There are three species of bluefin tunas: the Atlantic (Thunnus Thynnus the largest and most endangered), Pacific (T. orientalis), and Southern (T. maccoyi). More distant relatives in the genus Thunnus are the bigeye tuna (T. obesus) and the yellowfin tuna (T. albacares).
The Atlantic bluefin tuna Thunnus Thynnus is the largest tuna and the largest species in the mackerel family that can live for 40 years. It is native to both the western and eastern Atlantic. One spawning ground exists in the western Mediterranean, particularly in the area of the Balearic islands. Their other important spawning ground is the Gulf of Mexico.
Bluefins are tremendous predators, already seeking out schools of fish like herring, mackerel, and even eels just after birth. With their superb vision they hunt by sight, while their torpedo-shaped body and retractable fins made for speed allow them to reach almost 80 km per hour. Their specialized blood vessel system (called a countercurrent exchanger) serves to maintain a body temperature that is higher than the surrounding. Which facilitates movements such as rapid and powerful turnings of the body. To keep the blood oxygenated they swim constantly with their mouth open.
Atlantic bluefin tuna has been hunted down massively since it became the most highly prized fish used in Japanese raw fish dishes like sushi and sashimi. In the restaurants in southern Spain (e.g. in the tuna paradise Barbate) the Atún Rojo is also a local delicacy. A small tin of tuna costs around 20 Euro. Driven by such high prices, fishermen have developed more refined techniques like anchored nets to catch tuna, with the fish disappearing as a result. Although tuna do provide food and livelihoods for people, they are more than just seafood. Tuna is a top predator in the marine food chain, maintaining a balance in the ocean environment
About 80% of the caught Atlantic and Pacific bluefin tunas are consumed in Japan, where it is considered as an ultimate delicacy. Most catches of the Atlantic bluefin tuna are taken from the Mediterranean Sea which is the most important bluefin tuna fishery in the world. 15 years ago the future for tunas looked really bad, reaching a low point in 2012. A report of the International Commission for the Conservation of Atlantic Tunas (ICCAT) concluded that only 10% remained of the population that existed 50 years ago. The catch quota was then brought back in 2008 from 26.750 tons to 12.900 tons worldwide for a 4 year period, restricting the catch to species larger than 30 kilos.
However illegal catch continued in the waters of Southern Spain. And when the tuna populations started to grow again, the ICCAT decided, under pressure from member countries such as Japan, France, and Italy, to increase the catch quota of bluefin tuna from 24,000 tons annually to 36,000 tons annually. Scientists and conservationists fear that increasing the catch limit will set back the bluefin recovery effort enacted in 2008 after stocks of the prized sushi fish have badly overexploited in the Mediterranean and Atlantic.
The narwhals (Monodon Monoceros) live year-round in the Arctic waters around Greenland, Canada, and Russia. They have been harvested for hundreds of years by Inuit people in northern Canada and Greenland for meat and ivory, and a regulated hunt continues. It is one of two living species of whale in the family Monodontidae, along with the beluga whale. Narwhals can live up to 50 years and grow to 6 m long. Their pigmentation is a mottled pattern, with blackish-brown markings over a white background. They are darkest when born and become whiter with age. During the winter, narwhals make some of the deepest dives recorded for a marine mammal, diving to at least 800 meters, several times per day, with some dives reaching 1,500 meters.
Normally, the canine tooth only on the left side of the upper jaw becomes a tusk. The tusk is actually an enlarged spiraled ivory tooth with sensory capability (somewhat like a feeler) and up to 10 million nerve endings inside, that can grow as long at 10 feet. Rarely, males develop two tusks. Only about 15 percent of females grow a smaller tusk.
