
| 2021 | 33 | 21 | 53 | 555 | 63 | 11 | 629 |
| 2022 | 37 | 28 | 59 | 667 | 73 | 19 | 759 |
| 2023 | 34 | 30 | 79 | 536 | 127 | 20 | 683 |
| 2024 | 35 | 25 | 58 | 669 | 114 | 23 | 806 |
Catch per unit of effort (CPUE)
CPUE estimates of dab in Icelandic waters are not considered representative of stock abundance as changes in fleet composition, technical improvements, and differences in gear setup among other things have not been accounted for when estimating CPUE.
Non-standardised estimate of CPUE in demersal seine (kg/set) is calculated as the average weight of dab in sets in which dab was more than 10% of the catch. CPUE decreased rapidly from more than 300 kg per set in 2002 to less than 120 kg per set in 2007. Since 2011 CPUE has fluctuated around 200 kg per set (Figure 6). CPUE of demersal trawl (kg/hour), in hauls where dab is more than 10% of the catch, increased in years 2011-2012, peaked at around 225 kg/hour in 2012 and deceased to 75-100 kg/hour in 2018 and has been at that level since (Figure 6).
Data available
Sampling of biological data from main gear (demersal seine) in commercial catches is considered acceptable. The sampling does cover the spatial and temporal distribution of catches, from main gear (Figure 7 og Figure 8). The sampling coverage by gear in 2024 is shown in Figure 8 and overview of the number of samples is shown in Figure 7. Note that in Figure 8, one tow from which samples were taken, was incorrectly recorded in the logbooks and appears on land.
Length composition
For the years 1993-2001, the average length of dab in samples from landed catch was between 28.5-29.7 cm, lowest in 2001. In the following years the average length has been around 30-33 cm (Figure 9), with clear shift towards larger fish in last five years.
Age composition
Annually 200-500 otoliths are collected from demersal seine catches of dab (Table 3). The commercial catch consists mainly of 5-9 year old dab, and fish older than 9 years old are rarely seen in the fishery (Figure 10 and Figure 11).
| Year | Sampling_DSE | n_DSE |
|---|---|---|
| 2000 | 21 | 525 |
| 2001 | 23 | 600 |
| 2002 | 23 | 725 |
| 2003 | 22 | 1 040 |
| 2004 | 16 | 800 |
| 2005 | 10 | 450 |
| 2006 | 6 | 275 |
| 2007 | 15 | 725 |
| 2008 | 14 | 700 |
| 2009 | 11 | 500 |
| 2010 | 7 | 350 |
| 2011 | 10 | 500 |
| 2012 | 10 | 500 |
| 2013 | 7 | 350 |
| 2014 | 13 | 500 |
| 2015 | 15 | 525 |
| 2016 | 9 | 350 |
| 2017 | 4 | 100 |
| 2018 | 6 | 200 |
| 2019 | 3 | 75 |
| 2020 | 3 | 150 |
| 2021 | 5 | 175 |
| 2022 | 18 | 360 |
| 2023 | 15 | 300 |
| 2024 | 23 | 460 |
Catch curves and estimated year-class mortality
Catch curves based on catch data are shown in Figure 12. Analysis of catch curves for year classes 1984–2019 indicates that the total mortality rate (Z) in the stock is around 0.85. They show that most fish enter the fishery by age six and then disappear quickly from the catch. This steep drop with age means that total mortality, mainly due to fishing, is very high i.e. often around Z = 0.85 or more. The grey lines show how catch would decline with different mortality levels. When the black lines match the steepest grey lines, it confirms that fishing pressure on the stock is strong.
Figure 13 shows estimated total mortality (Z) for each year class (4+ age) of dab in 5a, based on catch-at-age data. The estimates are mostly above 1 from the late 1980s through the early 2000s, indicating very high mortality, likely driven largely by fishing. Since around 2010, the mortality estimates have declined, but values still suggest moderate to high fishing pressure. These patterns match the catch curves shown earlier, where the steep decline in catch with age also pointed to strong fishing impacts.
Survey data
The Icelandic spring groundfish survey (hereafter spring survey, SMB), which has been conducted annually in March 1985-2023, covers the most important distribution area of the dab fishery. In addition, the Icelandic autumn groundfish survey (hereafter autumn survey, SMH) 1996-2022. The autumn survey was not conducted in 2011. The spring survey is considered to measure changes in abundance/biomass better than the autumn survey. It does not, however, adequately cover the main recruitment grounds for dab as recruitment takes place in shallow water which is not covered by the bottom trawl surveys. In addition to the spring and autumn surveys a designated flatfish survey with beam trawl started in 2016 and expanded in 2017-2019 to cover of the recruitment grounds of dab and other flatfish species (Thorlacius et al. 2024). This survey was discontinued in 2023.
