Plaice

Pleuronectes platessa


Technical report
Published by

Marine and Freshwater Research Institute, Iceland

Published

6 June 2025

Key signals

  • Biomass indices from stock surveys were very high before 1990, then declined rapidly until 2000, but have increased since slowly and reached have reached a equilibrium. The biomass of large plaice (≥47 cm) peaked in 2024.

  • The recruitment index (<20 cm) has remained low but stable since 1998, with a few peaks. The SMB does not adequately cover the nursery grounds of plaice.

  • In both survey and catch data, the peak of the length distribution has gradually shifted to the right toward larger fish, possibly both reflecting reduced fishing pressure on smaller individuals (catch) and period of change in the recruitment.

  • Data from surveys and fisheries indicate that plaice in 5a is currently in good condition. This is also confirmed by the stock assessment.

  • There is considerable uncertainty in the assessment model due to limited information on recruitment in the surveys.

General information

Plaice (Pleuronectes platessa) is found on the continental shelf surrounding Iceland, with highest abundance in the southwest and west. It primarily inhabits sandy or muddy substrates at depths ranging from the coastline down to 200 meters, and occasionally deeper (Jónsson and Pálsson 2013).

Plaice exhibit sexual dimorphism: females grow larger than males and reach maturity at a larger size. Few males exceed 45 cm in length, whereas a similar proportion of females exceed 55 cm. Size at sexual maturity also differs by sex — about 50% of males are mature at 33 cm, while females reach that level at around 38 cm. Spawning occurs mainly at depths of 50–100 meters in the relatively warm waters south and west of Iceland, with limited spawning activity off the northwest and north coasts [Sigurðsson 1989; Solmundsson, Palsson, and Karlsson (2005)].

After metamorphosis, 0-group juveniles settle in shallow coastal waters, typically just below the tidemark during their first summer (Hjorleifsson and Palsson 2001; Gunnarsson, Jonasson, and McAdam 2010).

Genetic studies (Le Moan, Bekkevold, and Hemmer-Hansen 2021; Hoarau et al. 2004) indicate that plaice on the Icelandic and Faroese shelves are genetically distinct from those elsewhere. Tagging experiments have shown that plaice remain on the Icelandic continental shelf, although they move between regions (Solmundsson, Palsson, and Karlsson 2005). Combined with observed seasonal shifts in distribution between spring and autumn surveys, these findings suggest little variation in population structure within Icelandic waters.

Fishery

Landings trends

Plaice landings in Division 5.a peaked historically at around 14.5 thousand tonnes in 1985. Since then, landings have declined and stabilized over the past two decades, typically ranging between 5 and 8 thousand tonnes annually (Figure 1). In 2024, landings were estimated at approximately 7566 thousand tonnes (Figure 1 and Table 1).

Before the expansion of the Icelandic Exclusive Economic Zone (EEZ) to 200 nautical miles in 1977, foreign vessels accounted for a substantial share of plaice landings (Figure 1). They dominated the fishery prior to World War II, but during the war, the Icelandic fleet increased its catches. From around 1960 onward, most plaice landings have been by Icelandic vessels.

The main fishing grounds for plaice are located on the southwestern, western, and northwestern part of the Icelandic shelf, with smaller fishing areas in the southeast and several fjords in the north (Figure 2; Figure 3). Since 2000, the spatial distribution of the plaice fishery has remained relatively stable, with around 60% of the catch taken from the western and northwestern shelf (Figure 3).

Plaice is typically caught in relatively shallow waters, with 60–80% of the catch taken at depths between 21 and 80 meters (Figure 4).

Demersal seine is the primary gear used (Figure 5), accounting for 59–71% of the catch since 2011, followed by demersal trawl (23–37%). A small share is also caught with gillnets and longlines (Table 1). Seiners dominate the coastal fishery, while trawlers operate in deeper and more offshore areas.

Since 2000, the number of vessels reporting plaice catches annually has declined, while total catches have increased in recent years (Figure 6 and Table 1). The most notable decrease has occurred in the demersal seine fleet, which fell from 125 vessels in 2000 to 36 in 2024. The number of trawlers has also gradually declined over the same period, reaching around 60 vessels in 2024.

