HERRING

General information

The Icelandic summer-spawning herring (Clupea harengus) is a pelagic fish that can be found all around the country. 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).

Surveys

Description

The scientific data used for assessment of the Icelandic summer-spawning (ISS) herring stock derives from annual acoustic surveys, which have been ongoing since 1973 (Figure 1). These surveys are conducted in October–January and March–April. The surveyed area each year is decided based on available information on the distribution of the stock in the previous and the current year, which include information from the fishery. Thus, the survey area varies spatially as the survey is focused on the adult and incoming year classes but is usually considered to cover the whole stock each year. However, in the winter 2023/2024 the autumn survey did not manage to cover the recently growing portion of the stock that resides in the east and are therefore lacking from the survey index. The reason for this unsuccessful survey is due to increasing ISSH and Norwegian spring spawning herring (NSSH) mixing in the east of Iceland in recent years. To separate the measurements of the stocks, the autumn survey was delayed by several weeks in hopes that the NSSH had migrated out of Icelandic waters and ISSH would remain in the area. When surveying the area, no herring was found in the east and it is assumed that the ISSH component accompanied the NSSH migrating east, at least to some extent. The autumn survey also targets juvenile herring on the south coast of Iceland and this to resulted in no herring observed. A later survey, targeting migrating capelin, did successfully measure herring juveniles in the southeastern areas and results from that survey were used. Further details about the surveys can be found in the survey reports (Bjarnason, 2024).

Thus, the acoustic index for adult part of the Icelandic summer-spawning herring in the winter 2023/2024 derives from one dedicated survey on RV Bjarni Sæmundsson in the end of March 2024 (B4-2024) and from a capelin survey (AH3-2024 and AMM3-2024) in the south-east of Iceland in February 2024.

In addition to getting an acoustic estimate on the adult part and on juveniles at age 2 (juvenile survey for age 1 was not conducted in the year 2023), the objective was also to get an estimate of the prevalence of Ichthyophonus infection in the stock. The instrument and methods in the surveys were the same as in previous years. The biological sampling in the survey is detailed in Table 1.

As a part of the 2024 WKICEHER workshop assessment (ICES 2024), the infection mortality was estimated by the Muppet model in a similar way as done by 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 (2008-2023) on the basis of the interannual 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. It means that instead of a sort of subjective approach, a simpler approach was taken. The resulting multiplier for the years 2008-2023, and for the coming years until revised again, is 0.22.

Results

The fishable part of the Icelandic summer-spawning herring stock was observed only in one area, west of Iceland in Kolluáll/Snæfellsnes in the end of March 2024 (Figure 2). The total acoustic estimate came to 2.51 billion in numbers and the total biomass index was 432 kt. The fishable part of the stock (≥ 27 cm) accounted for 60% in number and 88% of the biomass, or 380 kt.

The annual survey aiming for the abundance of herring juveniles east and southeast of Iceland took place in November 2023 and was unsuccessful as stated above. The abundance of herring east of Iceland has varied since measurements started but has been high in recent years (Figure 1). Consequently, a potentially significant portion of the adult herring stock was not measured and, as a result, was not included in the acoustic index this year. An acoustic survey targeting capelin was conducted in February 2024 on the continental slope of southeast Iceland, in the same areas as the autumn survey, where juvenile herring was measured and results from that survey was used in this years assessment(Figure 2). Estimates of the infection rate caused by Ichthyophonus was <4.1% for ages 2-4 and 4-25% for ages 5-12. There are still new infections taking place as seen with the younger ages, so infection mortality is assumed to take place in 2024, like in previous years (Figure 3).

Fishery

The total catch in the 2023/2024 season was 94 422 tonnes (Table 2, Figure 4 and 5). This also includes the by-catch of herring in the mackerel and Norwegian spring-spawning herring fisheries in June - November 2023, and the part that was caught in June-August belongs to the previous fishing season. The recommended TAC for the 2023/2024 fishing season (September-August; ICES 2018) and the TAC (Regulation No. 672, 2 July 2020) was 92 634 tonnes (Table 2). Traditional catches in wintering grounds west of the country in September-December amounted to 66 396 tonnes while 28 022 tonnes were caught as bycatch in the mackerel and Norwegian spring-spawning herring fishery in the east in June-November.