Figure 14 shows trends in various biomass indices and a recruitment index based on abundance of dab smaller than 20 cm. In the spring survey, total biomass index and the biomass index for dab larger than 25 cm (harvestable part of the stock) have been at lowest level in the time series since 2006, following high indices in 2001-2003 (Figure 14). In the autumn survey the biomass indices have been low since 2003 (Figure 14). There is some consistency between the spring and autumn surveys in recent years regarding recruitment indices, as the last small peak was registered in 2013 in both surveys. The SMB recruitment index was however, at a historic low in 2020 and 2022. All indices are at very low level with no recruitment visible.
Dab was most abundant in the west in the spring survey in 2025 (Figure 15). In 1986-2004 a considerable part of the biomass was measured in the southeast. After 2004 this changed, and very little has been observed in this area ever since, suggesting a possible change in the spatial distribution of dab around the country (Figure 16). Biomass in the west area has increased over the same time period. In 2009, several shallow stations were added to the SMB in the northwest region, where increased dab catches were observed. These stations have contributed to an increase in the index for this area. Dab was mainly observed in the southeast, west and northwest of Iceland in the 2024 autumn survey (Figure 15). Abundance is patchy, and most of the observed dab came from a few large tows. Comparable changes in spatial distribution of dab are observed in the autumn and spring surveys (Figure 16).
The average length of dab in the first two years in the spring survey was 28.2 cm (Figure 17). From 1987 to 2002 the average length declined to around 23 cm and stayed at that level for almost a decade. In the years 2004–2006, the average length dropped to approximately 21 cm, which corresponds with an increase in recruitment indices during this period. Since 2013 the average length has gradually increased to 25 cm and has remained at that length since. Data from the autumn survey tells a similar story, with a marked increase in average size of dab in most recent years (Figure 17).
Stock assessment
Management
The Ministry of Industries is responsible for management of the Icelandic fisheries and implementation of legislation. Scientific advice on dab fisheries is provided by the Marine and Freshwater Research Institute. Dab was included in the ITQ system in the 1997/1998 quota year and as such subjected to TAC limitations (Figure 22). For most of the fishing years up to 2004/2005, the TAC was set according to recommendations, but for the fishing years 2005/2006 to 2012/2013, TAC was significantly higher than recommendations. The quota area for dab has been specified as the area from Snæfellsnes south and east to Stokksnes. Since 2016, the MFRI has recommended that the defined quota area should be abolished, and all dab fishing grounds be under TAC limits. This came into effect in the year 2022.
Figure 23 shows the net transfers for dab in the Icelandic ITQ-system. The net transfer has nearly always been from dab to other species, the amount ranging from 5 to almost 60% of the allocated quota of the respected quota year. In the last four fishing years however for the fist time, around 2-150% was transferred to dab from other species. Transfer of dab quota from one quota year to the next is usually in the range of 7-15%. However, in 2014/15-2019/20, it was much higher with 2014/15 and 2016/17 quota years especially prominent. In 2023/24, approximately 25% of the quota was transferred between fishing years (Figure 23).
Management considerations
The indices for dab have been low for an extended period (Figure 14), and this year the stock biomass index is at the management threshold (fig-rfb_index). Recruitment has been poor since 2015, and there is considerable uncertainty about the strength of incoming year classes to the fishable stock. There are also indications that significant overfishing has occurred in the dab stock (Figure 12 and Figure 13). These factors provide strong indications that the stock is in decline — or may already have collapsed.
References
ICES. 2025. ICES Guidelines - Advice rules for stocks in category 2 and 3. Version 3. ICES Guidelines and Policies - Advice Technical Guidelines. 31 pp. https://doi.org/10.17895/ices.pub.28506179
Thorlacius, Magnús, Valur Bogason, Jónas Páll Jónasson, Bylgja Sif Jónsdóttir, Elzbieta Baranowska, and Guðjón Már Sigurðsson. 2024. “Grunnslóðaleiðangur 2016-2022.” Marine & Freshwater Research Institute, Iceland; Marine & Freshwater Research Institute, Iceland. https://www.hafogvatn.is/static/research/files/hv2024_04_l01022024.pdf
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 dab 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 19). 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 18). Note that b cannot exceed a value of 1.
The multiplier m is based on stock growth. The von Bertalanffy growth parameter K for dab is 0.3, therefore m = 0.9 (ICES 2025).
The application of rfb-rule
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 dab is 1100 and was recorded in the year 2015, therefore, Itrigger = 1.4 * 1100 (Figure 18). The biomass index in 2025 was 1595, which is just at Itrigger leading to b = 1.
m is the tuning parameter and for slow growing species (with von Bertanlaffy 0.2 < K < 0.32), m equals to 0.9.