Figure 1: Plaice in 5a. Recorded landings since 1903.
Figure 2: Plaice in 5a. Spatial distribution of catches by all gears according to Icelandic logbooks.
Figure 3: Plaice in 5a. Changes in spatial distribution of plaice catches as recorded in Icelandic logbooks.
Figure 4: Plaice in 5a. Depth distribution of plaice from demersal trawl and seine according to Icelandic logbooks.
Figure 5: Plaice in 5a. Landings in tonnes and percent of total by gear and year. Data from the Directorate of Fisheries
Figure 6: Plaice in 5a. Number of vessels (all gear types) accounting for 95% of the total catch annually since 1994. Left: Plotted against year. Right: Plotted against total catch. Data from the Directorate of Fisheries.
Table 1: Plaice in 5a. Number of Icelandic vessels landing plaice, and all landed catch divided by gear type.
Year Nr. demersal seiners Nr. bottom trawlers Nr. Other Demersal seine Bottom trawl Other Total catch
2000 125 134 450 3 070 1 747 442 5 259
2001 95 127 521 2 924 1 402 600 4 926
2002 96 118 420 3 426 1 270 446 5 142
2003 95 115 389 3 590 1 295 372 5 257
2004 95 108 399 4 037 1 375 294 5 706
2005 88 105 352 3 909 1 638 255 5 802
2006 87 99 365 3 720 2 449 212 6 381
2007 82 98 355 3 311 2 232 267 5 810
2008 80 93 322 3 836 2 605 285 6 726
2009 70 85 315 3 889 2 125 310 6 324
2010 61 85 328 3 647 2 038 299 5 984
2011 55 80 357 3 021 1 655 267 4 943
2012 54 88 374 4 079 1 410 437 5 926
2013 56 87 317 4 040 1 583 364 5 987
2014 45 74 317 4 239 1 380 308 5 927
2015 50 74 319 4 403 2 001 350 6 754
2016 44 73 280 4 896 2 120 430 7 446
2017 48 71 283 4 579 1 765 351 6 695
2018 47 66 257 5 584 2 436 321 8 341
2019 44 63 276 4 287 2 231 316 6 834
2020 41 65 213 4 682 2 474 350 7 506
2021 37 63 238 4 719 3 604 355 8 678
2022 40 62 210 4 307 2 743 227 7 277
2023 41 56 207 3 955 2 504 237 6 696
2024 36 57 197 4 566 2 674 327 7 567

Data available

Biological sampling from the main gears (demersal seine and bottom trawl) in commercial plaice catches is generally considered good. It provides satisfactory spatial and temporal coverage of the fishery. An overview of the number of samples month is provided in Figure 7 and sampling coverage by gear in 2024 is shown in Figure 8.

Figure 7: Plaice 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: Plaice in 5a. Fishing grounds in 2024 as reported in Icelandic logbooks (colours) and positions of samples taken from landings (asterisks) by main gear types.

Landings and discards

All landings in Division 5.a before 1982 are derived from the ICES STATLANT database. This also applies to foreign landings in 5.a up to 2005. Between 1982 and 1993, landings by Icelandic vessels were collected by the Fisheries Association of Iceland. From 1994 onward, landings by Icelandic vessels are provided by the Icelandic Directorate of Fisheries.

Foreign landings (primarily by Norwegian and Faroese vessels) were recorded by the Icelandic Coast Guard until 2014; after that, they have also been recorded by the Directorate of Fisheries.

Discarding is banned by law in the Icelandic fishery. Discard rates for plaice have been estimated as negligible since at least 2001 (MRI 2016). Various measures in the management system, such as converting quota shares between species, are widely used by the fleet and are believed to discourage discarding in mixed fisheries.

To further prevent high grading and quota mismatches, vessels are allowed to land catches that exceed their quota. These landings are not counted against the vessel’s allocated quota, provided the proceeds from the sale go to the Fisheries Project Fund (Verkefnasjóður sjávarútvegsins).

Length composition

An overview of available length measurements from Division 5.a is provided in Table 2. Most measurements come from the two main fleet segments — trawls and demersal seines (Figure 9).

The size distribution of plaice caught by these gears has remained relatively stable, with most fish ranging between 35 and 55 cm in length. However, there has been a gradual shift toward larger sizes in the catch, reflecting an increase in the effective minimum landing size in both fleets. As a result, the average length in commercial catch samples has increased from 35 cm in 1991 to 43.1 cm in 2016.

