TUSK in ICES Division 5.a and Subarea 14

Key signals

Total stock biomass observed in the spring survey declined steadily from 1985 to 2001. It subsequently increased and remained at relatively high levels until 2020, when it reached its lowest point in the time series. Since then, biomass has increased again and is now at one of the highest levels observed.
Abundance of small tusk (<30 cm) in the, spring survey peaked in 2007, followed by a sharp decline until 2012. Since then, abundance has increased steadily and is now at its highest level in the time series. This increase is also evident in the length distribution from the spring survey, where smaller tusk have become more frequent.
Spawning stock biomass (SSB) declined from 1981 to 2021, when the stock was just above B_lim. Since then it has shown a sharp increase and is now well above precautionary reference points.
Fishing mortality (F, ages 7–10) has been decreasing since 2010 and is now below the reference points F_MGT, F_PA and F_lim since.
Recruitment has been increasing since 2011, was the highest on record in the previous year, and remains at one of its highest observed levels.

General information

Tusk (Brosme brosme), also commonly called cusk, is a slow-moving demersal gadoid that lives solitary or in small aggregations on offshore stony or pebbly bottoms, mainly at depths shallower than 400 m. Around Iceland it is most abundant in southern and southwestern waters, where it inhabits primarily hard-bottom habitats along the shelf edge. The species feeds mainly on crustaceans, shellfish, and other demersal fish. In Icelandic waters tusk grows to sizes close to 100 cm and may attain ages approaching 20 years, although age determination of fish older than 10 years is considered uncertain. Tusk is a slow-growing and late-maturing species, reaching maturity at approximately 8–10 years of age and 45–60 cm in length. Spawning occurs mainly from April to July at depths of 200–400 m south and southwest of Iceland. The species is primarily caught in longline fisheries, with additional catches taken as bycatch in bottom trawls. Due to its life-history characteristics and the relatively high proportion of immature fish in catches, tusk is considered vulnerable to overexploitation, emphasizing the importance of precautionary management and protection of spawning habitats.

Fishery

Landings trends

Tusk in 5.a is caught in a mixed longline fishery, conducted in order of importance by Icelandic, Faroese, and Norwegian boats. Between 90- 370 Icelandic longliners report catches of tusk, but ~100 more vessels have small amounts of bycatch landings. Far fewer gillnetters and trawlers participate in the fishery. The number of longliners reporting tusk catches has been continually decreasing in the last decade. Most of tusk in 5.a, around 95% of catch, is caught by longlines, and this proportion has been relatively stable since 2000 (Table 1). Annual landings in ICES area 5.a and 14 since 1906 are shown in Figure 1.

Figure 1: Tusk. Annual landings in ICES area 5.a and 14 since 1906

Table 1: Tusk. Number of Icelandic boats with tusk landings, catch by gear (tonnes) and total catches in ICES area 5.a since 2000.

Year	nr. bottom trawl	nr. nets	nr. longlines	Bottom trawls	Net	Longlines	Other	Total catch
2000	121	175	370	101	44	4 564	404	5 114
2001	108	224	350	87	63	3 248	1 439	4 838
2002	105	174	304	88	93	3 722	1 659	5 563
2003	97	148	305	65	41	3 941	1 549	5 598
2004	90	130	303	92	28	3 007	1 703	4 830
2005	88	101	324	116	19	3 398	1 510	5 044
2006	86	82	338	100	40	4 912	1 549	6 601
2007	74	65	308	104	38	5 834	1 561	7 538
2008	75	59	255	126	42	6 762	1 699	8 629
2009	75	65	239	115	72	6 757	1 734	8 679
2010	70	62	228	97	52	6 761	2 066	8 976
2011	63	54	221	72	24	5 742	1 862	7 701
2012	65	68	228	64	13	6 255	1 540	7 872
2013	66	43	233	76	15	4 911	1 300	6 302
2014	62	43	249	87	18	6 045	13	6 163
2015	55	32	228	71	7	4 745	13	4 835
2016	59	32	206	61	6	3 420	7	3 494
2017	52	31	180	48	5	2 481	6	2 540
2018	55	27	158	83	8	2 841	7	2 940
2019	49	23	154	103	7	3 323	11	3 445
2020	55	23	126	108	31	3 037	10	3 187
2021	51	18	123	112	12	2 649	6	2 779
2022	51	26	109	110	17	2 446	3	2 577
2023	53	32	94	91	10	2 940	5	3 047
2024	52	18	91	81	8	2 055	4	2 149
2025	48	21	90	95	9	1 996	4	2 105

The main fishing grounds for tusk in 5.a as observed from logbooks are on the southeast, southwestern and western part of the Icelandic shelf (Figure 4 and Figure 5). The spatial distribution of catches in 5.a show a decreasing trend in the southeast until 2015, but this proportion has been increasing in the last 5 years (Figure 4 and Figure 5). The proportional catch in the northwest has also increased over the years. Most of the tusk caught in 5.a by Icelandic longliners is caught at depths less than 300 meters (Figure 3). Around 50–60% of tusk is caught on the southern and western parts of the shelf (Figure 4). Tusk in subarea 14 is caught mainly as a bycatch by longliners and trawlers. The main area where tusk is caught in subarea 14 is 63°–66°N and 32°–40°W (Figure 2).