All catches in the year 2023/2024 were caught in pelagic trawls (Figure 4). In the seasons 2007/2008 to 2012/2013, most of the catch (~90%) was caught in Breiðafjörður (Figure 5), but before that it was mainly caught off the south, southeast and east coasts. The fishing season 2013/2014 was an indication of changes in this pattern, with a smaller proportion 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 2023/24. Normally, the age of first recruitment to the fishery is age-3, which is fish at length around 26–29 cm.

Catch in numbers, weight and maturity

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. This year, the calculations were accomplished by dividing the total catch into three cells confined by season and area. In the same way, weight-at-length relationships derived from the length and weight measurements of the catch samples were used. Based on difference in length-at-age between the summer months and the winter, 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. 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 than in previous period (Figure 6). The large 2017- and 2018-year classes entered the catch as 4- and 5-year-olds. In the survey index these same year classes can be seen in recent years, although the 2017-year class was underrepresented as 5-year-old by the survey. In the survey index the 2019-year class is also predicted to be above average (Figure 6). The average weight by age obtained from the catch samples is shown in Figure 7. The fixed maturity ogives were used in this year’s assessment, where proportion mature-at-age 3 is set 20% and 85% for fish at age 4, while all older fish is considered mature.

Assessment

Analysis of input data

Examination of catch curves from survey indices for the year classes 1987-2018 (Figure 8) indicates that the total mortality signal (Z) in the fully recruited age groups is around 0.4. It is under the assumption that the effort has been the same the whole time. In recent years, the effort has changed a lot because of the infection and spatial distribution of the stock, and the mass mortality in 2012/2013 (Óskarsson et al. 2018b), which makes any strong deductions from the catch curves for those years less meaningful. Catch curves from catch data were also plotted using year classes 1989-2018 (Figure 9). Even if the total mortalities look at bit noisy for some year classes, they seem to be close to 0.4. There is an indication that the fish is fully assessable at age 3–5. Increased mortality in the stock due to Ichthyophonus cannot be detected clearly from the catch curves. However, considering that F was reduced drastically in the beginning of the outbreak, similar Z means an increased M during that period, representing infection mortality.

Assessment models and input data

In accordance with the recommendation from the 2024 WKICEHER workshop (ICES, 2024), a statistical catch at age model 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 10. The catch and survey data used were from 1987/88–2023/24. Other input data consisted of: (i) mean weight at age (Figure 7); (ii) fixed maturity ogive ; (iii) natural mortality, M, that was set to 0.1 for all age groups in all years, except for 2009–2023 where additional mortality was applied because of the Ichthyophonus infection (Figure 11; ICES 2024, Óskarsson et al. 2018a); (iv) proportion of M before spawning was set to 0.5; and (v) proportion of F before spawning was set to 0.

Model results

Model diagnostics.

Fits to the catch data and acoustic survey numbers-at-age indices can be found in Figure 12 and Figure 13. Catch data follow reasonably well with the model 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 and gets noticeably better in later years.

Observation error residuals (Figure 14) for the herring acoustic survey are generally higher in the period 2000-2010 than other parts of the data 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, compare to mainly easterly distribution before and overwintering in inshore areas there after (from ~2006–2012). These positive blocks could therefore reflect changes in catchability of the survey for these years.

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 15).

Stock overview.

Summary of the assessment is shown in Figure 16. The spawning stock biomass was large around 2007 but steadily declined until 2017 despite small catches. This decrease was due to the Ichthyophonus infection mortality in 2009–2018 in addition to small year classes entering the stock since around 2005, particularly the 2011–2014-year classes. The 2017- 2019-year classes are large, and indices from the last fishing season 2023/24 indicate that the 2020-year class will also be above average and will enter the fishable stock in autumn 2024 at age 4. Consequently, SSB has been growing since 2020 but declined again in this year’s assessment as the survey index does not depict the 2018- and 2019-year classes as large as previously thought. The information about recruitment is also poor which leads to high uncertainty around recruitment. 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.

Retrospective pattern

The assessment model had relatively low Mohn’s ρ statistic values for spawning stock biomass and fishing mortality and the recruitment values (Table below) are within the recommended by Carvalho et al. 2021. Mohn’s ρ statistical values quantify the extent of retrospective bias in stock assessment results and these low values indicate that the assessment model has relatively low retrospective bias.

R (age 2)	SSB	Fbar (5-10)
-0.03	-0.05	0.09

Analytical retrospective plots do not indicate any substantial deviations in assessment apart from the recruitment which has high uncertainty due to scarce information regarding recruiting year classes (Figure 17)).