Table 2: Plaice in 5a. Number of samples and length measurements from landed catch.
Year
Bottom Trawl
Danish Seine
Num. samples Num. lengths Num. samples Num. lengths
2000 33 4 261 49 7 185
2001 9 1 003 51 7 517
2002 18 2 392 69 11 263
2003 21 3 278 96 13 804
2004 28 3 834 150 21 216
2005 35 5 251 139 20 583
2006 60 8 102 135 19 222
2007 49 6 837 124 17 073
2008 77 11 359 129 17 471
2009 50 7 201 136 19 106
2010 62 9 608 126 17 387
2011 55 7 609 110 16 857
2012 39 5 723 129 18 329
2013 31 4 688 115 16 647
2014 21 2 531 53 7 271
2015 33 4 142 44 5 997
2016 32 4 757 58 8 075
2017 28 3 527 52 6 231
2018 24 3 506 43 5 666
2019 36 4 838 47 5 990
2020 27 2 788 24 3 031
2021 53 6 922 42 5 067
2022 34 4 507 26 3 211
2023 41 4 474 31 3 486
2024 51 7 085 33 4 190
Figure 9: Plaice in 5a. Commercial length distributions by gear and year

Age composition

Table 3 provides an overview of otolith sampling intensity by gear type in Division 5.a.

Between 2002 and 2005, fish aged 4–7 years made up approximately 60% of the landed plaice catch (by number). Since then, the proportion of these age classes has declined, averaging around 40–45% in the past five years. In recent years, the catch has included a higher proportion of older fish, with individuals aged 6–11 years being especially prominent in the most recent samples (Figure 10; Figure 11).

Table 3: Plaice in 5a. Number of samples and otoliths collected from landed catch.
Year
Bottom Trawl
Danish Seine
Num. samples Num. otoliths Num. samples Num. otoliths
2000 32 1 507 48 2 400
2001 7 350 45 2 250
2002 12 599 49 2 424
2003 11 550 63 3 149
2004 17 820 74 3 701
2005 20 1 000 61 3 036
2006 29 1 450 64 3 200
2007 30 1 500 64 3 199
2008 37 1 850 62 3 099
2009 25 1 250 64 3 180
2010 41 2 016 78 3 901
2011 50 2 452 84 4 200
2012 37 1 835 104 5 199
2013 27 1 350 101 5 010
2014 20 575 36 900
2015 27 670 32 800
2016 23 573 45 1 125
2017 22 550 39 974
2018 16 400 35 880
2019 19 476 30 750
2020 22 550 22 550
2021 49 1 225 36 900
2022 27 560 23 470
2023 31 620 30 598
2024 37 740 29 579
Figure 10: Plaice in 5a. Catch at age from the commercial fishery in Iceland waters. Bar size is indicative of the catch in numbers and bars are colored by cohort. Note varying y-axis scales.
Figure 11: Plaice in 5a. Catch at age from the commercial fishery in Icelandic waters. Biomass caught by year and age; bars are coloured by cohort.

Weight at age

Mean weight at age in commercial catches is shown in Figure 12 and Figure 13. Since 1995, mean weight at age has increased across all age groups, and has remained above the long-term average over the past decade.

Figure 12: Plaice in 5a. Catch weights from the commercial fishery in Icelandic waters.
Figure 13: Plaice in 5a. Mean weight at age in the catch from the commercial fishery in Icelandic waters. Bars are coloured by cohort.

Catch per unit of effort (CPUE) and fishing effort

Catch per unit of effort (CPUE) estimates for plaice in Icelandic waters are not considered representative of stock abundance, as they are not standardized for changes in fleet composition or technological improvements.

Non-standardized CPUE for demersal seine (kg/set) is calculated from sets where plaice made up more than 10% of the catch. These estimates increased gradually from about 120 kg/set in the early 2000s to over 450 kg/set in 2016 (Figure 14), and have fluctuated around that level in recent years.

For demersal trawl (kg/hour), using hauls where plaice constituted more than 10% of the catch, CPUE increased rapidly between 2000 and 2015, peaking at 260 kg/hour. Since then, CPUE has declined and was approximately 150 kg/hour in the most recent year.

Figure 14: Plaice 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 plaice and solid lines on all records where plaice is caught. Note that a change took place in September 1999, when all vessels were required to submit a catch logbook. Before that time, vessels smaller than 10 gross tons were exempt from this requirement.

