HERRING

Clupea harengus


Technical report
Published by

Marine and Freshwater Research Institute, Iceland

Published

12 June 2026

Key signals

  • The spawning-stock biomass (SSB) was high around 2007, but declined steadily until 2017 due to additional mortality from the Ichthyophonus infection and a series of below-average year classes recruiting to the stock.

  • The strong 2017–2020 year classes contributed to an increase in SSB after recruiting to the fishable stock. Since 2023, SSB has declined again as these cohorts have become older and subsequent recruitment has been weaker.

  • The total acoustic biomass index from the 2025/2026 surveys was 597 kt, corresponding to 2.2 billion individuals. This is lower than the record-high estimate in 2024/2025 and closer to the long-term average.

  • The juvenile survey was reinstated in 2025 after several years without a survey. The resulting age-1 index was above average and is used in the assessment, reducing uncertainty around recent recruitment.

  • Fishing pressure has been variable over the time series, with high values in the late 1980s followed by a general decline. The current harvest rate is within the range expected under the management plan.

  • Retrospective patterns are moderate in the current assessment, and Mohn’s rho values do not indicate major concern. However, recent peels for SSB and Fbar(5–10) should continue to be monitored.

General information

The Icelandic summer-spawning herring (Clupea harengus) is a pelagic fish that can be found all around Iceland. It lives in a wide range of depths from the surface down to a depth of 400m and at temperatures from 1-15°C (Jakobsson 2000). Its main wintering grounds have been either shallow or deep east or west of Iceland or shallow in the south (Jakobsson 1980, Óskarsson et al 2009). Herring spawns in July, and its spawning grounds can be found along the south and southwest coast of Iceland (Óskarsson and Taggart 2009, Jakobsson et al. 1969). After hatching of eggs at the bottom, larvae reach the north of the country by currents and the main nursery areas are found in fjords northwest and north of the country (Guðmundsdóttir et al. 2007).

Fishery

The total catch in the 2025/2026 season was 104 591 tonnes (Table 4, Figure 2). This also includes the by-catch of herring in the mackerel and Norwegian spring-spawning herring fisheries in June - November 2025. The recommended TAC for 2025 and the TAC (Regulation No. 672, 2 July 2020) was 103 367. Traditional catches in wintering grounds west of the country in September-December amounted to 73 644 tonnes while 30 776 tonnes were caught as bycatch in the mackerel and Norwegian spring-spawning herring fishery in the east in June-November (Figure 1).

Landing trends

Figure 1: Icelandic summer spawning herring in 5a. Seasonal total landings (in thousand tonnes) during 1947–2025, referring to the autumns, by different fishing gears from 1975 onwards).
Figure 2: Icelandic summer spawning herring in 5a. Seasonal total landings (in thousand tonnes) from 1947, referring to the autumns, by different fishing gears from 1975 onwards.

Spatial and temporal patterns

The herring fishing season has changed slightly in the last three decades as detailed in the stock annex. All catches in the year 2025/2026 were caught in pelagic trawls (Figure 2). In the seasons 2007/2008 to 2012/2013, most of the catch (~90%) was caught in Breiðafjörður (Figure 3), but before that it was mainly caught off the south, southeast and east coasts. During the 2013/2014 fishing season, this pattern began to change, with a smaller proportion of catch in Breiðafjörður; and since 2014/2015, most of the fishing has taken place in the west of the country. To protect juvenile herring (27 cm and smaller) in the fishery, area closures are enforced based on a regulation on herring fishing issued by the Ministry of Fisheries (No. 376, 8 October 1992). No closure was enforced in this herring fishery in 2025/2026. Normally, the age of first recruitment to the fishery is age-3, which corresponds to lengths of approximately 26–29 cm.

Figure 3: Icelandic summer spawning herring in 5a. The distribution of the fishery (in tonnes) for the period 2004-2024.

Data and sampling

The assessment of the age composition of the catch is based on samples from the catch of fishing vessels collected at sea by fishermen and catch information. An overview of the sampled otoliths is shown in Table 1. The geographical location of the catch and sampling in 2025/2026 is shown in Figure 4. This year, the calculations were accomplished by dividing the total catch into two cells confined by season and area. Weight-at-length relationships derived from the length and weight measurements of the catch samples were split using the same structure. Based on differences in length-at-age between the summer and winter months, two length-age keys were applied. Catch at age and total landings are available from the 1940s, but only those from 1980 are used in the assessment (Table 1). From trends in the catch at age, it is evident that the older age classes have been contributing to the catches in larger numbers since 2008 (Figure 5). The large 2017–2019 year classes entered the catch as 4- and 5-year-olds (Table 2).

