Sample Research paper on Atlantic Bluefin Tuna

Introduction

The Atlantic Bluefin tuna constitutes among the fastest, most attractive, and largest fish of the world. Their body shape is designed for speed and maximum endurance, and their coloring, shimmering silver-white on the bottom and metallic blue on the top, aids them in camouflaging easily within the waters both from below and above. The species is scientifically referred to as Thunnus thynnus, divided into two major stocks, the eastern, and western stocks. This species has captured the attention of quite a number of philosophers and scientists for decades. The attraction is drawn from its striking biological possessions, including their swimming speed and massive size (Anderson, 1990, 33).

Atlantic Bluefin tuna have also captured significant economic interest through their continued exploitation in the Mediterranean Sea for ages. Fisheries for this species date back thousands of years in the Mediterranean unlike in the western Atlantic where they only emerged in the 1950s and as much as they are currently known as to be one of the highly priced species of tuna, the western Atlantic did not have any market until the last quarter of the 1950s. The demand for Thunnus thynnus increased dramatically with the increase in demand for sashimi and sushi between the 1970s and 1980s, which translated, to an increase in the price of the species and a major decrease in their populations (Block et.al., 2001, 57).

Population structure and ecology

Habitat

Thunnus thynnus are inhabitant of the pelagic environment of the North Atlantic Sea and its adjacent waters, succinctly from the Black Sea to the Gulf of Mexico, and the Mediterranean Sea all the way from the northern part of Norway to the equator (National Research Council, 1994, 63). Among its tuna species, Atlantic Bluefin tuna is the only large pelagic fish that is a permanent residence of the temperate waters of the Atlantic. Information from archival tracking and tagging has attested that this type of species is capable of withstanding high temperatures of 30 degrees and lows of 3 degrees, maintaining its internal body temperature.

In the past, Thunnus thynnus was assumed to occupy the subsurface and surface of the coastal waters out of preference. This assumption has however been changed by ultrasonic telemetry, and archival tagging. Both adult and juvenile Atlantic Bluefin tunas are frequent deep divers, diving up to depths of between 500m to 1000m. Preferential gradients and ranges in sea temperatures are hypothesized to be the influential factor that determines movement and special distribution of the Atlantic Bluefin tunas. However, this species is less likely to realize the gradient in horizontal temperatures as vertical temperature gradients are higher compared to horizontal. Atlantic Bluefin tuna species migrate more often, and during a migration process, they could migrate over several thousand miles. However, as much as they are a highly migratory species, this species has been discovered to spawn around the same area, succinctly, the Mediterranean and Gulf of Mexico.

Biology

Maturity

Atlantic Bluefin tunas have been found to mature approximately within four years for those found within the Mediterranean, and the East Atlantic regions, attaining a maturity length of between 110cm-120cm and weigh up to a maximum of 35 kilograms, and approximately eight years for those found in the West Atlantic, attaining a maximum maturity length of 200cm and about 150kg in weight (Rooker et.al., 2008, 194). The disparity existing between the two sides of the north Atlantic has been used as a key criteria for stock separation of Thunnus thynnus. There has been further progress made in the process involved in maturity determination of Thunnus thynnus through measuring their hormonal concentration in their bloodstream. Given the maturity age difference between species found in the Mediterranean and West Atlantic, Thunnus thynnus is likely to support the discrete population hypothesis of the existence of two stocks, however, it is quite confusing due to the apparent mixing level (Rooker et.al., 2008, 194).

Reproduction

Atlantic Bluefin tuna, just like the rest of its tuna species, is iteroparous and oviparous. They have a synchronous oocyte type of development and also a multiple spawner, with spawning frequency being estimated to be between one and two days in the Mediterranean. The production of eggs by Atlantic Bluefin tunas, just like the rest of the fish family, appears to be dependent on age or size. For instance, a female between fifteen and twenty years is capable of carrying up to 45 million eggs while on the other hand, a five-year old female produces an average of five million eggs (Block et.al., 2005, 153).

Recent estimates from stereological quantification have estimated the average fecundity for Thunnus thynnus at 93 oocytes per gram of body mass for the eastern species. However, the general assumption that adult Atlantic Bluefin tunas spawn on yearly basis has been contradicted by findings from experiments of electronic tagging which have depicted that adult-size fish usually occur quite a distance from any known spawning ground during the spawning season(Block et.al., 2005, 153). The assumption is also countered by experiments in captivity, which has argued that an individual Thunnus thynnus might spawn ones in every two to three years.

Feeding

Atlantic Bluefin tuna larvae majorly feed on small zooplanktons just like majority of other marine fish, specifically copepoda naupli and copepods. Adult and younger Atlantic Bluefin tunas have been identified to be opportunistic feeders as most predators are. The diet of Thunnus thynnus would comprise of salps and jellyfish, and other sessile and demersal species such as sponges, octopus, and crabs. Generally, juveniles feed on cephalopods, crustaceans, and fish while adults majorly prey on fish, such as sand lance, herring, bluefish, anchovy, mackerel, sprat, and sardine. Under normal circumstance, the stomach content of an Atlantic Bluefin tuna would constitute of at least two fish pray species (Lutcavage et.al., 1999, 315).

