PLAICE

Pleuronectes platessa


Technical report
Published by

Marine and Freshwater Research Institute, Iceland

Published

7 June 2024

General information

Icelandic plaice (Pleuronectes platessa) is found on the continental shelf around Iceland with the highest abundance in the southwest and west of the island. It is mainly found on a sandy or muddy substrate, occurring at depths ranging from the coast down to 200 meters, sometimes even deeper (Jónsson and Pálsson 2013). Sexual dimorphism occurs in plaice, as females grow larger than males and mature at larger size. Only a small proportion of males become longer than 45 cm, but about the same proportion of females grow larger than 55 cm. Size at sexual maturity differs between the sexes, whereas at the length of 33 cm about half the males have reached maturity, but females reach that level at 38 cm length. Spawning occurs mostly at 50–100 m depth in the relatively warm waters south and west of Iceland, but there is small-scale spawning off the northwest and north coast (Sigurðsson, 1989 and Sólmundsson et al., 2005). After metamorphosis, the 0-group juveniles seek bottom in shallow waters and spend the first summer just below the tidemark (Hjorleifsson and Palsson 2001).

Genetic studies (Le Moan, Bekkevold, and Hemmer-Hansen 2021; Hoarau et al. 2004) suggest that plaice found on the Icelandic and Faroese shelf areas are genetically different from plaice found elsewhere. Sigurdsson (1982) observed long distance migrations to the Barents sea. Similar migrations were not observed in recent tagging studies in Icelandic waters (Solmundsson, Palsson, and Karlsson 2005) and the validity of these older observations are considered questionable (Sigurdsson pers. comm). Furthermore, the older observations are in conflict with the results from Le Moan, Bekkevold, and Hemmer-Hansen (2021).

Tagging data suggests considerable movement within Icelandic waters, this is in accordance with the observed distributional shifts between the spring and autumn surveys, and suggests that sub-stock structure for plaice in Icelandic waters is negligible.

Fishery

Main fishing grounds for plaice are in the west and southwest of Iceland, with smaller fishing areas in the southeast and several fjords in the north (Figure 1 and Figure 2). Demersal seine is the main fishing gear for plaice (59–71% since 2011) in area 5a followed by demersal trawl (23–37%), while a small proportion of the catch is taken in gillnets and longline (Figure 4). Seiners dominate the coastal plaice fishery, but trawlers catch them deeper and further offshore. Plaice fishing grounds in 2015–2023, as reported by mandatory logbooks, are shown in Figure 1.

Since 2000, the main fishing grounds of plaice have been on the southwestern, western and north-western part of the Icelandic shelf (Figure 2). Spatial distribution of the Icelandic plaice fishery has been relatively stable, with around 60% of the plaice caught on the western and north-western part of the shelf. In the last decade, reported catches have increased in the southwestern part but decreased again in the last 5 years to previous proportions. On the contrary, an increase in reported caches was observed in western and north-western part of the shelf during that period. Plaice is caught in relatively shallow water, with most of the catch (60–80%) taken at depths of 21–80 m (Figure 3).

Since 2000, the number of vessels reporting catches of plaice annually has decreased, whereas total catches have been increasing in the past few years. This decrease is most noticeable in the demersal seiner fleet, where the number dropped from 92 vessels in 2004, to 37 in 2021. The number of trawlers has remained relatively stable since 2010 (Table 1). Total annual catch of plaice has been relatively stable (4900–8300 t) over the last 20 years.