Figure 4: . Icelandic summer spawning herring in 5a. Fishing grounds last year as reported in logbooks (contours) and positions of samples taken from landings (crosses).
Table 1: Icelandic summer-spawning herring 5a. Number of samples and otoliths collected from landed catch.
Year Pelagic Trawl Num. stations Pelagic Trawl Num. samples Pelagic Trawl Num. otoliths Purse Seine Num. stations Purse Seine Num. samples Purse Seine Num. otoliths
1981 0 0 0 32 4 237 1 813
1982 0 0 0 43 4 875 2 461
1983 3 562 278 54 11 124 3 280
1984 1 100 100 37 3 562 2 799
1985 1 100 100 35 5 366 2 182
1986 0 0 0 40 4 452 3 226
1987 0 0 0 55 5 743 4 295
1988 0 0 0 46 4 626 4 318
1989 0 0 0 67 8 159 5 485
1990 2 166 23 55 5 404 4 450
1991 0 0 0 95 12 139 5 689
1992 0 0 0 113 19 456 4 528
1993 92 23 489 76 101 18 772 3 501
1994 3 505 23 57 9 812 2 450
1995 0 0 0 66 8 577 4 042
1996 5 668 183 50 5 828 3 463
1997 27 5 398 834 39 3 928 2 883
1998 11 2 031 148 39 6 156 1 443
1999 41 2 101 952 152 15 691 5 710
2000 55 7 943 1 569 129 9 362 5 536
2001 80 9 238 2 702 82 9 565 2 457
2002 83 12 230 1 481 134 22 220 1 834
2003 171 28 389 916 205 37 084 1 841
2004 74 4 093 1 770 291 76 214 3 960
2005 110 7 565 1 853 157 21 847 4 080
2006 44 4 983 1 393 118 20 152 2 217
2007 19 2 211 658 107 14 000 3 219
2008 17 2 724 43 170 18 053 5 252
2009 60 7 043 1 551 123 10 406 8 390
2010 39 4 130 1 539 60 7 471 6 291
2011 148 17 410 1 488 57 4 033 3 067
2012 32 3 887 193 76 6 513 3 519
2013 76 5 466 1 103 67 8 032 1 480
2014 113 10 434 2 432 2 256 200
2015 98 9 165 2 519 4 364 150
2016 133 11 729 3 369 1 50 50
2017 63 4 907 1 585 1 30 25
2018 62 5 334 1 139 0 0 0
2019 87 7 558 1 665 0 0 0
2020 73 7 409 1 400 0 0 0
2021 59 5 773 1 833 0 0 0
2022 79 7 464 2 194 0 0 0
2023 69 4 547 2 215 0 0 0
2024 55 3 340 2 234 0 0 0
2025 108 5 819 4 309 0 0 0
Table 2: Icelandic summer-spawning herring 5a. Catch in numbers (millions) (1981 refers to season 1981/1982 etc).
Age group
Year 2 3 4 5 6 7 8 9 10 11 12 13 14 15
1980 3,147 14,347 20,761 60,727 65,328 11,541 9,285 19,442 1,796 1,464 698 — 110 79
1981 2,283 4,629 16,771 12,126 36,871 41,917 7,299 4,863 13,416 1,032 884 760 101 62
1982 454 19,187 28,109 38,280 16,623 38,308 43,770 6,813 6,633 10,457 2,354 594 75 211
1983 1,475 22,499 151,718 30,285 21,599 8,667 14,065 13,713 3,728 2,381 3,436 554 100 —
1984 421 18,015 32,244 141,354 17,043 7,113 3,916 4,113 4,517 1,828 202 255 260 —
1985 112 12,872 24,659 21,656 85,210 11,903 5,740 2,336 4,363 4,053 2,773 975 480 581
1986 100 8,172 33,938 23,452 20,681 77,629 18,252 10,986 8,594 9,675 7,183 3,682 2,918 1,788
1987 29 3,144 44,590 60,285 20,622 19,751 46,240 15,232 13,963 10,179 13,216 6,224 4,723 2,280
1988 879 4,757 41,331 99,366 69,331 22,955 20,131 32,201 12,349 10,250 7,378 7,284 4,807 1,957
1989 3,974 22,628 26,649 77,824 188,654 43,114 8,116 5,897 7,292 4,780 3,449 1,410 844 348
1990 12,567 14,884 56,995 35,593 79,757 157,225 30,248 8,187 4,372 3,379 1,786 715 446 565
1991 37,085 88,683 49,081 86,292 34,793 55,228 110,132 10,079 4,155 2,735 2,003 519 339 416
1992 16,144 94,860 122,626 38,381 58,605 27,921 38,420 53,114 11,592 1,727 1,757 153 376 —
1993 2,467 51,153 177,780 92,680 20,791 28,560 13,313 19,617 15,266 4,254 797 254 — —
1994 5,738 134,616 113,290 142,876 87,207 24,913 20,303 16,301 15,695 14,680 2,936 1,435 244 195
1995 4,555 20,991 137,232 86,864 109,140 76,780 21,361 15,225 8,541 9,617 7,034 2,291 621 235
1996 717 15,969 40,311 86,187 68,927 84,660 39,664 14,746 8,419 5,836 3,152 5,180 1,996 574
1997 2,008 39,240 30,141 26,307 36,738 33,705 31,022 22,277 8,531 3,383 1,141 10,296 947 2,524
1998 23,655 45,390 175,529 22,691 8,613 40,898 25,944 32,046 14,647 2,122 2,754 2,150 1,070 1,011
1999 5,306 56,315 54,779 140,913 16,093 13,506 31,467 19,845 22,031 12,609 2,673 2,746 1,416 2,514
2000 17,286 57,282 136,278 49,289 76,614 11,546 8,294 16,367 9,874 11,332 6,744 2,975 1,539 1,104
2001 27,486 42,304 86,422 93,597 30,336 54,491 10,375 8,762 12,244 9,907 8,259 6,088 1,491 1,259
2002 11,698 80,863 70,801 45,607 54,202 21,211 42,199 9,888 4,707 6,520 9,108 9,355 3,994 5,697
2003 24,477 211,495 286,017 58,120 27,979 25,592 14,203 10,944 2,230 3,424 4,225 2,562 1,575 1,370
2004 23,144 63,355 139,543 182,450 40,489 13,727 9,342 5,769 7,021 3,136 1,861 3,871 994 1,855
2005 6,088 26,091 42,116 117,910 133,437 27,565 12,074 9,203 5,172 5,116 1,045 1,706 2,110 757
2006 52,567 118,526 217,672 54,800 48,312 57,241 13,603 5,994 4,299 898 1,626 1,213 849 933
2007 10,817 94,250 83,631 163,294 61,207 87,541 92,126 23,238 11,728 7,319 2,593 4,961 2,302 1,420
2008 10,427 38,830 90,932 79,745 107,644 59,656 62,194 54,345 18,130 8,240 5,157 2,680 2,630 1,178
2009 5,431 21,856 35,221 31,914 18,826 22,725 10,425 9,213 9,549 2,238 1,033 768 406 298
2010 1,476 8,843 22,674 29,492 24,293 14,419 17,407 10,045 7,576 8,896 1,764 1,105 672 556
2011 521 9,357 24,621 20,046 22,869 23,706 13,749 16,967 10,039 7,623 7,745 1,441 618 785
2012 403 17,827 89,432 51,257 43,079 51,224 41,846 34,653 27,215 24,946 15,473 13,631 2,556 236
2013 6,888 46,848 24,833 35,070 17,250 18,550 19,032 21,821 15,952 15,804 10,081 9,775 6,722 2,486
2014 — 3,537 53,241 50,609 70,044 34,393 22,084 22,138 13,298 17,761 7,974 4,461 2,862 1,746
2015 89 6,024 29,890 53,573 43,501 43,015 15,533 10,760 8,664 8,161 6,981 2,726 2,467 1,586
2016 72 10,740 25,575 29,908 41,952 25,823 24,925 9,516 7,734 6,088 4,284 7,154 3,108 826
2017 1,262 5,236 31,855 18,113 10,239 15,506 10,223 8,830 5,676 3,399 1,616 2,220 1,533 1,596
2018 — 8,911 19,642 34,284 16,847 12,376 17,161 6,978 7,379 3,482 1,713 1,153 2,159 489
2019 461 4,601 15,845 12,970 16,084 12,244 6,944 9,531 6,165 4,732 2,983 2,808 2,200 1,866
2020 384 23,603 15,956 22,572 16,333 19,385 11,071 7,098 6,241 3,035 3,359 4,505 1,567 1,129
2021 12,440 21,018 88,992 37,291 37,244 17,231 21,230 13,155 11,781 7,270 5,213 3,549 2,771 1,583
2022 — 23,108 90,765 86,093 26,757 25,604 11,495 14,534 6,998 6,916 4,226 3,817 2,711 1,651
2023 — 8,178 75,892 90,608 56,330 26,617 29,872 11,921 16,204 9,236 8,009 4,399 3,936 2,219
2024 — 1,623 23,825 55,317 70,855 62,492 16,547 16,852 13,537 7,271 7,138 4,593 5,159 4,016
2025 — 26,730 28,952 67,405 86,278 72,227 53,318 18,418 14,638 7,411 7,514 4,058 2,706 1,619
Figure 5: Icelandic summer spawning herring in 5a. Catch at age from commercial samples. Bars are coloured by year class.

