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What the sources say

Sea OtterEnhydra lutris

Every figure, record and mechanism on the Sea Otter profile was written from a source, and this is the working table behind it: 58 claims checked word for word against these sources. The left column is what the page says. The right column is the source's own words, copied, not paraphrased; a ... marks a cut and[brackets] an insertion. Open any source and check it yourself.

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Source
01

Alaska Department of Fish and Game

Sea Otter Species Profile. Alaska Department of Fish and Game.

The page saysThe densest fur of any mammal
The source saysSea otters have the densest hair coat of any mammal, with 800,000 to one million hairs per square inch
The page saysa rock tucked under one arm
The source sayswill use tools, such as rocks (which they store under their "arm pit" while diving), to crack open shells
The page saysUp to a million hairs per square inch, the densest fur of any mammal
The source saysSea otters have the densest hair coat of any mammal, with 800,000 to one million hairs per square inch
The page saysa skin pocket that carries a rock for cracking shells
The source sayswill use tools, such as rocks (which they store under their "arm pit" while diving), to crack open shellsNote: The pocket is ADFG's own "Sea otters have a pouch in their armpit to store and carry food."
The page saysSea urchins, crabs, clams, mussels, octopus, other marine invertebrates and fish
The source saysTheir main prey species include sea urchins, crabs, clams, mussels, octopus, fish, and other marine invertebrates.
The page saysin the North Pacific
The source saysCurrently sea otters can be found in Alaska, Russia, British Columbia, Washington, and California.Note: Every place ADFG lists borders the North Pacific.
The page saysthe largest member of the weasel family
The source saysThe sea otter is the largest member of the weasel family.
The page saysgrowing up to 5 feet (1.5 m) long
The source saysAdult sea otters may grow up to 5 feet in length.Note: 1.5 m is a conversion of the source's 5 feet.
The page saysMales weigh 80 to 100 pounds (36 to 45 kg) and can weigh more
The source saysThe males weigh 80-100 pounds but can weigh more than 100 pounds.Note: 36-45 kg is a conversion of the source's 80-100 pounds.
The page saysfemales are smaller at 50 to 70 pounds (23 to 32 kg)
The source saysThe females are smaller, weighing 50-70 pounds.Note: 23-32 kg is a conversion of the source's 50-70 pounds.
The page saysIt relies on a high metabolism and its plush coat instead
The source saysSea otters rely on their high metabolism and their plush coat to keep warm.
The page saysThe fur traps air, which provides 4 times as much insulation as the same thickness of fat would
The source sayswhich fluffs their fur and traps air. Their body heat warms the trapped air and further insulates them. The trapped air provides 4 times as much insulation as the same thickness of fat would provide.
The page saysA sea otter dives to the bottom to catch its prey
The source saysThey dive to the bottom to catch their prey and surface to eat their food.
The page saysIts main prey include sea urchins, crabs, clams, mussels, octopus and fish
The source saysTheir main prey species include sea urchins, crabs, clams, mussels, octopus, fish, and other marine invertebrates.
The page saysAt the surface it rolls onto its back, sets the food on its chest, and may crack a shell open with a rock it stored under its "armpit" during the dive
The source saysUpon surfacing, the otter will roll onto its back and place the food on their chests. They eat using their front paws and will use tools, such as rocks (which they store under their "arm pit" while diving), to crack open shells.
The page saysDives generally last one to two minutes, though an otter can hold its breath for more than five
The source saysThe dives generally last 1-2 minutes, but they can hold their breath for over 5 minutes.
The page saysRussian explorers began the sea otter fur trade in the 1700s
The source saysRussian explorers began the sea otter fur trade in the 1700s.
The page saysthe population rebounded dramatically in the 20th century
The source saysFollowing protection under the Fur Seal Treaty in 1911, the sea otter population rebounded dramatically in the 20th century.
The page saysIt relies on a high metabolism and a plush coat instead
The source saysSea otters rely on their high metabolism and their plush coat to keep warm.
The page saysA sea otter's fur, a dense underfur beneath longer guard hairs, has 800,000 to one million hairs per square inch, more than a human head holds in total
The source saysTheir fur is made up of a dense underfur, ranging in color from brown to black, and longer guard hairs. ... 800,000 to one million hairs per square inch (humans only have 20,000 hairs on their whole head)
The page saysGrooming fluffs that fur and traps air, and the trapped air provides 4 times as much insulation as the same thickness of fat would
The source saysWhen sea otters are not foraging, you often see them grooming their fur. They do this to dry their fur and remove salt crystals and excess oil, which fluffs their fur and traps air. Their body heat warms the trapped air and further insulates them. The trapped air provides 4 times as much insulation as the same thickness of fat would provide.
The page saysTo open its food
The source sayswill use tools, such as rocks (which they store under their "arm pit" while diving), to crack open shells
The page saysan otter that floats on its back, sets its food on its chest and cracks shells open with a rock
The source saysUpon surfacing, the otter will roll onto its back and place the food on their chests. They eat using their front paws and will use tools, such as rocks (which they store under their "arm pit" while diving), to crack open shells.Note: vibe.reason, #384

