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SCRS/2020/017 Collect. Vol. Sci. Pap. ICCAT, 77(2): 215-225 (2020)

FISHING CAPACITY ON ATLANTIC BLUEFIN BY PURSE SEINE VESSELS IN THE NORWEGIAN EEZ FROM 2014 TO 2019

Leif Nøttestad1*, Erling Boge1, Rune B. Mjørlund2

SUMMARY

ABFT perform long-distance migrations to Norwegian waters for feeding purposes. We found that the ABFT school size category counting only 1-5 individuals dominated in Norwegian waters with more than 50% of total observed schools from 2016 to 2018. Small school size, low density and highly dynamic behavior and pronounced long-distance feeding migrations may strongly affect the purse seine catch rate and fishing capacity. We estimated the catch rate and fishing capacity on (ABFT) ( thynnus) from purse seine vessels during the ABFT active feeding season between 2014 and 2019 within the Norwegian Exclusive Economic Zone (EEZ). The fishing capacity onboard purse seine vessels in Norway was estimated to be 20,6 tons. We therefore suggest that the estimated average fishing capacity for purse seine vessels inside Norwegian EEZ, is considerably lower than the estimated fishing capacity for similar sized purse seine vessels operating in the Mediterranean Sea. We discuss possible explanations behind lower fishing capacity of ABFT in the Norwegian EEZ during feeding, compared to higher fishing capacity of ABFT in the Mediterranean Sea during pre-spawning and spawning.

RÉSUMÉ

Le thon rouge de l’Atlantique (ABFT) effectue des migrations de longue distance vers les eaux norvégiennes à des trophiques. Nous avons constaté que la catégorie de taille des bancs d’ABFT comptant seulement 1-5 spécimens dominait dans les eaux norvégiennes avec plus de 50% du total des bancs observés de 2016 à 2018. La petite taille des bancs, la faible densité et le comportement très dynamique ainsi que les migrations trophiques prononcées sur de longues distances pourraient fortement affecter le taux de capture des senneurs et la capacité de pêche. Nous avons estimé le taux de capture et la capacité de pêche du thon rouge de l'Atlantique (ABFT) (Thunnus thynnus) capturé par les senneurs pendant la saison trophique active de l'ABFT entre 2014 et 2019 dans la zone économique exclusive (ZEE) norvégienne. La capacité de pêche à bord des senneurs en Norvège a été estimée à 20,6 tonnes. Nous pensons donc que la capacité de pêche moyenne estimée pour les senneurs dans la ZEE norvégienne est considérablement inférieure à la capacité de pêche estimée pour des senneurs de taille similaire opérant en mer Méditerranée. Nous discutons des explications possibles de la capacité de pêche inférieure de l'ABFT dans la ZEE norvégienne pendant la saison trophique, par rapport à la capacité de pêche supérieure de l'ABFT en mer Méditerranée pendant la période de pré-ponte et de ponte.

RESUMEN

El atún rojo del Atlántico realiza migraciones de larga distancia a aguas noruegas con fines alimenticios. Entre 2016 y 2018, hemos hallado que el tamaño de los cardúmenes de atún rojo del Atlántico es de solo 1-5 ejemplares en aguas noruegas, con más del 50 % de total de cardúmenes observados. El pequeño tamaño de los cardúmenes, la baja densidad y el comportamiento altamente dinámico, así como las pronunciadas migraciones de alimentación a larga distancia pueden afectar fuertemente a la tasa de captura y la capacidad de pesca del cerco. Hemos estimado la tasa de captura y la capacidad de pesca de atún rojo del Atlántico (Thunnus thynnus) de los cerqueros durante la temporada activa de alimentación del atún rojo del Atlántico entre 2014 y 2019 dentro de la Zona Económica Exclusiva de Noruega (ZEE). La capacidad pesquera a bordo de los cerqueros noruegos se estimó en 20,6 t. Por lo tanto,

1 Institute of Marine Research, Research Group on Pelagic , P. O. Box 1870, Nordnes, NO-5817 Bergen, Norway. Email: [email protected] 2 Directorate of , Department of Statistics, P. O. Box 185 Sentrum, 5804 Bergen, Norway, 215 sugerimos que la capacidad pesquera media estimada para los cerqueros dentro de la ZEE noruega es considerablemente inferior a la capacidad pesquera estimada para cerqueros de tamaño similar que operan en el Mediterráneo. Discutimos posibles explicaciones a esta capacidad pesquera inferior dirigida al atún rojo del Atlántico en la ZEE noruega durante la alimentación en comparación con la mayor capacidad pesquera dirigida al atún rojo del Atlántico en el Mediterráneo durante la temporada previa al desove y la temporada de desove.