Weight at age

Mean weight at age in the stock is shown in Figure 6. As stated in the stock annex, the mean weight-at-age of the stock is derived from catch samples and therefore represents both stock and catch weights. Stock weights of the older year classes have been generally decreasing in recent years.

Figure 6: Icelandic summer spawning herring in 5a. Mean weight by age from commercial samples.

Proportion mature

Fixed maturity ogives were used in this year’s assessment, as described in detail in the stock annex, where the proportion mature-at-ages 3 and 4 are set to 20% and 85% respectively, while all older fish are considered mature.

Length compositions

Length measurements are taken from the two main commercial fleets, i.e. pelagic trawls and purse seine (Table 1). Length distributions from these two fleets are shown in Figure 7. The sizes caught appear to be fairly stable, primarily catching herring in the size range 27–35 cm. Large cohorts can be seen entering the catches and moving the average length throughout the years.

Figure 7: Icelandic summer spawning herring in 5a. Commercial length distributions by gear and year.

Surveys data

The scientific data used to assess the Icelandic summer-spawning (ISS) herring stock derives from annual acoustic surveys (IS-Her-Aco-4Q/1Q). The surveys have been operating since 1973, although only data from 1988 is used to produce stock estimates (Table 3). These surveys are conducted from October–January and from March-April. The area surveyed each year is decided by evaluating available information on the distribution of the stock in the previous and current year, including information from the fishery. Thus, the survey area varies spatially between years because it is focused on the adult and incoming year classes, but it is usually considered to cover the whole stock each year.

Table 3: Icelandic summer-spawning herring 5a. Acoustic estimates (in millions) in the years 1987/88–2025/26 (age refers to the previous autumn, e.g., 1988 refers to the season 1987/1988). No survey was conducted in 1995.
Year
Age Group
3 4 5 6 7 8 9 10 11 12 13 14 15 Total
1988 115.5 401.2 858.0 308.1 57.1 32.5 70.4 36.7 23.6 18.4 24.3 10.1 8.8 1,964.8
1989 635.7 201.3 232.8 381.4 188.5 46.4 25.8 32.8 17.4 10.4 9.1 5.4 8.1 1,795.2
1990 138.8 655.4 179.4 278.8 593.0 179.7 22.2 21.8 13.1 9.9 2.0 - - 2,093.9
1991 403.7 132.2 258.6 94.4 191.1 514.4 79.4 37.6 9.4 12.6 - - - 1,733.3
1992 598.2 1,050.0 354.5 319.9 89.8 138.3 256.9 21.3 9.9 - 9.3 - 1.5 2,849.6
1993 267.9 830.6 729.6 158.8 130.8 54.2 96.3 96.6 24.5 1.1 1.1 3.4 - 2,394.9
1994 302.1 505.3 882.9 496.3 67.0 58.3 106.2 48.9 36.2 - 4.2 18.1 - 2,525.3
1995 - - - - - - - - - - - - - 0.0
1996 217.0 133.8 761.6 277.9 385.0 176.9 98.1 48.5 16.2 29.4 47.9 4.5 - 2,196.9
1997 33.4 270.7 133.7 468.7 269.9 325.7 217.4 93.0 55.5 39.0 30.0 53.2 31.5 2,021.6
1998 291.9 601.8 81.1 57.4 287.0 156.0 203.4 105.7 35.5 27.4 14.2 36.5 25.8 1,923.6
1999 100.4 255.9 1,081.5 103.3 51.8 135.2 70.5 101.6 53.9 17.4 13.6 2.6 13.0 2,001.0
2000 516.2 839.5 239.1 605.9 88.2 43.4 165.7 89.9 121.3 77.6 21.5 3.7 11.1 2,823.1
2001 190.3 967.0 1,316.4 191.0 482.4 34.4 15.7 37.9 14.3 15.4 14.7 1.7 3.3 3,284.5
2002 1,047.6 287.0 217.4 260.5 161.0 345.9 62.5 57.1 38.4 46.0 38.1 21.1 3.7 2,586.3
2003 1,731.8 1,919.4 553.1 205.7 262.4 153.0 276.2 99.2 47.6 55.1 18.8 24.4 25.5 5,372.2
2004 1,115.3 1,435.0 2,058.2 330.8 109.1 100.8 38.7 45.6 7.0 6.4 7.5 10.9 2.3 5,267.6
2005 2,417.1 713.7 1,022.3 1,046.7 171.3 62.4 44.3 10.9 23.9 12.7 - 1.9 11.1 5,538.5
2006 469.5 443.9 345.0 818.7 1,220.9 281.4 122.2 129.6 73.3 65.3 10.1 9.2 16.0 4,005.2
2007 110.0 608.2 1,059.6 410.1 424.5 693.4 96.0 123.7 48.8 1.0 - - 0.5 3,575.8
2008 90.2 456.8 289.3 541.6 309.4 402.9 702.7 221.6 244.8 14.0 22.1 68.1 12.9 3,376.4
2009 149.5 196.1 416.9 288.2 457.7 267.0 225.7 169.0 29.9 26.3 17.8 9.9 4.2 2,258.0
2010 151.1 315.9 490.7 554.8 271.4 327.3 149.1 83.9 156.9 36.7 13.6 8.5 7.0 2,567.0
2011 107.6 280.6 228.9 304.9 296.3 138.7 301.3 61.0 141.3 97.4 37.0 - 4.0 1,998.9
2012 705.0 978.0 436.0 290.0 281.0 246.0 149.0 175.0 83.0 104.0 94.0 21.0 5.0 3,567.0
2013 178.5 781.1 631.4 166.6 127.0 142.0 110.1 97.0 74.3 69.5 43.4 38.5 8.2 2,467.6
2014 16.0 314.9 218.7 345.0 151.7 132.8 120.7 118.3 89.5 74.6 48.7 44.6 42.8 1,718.3
2015 152.4 90.3 330.1 260.9 259.1 187.9 112.0 91.6 37.9 76.7 30.4 10.6 32.9 1,672.7
2016 381.9 164.2 174.5 312.4 225.8 215.2 93.7 62.8 75.3 42.0 15.7 26.8 25.6 1,815.8
2017 97.0 220.6 137.2 151.9 262.5 136.8 241.4 61.2 55.9 62.8 11.4 20.1 14.0 1,473.0
2018 32.7 22.9 95.1 171.7 201.9 319.9 209.2 255.3 75.8 34.5 83.5 54.9 53.5 1,611.0
2019 306.3 137.4 67.9 201.4 101.9 110.8 167.4 163.8 73.3 30.0 30.0 38.5 16.4 1,445.2
2020 1,525.4 229.8 158.6 103.6 211.1 98.8 53.7 59.5 42.2 37.2 21.3 15.1 11.4 2,567.8
2021 1,399.8 1,114.7 424.3 138.2 82.0 127.7 66.5 102.8 82.8 63.5 57.0 22.8 32.7 3,714.7
2022 629.4 655.5 400.6 153.3 237.1 179.0 174.2 81.6 83.9 82.7 32.9 46.8 21.8 2,778.9
2023 136.7 823.6 994.9 574.7 244.7 159.7 109.6 72.5 87.9 38.7 57.1 34.0 31.8 3,366.0
2024 482.5 242.4 296.4 294.8 273.5 194.3 99.0 90.1 47.8 10.0 48.0 27.7 23.5 2,130.0
2025 30.3 158.1 406.4 826.5 946.8 776.7 229.3 272.6 188.6 142.7 97.0 90.9 54.3 4,220.4
2026 316.0 159.6 314.6 462.7 330.8 264.9 141.9 68.3 16.4 81.3 25.0 11.5 3.9 2,197.0