Final Assessment and TAC advice

The assessment method was reevaluated in 2024 (ICES, 2024) and the stock is now assessed using a catch- at-age based assessment model (SAM). Ichthyophonus infection mortality was revaluated for the period 2009-2023, resulting in applying lower infection mortality than previously. In this update assessment, where the 2023/24 catch and survey data have been added to the input data, additional natural mortality was applied for 2024 because of the Ichthyophonus infection in the stock.

The effect of the SAM assessment model results in a downward revision of the reference biomass (age 4+) and in the spawning stock biomass compared to the 2023 assessment but includes uncertainty estimates which the NFT-Adapt model was lacking. The results from the assessment model also indicate a downward revision in stock size from last year, due to the large 2017-2019 year-classes, who have now entered fully into the fishery, are not perceived as strong as previously suspected. Spawning stock biomass for 2024 is estimated 412.1 kt and the reference biomass of age 4+ (B_Ref) is 428.2 kt in the beginning of the year 2024. As the SSB will be above MGT B_trigger = 273 kt, the advised TAC according to the Iceland Management Plan is HR_MGT × B_Ref = 0.19 × 428 249 = 81 637 tonnes.

Reference points and the management plan

Reference points

The exploitation rate of F_0.1 = F_MSY = 0.22 proved successful in managing the stock for about 30 years, despite biased assessments. At the 2024 WKICEHER workshop, the PA reference points for the stock were verified and revised (ICES 2024). On basis of the stock-recruitment relationship deriving from time-series ranging from 1947–2015, keeping B_lim = 200 kt was considered reasonable as the 2016 NWWG meeting (ICES, 2016) and the Study Group on Precautionary Reference Points for Advice on Fishery Management concluded also in February 2003. Other PA reference points were derived from B_lim and these data in accordance with the ICES Advice Technical Guidelines and became these: B_pa = 273 kt (B_pa = B_lim × e^1.645σ, where σ = 0.19); HR_lim = 0.34 (HR that leads to SSB = B_lim, given mean recruitment); HR_pa= 0.248 (HR leading to P (SSB > Blim) > 95% with MSY Btrigger).

Management plan

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 (F_MGT = 0.15), in accordance with the ICES MSY approach. One of these HCR was later adopted by 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 (B_ref, Y), a spawning stock biomass trigger (MGT B_trigger) is defined as 273 kt, and the harvest rate (HR_MGT) 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 (1 September of year Y to 31 August of year Y+1) is calculated as follows:

When SSB_Y is equal or above MGT B_trigger:

TAC_Y/y+1 = HR_MGT*B_Ref,y

When SSB_Y is below MGT B_trigger:

TAC_Y/y+1 = HR_MGT* (SSB_y/MGT B_trigger) * B_ref,y

In the MSE simulation, the ongoing Ichthyophonus epidemic was considered to continue and was accounted for. Consequently, this HCR is independent of 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.

State of the stock

The stock was at high levels around 2002 but showed a steady decline to 2017 despite a low fishing mortality. The reduction is a consequence of mortality induced by the Ichthyophonus outbreak in the stock in 2009–2011 and 2016–2018 in addition to small year classes entering the stock since around 2005, particularly the 2011–2014-year classes. The 2017- 2019-year classes are large and will be the foundation of the fishable stock in the coming years. Consequently, SSB has been growing since 2021, but these strong year-classes are not perceived as strong in the latest assessment, causing the SSB to shift downwards in 2024.

Short term forcast

Input data

The final SAM model which gave the number-at-age on 1 January 2024, was used for the prognosis. All input values for the prognosis are given in Table 3. Because of the expected Ichthyophonus mortality in the stock in the spring 2024, the SAM model outputs were reduced according to the infection ratios times 0.22 in accordance with the 2024 WKICEHER workshop results of added natural mortality (ICES 2024). The stock weights were estimated from the last year catch weights (see Stock Annex).

In summary, the basis for the stock projection is as follows: SSB (2024) = 412.1 kt; Biomass age 4+ (1 January 2024) = 428.2 kt; Catch (2023/24) = 94.4 kt; WF_5–10 (2024) = 0.253; HCR (2024) = 0.19.