KEYWORDS

Bluefin tuna, Norwegian waters, fishing capacity, feeding period, small schools

1. Introduction

Atlantic bluefin tuna (Thunnus thynnus) has been feeding in Norwegian waters for thousands of years based on archeological findings (Tangen 1999). Atlantic bluefin tuna (ABFT) has historically been both abundant and widely distributed in Norwegian waters during summer and autumn (Hamre 1957; Hamre and Thiews 1964; Nøttestad and Graham 2004; Fromentin and Powers, 2005; Cort and Nøttestad 2007; Nøttestad et al. 2017; ICCAT 2018; 2019).

Historically, the ABFT migrated annually to Norwegian waters far into the northernmost regions of the Northeast Atlantic past the polar circle during summer and autumn, feeding ferociously on various prey species (Hamre and Thiews 1964; Nøttestad and Graham 2004; Tangen et al., 2016; Nøttestad et al. 2016; 2017). ABFT swim to Norwegian waters to gain substantial weight through active feeding, due to the fact that the and Norwegian coast represent one of the world’s best “fridges” of high quality food for ABFT; around 10 million tonnes of widely distributed such as Northeast Atlantic (NEA) mackerel ( scombrus), ( harengus) and blue whiting (Micromesistius poutassou) (Skjoldal et al. 2004; Huse et al. 2012, Trenkel et al 2014, Nøttestad et al. 2017; ICES 2019).

The overall impression of ABFT school size, density and behavior in Norwegian waters after the comeback, based on visual observations and acoustic recordings, has been that ABFT are primarily organized in small and loose schools/shoals/aggregations (Nøttestad et al. 2017; Boge 2019). This coincide with small schools of prey species available during the same period, such as juvenile and adult NEA mackerel recorded in coastal and offshore areas during summer feeding (Nøttestad et al. 2016a, b). ABFT may have the ability to adjust their school size, density and speed/maneuverability to the corresponding school size, density and speed/maneuverability of their hunted prey. These selected prey species for ABFT are predominantly staying and feeding themselves on in the upper parts of the water column above the shallow thermocline established during summer (Nøttestad et al. 2019). ABFT are often attacking schooling prey at or near the surface during late summer with very high speed and dynamic swimming behavior.

The major aim of this study was to quantify catch rates and fishing capacity on ABFT using purse seine vessels in the Norwegian Exclusive Economic Zone (EEZ) from 2014 to 2019.

A secondary aim was to compare fishing capacity of purse seiners fishing in Norwegian waters with the fishing capacity of purse seiners fishing in the Mediterranean Sea. Furthermore, we discuss and explain why the catch rates and fishing capacity on ABFT caught in purse seine vessels are lower in the Norwegian EEZ during feeding in summer and autumn, compared to the higher catch rates and fishing capacity on ABFT caught in purse in the Mediterranean Sea during pre-spawning and spawning in spring and early summer.

2. Material and Methods

We have included two different kind of available data sources to quantify and explain the low catch rates of Atlantic bluefin tuna using purse seine vessels in Norwegian waters during the active feeding period in summer and autumn:

1) Visual and acoustic detections are included to estimate school size, density and behavior of Atlantic bluefin tuna.

216 2) Catch rates and fishing capacity using purse seine vessels from available catch statistics are included on Atlantic bluefin tuna using purse seine vessels in Norwegian waters.

Visual observations and acoustic detections of ABFT in Norwegian waters

Numerous observations of ABFT from inside the Norwegian EEZ were collected from 2014 to 2019, assuming a somewhat representative observation scheme over the magnitude and distribution in space and time. This is obviously a great challenge. Observations of ABFT made in recent years were collected from several sources and through several different platforms of communication. The scientific data included visual observations of ABFT at or near the surface onboard vessels during the fishing season as well as more opportunistic observations from various sources at sea; Institute of Marine Research, fishing fleet, reference fleet, fish farms, offshore installations and media including various social media (Boge 2019). We have also collected some available acoustic detections and recordings by multibeam sonars and echosounders of ABFT from 2014-2019, to estimate school size and density as well as behavior.