The acoustic index for the adult component of the Icelandic summer-spawning herring during the winter of 2025/2026 is based on two dedicated surveys conducted onboard RV Þórun Þórðardóttir: (1) A survey in October 2025 (Þ10-2025), targeting ISS herring mixing with Norwegian spring-spawning (NSS) herring in the east, and assessing recruitment (age 3–4) southeast of Iceland; (2) A survey (Þ4-2026) in late March that focused on estimating the fishable stock in the main overwintering area west of Iceland (Figure 8). In addition to obtaining an acoustic estimate of the adult component a juvenile survey for age 1 was conducted in the year 2025, producing a usable index for stock assessment. The survey also aimed to estimate the prevalence of Ichthyophonus infections in the stock. The instruments and methods in the surveys were the same as in previous years. Further details about the surveys can be found in Bjarnason (2026). The biological sampling in the survey is detailed in Table 4.

Survey Results

The fishable component of the Icelandic summer-spawning herring stock was observed in two main areas: west of Iceland in Kolluáll/Snæfellsnes at the end of March 2026, and east and southeast of Iceland in autumn 2025 (Figure 8). The total acoustic estimate from these two surveys was 2.2 billion individuals, with a corresponding total biomass index of 597 kt (Table 3 and Figure 9). When only considering age 3+, the three most numerous year classes were those from 2018 (15,5%), 2019 (21,1%) and 2020 (14.1% Table 3). The annual survey targeting the abundance of herring recruits in the eastern and south-eastern regions was conducted in October 2025. The survey area (Figure 8) was consistent with previous years. Herring distribution was more concentrated in the east, where Icelandic summer-spawning herring overlaps with Norwegian spring-spawning herring. This survey targets the younger portion of the stock on the southern shelf of Iceland; however, limited herring was detected in the southern areas, where only 15% of the surveyed biomass comprised individuals aged three years or younger. In contrast, the western survey focuses on assessing the older segment of the stock. More details on survey results are available in the NWWG working document on survey reports (Bjarnason, 2026)

