Plaice

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.