Estimation of catch rates and fishing capacity on ABFT in Norwegian waters

The data on ABFT was collected from commercial catch statistics at the Norwegian Directorate of Fisheries and ICCAT, as well as from a whole range of structured data on observations of ABFT from both visual and acoustic recordings. There was no standardization in methods used for treatment of fish prior to weighing, between the certified fishing vessels, or methods for measuring length of ABFT caught. The biological data was standardized for comparison analysis (see Boge 2019). The commercial catch statistics on ABFT from the Directorate of Fisheries in Norway as well as catch per set and catch per day has been used to calculate the catch rates and fishing capacity, whereas VMS tracking movements of licensed purse seine vessels have been used to calculate the effort in terms of days at sea.

Catch rates from purse seine fishing on ABFT have been estimated by calculating the Catch Per Unit Effort (CPUE). The catch is the amount of ABFT in kg and effort is given as a) number of purse seine sets or b) days at sea: a) Weight of catch per set (kg/set) b) Weight of catch per day (kg/day)

Catch per set and catch per day represent catch per unit effort (CPUE) on each purse seine vessel involved in the targeted fishing on ABFT from 2014 to 2019. These figures are compared to the corresponding catch rates derived from the task II data reported by the CPC’s fishing in the Mediterranean Sea in the period 2014 to 2018. The task II data for 2019 have not yet been reported by the CPCs.

Due to extremely low catch rates and fishing capacity involved during after the comeback of ABFT in Norwegian waters, we have not seen the purpose of performing any statistical analysis on the fishing capacity for longline vessels. The catch rates and fishing capacity have been zero for most longline vessels, except for one vessel catching one adult ABFT, based on the few and sporadic longline attempts that have been conducted so far in Norwegian waters from 2015 to 2019.

We organized the date of the first and last targeted catch of ABFT from each year (2014-2019) to study the temporal development in the Norwegian purse seine fishing for ABFT.

Statistical analyses were conducted in R-statistical software (R Development Core Team, 2013), where p < 0.05 was chosen as significant level with a 95% confidence interval for the relevant tests.

3. Results

In the following we present results of our different analysis. We will begin by presenting results on school size, density and feeding behavior of ABFT in Norwegian waters, followed by results on catch rates in the purse seine in Norwegian waters.

217 School size, density and feeding behavior of ABFT in Norwegian waters

Schools sizes on ABFT ranged from an estimated 1-5 individuals up to >100 individuals (Figure 1a, b). The ABFT school size category of 1-5 individuals based visual observations at the surface dominated with >50% of total schools counted from 2014 to 2018 (Figure 1a). Few schools/shoals of ABFT were estimated to be larger than 100 individuals compared to the other school size categories (Figures 1a and 1b).

Most observations of ABFT in Norwegian waters were observed in August and September for the years 2014- 2019 (Figure 2). We also observed quite a lot of ABFT in October as well as in November, whereas very few observations of ABFT were registered in July (Figure 2).

The ABFT schools feeding in Norwegian EEZ during summer and autumn are predominantly small and involve dynamic behavior sometimes highly visible at the surface (Figure 3).

A significant relationship was found between ABFT school size versus Julian day of observation (Figure 4). School size was affected by time of the year (df = 2, Deviance = 11.19, Residual df = 182, Residual Deviance = 241.32, p < 0.01), with an optimum time of year, defined by a significant negative 2nd order polynomial (z = 2.44, p < 0.02). The probability of encountering schools > 10 individuals was highest between mid-September to mid- October (Figure 4).

Catch rates in the purse seine fishery in Norwegian waters

We estimated catch rates (the total weight of fish caught by an amount of effort) on Atlantic bluefin tuna (ABFT) (Thunnus thynnus) from purse seine vessels licensed to target ABFT during the active feeding season (August to October) from 2014 to 2019 within the Norwegian EEZ. During these years, we have 10 observations from the targeted ABFT fishery, representing 8 unique purse seine vessels with a total catch of 144 tons of bluefin tuna (Figure 5). Six of the purse seine vessels had an overall length between 24 and 40 meters, and the remaining two vessels had an overall length over 40 meters.