Figure 8: The survey tracks of three acoustic surveys on Icelandic summer-spawning herring in the southeast and east (Þ10-2025, orange lines) and juvenile survey in the north (Þ10-2025-juvenile, purple lines) and in the west (Þ4-2026, blue lines) in 2025/26.
Table 4: Number of fish aged (number of scales) and number of samples taken in the annual acoustic surveys in the seasons 1987/88–2025/26 (age refers to the former year, i.e., the autumn). In 2024/2025, samples from the commercial fishery were used for the western survey.
Number of scales
N of samples
Year Age 2 Age 3 Age 4 Age 5 Age 6 Age 7 Age 8 Age 9 Age 10 Age 11 Age 12 Age 13 Age 14 Age 15+ Total Total West East
1987/88 11 59 246 156 37 28 58 33 22 16 23 10 5 8 712 8 1 7
1988/89 229 78 181 424 178 69 50 77 42 29 23 13 7 12 1412 18 5 10
1989/90 38 245 96 132 225 35 2 2 3 3 2 0 0 0 783 8 - 8
1990/91 418 229 303 90 131 257 28 6 3 8 0 0 0 0 1473 15 - 15
1991/92 414 439 127 127 33 48 84 5 3 0 2 0 0 1 1283 15 - 15
1992/93 122 513 289 68 73 28 38 34 6 2 2 6 0 0 1181 12 - 12
1993/94 63 285 343 129 13 15 7 14 11 0 1 3 0 0 884 9 - 9
1994/95 - - - - - - - - - - - - - - - - - -
1995/96 183 90 471 162 209 107 38 18 8 14 18 2 0 0 1320 14 9 5
1996/97 24 150 88 351 141 137 87 32 15 10 7 14 4 2 1062 11 4 7
1997/98 101 249 50 36 159 95 122 62 21 13 8 15 8 5 944 14 7 7
1998/99 130 216 777 72 31 65 59 86 37 22 17 5 6 11 1534 17 10 7
1999/00 116 227 72 144 17 13 26 26 27 10 8 2 1 0 689 7 3 4
2000/01 116 249 332 87 166 10 7 21 8 14 11 3 3 0 1025 14 10 4
2001/02 61 56 130 114 62 136 25 24 17 21 17 10 3 0 676 9 4 5
2002/03 520 705 258 104 130 74 128 46 26 25 13 15 10 1 2055 22 12 10
2003/04 126 301 415 88 35 32 15 17 3 4 4 6 1 1 1048 13 8 5
2004/05 304 159 284 326 70 29 17 5 8 4 0 3 3 0 1212 13 4 9
2005/06 217 312 190 420 501 110 40 38 26 18 5 5 5 7 1894 22 14 8
2006/07 19 77 134 64 71 88 22 4 2 2 0 0 0 1 484 6 4 2
2007/08 58 288 180 264 85 80 104 19 15 2 2 6 1 3 1107 17 13 4
2008/09 274 208 213 136 204 123 125 97 18 13 9 7 4 17 1448 29 19 10
2009/10 104 100 105 116 60 74 34 19 36 8 3 4 2 2 667 17 10 7
2010/11 35 74 102 157 139 61 119 22 52 36 13 0 1 0 811 11 8 3
2011/12 229 330 134 115 100 106 74 87 45 48 51 10 3 3 1335 15 9 6
2012/13 42 266 554 273 220 252 198 165 126 114 69 61 12 2 2370 60 55 5
2013/14 26 472 275 414 199 200 199 208 163 138 90 85 60 23 2552 45 37 8
2014/15 83 50 96 71 72 53 32 26 11 22 8 3 6 4 534 10 8 2
2015/16 229 112 131 208 148 123 47 32 32 22 13 7 12 4 1120 14 7 7
2016/17 66 164 122 137 202 117 169 43 50 44 14 15 9 4 1162 14 12 2
2017/18 35 58 82 77 75 101 65 77 29 11 27 18 8 9 672 10 5 5
2018/19 28 39 31 98 50 53 77 75 36 15 15 21 5 4 547 7 5 2
2019/20 265 143 94 48 101 60 43 54 45 43 27 26 20 6 975 10 5 5
2020/21 248 215 116 68 59 104 52 79 55 44 35 13 6 8 1102 13 5 8
2021/22 39 89 588 258 254 113 138 87 78 49 34 24 19 8 1890 12 5 7
2022/23 214 306 410 388 127 118 120 90 83 83 61 41 37 15 2093 13 4 9
2023/24 48 529 652 396 192 208 84 110 65 54 29 25 14 8 2414 9 6 3
2024/25 12 173 384 463 412 102 105 94 45 50 28 30 24 3 1928 10 4 6
2025/26 78 173 133 207 242 151 136 60 28 18 32 17 4 4 1283 11 4 7
Figure 9: Comparison of total acoustical biomass indices of Icelandic summer-spawning herring over the autumns 1973/74 to 2025/26 (referring to the autumns) for age 3+ in the west (blue), east (red) and total (green).

Numbers-at-age from the acoustic surveys since 1988 are shown in Figure 10. The acoustic survey indices show the same prominent year classes as seen in the catch (Figure 5) in recent years, although the 2017 year class was underestimated at age 5 in the surveys. Survey indices from 2025 are high, particularly for older age groups, which may reflect uncertainty in the measurements rather than a true increase in stock size.

Figure 10: Icelandic summer spawning herring in 5a. Survey numbers at age from acoustic surveys (1988-2026). Bars are coloured by year class.

A widespread ichthyophoniasis epizootic infection has occurred in the ISS herring stock since late 2008. The infection is caused by the parasite Ichthyophonus sp. Comprehensive analyses for the period 2008–2014 indicated that significant infection-related mortality occurred during the first three years after the outbreak began, 2009–2011, but not in the following years, 2012–2016 [@Oskarsson2018b]. The level of mortality was estimated through a series of runs of the NFT-Adapt assessment model. The best fit to the data was obtained when infection-related mortality was set equivalent to 30% of infected herring dying annually during the first three years of the outbreak.

This assumption is used in the stock assessment by adding infection-related mortality to the fixed natural mortality, \(M_{\text{fixed}} = 0.1\), for each age group and year:

\[ M_{\text{year}, \text{age}} = M_{\text{fixed}} + M_{\text{infected}, \text{year}, \text{age}} \times 0.3 \]

Survey abundance estimates and heart inspection data are used to estimate \(M_{\text{infected}}\).

At the 2024 WKICEHER workshop assessment (ICES 2024), the infection mortality was estimated by the Muppet model using a method similar to Óskarsson et al. (2018b). That model had been used previously also and returned the same multiplier as NFT-Adapt, or 0.3. The multiplier was estimated for the whole time series (2009-2023) on basis of the inter-annual estimates of infection prevalence by the different age groups. Different from the previous estimation, the infection mortality was assumed to have taken place in all years, also in the years 2012-2016. This was considered appropriate because thorough inspection on development of the infection stages and prevalence of the infection has not been done for recent years. The resulting multiplier for the years 2009-2023, and for the coming years until revised again, is 0.22. The revised M for the stock is visualized in Figure 11.