Results

SSB in the beginning of the fishing season 2024/25 (approximately the same time as spawning in July 2024) is estimated to be 412 136 kt, which is above MGT B_trigger of 273 kt. Consequently, advised TAC on basis of the new Management rule is 0.19 × Biomass 4+ (428 249 kt) = 81 367 kt. This results in F_W5–10 = 0.232 in 2024/25 and SSB = 401 000 kt in 2025. The results of different options are given in Table 4.

Uncertainties in the assessment and forecast

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 2008-2023 (M_{infected, age, year} multiplied by 0.22 (see Stock Annex). This quantification of the infection mortality based on Óskarsson et al. (2018b) and revised at the 2024 WKICEHER workshop (ICES, 2024), was considered to improve the assessment, and reduce its uncertainty. Worth noticing, increasing M has been shown to increase the historical perception of the stocks size but has minor impacts on the assessment of the final year and the resulting advice. Further uncertainty regarding the assessment is the estimate of recruiting year-classes. With no active juvenile survey (discontinued in 2018) the first glimpse of year-class strength is at age 3 in autumn. A dedicated juvenile survey would reduce this uncertainty.

Uncertainty in the forecast

It is important to notice that the advice for 2024/2025 fishing season deriving 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

The lack of stability between years in the herring stock assessment has often been a cause for concern. In particular, there was a tendency to overestimate the size of the stock. No assessment was made in 2005 due to data and model problems, and for the next two years ACFM rejected the assessment due to instability in the results of the assessment. The last five years have been more stable, and this year’s assessment the retrospective pattern for spawning stock size (SSB), F and recruitment are improved in the new SAM model (Figure 16), and the residuals also behave better (Figure 14). This together could be interpreted as evidence of a more reliable stock assessment.

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 2023/2024 was different from the previous seasons.

Instead of fishing near 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 none means size selective.

Since around mid-2000s, Icelandic summer-spawning herring has been caught to varying degree mixed in 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 contributes normally to less than 10% of the total annual catch but were as high as 30% in this year’s fishery and 42% last year (around 30 kt each year). Easterly distribution of the large incoming year classes from 2017, 2018 and 2019 explains this high level of bycatch, which contributed to 52% of the catches in the east. This is also reflected in the acoustic measurements where there is an increasing part of the stock in the east compared to west (Bjarnason, 2023).

Species interaction effects and ecosystem drivers

Regarding relevant research on species interaction, the main work relates to the increasing amount of Northeast Atlantic mackerel (NEAM) feeding in Icelandic waters after 2006 (Astthorsson et al., 2012; Nøttestad et al., 2016). Surveys in the summers since 2010 indicate a high overlap in spatial and temporal distribution of NEAM and Icelandic summer-spawning herring (Óskarsson et al., 2016). Moreover, the diet composition of NEAM in Icelandic waters showed a clear overlap with those of the two herring stocks, i.e., Icelandic summer-spawning herring and Norwegian spring-spawning herring (Óskarsson et al., 2016). Even if copepoda was important diet group for all the three stocks its relative contribution to the total diet was apparently higher for NEAM than the two herring stocks. Considering former studies of herring diet, this finding was unexpected, and particularly how little the copepoda contributed to the herring diet. This difference in the stomach content of NEAM and the two herring stocks indicated that there could be some difference in feeding ecology between them in Icelandic waters, where NE-AM preferred copepoda, or feed in the water column where they dominate over other prey groups, while the opposite would be for the herring and the prey Euphausiacea. Recent studies in the Nordic Seas have shown similar results (Langøy et al., 2012; Debes et al., 2012). The indication for difference in feeding ecology of the species is further supported by the fact that the body condition of the two herring stocks showed no clear decreasing trend since the invasion of NEAM started into Icelandic waters. On the contrary the mean weights-at-age (and at-length) of the summer spawners have been high after 2010 (Óskarsson, 2019b). It should though be noted that comparison of the diet composition of herring in recent years to earlier studies, mainly on NSS herring, indicate that the herring might have shifted their feeding preference towards Euphausiacea instead of Copepoda. That is possibly a consequence of increased competition for food with NEAM, where the herring is overwhelmed and shifts towards other preys. The Northwestern working group at ICES is not aware of documentations of strong signals from ecosystem or environmental variables that impact the herring stock and could possibly be a basis for implementing ecosystem drivers in the analytical basis for its advice. For example, recruitment in the stock has been positively, but weakly, linked to NAO winter index (North Atlantic Oscillation) and sea temperature (Óskarsson and Taggart 2010), while indices representing zooplankton abundance in the spring have not been found to impact the recruitment (Óskarsson and Taggart 2010) or body condition and growth rate of the adult part of the stock (Óskarsson 2008). Considering these relations derived from the historical data, relatively warm waters around Icelandic (Hafrannsóknastofnun 2016), and high positive NAO in recent years (NOAA 2021), it seems to be coming about with the 2017-2019 -year classes.