The average fishing capacity onboard purse seine vessels on ABFT for these years was 14,4 tons. After filtering out vessels with zero successful fishing operations each year, the average catch per vessel increases to 20,6 tons. During this period, only two purse seine vessels with an overall length greater than 40 meters participated in the ABFT fishery. The average catch from these two vessels was 6,5 tons.

The catches from the purse seine vessels targeting ABFT varies from 0,4 to 35 tons. The average catch in this period is 6,5 tons (Figure 5).

A total number of 150 fishing operations targeting ABFT in the Norwegian EEZ are recorded in the period 2014- 2019. Of the 150 fishing operations, only 22 (15 %) were successful. The corresponding average catch rate per successful fishing operation for purse seine vessels was estimated to be 6,5 tons and 1,0 ton when also including non-successful purse seine sets. By contrast, the catch per successful fishing operation in the Mediterranean Sea in this period was 14,9 tons.

In the Norwegian task II-reporting, the reported effort type is number of fishing days. When including the years with zero catch, i.e. also including the years 2014 and 2015, the total number of fishing days is 245. This corresponds to an average of 0,6 tons ABFT per fishing day. If we exclude 2014 and 2015, the average catch per fishing day increases to 0,9 tons ABFT. The corresponding catch of ABFT per fishing day calculated from the task II-data from the purse seine fishery in the Mediterranean Sea in 2014-2018 is 9,2 tons, which is significantly higher than the catch per fishing day observed in the Norwegian EEZ. The earliest day of catch in the targeted ABFT fishery in the Norwegian EEZ after 2014 is 24. August. The corresponding last day of catch is 25. September (Figure 6).

4. Discussion

Small school size and dynamic hunting behavior of ABFT may influence catch rates

The ABFT school size between 1-5 individuals dominated with >50% of total schools counted from 2016 to 2018 in Norwegian waters, based visual observations at the surface (see also Boge 2019). The school size may nevertheless vary from a few individuals to several hundred specimens depending on the season and area. The

218 schooling prey species such as juvenile and adult NEA mackerel as well as herring have advanced antipredator behavior including their maneuverability, confusion and dilution effect when swimming in synchronized and polarized schools (Pitcher and Parrish 1992; Nøttestad et al. 2004; Nøttestad et al. 2014). ABFT are probably capable to adjust their school size, density and speed/maneuverablility to the corresponding school size, density and speed/maneuverability of their hunted prey. Such phenomenon has been documented in predator-prey interactions at sea e.g. when large pods (n=22-46) of killer whales (Orcinus orca) are hunting for huge aggregations of herring during winter (Nøttestad et al. 2002), compared to small pods (n=8.2 ± 5.8) of killer whales involved when hunting for small and loose mackerel schools during summer in the Norwegian Sea (Nøttestad et al. 2014). Most predators including ABFT seem to attack their prey from below, where the prey is most vulnerable. This is partly because the predator is attacking from the deep dark sea, where it is easier for the predator to detect the prey than for the prey to detect the predator (Pitcher and Parrish 1992). Since most prey in Norwegian waters seem to be located in the upper part of the water column, the ABFT may become highly visible at the surface due to hunting for e.g. juvenile mackerel that are feeding themselves on zooplankton in the uppermost part of the water column above the stabilized thermocline (Nøttestad et al. 2016b).

The probability of encountering schools > 10 individuals was highest between mid-September to mid-October. Larger shoals have been observed in Norwegian EEZ, particularly later in the fishing season. The largest purse seine catches of ABFT taken so far after the comeback have been up to 191 individuals in September 2016. Nevertheless, this is the exception, although the probability of such encounters increased when time passed by to late autumn. This is probably related to the fact that during late autumn the zooplankton species migrates to deep water, and pelagic planktivorous fish species such as mackerel and herring gather in larger schools with higher density for increased protection when feeding is coming to an end for the season (Skjoldal et al. 2004). Mean school sizes observed in the Norwegian EEZ is, however, small compared to the mean school sizes (n > 100) observed of adult bluefin tuna in the Mediterranean Sea (Alemany et al., 2019; ICCAT 2019).