Figure 2: Tusk. Distribution of catches in ICES area 14 from 1999-2023

Figure 3: Tusk. Depth distribution of catches in ICES area 5.a since 2000, as reported in logbooks by the Icelandic fleet. All gears combined.

Figure 4: Tusk. Catch distribution and proportions by region in ICES area 5.a since 2000, according to logbooks. All gears combined.

Figure 5: Tusk. Geographical distribution (tonnes) of the Icelandic longline fishery since 2006, as reported in logbooks by the Icelandic fleet.

Landings and discards

Total annual landings from ICES Division 5.a were 2105 tonnes in 2025 (Table 2). Since 2010, landings have steadily declined, dropping by 76%. This is contrary to the trend in landings from year 2000 to 2010 when annual landings gradually increased from approximately 5000 tonnes to 9000 tonnes (Figure 1). The foreign catch (mostly vessels from the Faroe Islands, but also from Norway) of tusk in Icelandic waters has always been considerable. Until 1990, between 40-70% of the total annual catch from ICES Division 5.a was caught by foreign vessels, mainly vessels from the Faroe Islands. This proportion has reduced since and has been 10-30% since 1991 (Table 2). Landings of Icelandic fishing vessels are registered by the Directorate of Fisheries, whereas landings of Norwegian and Faroese vessels are registered by the Icelandic Coast Guard. Discarding in the Icelandic demersal fishery is prohibited by law. Limited information is available on discarding of tusk in longline fisheries, but it is considered to be very low (<1%) (WGDEEP, 2011:WD02). Fisheries management measures, such as species flexibility within the quota system, are considered to reduce discarding in mixed fisheries.

Landings in Division 14 have always been low compared to 5.a, and before 2010 they rarely exceeded 100 t (Table 3). However, 1598 tonnes were caught in 2015, after which catches have been consistently substantial. In the fishing year 2015/2016, landings in 5.a were 900 tonnes and as the Icelandic TACs were relatively low during this period, this constituted over 25% of TAC. Landings data from section 14 reported by the Greenland Institute of Natural Resources also reflect this trend. In 2019, around 566 tonnes were caught in the 14.b mainly by Norwegian, Faroese and Greenlandic vessels (Table 3). This has however increased in 2025 to about 457 tonnes.

Discarding is banned in the Icelandic fishery. There is no available information on discarding of tusk in ICES area 14.

Table 2: Tusk. Nominal landings (tonnes) by country in ICES area 5.a since 1980

Year	Iceland	Faroe Islands	Germany	Norway	United Kingdom	Total landings
1980	3 109	2 873	0	928	0	6 910
1981	2 864	2 624	0	1 025	0	6 513
1982	2 801	2 410	0	666	0	5 877
1983	3 468	4 046	0	772	0	8 286
1984	3 430	2 008	0	254	0	5 692
1985	3 064	1 885	0	111	0	5 060
1986	2 549	2 811	0	21	0	5 381
1987	2 987	2 638	0	19	0	5 644
1988	3 087	3 757	0	20	0	6 864
1989	3 158	3 908	0	10	0	7 076
1990	4 821	2 475	0	0	0	7 296
1991	6 449	2 286	0	0	0	8 735
1992	6 432	1 567	0	0	0	7 999
1993	4 086	1 333	0	0	0	5 419
1994	4 065	1 217	0	0	0	5 282
1995	5 151	1 168	1	0	0	6 320
1996	5 548	907	1	3	0	6 470
1997	4 816	579	0	0	0	5 395
1998	4 130	1 080	1	0	0	5 211
1999	5 821	1 041	2	391	2	7 257
2000	4 727	10	0	374	3	5 114
2001	3 397	1 150	1	285	5	4 838
2002	3 910	1 279	0	372	2	5 563
2003	4 024	1 198	1	373	2	5 598
2004	3 135	1 478	1	214	2	4 830
2005	3 539	1 157	4	303	41	5 044
2006	5 054	1 244	2	299	2	6 601
2007	5 987	1 250	0	300	1	7 538
2008	6 934	1 398	0	298	0	8 630
2009	6 953	1 516	0	210	0	8 679
2010	6 919	1 794	0	263	0	8 976
2011	5 847	1 655	0	198	0	7 700
2012	6 344	1 310	0	217	0	7 871
2013	4 992	1 118	0	192	0	6 302
2014	5 057	681	0	425	0	6 163
2015	4 059	578	0	198	0	4 835
2016	2 704	488	0	302	0	3 494
2017	1 887	438	0	216	0	2 541
2018	2 160	454	0	326	0	2 940
2019	2 622	506	0	316	0	3 444
2020	2 380	536	0	271	0	3 187
2021	2 109	283	0	388	0	2 780
2022	1 956	264	0	357	0	2 577
2023	2 414	322	0	311	0	3 047
2024	1 539	487	0	123	0	2 149
2025	1 790	315	0	0	0	2 105

Table 3: Tusk. Nominal landings (tonnes) by country in ICES area 14 since 1978.