Table 1: Plaice in 5a. Number of Icelandic vessels landing plaice, and all landed catch divided by gear type.
Year Nr. Danish Seine Nr. Bottom Trawl Nr. Other Danish Seine Bottom Trawl Other Total catch
2000 125 134 450 3070 1747 442 5259
2001 95 127 521 2924 1402 600 4926
2002 96 118 420 3426 1270 446 5142
2003 95 115 389 3590 1295 372 5257
2004 95 108 399 4037 1375 294 5706
2005 88 105 352 3909 1638 255 5802
2006 87 99 365 3720 2449 212 6381
2007 82 98 355 3311 2232 267 5810
2008 80 93 322 3836 2605 285 6726
2009 70 85 315 3889 2125 310 6324
2010 61 85 328 3647 2038 299 5984
2011 55 80 357 3021 1655 267 4943
2012 54 88 374 4079 1410 437 5926
2013 56 87 317 4040 1583 364 5987
2014 45 74 317 4239 1380 308 5927
2015 50 74 319 4403 2001 350 6754
2016 44 73 280 4896 2120 430 7446
2017 48 71 283 4579 1765 351 6695
2018 47 66 257 5584 2436 321 8341
2019 44 63 276 4287 2231 316 6834
2020 41 65 213 4682 2474 350 7506
2021 37 63 238 4719 3604 355 8678
2022 40 62 210 4307 2743 227 7277
2023 41 56 208 3939 2456 237 6632

Figure 1: Plaice in 5a. Spatial distribution of catches by all gears according to Icelandic logbooks.

Figure 2: Plaice in 5a. Changes in spatial distribution of plaice catches as recorded in Icelandic logbooks.

Figure 3: Plaice in 5a. Depth distribution of plaice from demersal trawl and seine according to Icelandic logbooks.

Figure 4: Plaice in 5a. Landings in tonnes and percent of total by gear and year.

Data available

Sampling of biological data from main gears (demersal seine and bottom trawl) in commercial catches is considered good in general. The sampling does cover the spatial distribution of catches to a satisfactory extent. The sampling coverage by gear in 2023 is shown in Figure 7 and overview of the number of samples is shown in Figure 8. Due to the COVID-19 pandemic in 2020, researchers from MRFI and inspectors from Directorate of Fisheries in Iceland had difficult time obtaining necessary samples for biological measurements from the fisheries, therefore sampling locations and numbers were fewer than usual during this year.

Figure 7: Plaice in 5a. Fishing grounds in 2023 as reported in Icelandic logbooks (colours) and positions of samples taken from landings (asterisks) by main gear types.

Figure 8: 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.

Landings and discards

All landings in 5.a before 1982 are derived from the STATLANT database, and also all foreign landings in 5.a to 2005. The years between 1982 and 1993 landings by Icelandic vessels were collected by the Fisheries Association of Iceland (Fiskifélagið). Landings after 1994 by Icelandic vessels are given by the Icelandic Directorate of Fisheries. Landings of foreign vessels (mainly Norwegian and Faroese vessels) are given by the Icelandic Coast Guard prior to 2014 but after 2014 this are also recorded by the Directorate (Figure 6). Discarding is banned by law in the Icelandic demersal fishery. Discard rates in the Icelandic fishery for plaice are estimated negligible at least since 2001 (MRI 2016). Measures in the management system such as converting quota share from one species to another are used by the fleet to a large extent and this is thought to discourage discarding in mixed fisheries. In addition to prevent high grading and quota mismatch the fisheries are allowed to land fish that will not be accounted for in the allotted quota, provided that the proceedings when the landed catch is sold will go to the Fisheries Project Fund (Verkefnasjóður sjávarútvegsins).

Length composition

An overview of available length measurements from 5.a is given in Table 2. Most of the measurements are from the two main fleet segments, i.e. trawls and demersal seine. Length distributions from the main fleet segments are shown in Figure 9. The sizes caught by the main gear types (bottom trawl and Danish seine) appear to be stable, primarily catching plaice in the size range between 35 and 55 cm. There has been a gradual shift towards larger fish in the length distribution in both fleets. As a result, the average length in the samples taken from commercial catch 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 - nr. samples Bottom Trawl - nr. lengths Demersal Seine - nr. samples Demersal Seine - nr. lengths
2000 33 4261 49 7185
2001 9 1003 51 7517
2002 18 2392 69 11263
2003 21 3278 96 13804
2004 28 3834 150 21216
2005 35 5251 139 20583
2006 60 8102 135 19222
2007 49 6837 124 17073
2008 77 11359 129 17471
2009 50 7201 136 19106
2010 62 9608 126 17387
2011 55 7609 110 16857
2012 39 5723 129 18329
2013 31 4688 115 16647
2014 21 2531 53 7271
2015 33 4142 44 5997
2016 32 4757 58 8075
2017 28 3527 52 6231
2018 24 3506 43 5666
2019 36 4838 47 5990
2020 27 2788 24 3031
2021 53 6922 42 5067
2022 34 4507 26 3211
2023 41 4474 31 3486

