Drs. Leif Nøttestad and Øyvind Tangen are scientists at the Norwegian and North Sea Ecosystem and Fish Stocks group at the Norwegian Institute of Marine Research, specializing in trophic interactions and behavior of fish and mammals. This series finds different perspectives to problems facing bluein tuna and large pelagics. -Dr. Molly Lutcavage, Large Pelagics Research Center Director

Atlantic bluefin tuna in Norway: where have all the tuna gone?
Leif Nøttestad and Øyvind Tangen, the Norwegian Institute of Marine Research

The north Atlantic bluefin tuna, Thunnus thynnus, is a member of the mackerel family (Scombridae) and regarded as one of the most highly evolved fish species and are among the most powerful, as well as largest fish inhabiting the ocean. Preying on mackerel, herring, whiting, and squid, giant bluefin may reach weights of more than 500 kilograms, grow as long as 3-4 meters, and live to 30 years of age. Tuna, originating from the Greek word meaning "to rush," swim at speeds approaching 90 km/h.

The Norwegian fishery on Atlantic bluefin tuna

Tuna has existed in Norway for thousands of years, based on archeological findings. The Norwegian bluefin tuna fishery had a slow start in the beginning of the last century. The fishermen had problems finding a practical way to catch the large and fast swimming tuna. Several methods were tried out during the 1920's. Smaller fishing boats were catching tuna in the beginning using large sized hooks or harpoon guns and to some extent experimented with purse seine. A tuna seine inspired from American fishing gear technology was later constructed and then modified several times. The big breakthrough in the fishery came in the northern part of Norway in 1948 when a new type of purse seine was constructed. Within a few years the bluefin tuna fishery became an important fishery in Norway during the months of July-October when the tuna migrated to productive northern waters for active feeding (see photo).

The tuna arrived along the Norwegian coast in several runs with the timing and migration pattern depending on the size and age composition of the schools. Fish from both the western and eastern bluefin tuna stock were feeding and caught in Norwegian waters. Large tuna up to 500 kg were caught in the north, while medium sized adult tuna (<100 kg) dominated further south. In the 1950's the southwestern part of the country became the major fishing area and the annual catches peaked up to 14 000 tons. In some years more than 350 seiners (max. 470) participated in this fishery along the Norwegian coast (Figure 2). These vessels cooperated with assistant vessels and vessels used as fish carriers. A total of 3500 - 4000 fishermen were then involved in the tuna fishery. During the 1960's and 1970's it became obvious that the tuna stock was overfished. There were two important periods with respect to migrations affecting the Norwegian fishery, from 1950 to 1962 and from 1963 to 1985.

The first period is categorised by a number of age groups being present in Norwegian waters. In the early period, several year classes from the 1940's (1945, 1946, 1948, 1950 and 1952) provided high catches along the whole coast of Norway up to 71ºN. The older fish, up to 200 kg arrived in early July and migrated to the north Norwegian coast. After feeding there for some 3-4 weeks the fish migrated quickly southwards into the bank area of the North Sea . The youngest individuals, up to 70 kg migrated to the south-eastern part of Norway , while most medium sized tuna remained in south-western Norway from July to October (see left side of map). During this period, the fishery gradually decreased in range, and by the early 1960's very few fish were caught north of 62ºN, presumably due to the demise of the year classes from the early 1940's. From about 1956 onwards, the fishery relied mostly on the strong year classes from 1950 and 1952.

From 1963 onwards, the fishery in Norway consisted also mainly of the 1950 and 1952 year classes, while the largest tuna (from 1940's) were not present in sufficient numbers for a viable fishery of the northern areas off the Norwegian coast, and the majority of catches were taken south of 62ºN (see right side of map).

Following the strong year classes of 1950 and 1952 there appears to have been very little recruitment to the stock, except for the 1954 year class, which seems to be the last years class migrating north to some extent. This has resulted in a decline in the annual catch and a change in the migration pattern of the fish. From 1965 onwards the vast majority of tuna catches were taken in the districts of Sogn and Fjordane and Hordaland in Southwestern Norway . Thus, the stock size of tuna declined at the same time as the distribution area and migration pattern along the west coast was drastically reduced due to the diminished stock size. Fewer year classes were also present in the catches, indicating lack of proper recruitment providing less profitable tuna fishery. Norway basically experienced a stock and range collapse on the Atlantic bluefin tuna stock in the mid 1960's. The average weight of tuna caught in Norwegian waters increased from <100 kg in the early 1950's to >350 kg in the late 1970's, showing the year by year increase in growth rate of individuals especially from the 1950 and 1952 year classes (see photo left) . The bluefin tuna had spent their summers in Norwegian waters for thousands of years. In 1986 only 70 tuna were caught, and the Norwegian tuna fishery was a part of the history.

Norway's membership and role in ICCAT

Norway became a full member of the International Commission for the Conservations of Atlantic Tunas (ICCAT) in March 2004. It had a central role in science and fishing of Atlantic bluefin tuna up until around 1970; providing detailed catch statistics starting in 1950 comprising of individual fish weight and total numbers caught by purse seine set in each fishing region around the Norwegian coast for the period. During the last decades very few adult Atlantic bluefin tuna has been migrating and feeding in the highly productive northern ecosystems such as the Norwegian Sea . Norway claims that this situation is indicative of the unhealthy state of the bluefin tuna population, represents long-term sign of considerable growth over-fishing, signalling that the bluefin tuna population is not managed in a sustainable way.

Explanations for the disappearance of bluefin tuna in Norwegian waters

There exist several plausible mechanisms and explanations of why we do not observe any bluefin tuna anymore in highly productive Norwegian waters. The most obvious and probably most important long-term factor has been substantial overfishing for decades on both juvenile and large adult tuna. Variable recruitment and year class strength seems to be a major driving force influencing distribution and migration patterns, but little knowledge is available. Learning processes and information transfer between tuna year classes may play a significant role in establishing large-scale migration patterns in tuna as clearly observed in Atlantic herring. Prey abundance and feeding opportunities, as well as physical driving forces such as decadal climate variability and temperature fluctuations in Northeast Atlantic ecosystems should effect both population level and distribution patterns.

Possible solutions to the dramatic situation of Atlantic bluefin tuna

We emphasise how the overall population size, health and production for a future sustainable and viable fisheries of the East Atlantic bluefin tuna stock can be improved by; 1) significantly reduce overall fishing effort and quotas in the short term, especially on the vulnerable spawning grounds, to benefit a much higher sustainable medium- and long-term yield, 2) increase minimum landing size to protect juveniles, and 3) reduce transfer of old and large bluefin tuna to large pens for fattening in order to protect individuals with the highest reproduction potential and spawning success within the population. A substantial and effective control and enforcement regime need to be put in place by each member state in ICCAT to ensure that the bluefin tuna is managed in a responsible and sustainable manner for the benefit of future generations.