Year	Faroe Islands	Norway	Iceland	Russia	Spain	Greenland	Germany	United kingdom	Total landings
1978	0	38	0	0	0	0	47	0	85
1979	0	0	0	0	0	0	27	0	27
1980	0	0	0	0	0	0	13	0	13
1981	110	0	0	0	0	0	10	0	120
1982	0	0	0	0	0	0	10	0	10
1983	74	0	0	0	0	0	11	0	85
1984	0	58	0	0	0	0	5	0	63
1985	0	0	0	0	0	0	4	0	4
1986	33	0	0	0	0	0	2	0	35
1987	13	0	0	0	0	0	2	0	15
1988	19	0	0	0	0	0	2	0	21
1989	13	0	0	0	0	0	1	0	14
1990	0	7	0	0	0	0	2	0	9
1991	0	68	0	0	0	0	2	1	71
1992	0	120	3	0	0	0	0	0	123
1993	0	39	1	0	0	0	0	0	40
1994	0	16	0	0	0	0	0	0	16
1995	0	30	0	0	0	0	0	0	30
1996	0	157	0	0	0	0	0	0	157
1997	0	9	10	0	0	0	0	0	19
1998	0	12	0	0	0	0	0	0	12
1999	0	8	0	0	0	0	0	0	8
2000	0	11	11	0	3	0	0	0	25
2001	3	69	20	0	0	0	0	0	92
2002	4	30	86	0	0	0	0	0	120
2003	0	88	2	0	0	0	0	0	90
2004	0	40	0	0	0	0	0	0	40
2005	7	41	0	8	0	0	0	0	56
2006	3	19	0	51	0	0	0	0	73
2007	0	40	0	6	0	0	0	0	46
2008	0	7	0	0	0	33	0	0	40
2009	12	5	0	11	0	15	0	0	43
2010	7	5	0	0	0	0	0	0	12
2011	20	24	131	0	0	0	0	0	175
2012	33	46	174	0	0	0	0	0	253
2013	2	24	401	0	0	0	0	0	427
2014	145	35	0	0	0	74	0	0	254
2015	759	55	0	0	0	784	0	0	1 598
2016	243	178	0	0	0	182	3	0	606
2017	281	141	0	0	0	358	0	0	780
2018	345	228	0	0	0	108	0	0	681
2019	41	458	0	0	0	66	1	0	566
2020	64	114	0	0	0	45	2	0	225
2021	260	380	0	0	0	59	2	0	701
2022	35	558	0	0	0	87	1	0	681
2023	170	479	0	0	0	115	0	0	764
2024	287	477	0	0	0	174	1	0	939
2025	62	303	0	0	0	92	0	0	457

Data and sampling

In general sampling is considered appropriate from commercial catches from the main gear (longlines), although the quantity of samples has decreased substantially in the last decade (Table 5). The sampling does seem to cover the spatial distribution of catches for longlines and trawls. Similarly, sampling does seem to follow the temporal distribution of catches. The sampling coverage in 2025 is shown in Figure 6.

Figure 6: Tusk. Ratio of catch samples by month (bars) compared with proportion landings by month (black line) split by year and main gear types. Numbers above the bars indicate number of samples by year, month and gear from commercial catches of Icelandic vessels fishing in ICES area 5.a from 2022-2025

Length compositions

No length composition data from commercial catches in Greenlandic waters are available. An overview of available length measurements from 2000 to 2025 in division 5.a is given in Table 4. Most of the measurements are from longlines were annual number of measurements ranges from 682 to 21451 individuals. Measurements generally increased from during the first decade from about 3000 fish to a peak of 21 thosand fish in 2008 and 2009. Since then, measurements have declined and have been below 3000 since 2017 with 1719 fish measured in 2025. Length distributions from the longline fishery is shown in Figure 7.

Figure 7: Tusk. Length distributions from Icelandic commercial longline catches.

Table 4: Tusk. Number of available length measurements from the Icelandic commercial catches in ICES area 5.a since 2000.