Figure 9: Plaice in 5a. Commercial length distributions by gear and year

Age composition

Table gives an overview of otolith sampling intensity by gear types in 5.a. From 2002 to 2005, a large proportion of the landed plaice consisted of 4-7 years old fish, making up approximately 60% of the landed catch in numbers. The proportion of these age classes has decreased, and in the last five years, it has been around 40-45%. In recent years, more older plaice have been caught, with 6-8 years old fish being prominent in the most recent catches (Figure 10).

Table 3: Plaice in 5a. Number of samples and otoliths collected from landed catch.
Year Bottom Trawl - nr. samples Bottom Trawl - nr. otol Demersal Seine - nr. samples Demersal Seine - nr. otol
2000 33 1507 49 2400
2001 9 350 51 2250
2002 18 599 69 2424
2003 21 550 96 3149
2004 28 820 150 3701
2005 35 1000 139 3036
2006 60 1450 135 3200
2007 49 1500 124 3199
2008 77 1850 129 3099
2009 50 1250 136 3180
2010 62 2016 126 3951
2011 55 2452 110 4200
2012 39 1835 129 5199
2013 31 1350 115 5010
2014 21 575 53 900
2015 33 670 44 800
2016 32 573 58 1125
2017 28 550 52 974
2018 24 400 43 880
2019 36 476 47 750
2020 27 550 24 550
2021 53 1225 42 900
2022 34 560 26 470
2023 41 620 31 598

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.

Weight at age

Mean weight at age in commercial catches is shown in Figure 11. Mean weight at age has been increasing in all age groups since 1995.

Figure 11: Plaice in 5a. Catch weights from the commercial fishery in Icelandic waters.

Natural mortality

No information is available on natural mortality. For assessment and advisory purpose, the natural mortality is set to 0.15 for all age groups.

Catch, effort and research vessel data

Catch per unit of effort (CPUE) and effort data from commercial fisheries

CPUE estimates of plaice in Icelandic waters are not considered representative of stock abundance as changes in fleet composition and technical improvements have not been accounted for when estimating CPUE. Non-standardised estimates of CPUE in demersal seine (kg/set) is calculated as the total weight in sets in which plaice was more than 50% of the catch. CPUE gradually increased from 250 kg/set to over 700 kg/set in 2016 (Figure 12). CPUE of plaice in demersal seine has been fluctuating around that level in the last few years. CPUE of demersal trawl (kg/hour), in hauls where plaice is more than 50% of the catch, remained relatively stable between 150-200 kg/hour until 2005. CPUE of plaice has in trawl, like in the demersal seine fishery, gradually increased from 200 kg/hour in 2004 to just under 600 kg/hour in 2015 and has fluctuated between 400 and 600 kg/hour since.

Figure 12: Plaice in 5a. Catch per unit of effort in the most important gear types. The dashed lines are based on locations where more than 50% of the catch is plaice and solid lines on all records where plaice is caught.

Icelandic survey data

Information on abundance and biological parameters from plaice in 5.a is available from two surveys, the Icelandic groundfish spring survey and the Icelandic groundfish autumn survey. The Icelandic spring groundfish survey, which has been conducted annually in March since 1985, covers the most important distribution area of the plaice fishery. In addition, the Icelandic autumn groundfish survey was commenced in 1996. 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 plaice, as recruitment takes place in shallow water in habitats unsuitable for demersal trawling. In addition to these two major surveys, there was a designated flatfish survey with beam trawl, conducted annually in July/August from 2016 to 2022, with the aim to cover most of the recruitment grounds of plaice and other flatfish species. This survey was discontinued in 2023.