Growth

Atlantic Bluefin tuna juveniles experience faster growth compared to adults lengthwise. An infant born in June will have attained a length of approximately between thirty to forty centimeters and a weight of 1kg by October. One year down the line, the fish will have attained approximately 4kg and a length of 60 cm (Mather et.al., 1995, 176). Young Atlantic Bluefin tunas are relatively slimmer compared to the adults who are often larger and thicker. Thunnus thynnus attains an approximate length of between 200 cm at 10 years of age, weighing approximately 150kg. At 20 years of age, this species would weigh at approximately 400kg achieving an average length of 300cm, which varies between individuals (Mather et.al., 1995, 176). Allegedly, species of Thunnus thynnus located in the West have been reported to experience a faster growth rate and attain much larger sizes and weight as compared to species found in the Mediterranean and East. A couple of studies have also indicated faster growth in the male Atlantic Bluefin tunas compared to the females (Mather et.al., 1995, 176).

Natural mortality

Atlantic Bluefin tunas have a longer lifespan, which gives them a lesser variable of instantaneous annual rate compared to fish with shorter lifespan. Instantaneous annual rate is observed in younger Atlantic Bluefin tunas than during the start of adulthood. The instantaneous annual rate differs depending on the population density, sex, size, environment, and predation (Fromentin, 2006, 215). Predation for Thunnus thynnus results from other bigger pelagic fish such as killer whales and sharks. Recent studies have observed killer whales to flock the Straight of Gibraltar’s entrance on the onset of the spawning migration of Atlantic Bluefin tunas. The proportion of males in catch samples of large individuals was also observed to be higher which may be an indication of lower growth for the females or higher instantaneous annual rate(Fromentin, 2006, 215).

Dangers facing the species

Thunnus thynnus are certainly facing a major threat to their existence. Unlike the pacific Bluefin tunas that are not endangered, Atlantic and Southern Bluefin tunas are highly at risk of being extinct due to overfishing (NOAA). This has shifted attention to the conservation and preservation of this species. Their stocks have declined drastically with a 96% margin and approximately 90% of the specimen caught are juveniles that have not reproduced yet (Fromentin & Powers, 2005, 71). The high risk of their extinction is pushed majorly by two factors: increased demand for sushi and the high price tag placed on the species. The rarity of this species saw one sold recently in Japan for over one million Euros (Skerry, n.d., 1).

Conclusion

The Atlantic Bluefin tuna constitutes among the fastest, most attractive, and largest fish of the world, which has earned them a major economic interest and attention. Their quality, high fat content, and delicious taste among other factors has also seen their demand increase drastically. As a result, this species lives under the threat of extinction. Among other factors, increase in demand for sushi by Japanese sushi lovers and the extremely high price tag placed on this species adds up to reasons contributing to their extinction.

References

Anderson, A., 1990. The Atlantic Bluefin tuna – yesterday, today and tomorrow. Point Pleasant, N.J., Ocean Sport Fishing.

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Block, B. A., Dewar, H., Blackwell, S. B., Williams, T. D., Prince, E. D., Farwell, C. J., … & Fudge, D., 2001. Migratory movements, depth preferences, and thermal biology of Atlantic Bluefin tuna. Science293(5533), 1310-1314.

Block, B. A., Teo, S. L., Walli, A., Boustany, A., Stokesbury, M. J., Farwell, C. J., … & Williams, T. D., 2005. Electronic tagging and population structure of Atlantic Bluefin tuna. Nature434(7037), 1121-1127.

Fromentin, J. M., 2006. Atlantic Bluefin tuna. ICCAT Manual, 5.

Fromentin, J. M., & Powers, J. E., 2005. Atlantic Bluefin tuna: population dynamics, ecology, fisheries and management. Fish and Fisheries6(4), 281-306.

Lutcavage, M. E., Brill, R. W., Skomal, G. B., Chase, B. C., & Howey, P. W., 1999. Results of pop-up satellite tagging of spawning size class fish in the Gulf of Maine: do North Atlantic Bluefin tuna spawn in the mid-Atlantic? Canadian Journal of Fisheries and Aquatic Sciences56(2), 173-177.

Mather, F. J., Mason, J. M., & Jones, A. C., 1995. Historical document: life history and fisheries of Atlantic Bluefin tuna (No. 8986). US Dept. of Commerce, National Oceanic and Atmospheric Administration, National Marine Fisheries Service.

National Research Council (U.S.), 1994. An assessment of Atlantic Bluefin tuna. Washington, DC, National Academy Press.

NOAA – FishWatch: Western Atlantic Bluefin Tuna, n.d. [Online] Available at:

Rooker, J. R., Secor, D. H., De Metrio, G., Schloesser, R., Block, B. A., & Neilson, J. D., 2008. Natal homing and connectivity in Atlantic Bluefin tuna populations. Science322(5902), 742-744.

Skerry, B. J., n.d. Bluefin Tuna. [Online] Accessed at

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