Our study strongly suggests lower fishing capacity of ABFT caught in the Norwegian EEZ during feeding in summer and autumn (see Boge 2019), compared to the higher fishing capacity of ABFT caught with purse seine vessels in the Mediterranean Sea during pre-spawning and spawning in late spring /early summer (ICCAT 2018; 2019). Plausible reasons behind low documented fishing capacity, high number of non-successful purse seine sets, and long searching times involved in the ABFT fishing in Norway are probably related to predominantly small schools of ABFT with unpredictable hunting behavior which lower the availability for the purse seine vessels during the feeding season in Norway. Furthermore, bad weather conditions during part of the fishing season in make it difficult to observe and catch ABFT, thus reducing the catch rate of ABFT in Norwegian waters.

After 2014, the average catch per purse seine vessel targeting ABFT in the Norwegian EEZ is 14,4 tons. If we exclude vessels with zero catch of ABFT, the average catch increases to 20,6 tons. Both figures are much lower than the catch rates defined by the SCRS in 2009, which are 49,78 tons for vessels with an overall length between 24 and 40 m and 70,7 tons for vessels with an overall length over 40 m.

The catch rates defined by the SCRS in 2009 distinguishes between purse seine vessels with an overall length under and over 40 meters. With only two Norwegian purse seine vessels over 40 meters participating in the ABFT fishery, there is not enough data to calculate catch rates for purse seine vessels over 40 meters. The dynamic behavior and small schools of ABFT will not favor larger purse seine vessels, and we therefore suggest that the average catch of 20,6 tons should apply to all purse seine vessels when fishing in Norwegian waters.

ICCAT recommendation 19-04 paragraph 29 states that the purse seine fishing season in the Norwegian Economic Zone and in the Icelandic Economic Zone shall be from 25 June to 15 November. It could be argued that the lower catch rates are compensated by a longer fishing season for the Norwegian purse seine vessels. However, there are significant costs in the ABFT fishery (observer fees, fuel costs from the long searching time, etc.) which means that it is only economically profitable to fish for ABFT when the catch rates are at a certain level. The vessels also have quotas in other fisheries. The mackerel fishery, which typically starts in the last part of September, is an important fishery for many of these vessels. Lastly, the weather along the Norwegian coast in the autumn will also reduce the number of potential fishing days.

Challenges for ABFT purse seine fishing in Norwegian waters

ABFT may perform both fast swimming and long-distance migrations in Norwegian waters with the largest individuals migrating furthest to the north (Nøttestad et al. 1999; Nøttestad and Graham 2004; ICCAT 2018) The migration pattern of an ABFT tagged west of Bergen in 2018, showed quite large distances travelled by the ABFT within rather limited period of time (Ferter et al. 2018). This indicate that ABFT may exhibit considerable spatial

219 movements in search for suitable prey such as pelagic schooling fish (NEA mackerel, herring, sand lance and ) along the coast of Norway (Cort and Nøttestad 2007; Tangen et al. 2016; Nøttestad et al. 2017). ABFT being constantly on the move for food will of course make it more difficult to observe, locate and successfully catch the ABFT in Norwegian waters using purse seine vessels.

Decades have passed since most Norwegian fishermen last time were involved in fishing for ABFT in Norway, due to the disappearance of ABFT in the 1980s (Nøttestad and Graham 2004; Nøttestad et al. 2017). Limited knowledge and experience when fishing for ABFT in Norway after the comeback from around 2012, among skippers and crew onboard purse seine vessels, may to some extent explain the low catch rates and low fishing capacity found from 2014 to 2019. Nevertheless, the inherent highly dynamic behavior and small schools of ABFT dominating the picture in Norwegian waters from August onwards, are probably the major challenge the fishermen in Norway are facing when it comes to catching bluefin tuna. On the other hand, it has been shown that targeting smaller schools of herring with purse seine, reduces crowding densities and thereby also stress to the fish, ensuring higher meat quality (Tenningen et al. (2012). This is also likely to apply catching small schools of ABFT, by maintaining the high quality of the fish, which is also ensured through faster handling and proper storage of the catch onboard each purse seine vessel.