Figure 13 shows trends in various biomass indices and a recruitment index based on abundance of plaice smaller than 20 cm. Survey length-disaggregated abundance indices are shown in Figure 16 and changes in spatial distribution in Figure 14. Total biomass index of plaice and plaice larger than 30 cm (harvestable part of the stock), decreased rapidly in the first years of the spring survey and were at the lowest level in 1997–2002. In 2003–2016 the indices gradually increased and stabilized. Since 2017 there have been minor annual fluctuation in the indices, but they are still fairly stable. This year’s spring survey biomass index is in correspondence with the biomass from early 1990. The indices are now only one-third to half of what they were in the first four years of the time series. The index of plaice larger than 50 cm in the spring survey also decreased to lowest levels in 1997-2002 but has increased and has been in recent years at similar level as in the beginning of the time series. The index of juvenile abundance (<20 cm) has maintained at the low level since 1998 with occasional small peaks. Trends in the autumn survey are similar to those observed from the spring survey, but standard deviations in the measurements are higher.

Figure 13: Plaice 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 14: Plaice in 5a. Changes in geographical distribution of the survey biomass.

Figure 15: 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 16: Plaice in 5a. Length distributions from the spring and autumn surveys.

Mean weight at age in SMB is shown in Figure 17. Mean weights at age from SMB are also used as mean weight at age in the spawning stock, approximated from lengths. For stock weights for age 9 are smoothed using a running 3-year average. Prior to 1985 the stock weights are assumed fixed at 1985 levels.

Maturity-at-age data from SMB are given in Figure 18. Based on guidelines from PGCCDBS (ICES, 2017) it was decided to use mature females as the basis for maturity at age. Prior to 1985 the proportion mature is assumed fixed at 1985 levels. Maturity at age is estimated from yearly maturity at length ogives estimated using logistic regression treating individuals as fixed effects. Maturity at age was smoothed with a 3-year running average.

Plaice is mainly caught in the northwest area as well as on the main spawning grounds off the western part of the country in the spring survey and on the species main feeding grounds in the northwest of the country in the autumn survey (Figure 15). Spatial distribution of plaice catch in the surveys shows some temporal changes, particularly between catches taken in the west and northwest areas (Figure 14). This could be due to annual variation in timing of plaice moving to their traditional spawning grounds in the western part of the country as the survey takes place around that same time every year.

Figure 17: Plaice in 5a. Weight at age observed in the spring survey.

Figure 18: Plaice in 5a. Maturity at age observed in the spring survey.

Figure 19: Plaice in 5a. Age disaggregated indices in the Spring Survey, colored by cohorts.

Data analyses

Analytical assessment

This stock was benchmarked for the first time in April 2022 (WKICEMP 2022), and was first assessed the same year.

The assessment model for plaice in 5a is a statistical catch at age model based on:

  • commercial catch-at-age data from 1979 onwards
  • the Icelandic spring groundfish survey from 1985
  • Recruitment estimated at age 3 every year.

Model setup and settings are described in the Stock Annex (ICES 2022). The maximum age of the model is 12, which is considered a plus group.

For assessment and advisory purpose, the natural mortality is set to 0.15 for all age groups.

Model fit

The model fit to survey indices and catch at age data are shown Figure 20. Generally, except for the youngest age classes, the SAM model follows the catch-at-age and spring survey data.

Figure 20: Plaice in 5a. Model fit to survey indices and catch at age data

Stock overview

Population dynamics of plaice estimated by this model (Figure 21) show a clear reduction in the level of recruitment (at age 3) in 1993, and subsequently we see an increase in fishing mortality and reduction in total catches. Spawning stock biomass (SSB) was at its lowest value at the turn of the century. In recent years recruitment is seen to be stable at the post 1993 levels whereas fishing mortality has been reduced and SSB increased. Catches have remained stable, slightly decreasing.

Figure 21: 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.