Figure 11: Natural mortality at age for the different years (refers to the autumn) where the deviation from the fixed M = 0.1 is due to the *Ichthyophonus* infection. The period 1987-2008 M=0.1

The prevalence of the Ichthyophonus infection in the stock in 2025/26 was estimated in a same way as has been done since the initiation of the infection in the autumn 2008 (Óskarsson and Pálsson, 2018). The prevalence of infection shows a declining trend for all age classes for the past decade. The infection rate for the younger year classes (age 2-5) seems to be low, or <4% in the west (Figure 12). There are still new infections taking place as seen with the younger ages, so infection mortality is assumed to take place in 2026, like in previous years. Thus, in the stock prognosis (Section 11.6), the abundance estimates from the final year of the assessment (1 January 2026) is lowered by this additional M as done in assessments for the past years. The level of M should then follow the results of the WKICEHER workshop (2024), where age specific Minfected (estimated from the catch samples; Figure 12) is multiplied by 0.22 and the fixed M (0.1) added to it. The M for 2025 (Table 11.3.2.1) should be used in the prognosis in 2026 and in the analytical assessment from 2024 and onwards, until better more reliable estimates become available.

Figure 12: Icelandic summer spawning herring in 5a. The prevalence of the Ichthyophonus infection for each year-class 1996-2022. Estimated from catch samples in the west and, when available, samples from the acoustic survey in the east of Iceland.

Stock assessment

Model and data inputs

In accordance with the recommendation from the 2024 WKICEHER workshop (ICES, 2024), a statistical catch at age model (SAM) was adopted for the 2024 assessment and the reference points were updated. The estimated SAM model parameters are described in the stock annex and illustrated in Figure 13. The catch and survey data used were from 1987/1988–2025/2026. Other input data and model settings consisted of: (i) mean weight at age (Figure 6); (ii) a fixed maturity ogive; (iii) natural mortality (M) was set to 0.1 for all age groups in all years prior to 2009, and from 2009–2025, an additional mortality term was applied because of the Ichthyophonus infection (Figure 11; ICES 2024, Óskarsson et al. 2018a); (iv) the proportion of M before spawning was set to 0.5; and (v) the proportion of F before spawning was set to 0.

Figure 13: Illustration of estimated model parameters.

Diagnostics and fit

Fits to the catch data and acoustic survey numbers-at-age indices can be found in Figure 14 , Figure 15 and Figure 16. The model follows the catch data reasonably well, but younger age groups (<5) are not as well described by the model as the older age groups. For the herring survey data, the model fit is best for age groups 4-13. In 2025 survey year, the model fit does not reach the relatively high indices of age 6 and older. The model follows the juvenile survey reasonably well.

Figure 14: Assessment model fit to the catch numbers at age (2-15) data.
Figure 15: Assessment model fit to the acoustic survey numbers at age (3-15) data.
Figure 16: Assessment model fit to the juvenile acoustic survey numbers at age 2

Observation error residuals (Figure 17) for the herring acoustic survey are generally higher in the period 2000-2010 than other parts of the time series, underlining the inaccuracies in the survey at that time. Positive residuals, where the model estimates are smaller than seen in the survey, can be seen for 1994- and 1999-year classes for almost all age groups and negative residuals for the 2001- and 2003-year classes. Year blocks of positive residuals are apparent for the years ~2000 to 2006 (i.e. referring to 1 January). During these years, the stock was overwintering in offshore areas off the east and west coast, prior to this period the stock was mainly distributed in the east, and from 2006–2012 the stock was overwintering in inshore areas. These positive blocks could therefore reflect changes in catchability of the survey for these years. In 2025, a positive block for all ages, except age 3, can be seen due to a high survey index that year.

Figure 17: Observation error residuals from the assessment model

Since 2020, a series of positive survey residuals can be seen for recruiting herring, this is due to large year classes entering the stock. The residuals from the catch show no definitive trend other than that they are higher in earlier years of the series. Process residuals showed only a minor trend (Figure 18).

Figure 18: Joint sample residuals log(N) and residual catch from the assessment model

Results

Summary of the assessment is shown in Figure 19. The spawning stock biomass was large around 2007 but steadily declined until 2017 despite small catches. This decrease was due to mortality from the Ichthyophonus infection from 2009–2018, and also due to a number of small year classes entering the stock since 2005, particularly the 2011–2014-year classes. The 2017–2020 year classes are large, and since their recruitment to the fishable stock in 2020, the SSB fallowed an upward trajectory. Since 2023, the SSB has been declining due to lower year class strenght of recruiting herring. The information about recruitment has been poor in recent years which leads to high uncertainty in model estimates. However, the most recent recruitment estimate is based on juvenile survey that shows above average 2024 year class. Fishing mortality of herring in 27.5a has been quite variable since 1980, reaching a peak in late 80‘s and gradually reducing in the following years.

Figure 19: Assessment model results of population dynamics overview: estimated catch, recruitment (age 2), weighted average fishing mortality over ages 5–10 (Fbar), harvest rate of the reference biomass, and spawning stock biomass (SSB) with reference biomass (B4+ in red). Note that the mass mortality in Kolgrafafjörður in the winter 2012/13 is included in harvest rate.

The retrospective analysis indicates a moderate retrospective pattern in the assessment. Mohn’s ρ is positive for fishing mortality, Fbar(5–10), and recruitment, R(age 2), with values of 0.1119 and 0.1364, respectively, while spawning stock biomass has a negative value, ρ = −0.1009. Overall, the peels are generally within, or close to, the confidence intervals of the terminal assessment (Figure 20). The main deviations occur in the two most recent peels for SSB and Fbar(5–10), which fall outside the confidence intervals and indicate some uncertainty in the terminal estimates. Recruitment estimates remain uncertain due to limited information on incoming year classes, but uncertainty is lower than in previous assessments because of the recent juvenile survey.

Figure 20: Retrospective analyses: estimated catch, average fishing mortality over ages 5 – 10 (Fbar), recruitment (R (age 2)), and spawning stock biomass (SSB).

Short-term projections

The final SAM model, providing number-at-age on 1 January 2026, was used for the prognosis. To account for expected Ichthyophonus-related mortality in spring 2026, SAM outputs were adjusted using the estimated infection ratios, following the additional natural mortality approach from WKICEHER 2024 (ICES 2024). Stock weights were estimated from the previous year’s catch weights (see Stock Annex).