Year	Bottom trawl	Long lines	Danish seine	Gillnets
2000	0	2 995	0	0
2001	0	3 097	0	0
2002	0	2 843	0	0
2003	0	8 444	0	0
2004	150	3 809	0	0
2005	21	5 820	0	0
2006	472	4 861	0	0
2007	150	11 936	0	167
2008	0	20 963	0	0
2009	0	21 451	0	0
2010	0	9 084	0	0
2011	0	8 159	0	0
2012	150	11 867	0	0
2013	0	6 469	150	0
2014	0	11 748	0	0
2015	0	4 821	0	0
2016	0	4 844	0	0
2017	0	1 710	0	0
2018	0	2 781	0	0
2019	0	2 952	0	0
2020	1	2 336	0	0
2021	0	1 499	0	0
2022	83	682	0	0
2023	0	2 671	0	0
2024	120	1 748	0	0
2025	0	1 719	0	0

Age composition

Table 5 gives an overview of otolith sampling intensity by gear types from 1985 to 2025 in 5.a. Since 2010, considerable effort has been put into ageing tusk otoliths, so now aged otoliths are available from 1981–1983, 1985, 1987 and 1991–2025. The age data are used as input for the SAM assessment. It is expected that the effort in ageing of tusk will continue. Catch at age per year is shown in Figure 8 and by cohort per year in Figure 9.

Figure 8: Tusk. Catch at age from the commercial fishery in ICES area 5.a. Bar size is indicative of the catch in numbers and bars are coloured by cohort.

Figure 9: Tusk. Catch at age from the commercial fishery in ICES area 5.a since 1981. Biomass (tonnes) caught by year and age; bars are coloured by cohort.

Table 5: Tusk. Number of available otoliths from Icelandic commercial catches and the Icelandic Spring survey (SMB) and the number of aged otoliths in ICES area 5.a since 1985

Year	No. samples (Bottom trawl)	No. samples (Long line)	No. otoliths (Bottom trawl)	No.otoliths (Long line)	No. samples (Survey)	No. otoliths (Survey)
1986	3	1	0	0	246	350
1987	5	4	0	0	286	197
1988	0	0	0	0	274	192
1991	0	4	0	0	295	404
1992	1	4	0	101	284	364
1993	0	9	0	400	265	351
1994	2	15	0	1 426	261	299
1995	2	16	35	1 004	217	684
1996	0	14	0	1 299	208	270
1997	3	10	200	981	227	323
1998	0	13	0	1 227	208	281
1999	0	24	0	1 350	201	272
2000	0	19	0	849	233	322
2001	0	19	0	849	209	282
2002	0	21	0	851	210	303
2003	0	47	0	900	234	345
2004	1	28	50	500	234	422
2005	1	34	0	600	266	488
2006	4	30	150	750	287	499
2007	1	68	50	1 100	294	483
2008	0	110	0	1 600	287	489
2009	0	108	0	1 350	286	453
2010	0	58	0	1 449	248	378
2011	0	43	0	1 400	274	738
2012	1	70	50	1 700	286	771
2013	0	35	0	1 100	278	744
2014	0	62	0	620	243	585
2015	0	35	0	555	263	614
2016	0	28	0	290	261	689
2017	0	14	0	160	247	580
2018	0	23	0	180	251	560
2019	0	29	0	330	252	721
2020	1	18	0	290	251	657
2021	0	13	0	270	278	827
2022	1	8	20	160	315	906
2023	0	22	0	355	305	905
2024	1	13	20	200	305	954
2025	0	16	0	220	360	1 276

Catch weight at age

Weight-at-age per cohort from catch in 5.a is shown in Figure 10. During the last decade, catch weights of three year old had been stable and around the average, whereas the other age groups show more variability between years. The three oldest year classes are the most common in the catch, and recently, younger tusk has become less common in catch (Figure 10). No data are available from area 14.

Figure 10: Tusk. Mean weight at age in the catch from the commercial fishery in ICES area 5.a. Bars are coloured by cohort.

Icelandic survey data (ICES Subarea 27.5a)

Information on abundance and biological parameters from tusk in Icelandic waters is available from the Icelandic groundfish survey in the spring (SMB) and the Icelandic autumn survey (SMH). In addition, a gillnet survey is conducted in areas closer inshore every April during cod spawning periods, designed to sample the cod spawning stock. A detailed description of the Icelandic spring, autumn groundfish surveys and the gillnet surveys are given in the stock annex (ICES 2023). The Icelandic spring groundfish survey, which has been conducted annually in March since 1985, covers the most important distribution area of the tusk fishery. In the years between 1996 and 2006 the Icelandic spring survey did not cover the Faroe ridge area. A considerable proportion of the catches, roughly 20%, in the survey is caught in that area, although variable between length groups. To allow for consistency within the indices the values of the indices were scaled according the median proportion caught in the area. In addition, the autumn survey commenced in 1996 and expanded in 2000; however, a full autumn survey was not conducted in 2011 and therefore the results for 2011 are not presented. Figure 11 shows a recruitment index and the trends in various biomass indices and the survey index at age from the spring survey is shown in Figure 14. Figure 12 shows the spatial distribution of biomass index on the continental shelf and the length distribution from the autumn and spring survey is shown in Figure 13. Since 2014, the survey indices of younger tusk have been increasing. This is also apparent in the length distribution from the spring survey, where smaller tusk have become more frequent.