Survey data

Information on the abundance and biological characteristics of plaice in Division 5.a is available from two main surveys: the Icelandic spring groundfish survey and the Icelandic autumn groundfish survey.

The spring survey has been conducted annually in March since 1985 and covers the primary distribution area of the plaice fishery. The autumn survey began in 1996 but was not conducted in 2011. While the spring survey is considered more reliable for detecting changes in overall abundance and biomass, it does not adequately cover the main recruitment areas, which are located in shallow habitats unsuitable for demersal trawling.

To address this limitation, a dedicated flatfish survey using a beam trawl was carried out annually in July/August from 2016 to 2022, targeting recruitment grounds for plaice and other flatfish species (Thorlacius et al. 2024). This beam trawl survey was discontinued after 2022.

Figure 15 shows trends in various biomass indices and a recruitment index based on the abundance of plaice smaller than 20 cm.The total biomass index of plaice, as well as the biomass of fish larger than 30 cm (representing the harvestable portion of the stock), declined sharply during the early years of the spring survey and reached a minimum between 1997 and 2002. From 2003 to 2016, the indices increased gradually and then stabilized. Since 2017, annual fluctuations have been relatively minor. The most recent spring survey index corresponds to biomass levels observed in the early 1990s and remains at only one-third to half of the levels recorded during the first four years of the time series.

The index of plaice larger than 47 cm also declined to its lowest point in 1997–2002, but has since increased and, in recent years, reached levels comparable to the beginning of the time series, with a new peak in 2024.

The recruitment index (plaice <20 cm) has remained at a low level since 1998, with only occasional small peaks. However, as noted, neither the spring nor the autumn survey provides an accurate measure of recruitment.

Trends in the autumn survey mirror those of the spring survey, although with higher variability (i.e., larger standard deviations).

Plaice is mainly caught in the northwest and on the main spawning grounds off the west coast during the spring survey in 2025, and on the species’ primary feeding grounds in the northwest during the autumn survey in 2024 (Figure 16). The spatial distribution of plaice catch in surveys shows some temporal variability, particularly in the relative catches between the west and northwest regions (Figure 17). This may reflect annual variation in the timing of plaice migration to their traditional spawning grounds in the west, as the spring survey coincides with this migration period each year.

The length distribution of plaice in the spring survey (Figure 18) has shifted toward larger sizes, consistent with trends observed in the landed catch. The average length of plaice increased from 33.5 cm in 1995 to 42 cm in 2022. Data from the autumn survey (SMH) show a similar pattern, with a clear increase in average length over time.

Mean weight at age in the spring groundfish survey (SMB) is shown in Figure 20 and Figure 21. Over the past decade, weight-at-age estimates from SMB have been above average for nearly all year classes. These values are also used to estimate mean weight at age in the spawning stock, derived from length data. Stock weights for age 9 are smoothed using a 3-year running average, and for years prior to 1985, stock weights are assumed constant at 1985 levels.

Maturity-at-age data from SMB are presented in Figure 22 and Figure 23. Following ICES PGCCDBS (2017) guidelines, maturity at age is based on mature females. For years before 1985, the proportion mature is assumed fixed at 1985 levels. Maturity at age is estimated from annual maturity-at-length ogives using logistic regression, with individuals treated as fixed effects. These values are then smoothed using a 3-year running average. In the years 2007–2008, a shift occurred in maturity at age and plaice began to mature earlier than during the period 1985–2007.

Figure 15: Plaice in 5a. Indices in the spring Survey (March) 1985 and onwards (line shaded area) and the autumn survey (October) (point ranges).
Figure 16: Plaice in 5a. Location of plaice in the most recent spring (SMB) and the autumn (SMH) surveys, bubble sizes are relative to catch sizes.
Figure 17: Plaice in 5a. Changes in geographical distribution of the survey biomass.
Figure 18: Plaice in 5a. Length distributions from the spring and autumn surveys.
Figure 19: Plaice in 5a. Age disaggregated indices in the Spring Survey, colored by cohorts.
Figure 20: Plaice in 5a. Weight at age observed in the spring survey.
Figure 21: Plaice in 5a. Stock weights from the spring survey in Icelandic waters. Bars are coloured by cohort.
Figure 22: Plaice in 5a. Maturity at age observed in the spring survey.
Figure 23: Plaice in 5a. Maturity at age in the survey. Bars are coloured by cohort. The values are used to calculate the spawning stock.

