Dab

Limanda limanda


Technical report
Published by

Marine and Freshwater Research Institute, Iceland

Published

6 June 2025

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).

Figure 1: Dab in 5a. Spatial distribution of catches by all gears according to Icelandic logbooks.
Figure 2: Dab in 5a. Changes in spatial distribution of dab catches as recorded in Icelandic logbooks.
Figure 3: Dab in 5a. Depth distribution of dab from demersal trawl and seine according to Icelandic logbooks.
Figure 4: Dab in 5a. Landings in tonnes and percent of total by gear and year.
Table 1: Dab in 5a. Number of Icelandic vessels landing dab, and all landed catch divided by gear type.
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
Figure 5: Dab in 5a. Number of vessels (all gear types) accounting for 95% of the total catch annually since 1994. Left: Plotted against year from 1994. Right: Plotted against total catch. Data from the Directorate of Fisheries.

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).

Figure 6: Dab in 5a. Catch per unit of effort in the most important gear types. The dashed lines are based on locations where more than 10% of the catch is dab and solid lines on all records where dab is caught.

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.

Figure 7: Dab in 5a. Ratio of samples by month (bars) compared with landings by month (solid black line) split by year and main gear types. Numbers above the bars indicate number of the samples by year, month and gear.
Figure 8: Dab in 5a. Fishing grounds in 2024 as reported in Icelandic logbooks (colours) and positions of samples taken from landings (asterisks) by main gear types.

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.

Table 2: Dab in 5a. Number of samples and length measurements from landed catch.
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
Figure 9: Dab in 5a. Commercial length distributions by gear and year.

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).

Table 3: Dab in 5a. Number of samples and otoliths collected from landed catch.
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
Figure 10: Dab in 5a. Estimated age distribution of landed catch based on landings and otoliths collected from landed catch.
Figure 11: Dab in 5a. Catch at age from the commercial fishery in Icelandic waters. Biomass caught by year and age; bars are coloured by cohort.

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.

Figure 12: Dab in 5a. Catch curves (log2 of catches) from catch data by year classes 1989–2018. Grey lines correspond to Z = 0.85.
Figure 13: Dab in 5a. Estimated mortality of cohors based on aged catch data.

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).

Figure 14: Dab in 5a. Indices in the spring survey (March) 1985 and onwards (line shaded area) and the autumn survey (October) (point ranges). On left y-axis is shown scale for the spring survey and on the right y-axis for the autumn survey.
Figure 15: Dab in 5a. Location of dab in the most recent spring (SMB) and the autumn (SMH) surveys, bubble sizes are relative to catch sizes.
Figure 16: Dab n 5a. Changes in geographical distribution of the survey biomass.