The harvest rate estimated from the assessment for 2025 was 0.21. The spawning-stock biomass in 2026 was estimated at 409 080 tonnes, and recruitment at age 2 in 2026 was estimated at 894 213 thousand individuals. Recruitment at age 2 in 2027 was assumed to be 603 988 thousand individuals, based on the average recruitment over the past 10 years. The reported catch from June 2025 to the end of April 2026 was 104 591 tonnes.

Quantity Value Notes
Harvest rate, HR (2025) 0.21 Harvest rate from the assessment
SSB (2026) 409 080 t From the assessment
Recruitment age 2 (2026) 894 213 From the assessment; thousands
Recruitment age 2 (2027) 603 988 Average of the past 10 years; thousands
Total catch (2025) 104 591 t Reported catch from June 2025 to the end of April 2026

Prognosis results

The management plan harvest rate of 0.19 was applied to the forecast biomass. This gives an advised total catch of 80 307 tonnes for 2026. Under this scenario, the biomass of age 4+ in 2027 is projected to be 439 380 tonnes and the spawning-stock biomass in 2027 is projected to be 423 579 tonnes.

This corresponds to a 4% increase in SSB and a 22% decrease in advice compared with the previous advice.

Basis Total catch (2026) HR (2026) Biomass age 4+ (2027) SSB (2027) % SSB change % advice change
Management plan 80 307 t 0.19 439 380 t 423 579 t 4 -22

Reference points & advice basis

Precautionary approach reference points

The exploitation rate \(F_{0.1} = F_{\text{MSY}} = 0.22\) proved successful in managing the stock for about 30 years, despite biased assessments. At the 2024 WKICEHER workshop, the precautionary approach (PA) reference points for the stock were verified and revised (ICES 2024).

On the basis of the stock–recruitment relationship derived from a time series spanning 1947–2015, keeping \(B_{\text{lim}} = 200\) kt was considered reasonable at the 2016 NWWG meeting (ICES 2016) and by the Study Group on Precautionary Reference Points for Advice on Fishery Management in February 2003.

Other PA reference points were derived from \(B_{\text{lim}}\) and these data in accordance with the ICES Advice Technical Guidelines:

  • \(B_{\text{pa}} = 273\) kt, calculated as \(B_{\text{pa}} = B_{\text{lim}} \times e^{1.645\sigma}\), where \(\sigma = 0.19\).
  • \(HR_{\text{lim}} = 0.34\), defined as the harvest rate that leads to \(SSB = B_{\text{lim}}\), given mean recruitment.
  • \(HR_{\text{pa}} = 0.248\), defined as the harvest rate leading to \(P(SSB > B_{\text{lim}}) > 95\%\) with \(MSY\ B_{\text{trigger}}\).

MSY based reference points:

At the NWWG meeting in 2011, exploratory work using the HCS program version 10.3 (Skagen, 2012) was used to evaluate possible reference points based on the MSY framework. These reference points could later provide a basis for a management plan and harvest control rule (ICES, 2011b).

A number of runs were carried out with varying settings. The results indicated that the MSY framework was consistent with the currently used precautionary reference points, suggesting that the currently used \(F_{0.1} = 0.22\) could be a valid candidate for \(F_{\text{MSY}}\).

During Management Strategy Evaluations (MSEs) for the stock in April 2017 and again in March 2024 (ICES, 2024, 2017b), simulation results from both evaluations concluded that \(F_{\text{MSY}} = 0.22\) is appropriate.

Management

A Management Strategy Evaluation (MSE) for the stock took place in March 2024 (ICES, 2024). Three different HCRs were tested and all of them were considered precautionary, and, except for the advisory rule applied at that time (FMGT = 0.15), in accordance with the ICES MSY approach. One of the HCRs was later adopted by the Icelandic Government as a Management plan for the stock. This HCR is based on reference biomass of age 4+ in the beginning of the assessment years (Bref, Y), a spawning stock biomass trigger (MGT Btrigger) equal to 273 kt, and the harvest rate (HRMGT) is set as 19% of the reference biomass (age 4+) in the beginning of the assessment year. In the assessment year (Y) the TAC in the next fishing year (1st September of year Y to 31st August of year Y+1) is calculated as follows:

When SSBY is equal or above MGT Btrigger:

TACY = HRMGT*BRef,y

When SSBY is below MGT Btrigger:

TACY = HRMGT* (SSBy/MGT Btrigger) * Bref,y

In the MSE simulation, the ongoing Ichthyophonus epidemic was considered to continue and was accounted for. Consequently, this HCR is independent of the estimated level of Ichthyophonus mortality and requires no further action during such epidemics.

The distribution of the realized harvest rate when the HCR is followed showed that the 90% expected range are within a harvest rate of 0.099–0.22. The recent realized harvest rates are within the above range. In 2025, the Icelandic authorities changed the fishing year for this stock to a calendar year (previously 1 September of year Y to 31 August of year Y+1) (Government of Iceland, 2026).

Management considerations

Uncertainty in the assessment

There are number of factors that could lead to uncertainty in the assessment. Two of them are addressed here. Additional natural mortality caused by the Ichthyophonus infection was set for the whole infection period 2009-2026 (Minfected, age, year multiplied by 0.22 (see Stock Annex). This quantification of the infection mortality is based on Óskarsson et al. (2018b) and was revised at the 2024 WKICEHER workshop (ICES, 2024), and was considered to improve the assessment and reduce its uncertainty. It is worth noting that increasing M has been shown to increase the historical perception of the stock’s size but has minor impacts on the assessment of the final year and the resulting advice. There have been technical issues with the acoustic surveys in 2024-2025 that results in increased uncertainties around the population estimates. Further uncertainty regarding the assessment is the estimate of recruiting year classes. The juvenile survey, which had not been conducted since 2018, was reinstated in autumn 2025 and now provides information on year-class strength at age 1. This reduces uncertainty in the assessment, although recruitment estimates remain uncertain until the year classes are observed at older ages.

Uncertainty in the forecast

It is important to note that the advice for the 2026 fishing season that is derived from the Management plan is independent of the forecast and its uncertainty as it is only based on the reference biomass in the beginning of the assessment year. The uncertainty in the assessment mentioned above related to the apparent new infection in the stock and size of the recruiting year classes, apply also for the forecast.