What is the function of the strange rapier-like tusk of the male narwhal: is it a weapon or rather a signal? The general scientific consensus is that the narwhal tusk is not directly necessary for survival but serves as a sexual trait, much like the manes of a lion, or the feathers of a peacock Perhaps the sensors in the tusk have also a communicative function, the reason why some males have been seen ‘tusking’: that is crossing the tusks like rapiers in a simulated fight. Based on the disproportional growth and large variation in male tusk length biologist Zackary Graham from Arizona and colleagues found morphological evidence that narwhal tusks are indeed sexually selected during male-male contests. Parts of the body that are sexually selected are often disproportionally larger. The variation is tusk size among male narwhals is indeed much larger (between 45-250 cm) than the tail (or fluke, between 45-90 cm). Large tusks thus benefit male narwhals in sexual acts, probably signaling ’I am bigger than you'', and avoid potentially dangerous fights by impressing rivals (see insert). Source: Royal Society Biology Letters, March 2020
The green sea turtle (Chelonia mydas) is listed as endangered by the IUCN and CITES and is protected from exploitation in most countries where the species nests. Factors threatening its existence are direct harvesting for the consumption of its flesh, boat strikes, egg poaching, and habitat destruction, such as the sandy beaches that form their egg nesting sites. Worldwide, hundreds of thousands of sea turtles a year are accidentally caught in shrimp trawl nets, on longline hooks, and in fishing gillnets. The dramatic increase of tourism on the Carribean island also had a negative impact on the turtle populations.
Turtles may swim more than 2,600 kilometers to reach their spawning grounds, and then return to the beaches on which they were born to lay their own eggs. The green turtle is found on both the Caribbean and Pacific coastlines and can reach a weight of up to a hundred and eighty kilograms. The extensive, shallow continental shelf of eastern Nicaragua is home to hundreds of thousands, possibly millions, of green turtles that forage on the abundant seagrass that grows there. The turtles help to improve the health and growth of the seagrass beds. Although trade in turtles is prohibited, Nicaraguan law still allows the subsistence use of green turtles, and local demands from coastal Indian inhabitants have supplanted the historical export demand. Here the demand for its meat is estimated to involve approximately eleven thousand killings each year. Hunting these turtles is affecting the turtle populations, as the green turtle takes between twenty to fifty years to reach sexual maturity (they may live on to 80 years) and without sexually mature adults, the numbers of these turtles could decline rapidly.
The name porpoise is a strange assembly of fish and swine. It’s stocky body and bulbous head with a small blunt snout was probably why the species was called porcopiscus in Latin which is a compound of porcus (pig) and piscis (fish). The species' taxonomic name, Phocoena, is derived from the Greek phōkaina, which in turn comes from φώκη (phōkē) or seal. Suggesting that the ancients might have mistaken the porpoise for a seal.
Left: Harbor porpoise. Picture by F.Graner
Porpoises are small toothed whales (Odontoceti) that are closely related to oceanic dolphins. The porpoise, however, has always remained more mysterious than its relative the dolphin. Perhaps because they are less wide-spread over the world than the dolphins, and are also not easily spotted in the open sea. Porpoises rarely jump out of the water like dolphins and even then you are not likely to see more than the top part of its back with its small triangular dorsal fin when surfacing for a breath of air. Porpoises also have a compact body shape with a stiff neck while the dolphins have a long beak and flexible head.
Seven extant species of porpoises The porpoises belong to the order of the toothed whales, consisting of around 70 species. Odontocetes feed largely on fish and squid, not rely on their sense of sight, but rather on their sonar to hunt prey. They echolocate by creating a series of clicks emitted at various frequencies. There are now only seven extant species of porpoises that fall in the family of Phocoenidae. They are in respective order:
- Genus Phocoena (four species): the harbor porpoise, vaquita, spectacled porpoise, and Burmeister’s porpoise
- Genus Neophocaena (two species): the finless porpoise and narrow-ridged finless porpoise
- Genus Phocoenoides (one species), the Dall’s porpoise
As said, porpoises are not widespread, with some species specializing near the polar regions, usually near the coast. The most frequent species, the harbor porpoise (Phocoena phocoena) lives in the shallow, relatively cold northern coastal seas. One believes that there were once huge populations living in these waters, when their favorite food, the anchovies, was still abundant. The reason why the species was called harbor porpoises was that they probably often followed the fishing boats into the harbors. Because it is most commonly found in bays, estuaries, harbors, and fjords the species is particularly vulnerable to gillnets and fishing traps, pollution, and other types of human disturbance, such as underwater noise.