Analytical retrospective

The proposed model had low Mohn’s \(\rho\) statistic values for spawning stock biomass, fishing mortality, and recruitment. Analytical retrospective plots do not indicate any substantial deviations in assessment (Figure 22). These Mohn’s \(\rho\) values are well within the range recommended by Carvalho et al. (2021).

Figure 22: Plaice in 5a. Analytical retrospective estimates of SSB, catch, F and recruitment. Mohns rho is indicated in the bottom right corner.

Short-term forecast

Short term projections are performed using the standard procedure in SAM using the forecast function. Three year averages are used for stock and catch weights, and maturity. From this projection the advice is derived. The advice is based on the Icelandic fishing year starting in September each year. This causes a mismatch between the assessment model, which is based on the calendar year. So in order to provide advice for the fishing year, the standard projection procedure in SAM will need to be adapted to accommodate these differences. So given the assessment in year \(y\) the 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
fbar rec ssb catch
2024 0.277 16236 20279 7322
2025 0.300 16462 19369 7852
2026 0.300 16462 18638 7554

Management

The Ministry of Food, Aggriculture and Fisheries is responsible for management of the Icelandic fisheries and implementation of legislation. The Ministry issues regulations for commercial fishing for each fishing year (1. September – 31. August), including an allocation of the TAC for each stock subject to such limitations. Plaice was included in the ITQ system in the 1991/1992 quota year and as such subjected to TAC limitations. For the first six years, the TAC was set higher than recommended by Marine Research Institute (MRI), but this practice stopped in the 2010/2011 quota year (Table 5)). One reason for this practice was that no formal harvest rule existed for this stock. Through this time period the landings have been fluctuating between the over- or undershoot of the set TAC. This is related to the management system that allows for transfers of quota share between fishing years and conversion of TAC from one species to another (species transformation).

Figure 23 shows the net transfers in the Icelandic ITQ-system. From 2002-2008 there was a net transfer of other species being transferred to plaice quota (positive values in the figure). However, from 2009-2015, this was reversed, and plaice quota was transferred to other species. In recent years species-transfer of quota trough plaice has been low, except for 2020/2021 when around 1500 t were transferred from quotas of other species to plaice. Net transfer of plaice quota between fishing years has varied between years, and ranges from 10 to -12%.

Figure 23: Plaice in 5a. An overview of the net transfers of quota between years and species transformations in the fishery in 5a.

Table 5: Plaice in 5a. National and ICES advice and official landings. All weights are in tonnes.
Year Rec. TAC National TAC Catch
1991/92 10 000 11 000 10 157
1992/93 10 000 13 000 15 474
1993/94 10 000 13 000 12 465
1994/95 10 000 13 000 11 320
1995/96 10 000 13 000 11 197
1996/97 10 000 12 000 10 516
1997/98 9000 9000 8241
1998/99 7000 7000 7711
1999/00 4000 4000 4975
2000/01 4000 4000 4946
2001/02 4000 5000 4420
2002/03 4000 5000 5427
2003/04 4000 4500 5861
2004/05 4000 5000 6193
2005/06 4000 5000 5659
2006/07 5000 6000 6144
2007/08 5000 6500 6624
2008/09 5000 6500 6368
2009/10 5000 6500 6389
2010/11 6500 6500 4846
2011/12 6500 6500 5819
2012/13 6500 6500 5935
2013/14 6500 6500 6036
2014/15 7000 7000 6230
2015/16 6500 6500 7612
2016/17 7330 7330 6373
2017/18 7103 7103 8208
2018/19 7132 7132 7096
2019/20 6985 6985 7177
2020/21 7037 7037 9082
2021/22 7805 7805 7306
2022/23 7663 7663 7264
2023/24 7830 7830   

Management considerations

All the signals from commercial catch and survey data indicate that plaice in Iceland is at present in a good state. This is also confirmed in the assessment. Considerable uncertainty is present in the model due to limited information on recruitment from the Spring survey.

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.
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.
Sigurdsson, Adalsteinn. 1982. “Long Distance Migration of Plaice (Pleuronetes Platessa l.).” Rit Fiskideildar 4: 27–31.
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.