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Fishing Capacity on Atlantic Bluefin Tuna by Purse Seine Vessels Fishing in the Norwegian Eez from 2014 to 2019

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Fishing Capacity on Atlantic Bluefin Tuna by Purse Seine Vessels Fishing in the Norwegian Eez from 2014 to 2019 SCRS/2020/017 Collect. Vol. Sci. Pap. ICCAT, 77(2): 215-225 (2020) FISHING CAPACITY ON ATLANTIC BLUEFIN TUNA BY PURSE SEINE VESSELS FISHING 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 Atlantic bluefin tuna (ABFT) (Thunnus 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 fins 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 Fish, P. O. Box 1870, Nordnes, NO-5817 Bergen, Norway. Email: [email protected] 2 Directorate of Fisheries, 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 Norwegian Sea and Norwegian coast represent one of the world’s best “fridges” of high quality food for ABFT; around 10 million tonnes of widely distributed pelagic fish such as Northeast Atlantic (NEA) mackerel (Scomber scombrus), herring (Clupea 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 zooplankton 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 seine fishing 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).
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