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33. Óskarsson, G.J., Pálsson, J., and Gudmundsdottir, A. 2018a. An ichthyophoniasis epizootic in Atlantic herring in marine waters around Iceland. Can. J. Fish. Aquat. Sci. dx.doi.org/10.1139/cjfas-2017-0219.

34. Skagen, D. 2012. HCS program for simulating harvest control rules. Program description and instructions for users. Version HCS12_2. Available from the author.

35. NOAA 2021: National Oceanic and Atmospheric Administration, National weather service – Climate prediction center http://www.cpc.ncep.noaa.gov/products/precip/CWlink/pna/nao.shtml.

Tables

Table 1: Number of fish aged (number of scales) and number of samples taken in the annual acoustic surveys in the seasons 1987/88–2023/24 (age refers to the former year, i.e. autumns). In 2000 seven samples were used from the fishery.

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

Table 2: Landings, catches, recommended TACs, and set National TACs in thousand tonnes. After 2011/2012, landings and catches include bycatch of Icelandic summer-spawning herring in the mackerel and Norwegian spring spawning herring fishery during the preceding summer (i.e., from the fishing season before in June–August).

Year/Fishing year	Landings	Catches	Recom. TACs	Nat. TACs
1972	0.310	0.310
1973	0.254	0.254
1974	1.275	1.275
1975	13.280	13.280
1976	17.168	17.168
1977	28.925	28.925
1978	37.333	37.333
1979	45.072	45.072
1980	53.268	53.268
1981	39.544	39.544
1982	56.528	56.528
1983	58.867	58.867
1984	50.304	50.304
1985	49.368	49.368	50.0	50.0
1986	65.500	65.500	65.0	65.0
1987	75.000	75.000	70.0	73.0
1988	92.800	92.800	90.0	90.0
1989	97.300	101.000	90.0	90.0
1990/1991	101.600	105.100	80.0	110.0
1991/1992	98.500	109.500	80.0	110.0
1992/1993	106.700	108.500	90.0	110.0
1993/1994	101.500	102.700	90.0	100.0
1994/1995	132.000	134.000	120.0	120.0
1995/1996	125.000	125.900	110.0	110.0
1996/1997	95.900	95.900	100.0	100.0
1997/1998	64.700	64.700	100.0	100.0
1998/1999*	87.000	87.000	90.0	70.0
1999/2000	92.900	92.900	100.0	100.0
2000/2001	100.300	100.300	110.0	110.0
2001/2002	95.700	95.700	110.0	125.0
2002/2003**	96.100	96.100	125.0	105.0
2003/2004**	130.700	130.700	105.0	110.0
2004/2005	114.200	114.200	110.0	110.0
2005/2006	103.000	103.000	110.0	110.0
2006/2007	135.000	135.000	130.0	130.0
2007/2008	158.900	158.900	130.0	150.0
2008/2009	151.800	151.800	130.0	150.0
2009/2010	46.300	46.300	40.0	47.0
2010/2011	43.500	43.500	40.0	40.0
2011/2012	49.400	49.400	40.0	45.0
2012/2013	72.000	72.000	67.0	68.5
2013/2014	72.000	72.000	87.0	87.0
2014/2015	95.000	95.000	83.0	83.0
2015/2016	69.700	69.700	71.0	71.0
2016/2017	60.400	60.400	63.0	63.0
2017/2018	35.000	35.000	39.0	39.0
2018/2019	40.700	40.700	35.1	35.1
2019/2020	30.000	30.000	34.6	34.6
2020/2021	36.100	36.100	35.5	35.5
2021/2022	70.100	70.100	72.2	72.2
2023/2024	94.400	94.400	92.6	92.6
2024/2025			81.3	81.3

:* TAC was decided 70 thousand tonnes but because of transfers from the previous quota year the national TAC became 90 thousand tonnes. **Summer fishery in 2002 and 2003 included.