Figure 11: Tusk. Total biomass indices, biomass indices larger than and including 40 cm, biomass indices larger than and including 60 cm and abundance indices smaller than and including 30 cm. The lines with shaded areas show the spring survey (SMB) index from 1985 and the points with the vertical lines show the autumn survey (SMH) from 1996. The shaded area and vertical lines indicate +/- standard error.

Figure 12: Tusk. Estimated annual survey biomass in the spring survey (SMB) by year since 1985, and the autumn survey since 1996 from different parts of the continental shelf (upper figure) and as proportions of the total (lower figure)

Figure 13: Tusk. Length distributions from the spring survey (SMB) since 1985 and the autumn survey (SMH) since 1996.

Figure 14: Tusk. Age disaggregated indices in the autumn survey (SMH) since 1996 and the spring survey (SMB) since 1985. Bar colours indicate cohorts. Note different scales on y-axes.

Stock weight at age

Mean stock weight at age per cohort in the spring survey is shown in Figure 15. The values are obtained from the spring survey in March and are also used as mean weight at age in the spawning stock for the assessment. Mean weight of the oldest year classes gradually increased since the early 2000s, whereas the mean weight at age of younger tusk is more variable between years.

Figure 15: Tusk. Stock weights from the spring survey (SMB) in Icelandic waters since 1985. Bars are coloured by cohort

Stock maturity at age

Maturity at age data is taken from the autumn survey and calculated based on maturity at length for each year and length distributions of fish assigned to each age. From 1994 to 2000, the proportion mature at age increased gradually in age groups 5 to 10, but steadily declined after until the year 2015. Since then, there has been an upward trend in the proportion of individuals reaching maturity at older ages, with maturity approaching the mean (Figure 16 and Figure 17). The spring survey data is not used because maturation patterns appeared to occur at larger fish and differed between sexes.

Figure 16: Tusk. Maturity at age in the autumn survey (SMH). Bars are coloured by cohort. The values are used to calculate the spawning stock.

Figure 17: Tusk. Proportion mature at length from the autumn survey (SMH). Black line is year 2025

Other surveys

German survey data (ICES Subarea 27.14)

The German groundfish survey was initiated in 1981 and is conducted annually in autumn. It was originally designed to monitor cod stocks but samples the entire demersal fish community down to depths of 400 m. The survey follows a stratified random design, where hauls off both West and East Greenland are allocated among strata according to both area and historical cod abundance, with equal weighting given to each factor. Standard towing duration is 30 minutes at a speed of 4.5 kn. (Rätz, 1996). The trend in the German survey catches is similar to those observed in surveys in 5.a. It should, however, be noted that the data presented in Figure 18 is based on total number caught each year so it can’t be used directly as an index from East Greenland. Length distributions from the survey in recent years are shown in Figure 19.

Figure 18: Tusk. Biomass and abundance estimates from the Walter Herwig survey in ICES area 14 from 1981 to 2015. The data display the total number caught (abundance) which is then converted to biomass

Figure 19: Tusk. Length distributions from the Walter Herwig survey in ICES area 14 from 1991 to 2025 (5 cm bins)

Greenland survey data (ICES Subarea 27.14)

The Greenland Institute of Natural Resources conducted a stratified bottom trawl survey in East Greenland (ICES 14b) from 1998 to 2016 at depths between 400 to 1500 m. (ICES, 2019). Survey results for tusk show a highly variable but increasing trend over recent years, so results from this survey will be monitored after it resumes in the future as a potential biomass index to be included in the tusk assessment.

Stock assessment

Landings data

There have been notable changes in the number of boats or the composition of the fleet participating in the tusk fishery in area 5.a since the year 2000 (Table 1). Catches decreased from around 9000 tons in 2010 to 2105 tonnes in 2025. This decrease is mainly because of reductions in landings by the Icelandic long line fleet, and to a lesser extent, Faroese and Norwegian landings (Table 2 and Table 3). This has resulted in less overshoot of landings relative to set TAC.