Assessment quality

For a period in the 2000s, there were concerns regarding the assessment because of retrospective patterns in the results. No assessment was provided in 2005 due to data and model problems, and in the following two years ACFM rejected the assessment because of the retrospective pattern. In the assessments in 2007–2009, an improvement in the pattern from NFT-Adapt was observed, while in 2010–2011 a retrospective pattern appeared again. This was related both to the high natural mortality associated with the Ichthyophonus infection and to new, more optimistic information on incoming year classes to the fishable stock, particularly the 2008 year class, as well as changes in the recent fishing pattern.

In recent years, retrospective patterns have generally remained moderate. In this year’s assessment, Mohn’s rho values indicate some retrospective uncertainty, but they are no longer at a level suggesting major concern. The values are 0.1119 for Fbar(5–10), 0.1364 for recruitment at age 2, and -0.1009 for SSB (Figure 20). Most peels are within, or close to, the confidence intervals of the terminal assessment, with the main deviations occurring in the two most recent peels for SSB and Fbar(5–10). The retrospective pattern is therefore less pronounced than in previous problematic periods and does not indicate a consistent overestimation of stock size together with underestimation of fishing mortality.

Residual patterns should nevertheless continue to be monitored, particularly for recent survey observations and incoming year classes. Overall, the current assessment shows a moderate retrospective pattern, but the Mohn’s rho values do not indicate the same level of concern as previously reported.

Changes in fishing technology and fishing patterns

There are no recent changes in fishing techniques that could lead to different catch compositions. The fishing pattern in the seasons 2014/2015 to 2025/2026 was different from the previous seasons. Instead of fishing only in a small inshore area off the west coast in purse seine, the directed fishery mainly took place in offshore areas west and east of the country. These changes are not considered to affect the selectivity of the fishery because the fishery is still targeting dense schools of overwintering herring in large fishing gears, getting huge catches in each haul and is by no means size selective.

Since around mid-2000s, Icelandic summer-spawning herring has been caught in a mixed (to varying degrees) fishery with the summer fishery for NE-Atlantic mackerel and Norwegian spring-spawning herring. Until that time, no summer fishery on this stock had taken place for decades. Part of this bycatch is on the stock components (e.g. juveniles and herring east of Iceland) that are not fished in the direct fishery on the overwintering grounds in the west. These bycatches are well sampled and normally contribute less than 10% of the total annual catch, but have been unusually high in recent years — accounting for 30% in 2022/2023, 42% in 2023/2024 (approximately 30 kt each year), and again 30% in 2024/2025 (18 kt) and 2025/2026 (30 kt). Easterly distribution of the large incoming year classes from 2017, 2018 and 2019 explains this high level of bycatch, which contributed to 50% of the catches in the east. This is also reflected in the acoustic measurements where measurements in the east have been increasing in recent years (Figure 9; Bjarnason, 2026).

The fishing pattern varies annually as noted in earlier sections and it is related to variation in winter distribution of the different age classes of the stock. This variation can have consequences for the catch composition, but it is impossible to provide a forecast about this variation.

Comments on the assessment

The assessment shows that the stock size was declining 2000–2018 due to a combination of mortality due to Ichthyophonus infection and a series of below average and poor year classes entering the stock. The 2017-2019 year classes which entered the reference biomass in autumn 2021-2023 caused an upward revision of the assessment but a downward revision of spawning biomass estimates and a reduction in the reference biomass is caused by reduced recruitment in recent years.

There is still evidence for new infection by Ichthyophonus in the stock in the winter 2025/26, even if it is less intense than in the years before. This meant applying additional infection mortality in the assessment. If the low levels of new infection in the recent four years, and the resulting low infection rate for the younger age groups, marks a cessation of the outbreak is unclear. This current outbreak adds uncertainty to the assessment and advice.

Ecosystem considerations

The reason for the outbreak of Ichthyophonus infection in the herring stock that was first observed in the autumn 2008 is not known but is probably the effect of interaction between environmental factors and distribution of the stock (Óskarsson et al. 2009). It includes that out-break of Ichthyophonus spores in the environment, which infect the herring via oral intake (Jones and Dawe, 2002), could be linked to the observed increased temperature off the southwest coast. Further research on the causes and origins of such an outbreak are ongoing at MFRI. It involves scanning for Ichthyophonus DNA in zooplankton species that the herring feeds on with PCR (Polymerase chain reaction) technique. Results from that work (MS thesis) can be expected in the in the near future, while preliminary results indicate that the source of the infection is widespread and is in various zooplankton groups and species. With respect to the impacts of the outbreak on the herring stock, recent analyses show that significant additional mortality took place over the first three years only (Óskarsson et al., 2018b), despite a high prevalence of infection for the past decade. For how long time this outbreak will last is unknown as this is basically an unprecedented outbreak. The signs of the infection that is found in the stock will most likely remain for some years, even if no new infection will occur, and then decrease and disappear over some years as new year classes replace the older ones. The observed new infection, even if at a relatively low level, will however delay this process.

The Northwestern working group at ICES is not aware of any strong, well-documented ecosystem or environmental signals that consistently influence the dynamics of the Icelandic summer-spawning herring stock, and which could therefore justify incorporating ecosystem drivers into the analytical basis for scientific advice. While some weak positive correlations have been identified—such as between recruitment and the winter North Atlantic Oscillation (NAO) index or sea temperature (Óskarsson and Taggart, 2010)—other potential indicators, like spring zooplankton abundance, have shown no significant effect on recruitment (Óskarsson and Taggart, 2010), nor on the body condition or growth rate of adult herring (Óskarsson, 2008). Based on these historical relationships, combined with relatively warm sea temperatures around Iceland (MRI, 2016) and persistently high NAO values in recent years (NOAA NAO Index), an earlier assessment (ICES, 2021) suggested the likelihood of strong recruitment. This appears to be supported by the emergence of strong year classes in 2017–2019.

However, there are early indications that the NAO index may now be entering a weakening phase. If this trend continues, it could lead to a shift in oceanographic conditions around Iceland potentially cooler waters and altered plankton dynamics which may negatively impact herring recruitment in the coming years. This highlights the need for ongoing monitoring of environmental drivers, even if they are not yet formally included in assessment models, to better anticipate potential changes in stock productivity under shifting climate conditions. All general ecosystem consideration with respect to the stock can be found in the Ecosystem Overview for the Icelandic Ecoregion (ICES, 2017a).

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