Catch statistics from targeted fishing and bycatches of ABFT from other countries/parties during the last few years in the northern part of the Northeast Atlantic, indicate the same presence of predominantly small sized schools of ABFT. This furthermore suggest that small school sizes with low densities dominate in the northern part of the Northeast Atlantic during the summer and autumn. This include ABFT found in Icelandic (Olafsdottir and Ingimundardottir 2003) and Greenland waters (MacKenzie et al. 2014). Norway is the only country in the Northeast Atlantic fishing ABFT with purse seine vessels. Other countries/parties fishing ABFT with purse seine vessels in ICCAT, are predominantly found inside the Mediterranean Sea.

Future perspectives

Progress need to be done in Norway to further develop and improve the skills and experience of skippers and crew during ABFT purse seine fishing operations, to better understand and tackle the dominating behavior of ABFT in Norwegian waters during their feeding period, characterized by schools being small, loose and very dynamic with high speed and maneuverability.

5. Acknowledgements

We would like to thank Geir Huse and Bjørn Erik Axelsen at the Institute of Marine Research in Norway and Per Sandberg at the Directorate of Fisheries in Norway for useful and constructive comments and suggested changes to this document.

220 References

Alemany, F., S. Tensek, and A. Paga Garcia 2020. ICCAT Atlantic-Wide Research Programme for bluefin tuna (GBYP) activity report for the phase 8 and the first part of phase 9 (2018-2019). Collect. Vol. Sci. Pap. ICCAT, 76(2):521-566. Boge, E. 2019. The return of the Atlantic bluefin tuna to Norwegian waters. Master thesis in Fisheries Biology and Management, University of Bergen, Norway. 84 p. Cort, J. L. and Nøttestad, L. 2007. Fisheries of bluefin tuna (Thunnus thynnus) spawners in the Northeast Atlantic. ICCAT Collective Volume of Scientific Papers, 60: 1328-1344. Ferter, K., Tracey, S., Hinriksson, J., Bjelland, O., Onandia, I. and Nøttestad, L. 2018. Tagging of Atlantic bluefin tuna (Thunnus thynnus) with pop-up satellite archival tags (PSAT) in western Norway during 2018. Institute of Marine Research, Norway, 12p. Fromentin, J. M. and Powers, J. E. 2005. Atlantic bluefin tuna: population dynamics, ecology, fisheries and management. Fish and Fisheries 6: 281-306. Hamre, J. 1957. Atlantic bluefin tuna. Master thesis in Biology, University of Bergen, Norway. 74 p. (in Norwegian). Hamre, J. and Thiews, K. 1964. Report of the Bluefin Tuna Working Group. On the Size composition of Tuna Catches from 1956-1962. In: Fredriksson, A. (ed.). ICES Statistical Newsletters, 43 p. Huse, G., Holst, J. C., Utne, K., Nøttestad, L., Melle, W., Slotte, A., Ottersen, G., Fenchel, T. and Uiblein, F. 2012. Effects of interactions between fish populations on ecosystem dynamics in the Norwegian Sea–results of the INFERNO project. Marine Biology Research, 8, 415-419. ICCAT 2018. Report of the Standing Committee on Research and Statistics (SCRS). Spain, Madrid, 1 to 5 October 2018, ICCAT Collective Volume of Scientific Papers. 469 p. ICCAT 2019. Report of the Standing Committee on Research and Statistics (SCRS). Spain, Madrid, 30 September to 4 October 2019, ICCAT Collective Volume of Scientific Papers. 454p. ICES. 2019. Working Group on Widely Distributed Stocks (WGWIDE). ICES Scientific Reports. 1:36. 948 pp. http://doi.org/10.17895/ices.pub.5574. MacKenzie, B. R., Payne, M. R., Boje, J., Høyer, J. L. and Siegstad, H. 2014. A cascade of warming impacts brings bluefin tuna to Greenland waters. Global change biology, 20, 2484-2491. Nøttestad, L., J. Giske, J.C. Holst, and G. Huse 1999. A length-based hypothesis to explain feeding migrations in pelagic fish. Canadian Journal of Fisheries and Aquatic Sciences 56 (Supplement I):26-34. Nøttestad, L., Fernö, A. and Axelsen, B. E. 2002. Digging in the deep: killer whales' advanced hunting tactic. Polar Biology: 25, 939-941. Nøttestad, L. and Graham, N. Preliminary overview of the Norwegian fishery and science on Atlantic bluefin Tuna (Thunnus thynnus). Scientific report from Norway to ICCAT Commission meeting in New Orleans, USA, 2004. pp.15-21. Nøttestad, L. and Graham, N. Lack of Atlantic bluefin tuna (Thunnnus thynnus) observations off Norway: Why did bluefin tuna does not enter Norwegian waters in 2005. Scientific report from Norway to ICCAT Commission meeting in Sevilla, Spain, 2005. pp.14-20. Nøttestad L., Sivle, L. D., Krafft, B. A., Langård, L., Anthonypillai, V., Bernasconi, M., Langøy, H., and Fernø, A. 2014: Prey selection of offshore killer whales Orcinus orca in the Northeast Atlantic in late summer: spatial associations with mackerel. Marine Ecology Progress Series, 499: 275-283. DOI:10.3354/meps10638. Nøttestad, L., Utne, K. R., Óskarsson, G. J., Jónsson, S. Þ., Jacobsen, J. A., Tangen, Ø., Anthonypillai, V., et al. 2016. Quantifying changes in abundance, biomass and spatial distribution of Northeast Atlantic (NEA) mackerel (Scomber scombrus) in the Nordic seas from 2007 to 2014. ICES Journal of Marine Science, 73: 359-373.