Analytical assessment

Plaice in Division 5.a was benchmarked for the first time in April 2022 (WKICEMP 2022) and was assessed by ICES for the first time the same year.

The assessment is conducted using a state-space stock assessment model (SAM), which is a statistical catch-at-age model informed by:

  • Commercial catch-at-age data from 1979 onward (Figure 10)
  • Icelandic spring groundfish survey data from 1985 (Figure 19)
  • Recruitment estimated annually at age 3

Model configuration and settings are detailed in the Stock Annex (ICES 2022). The model uses age 12 as the maximum age group, treated as a plus group.

No direct information is available on natural mortality. For both assessment and advisory purposes, natural mortality is assumed to be 0.15 across all age groups.

Model fit

Model fits to the survey indices and catch-at-age data are shown in Figure 24. In general, the SAM model tracks both the catch-at-age data and the spring survey indices well, except for some deviations in the youngest age classes.

Residual diagnostics do not indicate any clear patterns or trends in either the observation or process residuals (Figure 25; Figure 26).

Figure 24: Plaice in 5a. Model fit (lines) to survey indices (dots) and catch at age (dots) data
Figure 25: Plaice in 5a. Model residuals from the assessment model. Red circles indicate where the model estimates are higher than the observed while blue indicate models estimates lower than observed.
Figure 26: Plaice in 5a. Process error residuals of the SAM model.

Model results

The results of this year’s assessment are shown in Figure 27. The model indicates a marked decline in recruitment at age 3 beginning in 1993, followed by an increase in fishing mortality and a decline in total catches. Spawning stock biomass (SSB) reached its lowest level around the turn of the century.

In recent years, recruitment has remained stable at the lower post-1993 level, while fishing mortality has declined and SSB has increased. Catches have remained relatively stable, with a slight downward trend.

Figure 27: Plaice in 5a. Estimates of spawning stock biomass, fishing mortality (weighted average of ages 5 to 10), recruitment and landings from the best model. Black line represents the point estimates and blue ribbon the 90\% confidence intervals.

Retrospective Analysis

The analytical retrospective pattern, based on a five-year peel, is shown in Figure 28. The retrospective plots do not indicate any substantial deviations in the assessment.

Mohn’s \(\rho\) values for spawning stock biomass, fishing mortality, and recruitment are low and fall well within the acceptable range recommended by Carvalho et al. (2021).

Figure 28: Plaice in 5a. Analytical retrospective estimates of SSB, catch, F and recruitment. Mohns rho is indicated in the top left corner.

Short-term forecast

Short-term projections are carried out using the standard procedure in SAM via the forecast function. Three-year averages are applied for stock weights, catch weights, and maturity. The projections form the basis for the catch advice.

The advice is aligned with the Icelandic fishing year, which begins in September, whereas the assessment model operates on a calendar-year basis. This mismatch requires adaptation of the standard SAM projection procedure to accommodate the offset between assessment and advisory years.

Given the assessment in year \(y\), interim year catches are based on the following fishing mortality:

\[ F_{y} = \left(\frac{8}{12}F_{sq} + \frac{4}{12} F_{mgt}\right) \]

and therefore the total catches for year \(y\) will be:

\[ C_{y} = \frac{F_{y}}{F_{y} + M} \left(1 - e^{-(F_{y} + M)}\right)B_{y} \]

and the part of the catch in the fishing year y-1/y will be

\[ \frac{\frac{8}{12}F_{sq}}{\left(\frac{8}{12}F_{sq} + \frac{4}{12} F_{mgt}\right)} C_y \]

and the catch in fishing year y/y+1 will be:

\[ C_{y/y+1} = \frac{\frac{4}{12}F_{mgt}}{\left(\frac{8}{12}F_{sq} + \frac{4}{12} F_{mgt}\right)} C_y + \frac{8}{12}C_{y+1} \]

where

\[ C_{y+1} = \frac{F_{mgt}}{F_{mgt} + M} \left(1 - e^{-(F_{mgt} + M)}\right)B_{y} \]

The results from the short term prognosis are shown in Table 4.

