Species in Peril:
Bluefin, the largest tuna
The Atlantic bluefin tuna has small yellow fins with black edges and silver dots or bands on their bellies. It can grow up to 900kg and more than three meters and lives in the open seas. It never stops swimming to ensure getting enough oxygen from the water and helps to create the heat that keeps the fish’s temperature similar to the surrounding water. Its swimming speed is 43 miles per hour which help the fish to migrate from cold water where it feeds to warm water where it spawns. The bluefin tuna is hugely targeted in fishing because of its increasing use of it in the sushi market. This has risen the prices of the fish where a single fish can be sold for thousands of dollars.
This tuna lives in the Atlantic Ocean specifically in the north. It also lives in the seas near the Atlantic Ocean mainly in the Mediterranean Sea but also in the black sea and the Gulf of Mexico. Bluefin tuna mainly stays at the sea surface, but it can dive deep to 500-1000m. It prefers to stay at the surface because it is harder to forage and to control body temperature in deep water. Its movement is controlled by the differences in temperatures, as well as feeding availability.
Biological features of bluefin tuna:
Maturity is reached at 4 years old in the east Atlantic and Mediterranean Sea, but it matures at 8 years old in the west Atlantic like the pacific and southern bluefin tuna. Its length at maturity varies from east to west Atlantic, with east being 110–120 cm/30–35 kg and west being 200 cm/150 kg.
This tuna is a multiple-spawning fish that spawn every year during the spawning season which is from mid-April to June. It spawns eggs and the number of eggs depends on the size and the age of the female. A 5-year-old female produces about 5 million eggs compared to a 15–20 years female that can carry up to 45 million eggs. Fertilization of the eggs happens in the water and after two days of incubation without parental care, hatching happens.
Larvae feed on small zooplankton, but juveniles and adults feed on a wide variety of prey and are able to adapt to whatever food becomes available. Bluefin tuna juveniles grow about 30cm per year which is slower than other tuna species. Then growth in length becomes less in adults than in juveniles, while growth in weight increases.
It is a long-lived fish which means its natural mortality is low, and its higher during juvenile stages. Predation of bluefin tuna is from large pelagic sharks and killer whales. Males are higher in fish nets than females and this causes lower growth for females.
Threats to bluefin tuna
Bluefin tuna fishing is very ancient, but it increased dramatically after the use of traps in commercial fisheries. Fishing was mainly targeting juveniles until the late 90s when large fish starts to be in demand, especially by Japan. The development of the bluefin tuna market affected the fishing equipment and techniques to be more powerful. This development also caused an expansion in fishery areas which yields catches of 40,000-50,000 tons a year. As a result, the bluefin tuna population decreased and is labeled endangered. One of the solutions to protect it is putting a quota for catches of only 2,500 tons and a size limit of 6.4 kg. other solutions that the International Commission for the Conservation of Atlantic Tunas (ICCAT) has used to protect bluefin tuna are increasing the minimum size and ensure transfer of fish to cages from nets.
Other threats to bluefin tuna are oil spills that restrict the habitat of the fish and cause harm on the health of species. Climate change can also have an effect on the habitat of bluefish tuna. The change in the temperature of the water that gets warmer would affect the migration and spawning of the Atlantic bluefin tuna.
7-10 INTERESTING FACTS
- Have the widest geographical distribution among species of tuna.
- Can live in a range of temperatures from 3 °C to 30 °C because it has a stable body temperature.
- Adults do not feed during the spawning season.
- The largest is 678.58kg and it was caught in 1979.
- Bluefin tuna are easily affected by fishing because of their slower growth, later maturity, shorter spawning season, larger size, and longer life span.
- The International Commission for the Conservation of Atlantic Tunas (ICCAT) was founded in 1966.
- Its eyes are the smallest among other tuna species.
limited parts of sub-Saharan Africa
Reasons for Population Decline
Poaching and illegal trafficking of their horns