Length data

There are no marked changes in the length compositions from catches since 2000 (Figure 7). Length distributions from spring survey show a distinct large cohort (Figure 13), or series of consecutive cohorts, appearing in 2014, growing through time, and just beginning to reach fished sizes approximately two years ago. This recruitment peak appears to follow a recruitment low that can also be traced through the length distribution from 2014 and can still be observed this year as slightly lower-than-average frequencies of tusk in the > 45-50 cm range (Figure 17). According to the available length distributions and information on maturity only around 29% of catches in abundance and 44% in biomass are mature. The reason for this is unknown but given the lack of distinctive cohort structure in the data the first explanation might be a lack of consistency in ageing. Also, tusk have experienced a reduction in fishing mortality over the latter half of this range. Reasons such as difference in sampling, temporal or spatial are highly unlikely.

Survey index data

At WGDEEP 2011 the Iceland-Faroe Ridge was included in the survey index when presenting the results from spring survey for tusk in area 5.a. The total biomass index and the biomass index for tusk larger than 40 cm (reference biomass) decreased substantially until the year 2000, but increased again until around the year 2011 (Figure 11). The index decreased again until 2020 but has since been increasing and is now the highest in the time-series. The index of tusk larger than 60 cm (spawning–stock biomass index), which has gradually increased since 1995. The index of juvenile abundance (<30 cm) decreased by a factor of six between the 2006 survey when it peaked and the 2013 survey when it was at its lowest observed value. Since 2013 juvenile index has increased. The index excluding the Iceland-Faroe Ridge shows similar trends as described above. The results from the shorter autumn survey show a juvenile abundance index that is more or less at a constant and much lower level compared to spring survey juvenile index. When looking at the spatial distribution from spring survey, more than 50% of the index is from the NE area (Figure 12). However only around 14% of the catches are caught in this area (Figure 4 and Figure 5). Due to a labour strike, the autumn survey did not take place in 2011.

Catch, effort and research vessel data

The CPUE estimates of tusk in 5.a are not considered representative of stock abundance.

CPUE estimations have not been attempted on available data from 14.

Analytical assessment using SAM

From 2010-2021, a Gadget model (Globally applicable Area Disaggregated General Ecosystem Toolbox, see www.hafro.is/gadget) was used for the assessment of tusk in 5.a (See stock annex for details, ICES 2023). In 2022, Tusk in 5.a and 14 was re-assessed as the previously benchmarked Gadget model had begun to show great instability in retrospective patterns in recent years. As a part of a Harvest Control Evaluation requested by Iceland, the stock was benchmarked in the year 2022 (ICES 2022a) which resulted in changes in the assessment method and updated reference points. Model setup and settings are described in the stock annex (ICES 2023).

Data used by the assessment and model settings

Data used for tuning and the model configuration are given in the stock annex (ICES 2023).

Model fit

Model results are shown in Figure 24 The model fit to survey indices and catch are shown in Figure 20, Figure 21, Figure 22 and Figure 23. Generally, the model closely follows spring survey data, which are in good agreeance. The autumn survey and catch are noisier but generally follows the same pattern. Fits to the gillnet survey (age 10 abundance) are much noisier. An overview of model parameter estimates are shown in Figure 27.

Figure 21: Tusk. Model fit to the autumn survey indices. Black dots are observed values and the black line is the model fit.

Figure 22: Tusk. Model fit to the gillnet survey indices. Black dots are observed values and the black line is the model fit.

Figure 23: Tusk. Model fit to catch. Black dots are observed values and the black line is the model fit.

Model results

Estimated spawning stock biomass (SSB) displays a declining trend from 1979 to 1995, although prior to 1985 the model is informed by very little data, so uncertainty is high. SSB in the period 1995-2015 was steady, with a gradual decline thereafter that continued until 2021, when biomass levels began increasing again. This pattern is likely due to a distinctive low point in recruitment in 2011-2012, which has since then increased to relatively high levels. Therefore, given moderate fishing levels, spawning stock biomass is expected to increase over the next several years due to large cohorts being recruited into the fishery. The previous peak in recruitment (2004-2005) likely did not increase spawning stock biomass levels substantially during years 2008-2010 due to higher fishing rates and catch values during those years, when these fish would have been entering the fishery (Figure 24).

Observation and process residuals show slight trends in autocorrelation and some blocks of time where the model was consistently over- or underestimating the model (Figure 25 and Figure 26). However, a better model configuration could not be found at the last benchmark that would remove these patterns, and similar model configurations gave similar model results (ICES 2022a, ICES 2022b). Process variance is therefore rather high in this model, indicating high uncertainty in true population dynamics, due to greater uncertainty in input data.

Figure 25: Tusk. Observation error residuals of the SAM model.

Figure 26: Tusk. Process error residuals of the SAM model.

Figure 27: Tusk. Illustration of estimated model parameters.

Figure 28: Tusk. Comparison of the modelled survey indices (solid lines) and observed survey indices (dots).

Figure 29: Tusk. Comparison of the modelled gillnet survey indices (solid lines) and observed gillnet survey indices (dots).