Table 4: Plaice in 5a. Results from the short term prognosis
Year F(5-10) Recruitment SSB Catch
2025 0.268 16529 20733 7105
2026 0.300 16586 19743 7825
2027 0.300 16586 18906 7522

Management considerations

All available indicators from commercial catch and survey data suggest that the plaice stock in Icelandic waters is currently in good condition. This conclusion is supported by the results of the analytical assessment.

However, there is considerable uncertainty in the model, primarily due to limited information on recruitment from the spring survey.

Management

The Ministry of Industries is responsible for the management of Icelandic fisheries and the implementation of relevant legislation. Each fishing year (1 September – 31 August), the Ministry issues regulations for commercial fishing, including the allocation of total allowable catch (TAC) for stocks subject to such limitations.

Plaice was incorporated into the individual transferable quota (ITQ) system in the 1991/1992 fishing year and has since been subject to TAC restrictions. For the first six years, the TAC for plaice was set higher than the levels recommended by the Marine and Freshwater Research Institute (MFRI) (Figure 29). This was partly due to the absence of a formal harvest control rule for the stock and the flexibility within the management system, which allows for both inter-annual quota transfers and species transformations (i.e., converting TAC between species).

Figure 30 shows net quota transfers involving plaice in the Icelandic ITQ system. Between 2002 and 2008, there was a net transfer of quota from other species into plaice (positive values). From 2009 to 2015, this trend reversed, with plaice quota being transferred to other species. In recent years, species transfers involving plaice have been minimal, except in 2020/2021 when approximately 1 500 tonnes were transferred into the plaice quota from other species. Net transfers of plaice quota between fishing years have varied annually, ranging from +10% to –12%.

Figure 29: Plaice in 5a. Comparison of advice, quota and total catch.
Figure 30: Plaice in 5a. An overview of the net transfers of quota between years and species transformations in the fishery in 5a.

References

Carvalho, Felipe, Henning Winker, Dean Courtney, Maia Kapur, Laurence Kell, Massimiliano Cardinale, Michael Schirripa, et al. 2021. “A Cookbook for Using Model Diagnostics in Integrated Stock Assessments.” Fisheries Research 240: 105959.
Gunnarsson, Björn, Jonas P Jonasson, and Bruce J McAdam. 2010. “Variation in Hatch Date Distributions, Settlement and Growth of Juvenile Plaice (Pleuronectes Platessa l.) in Icelandic Waters.” Journal of Sea Research 64 (1-2): 61–67.
Hjorleifsson, Einar, and Jonbjorn Palsson. 2001. “Settlement, Growth and Mortality of 0-Group Plaice (Pleuronectes Platessa) in Icelandic Waters.” Journal of Sea Research 45 (3-4): 321–24.
Hoarau, G, AM-T Piquet, HW Van der Veer, AD Rijnsdorp, WT Stam, and JL Olsen. 2004. “Population Structure of Plaice (Pleuronectes Platessa l.) in Northern Europe: A Comparison of Resolving Power Between Microsatellites and Mitochondrial DNA Data.” Journal of Sea Research 51 (3-4): 183–90.
ICES. 2022. “Stock Annex: Plaice (Pleuronectes Platessa) in Division 5.a (Iceland Grounds).” International Council for the Exploration of the Seas; ICES publishing. https://doi.org/10.17895/ices.pub.20132660.v1.
Le Moan, Alan, Dorte Bekkevold, and Jakob Hemmer-Hansen. 2021. “Evolution at Two Time Frames: Ancient Structural Variants Involved in Post-Glacial Divergence of the European Plaice (Pleuronectes Platessa).” Heredity 126 (4): 668–83.
MRI. 2016. “Mælingar á brottkasti þorsks og ýsu (e. Measurments of discards of Cod and Haddock), 2014–2016, Reykjavik, Iceland.” Vol. 3. Marine & Freshwater Research Institute, Iceland; Marine Research Institute, Iceland. https://www.hafogvatn.is/static/research/files/fjolrit-183pdf.
Solmundsson, Jon, Jonbjorn Palsson, and Hjalti Karlsson. 2005. “Fidelity of mature Icelandic plaice (Pleuronectes platessa) to spawning and feeding grounds.” ICES Journal of Marine Science. https://doi.org/10.1016/j.icesjms.2004.11.012.
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.