In past decades, pollution, particularly PCBs, caused a sharp decline in the population of harbor porpoises along all of the coastal areas of the southern North Sea. Porpoises were also highly affected by bycatch. Many porpoises, mainly the vaquita, are subject to great mortality due to gillnetting. The vaquita is a species of porpoise endemic to the northern part of the Gulf of California that is on the brink of extinction. The Dall's porpoise from the northern Pacific is still extensively harvested for meat in Japan. Stranded porpoises often got killed by drowning after they became entangled in fishing nets. The ‘pinger'', a loudspeaker tied to a floating buoy, is used by fishermen to keep the harbor porpoises away from their boats. But these devices could also serve to tell the porpoise that ’dinner is ready’, so rather attracting the animals than keeping them at a distance. Along the northern Dutch coast porpoises have been found with bites resembling the tooth of the grey seals, which often show up in the North Sea. Suggesting that adult grey seals are true predators, even preying on porpoises. The good news is that in the open North Sea the porpoises now seem to be back again, feeding mostly on herring, sprat and mackerel, and even smaller species like gobies. The estimate is that there are presently living around 250.000 species living in the North Sea, making them the most common cetacean in these waters.
Hibernation is a state of constant hypothermia (low body temperature) and low metabolism. This can often take a long period. The evolutionary advantage of hibernation is that a non-migratory mammal can survive during the winter without having to spend energy searching for food, which is then difficult to find. There are facultative hibernators entering hibernation only when either cold-stressed, food-deprived, or both, and obligate hibernators, who enter hibernation regardless of ambient temperature and access to food.
Fishes and reptiles Body temperature in fresh- and saltwater fishes including larger predators like sharks reflects the temperature of their watery environment: they are ectothermic. They do not hibernate in the strict sense because they cannot actively down-regulate their body temperature or their metabolic rate. However, some species experience decreased metabolic rates called dormancy, associated with colder environments and/or low oxygen availability (hypoxia). Water also makes a good shelter for freshwater fishes as well as reptiles such as frogs and turtles. When the weather gets cold, they move to the bottom of lakes and ponds. There, they hide under rocks, logs or fallen leaves or may even bury themselves in the mud where they become dormant. Cold water holds more oxygen than warm water, and the frogs and turtles can breathe by absorbing it through their skin. The same holds for the common goldfish (Carassius auratus) in domestic ponds that are able to survive in temperatures below 10 degrees Celsius, even when the pond is covered with ice, as long there is some oxygen available. With European winters becoming increasingly mild, goldfish may delay or even skip their annual period of a dormant state as long a there remains some food available from the surface.
Sea mammals A different situation holds for sea mammals. Whales are warm-blooded (endothermic) and will keep a high body temperature that does not change in the colder water. In order for whales to keep warm in cold/polar climates they have developed a thick layer of insulating blubber, which protects against freezing winds and icy water. Whales also profit from migration to colder oceans where food like krill is more abundant. Manatees, however, that lack the protective blubber do migrate from the sea to warmer water in winter, often found in inshore freshwater springs, or even power plants along the shore.
Climate change and hibernation To what extent do rising winter temperatures affect hibernation and animal's chances of survival? If the bees, hedgehogs or bats get out of their winter rest too early due to high winter temperatures, then there is far too little food (e.g. insects) available. When two periods of frost are separated by one warm week, the hedgehog is in trouble. When it awakes, it is hard for the animal to get back to sleep. And a hedgehog that is awake but unable to find food will not survive in the cold. Even the state of dormancy, accompanied by minimal use of the body resources and slowing down of physiological functions (think of our goldfish) could be essential, not so much to overcome a temporary shortage of food or lower body temperature, but as a rest period allowing recuperation and recovery of metabolic functions.