221 Nøttestad, L., Tangen, Ø., Rong Utne, K. and Hamre, J. 2017. The comeback kid: Atlantic bluefin tuna (Thunnus thynnus) returning to highly productive feeding grounds off Norway. Institute of Marine Research, 14 p. Nøttestad, L., Ólafsdóttir, A.H., Anthonypillai, V. Homrum, E., Jansen, T.; Wieland K. et al. 2019. Cruise report from the International Ecosystem Summer Survey in the Nordic Seas (IESSNS) 28th June – 5th August 2019. Working Group Document to ICES Working group on Widely Distributed Stocks (WGWIDE, No. 5). Spanish Institute of Oceanography (IEO), Santa Cruz, Tenerife, Canary Islands 28. August – 3 September 2019. 51 pp. Olafsdottir, D. and Ingimundardottir Th. 2003. Experimental fisheries for bluefin tuna (Thunnus thynnus) within the Icelandic EEZ in 1996-2002. SCRS/2002/142. Collective Volume Scientific Paper 55(3): 1242-1253. Pitcher T.J., Parrish J.K. 1993. Functions of shoaling behaviour in . In: Pitcher T.J. (ed) Behaviour of , 2nd edn. Chapman & Hall, London, p 363−439. Tangen, M. 1999. Størjefisket på Vestlandet, Eide Forlag. 163 p. (In Norwegian). Tangen, M., Hamre, J., Johnsen, E., Nakken, O., Nedreaas, K., Tangen, Ø. and Ågotnes, P. 2016. Tobis ved Vestlandet og i Nordsjøen 1950-1990. 20 p. (In Norwegian). Tenningen, M., Vold, A. and Olsen, R. E. 2012. The response of herring to high crowding densities in purse- seines: survival and stress reaction. ICES Journal of Marine Science, 69: 1523-1531. Trenkel, V., Huse, G., MacKenzie, B., Alvarez, P., Arrizabalaga, H., Castonguay, M., Goñi, N., Grégoire, F., Hátún, H. and Jansen, T. 2014. Comparative ecology of widely distributed pelagic fish species in the North Atlantic: implications for modelling climate and fisheries impacts. Progress in Oceanography, 129: 219-243.

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Figure 1. a) Percentage of Atlantic bluefin tuna (ABFT) school size categories observed from 2014 to 2019. B) Number of ABFT school size categories observed from 2014-2019.

Figure 2. Number of observations of Atlantic bluefin tuna (ABFT) in Norwegian waters per month from 2014 to 2019.

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Figure 3. Large adult Atlantic bluefin tuna (ABFT) hunting for juvenile mackerel at the surface along the coast of Norway. Photo: Enrico Wyrwa.

Figure 4. Probability of encountering school sizes > 10 fish of ABFT by days during the year, where day 200 represent the start (July) and day 350 the end (December) of the ABFTs feeding season in Norwegian waters. The blue line represents the best model that includes a significant 2nd order polynomial. The shaded area represents the 95% confidence interval for the model line (Boge 2019).

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Figure 5. Catches from purse seine vessels in 2014-2019. Each dot represents a catch.

Figure 6. Catches per day from purse seine vessels in 2014-2019.

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