Retrospective analysis

The results of an analytical retrospective analysis are presented (Figure 30). The analysis indicates generally consistent model results over the 5-year peel. Mohn’s ρ was estimated to be 0.026 for SSB, 0.075 for F, and -0.144 for recruitment. Recruitment indices generally tend to be uncertain as there are few repeated observations at larger sizes with which this influence can be tempered. However, the good fit to survey indices at age 1, (Figure 20 and Figure 21), suggests that recent recruitment estimates from this peak are reliable. In addition, a peak in these sizes of tusk followed by a sharp decline in 2020 are reflected in length distribution data as a rather large but steep peak in proportions of fish that have begun to shift right (to larger sizes) with no obvious new peaks of small sizes taking its place (Figure 8). Therefore, it is likely that the increase in biomass observed this year will continue in the next year or so.

Figure 30: Tusk. Retrospective plots illustrating stability in model estimates over a 5-year “peel” in data. Results of spawning stock biomass, fishing mortality F, and recruitment (age 1) are shown as well as the mohn´s rho values

Reference points

As part of the WKICEMP 2022 HCR evaluations (ICES 2022b), the following reference points were defined.

Table 6: Tusk. Reference points adopted from the last benchmark in 2022.

Approach	Reference point	Value	Basis
MSY approach	MSY Btrigger	4 800	Based on Bpa
	FMSY	0.23	Leads to long-term MSY, based on stochastic simulations (EqSim).
Management plan	MGT Btrigger	4 800	From the management plan
	FMGT	0.23	From the management plan
Precautionary approach	Blim	3 400	Lowest SSB (2016) where large recruitment was observed
	Bpa	4 800	Blim x e1.645×0.2
	Flim	0.44	Fishing mortality that in stochastic equilibrium will result in median SSB at Blim
	Fpa	0.23	Fp05, maximum F at which the probability of SSB falling below Blim is < 5%

The harvest control rule (HCR) for the Icelandic tusk fishery, which sets a TAC for the fishing year y/y+1 (September 1 of year y to August 31 of year y+1) based on a fishing mortality F_MGT of 0.23 applied to ages 7 to 10 modified by the ratio SSB\(_{y}\)/MGT B\(_{\text{trigger}}\) when SSB\(_{y}\) < MGT B\(_{\text{trigger}}\), maintains a high yield while being precautionary as it results in lower than 5% probability of SSB < B\(_{\lim}\) in the medium and long term. WKICEMP 2022 concluded that the HCR was precautionary and in conformity with the ICES MSY approach (ICES 2022b).

Management

The Icelandic Ministry of Industries is responsible for management of the Icelandic fisheries and implementation of legislation setting the annual Total Allowable Catch (TAC). Tusk was included in the Individual Transferrable Quota (ITQ) system in the 2001/2002 quota year and as such subjected to TAC limitations. At the beginning, the TAC was set as recommended by MFRI but thereafter had often been set higher than the advice. One reason is that no formal harvest advisory rule existed for this stock. Up until the fishing year 2011/2012, the landings, by quota year had always exceeded the advised and set TAC by 30-40%. However, since then the overshoot in landings has decreased substantially, apart from 2014/2015 when the overshoot was 34%. In recent years the TACs were not filled, until the past two years when the TAC has been exceptionally low (Table 7).

The reasons for the large difference between annual landings and both advised and set TACs are threefold: 1) It is possible to transfer unfished quota between fishing years; 2) It is possible to convert quota shares in one species to another; 3) The national TAC is only allocated to Icelandic vessels. All foreign catches are therefore outside the quota system. However, in recent years managers have to some extent taken into account the foreign catches when setting the national TAC (see below).

There are bilateral agreements between Iceland, Norway and the Faroe Islands (revised annually) related to fishing activity of foreign vessels in restricted areas within the Icelandic EEZ. Faroese longliners are allowed to fish for demersal species in the Icelandic EEZ and since 2011, around 15 Faroese longliners have operated in Icelandic waters, fishing mainly cod, haddock, ling, and tusk. The tusk advice given by MFRI and ICES for each quota year is, however, for all catches, including foreign catches.

Figure 31 shows the net transfers in the Icelandic ITQ system. During the 2005/2006–2010/2011 fishing years there was a clear net transfer of quota from other species to tusk. This pattern reversed in the following years, with net transfers becoming negative. A brief period of small positive transfers is again observed around 2015/2016–2016/2017. From 2017/2018 onwards, net transfers are predominantly negative, indicating that tusk quota was increasingly converted to other species. A positive spike is observed in 2020/2021, but in the most recent years, particularly 2022/2023 and 2023/2024, transfers are strongly negative, representing the largest net transfer of tusk quota to other species in the time series. Transfers between years were generally small and fluctuated around zero during the first part of the time series. However, from 2015/2016 onwards, transfers between years became increasingly negative, indicating that a substantial proportion of tusk quota was carried forward with reduced utilization. The strongest negative between-year transfers occurred in 2023/2024 and 2024/2025(Figure 31).

