Sea otter - Sea otter

A marine mammal belonging to the family Mustelidae in the order Carnivora of the class Mammalia. The head and body length of an adult is 1.2 meters, and the tail is 0.4 meters. It weighs 25 to 40 kilograms. Males are slightly larger than females. Its body shape is similar to that of a weasel, but its head is flattened from top to bottom and its body is long and cylindrical. The sensory hairs around its mouth are long and prominent. Its forelimbs are short and small. It has webbed feet between its toes, but is adept at grabbing objects. Its hind legs are large, with well-developed webbed feet that serve as swimming organs. Its tail is flat and pointed at the tip. Its body color varies greatly from individual to individual, ranging from black-brown to gray-brown, and is paler toward the head. It turns silvery white as it ages. It consists of a cotton-like undercoat and thick, long topcoat, with one topcoat and about 70 undercoats growing from each pore. The layer of air between the dense undercoat helps to keep it warm.

Sea otters are marine creatures, and although they may climb reefs, they usually live in the waters around rocky coastal areas. They typically rest or swim on their backs on the surface of the water. When doing so, they raise their heads and front legs above the water and use their hind legs and tail to propel themselves and balance their bodies. They also often rest with seaweed wrapped around their bodies. Sea otters live in the coastal waters of the North Pacific Ocean, from central California to Alaska, the Aleutian Islands, Kamchatka Peninsula, the Kuril Islands, Sakhalin, and Hokkaido, at depths of less than 40 meters, and do not migrate.

In the 18th and 19th centuries, they were hunted in large numbers for their high-quality fur, and were in danger of extinction in many areas. However, as a result of protection under an international treaty that began in 1911, the stock has gradually recovered, and in the 1970s the stock was estimated at about 140,000, and the distribution area has gradually expanded. Recently, they have been seen on the southern coast of Hokkaido. Females reach maturity in 3-4 years, and their breeding cycle is 2 years. The peak mating season is from October to November, and most births occur in May and June, but they can be seen mating and giving birth at any time of the year. The natural mortality rate is about 10%, and the maximum lifespan is estimated to be 23 years. They feed on shellfish, echinoderms, and arthropods, and they also use stones as tools to crack shells to eat.

Currently, hunting of sea otters is completely prohibited, and their fur is not commercially available. As the resource has recovered, the restrictions on capturing them for breeding have been relaxed, and recently aquariums across Japan have been importing them from the United States for viewing. As the resource has increased, sea otters have come into competition with fishermen. Sea otters have a rapid metabolism and prefer to prey on large quantities of highly commercially valuable abalone, bivalves, and sea urchins, causing harm to fishermen who make a living from these marine products.

[Masaharu Nishiwaki]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

哺乳(ほにゅう)綱食肉目イタチ科に属する海獣。成体の頭胴長は1.2メートル、尾は0.4メートル。体重は25～40キログラム。雄は雌よりやや大きい。イタチに似た体形であるが、頭部は上下に平圧された形で、胴は長く筒形である。口辺の感覚毛は長くて目だつ。前肢は短くて小さい。指の間には水かきがあるが、器用に物をつかむことができる。後肢は大きく、水かきが発達し、遊泳器官として適応している。尾は平たく、先端はとがる。体色は黒褐色から灰褐色まで個体差が大きく、頭部ほど淡い。年をとるにつれて銀白色に変わる。綿のような下毛と、太く長い上毛からなり、一つの毛穴から1本の上毛と約70本の下毛が生える。密に生えた下毛の間の空気の層が保温の役目をする。

　ラッコは海生で、岩礁に上ることもあるが、通常は海岸の岩礁域の海の中で生活し、背中を下にして水面で休んだり、泳いだりするのが特徴的である。その際には頭と前肢を水面に出し、後肢と尾で推進したり体のバランスをとったりする。また、海藻を体に巻き付けて休むことも多い。ラッコは北太平洋の中央カリフォルニアからアラスカ、アリューシャン列島、カムチャツカ半島、千島列島、樺太(からふと)（サハリン）、北海道にかけての沿岸の水深40メートル以浅の海に生活し、回遊はしない。

　良質な毛皮の生産を目的として、18世紀から19世紀にかけて乱獲され、各地で絶滅の危機にさらされたが、1911年（明治44）から国際条約によって保護した結果、資源はしだいに回復し、1970年代には約14万頭の資源量が推定され、分布域もしだいに拡大している。最近北海道の南岸でもみかけるようになった。雌は3、4年で成熟し、繁殖周期は2年である。交尾期は10～11月が盛期で、5～6月に出産する例が多いが、1年のいつでも交尾と出産がみられる。自然死亡率は約10％であり、最長寿命は23歳と推定される。餌(えさ)は貝類、棘皮(きょくひ)動物、節足動物で、石を道具に使って貝殻を割って食べることもする。

　現在、ラッコの猟獲は全面的に禁止され、毛皮は商品となっていない。資源が回復した結果、飼育のための捕獲が緩和され、最近では日本の各地の水族館でアメリカから輸入して観覧に供している。資源が増加するにつれて、ラッコは漁業者と競合するようになった。ラッコは新陳代謝が盛んで、商品価値の高いアワビ、二枚貝、ウニなどを好んで大量に捕食して、それらの水産物を利用して生活する漁業者に不利益を与えている。

［西脇昌治］

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