Table 3

Table 3: The input data used for prognosis of the Icelandic summer-spawning herring in the 2025 assessment: the predicted weights, the selection pattern, M, proportion of M before spawning, and the number-at-age derived from SAM.

Age (year class)	Mean weights (kg)	M	Maturity ogive	Selection pattern	Mortality prop. F	Mortality prop. M	Number at age (1 January 2024)
3 (2021)	0.154	0.100	0.20	0.13	0	0.5	568.6
4 (2020)	0.220	0.109	0.85	0.52	0	0.5	268.7
5 (2019)	0.262	0.107	1.00	1.00	0	0.5	393.0
6 (2018)	0.285	0.106	1.00	1.00	0	0.5	307.8
7 (2017)	0.307	0.114	1.00	1.00	0	0.5	208.5
8 (2016)	0.322	0.129	1.00	1.00	0	0.5	100.0
9 (2015)	0.331	0.139	1.00	1.00	0	0.5	69.4
10 (2014)	0.353	0.149	1.00	1.00	0	0.5	43.7
11 (2013)	0.363	0.136	1.00	1.00	0	0.5	44.8
12 (2012)	0.369	0.153	1.00	1.00	0	0.5	23.6
13 (2011)	0.377	0.159	1.00	1.00	0	0.5	22.9
14 (2010)	0.389	0.159	1.00	1.00	0	0.5	15.7
15 (2009)	0.388	0.159	1.00	1.00	0	0.5	10.3

Table 4: Catch options table for the 2024/2025 season according to the Management plan where the basis is: SSB (1 July 2024) 412.1 kt (accounted for Minfection in 2024); Biomass age 4+ (1 January 2024) is 428.2 kt; Catch (2023/24) 94.4 kt; HR (2023) 0.15, and WF5–10(2023) 0.253.

Rationale	Catches (2024/2025)	Basis	F (2024/2025)	Biomass of age 4+ (2025)	SSB (2025)	% SSB change*	% TAC change**
Aflaregla	81.4	HR = 0.19	0.232	406	401	-3	-14
MSY nálgun	94.6	HRMSY = 0.22	0.275	393	388	-5	>1
Engin veiði	0.0	F = 0	0.000	487	475	15	-100
HRpa	106.0	HRpa = 0.248	0.314	382	378	-8	13
HRlim	145.0	HRlim = 0.34	0.456	343	342	-15	54

*SSB 2025 relative to SSB 2024

**TAC 2024/25 relative to landings 2023/24

Figures

Figure 1. Comparison of total acoustical biomass indices of Icelandic summer-spawning herring over the autumns 1973/74 to 2023/24 (referring to the autumns) for age 3+ in the west (red), east (and south; blue) and total (black).

Figure 2. The survey tracks of three acoustic surveys on Icelandic summer-spawning herring in the southeast (AH3-2024 and AMM3-2024; younger part of the stock; red and blue) and in the west (B4-2024; adults; green) in 2023/24.

Figure 3.The prevalence of the Ichthyophonus infection for each year-class 1999-2020. Estimated from catch samples in the west and, when available, samples from the acoustic survey in the east of Iceland.

Figure 4. Seasonal total landings (in thousand tonnes) from 1947, referring to the autumns, by different fishing gears from 1975 onwards.

Figure 5. The distribution of the fishery (in tonnes) of Icelandic summer spawning herring for the period 1991-2023. For the years 2007-2010 the distribution in Breiðafjörður is also shown.

Figure 6. Catch at age from commercial samples (left) and survey numbers at age (right) from acoustic surveys (1973-2023). Bars are coloured by year class.

Figure 7. Herring mean weight by age from commercial samples.

Figure 8. Catch curves (log2 of catches) from survey data by year classes 1989–2018. Grey lines correspond to Z = 0.4.

Figure 9. Catch curves (log2 of indices) from catch data by year classes 1989–2018. Grey lines correspond to Z = 0.4.

Figure 10. Illustration of estimated model parameters.

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

Figure 12. Fit to the catch numbers at age (2-15) input data to the assessment model.

Figure 13. Fit to the acoustic survey numbers at age (3-15) input data to the assessment model.

Figure 14. Observation error residuals from the assessment model

Figure 15. Joint sample residuals log(N) and residual catch from the assessment model

Figure 16. 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.

Figure 17. Retrospective analyses: estimated catch, average fishing mortality over ages 5 – 10 (Fbar), recruitment (R (age 2)), and spawning stock biomass (SSB).