Table 7: Tusk. Recommended TAC, national TAC for Iceland, and catches. Landings of other nations before 2014 is by calender year

Fishing year	MFRI advice	National TAC	Catch Iceland	Catch other	Total catch
2010/2011	6000	6000	6235	1898	6235
2011/2012	6900	6900	5983	1606	5983
2012/2013	6700	6700	5555	1336	5569
2013/2014	6300	6300	4850	1360	5438
2014/2015	4000	4000	4136	1304	5440
2015/2016	3440	3440	3221	900	4121
2016/2017	3780	3780	1689	729	2418
2017/2018	4370	4370	2200	885	3085
2018/2019	3776	3776	2453	778	3231
2019/2020	3856	3856	2460	781	3241
2020/2021	2289	2289	2192	757	2949
2021/2022	2172	2172	1918	503	2421
2022/2023	4464	4464	2421	640	3061
2023/2024	5139	5139	1592	784	2376
2024/2025	5914	5914	1608	339	1947
2025/2026	7451	7451

Figure 31: Tusk. Net transfer of quota in the Icelandic ITQ system by fishing year since fishing year 2001/2002. Between species (upper): Positive values indicate a transfer of other species to tusk, but negative values indicate a transfer of tusk quota to other species. Between years (lower): Net transfer of quota for a given fishing year (may include unused quota). Displayed both in tonnes and as proportion on TAC.

Management considerations

Catches in area 14.b, and now 14.a also, increased from less than 100 tonnes annually from 1978 to 2014, to 1600 tonnes in 2015. Catches reduced after 2015 but have ranged from 225-934 tonnes since. The increase in catch during the last decade compared to previous 35 years is of concern. However, the signs from commercial catch data and surveys indicate that the total biomass of tusk in 5.a is stable. This is confirmed in the assessment. Recruitment in 5.a showed increasing levels from a low in 2011. A reduction in fishing mortality since 2010 has also led to harvestable biomass and SSB that seem to be either stable or slowly increasing. Due to the selectivity of the longline fleet catching tusk in 5.a and the species relatively slow maturation rate, a large proportion of the catches is immature (60% in biomass, 70% in abundance). The spatial distribution of the fishery in relation to the spatial distribution of tusk in 5.a as observed in spring survey may result in decreased catch rates and local depletions of tusk in the main fishing areas. Tusk is a slow growing late maturing species, therefore closures of known spawning areas should be maintained and expanded if needed. Similarly, closed areas to longline fishing where juvenile abundance is high were previously considered important to maintain but recent studies indicate that the rationale for some area closures aimed at protecting small tusk is no longer valid, and it has been proposed that these closures be lifted. At the same time, research has identified vulnerable habitats in other closed areas, which will remain under review.

Figure 32: Tusk. Comparison of the total allowable catch, adviced catch and total catch within the Icelandic EEZ.

References

ICES. 2011. “Report of the Working Group on the Biology and Assessment of Deep-Sea Fisheries Resources (WGDEEP), 2 March–8 March, 2011, Copenhagen, Denmark. ICES Cm 2011/Acom:17.” International Council for the Exploration of the Seas; ICES publishing.

ICES. 2012. “Report of the Working Group on the Biology and Assessment of Deep-Sea Fisheries Resources (WGDEEP), 28 March–5 April, 2012, Copenhagen, Denmark. ICES Cm 2012/Acom:17.” International Council for the Exploration of the Seas; ICES publishing.

ICES WGDEEP. 2019. WD05: Greenland deep-water survey at East Greenland (Division 14.b). Working document submitted to WGDEEP 2019.

ICES. 2022a. Iceland request for evaluation of a harvest control rule for tusk in Icelandic waters. In Report of the ICES Advisory Committee, 2022. ICES Advice 2022, sr.2022.6d, https://doi.org/10.17895/ices.advice.19625823

ICES. 2022b. Workshop on the evaluation of assessments and management plans for Tusk, tusk, plaice and Atlantic wolffish in Icelandic waters (WKICEMP). ICES Scientific Reports. Report. https://doi.org/10.17895/ices.pub.19663971.v1

ICES. 2023. Stock annex: Tusk (Brosme brosme) in Subarea 14 and Division 5.a (East Greenland, and Iceland grounds). ICES Stock Annexes. Report. https://doi.org/10.17895/ices.pub.23599668.v1

Rätz, H.-J. 1996. Relevance of some environmental parameters to distribution patterns of groundfish and implications for reasonable survey design: case study Atlantic cod off Greenland. NAFO Scientific Council Studies, 28: 73–78.