Source
02

Oregon Zoo

Sea otter. Oregon Zoo.

The page saysWild sea otters live between 15 and 20 years
The source saysWild otters live between 15 and 20 years.
The page saysUnlike most marine mammals, which have thick layers of fat to keep warm
The source saysUnlike most marine mammals, which have thick layers of fat to help keep them warm, the sea otter doesn't.
The page saysa dense underfur that a top layer of long, waterproof guard hairs helps keep dry
The source saysA top layer of long, waterproof "guard hairs" helps keep its underfur layer dry.
The page saysThe Densest Fur of Any Mammal
The source saysthe densest fur of any mammal in the world
The page saysA sea otter's underfur has 1 million hairs per square inch, while a human head holds about 100,000 hairs in total
The source saysIts short underfur has 1 million hairs per square inch. By comparison, we only have about 100,000 hairs in total on our heads!
The page saysThe otter grooms almost continuously, somersaulting, twisting and rubbing its fur to trap air bubbles against its skin
The source saysIt grooms itself almost continuously, doing somersaults, twisting, turning and rubbing its fur. All this activity traps air bubbles against its skin
The page saysIts front paws find and grab it, while its whiskers feel the movement of prey animals in murky water
The source saysA foraging sea otter uses its front paws to find and capture prey underwater. Its whiskers help it to feel the movement of prey animals in murky water.
The page saysIt tucks each catch into a pocket of loose skin under a foreleg and carries it up to eat
The source saysIt tucks its prey into one of the pockets of loose skin on the bottom side of each of its forelegs and carries it to the surface to eat.
The page saysIt usually feeds in water no deeper than 60 feet, but can dive to 300 feet or more if needed
The source saysA sea otter dives for food, usually in water no deeper than 60 feet, but it can hold its breath for 5 minutes and dive to 300 feet or more if needed.Note: Final pass: replaced ADFG's '5-250 feet' range, which the Oregon Zoo's current page contradicts.
The page saysIts metabolism also runs at two to three times the rate of other mammals its size, so it needs to eat about a quarter of its own weight every day
The source saysBecause its metabolism is 2-3 times the rate of other mammals its size, it needs to eat about 25% of its weight every day.

Source
03

Animal Diversity Web, University of Michigan

Enhydra lutris (sea otter). Animal Diversity Web, University of Michigan.

The page saysa keystone predator that stops urchins from overgrazing kelp forests
The source saysThey are considered a keystone species and play a major role in the community by controlling herbivorous invertebrates. Sea otters prey on sea urchins, thereby preventing sea urchins from overgrazing the kelp forest.
The page saysKelp forests and shallow coastal waters over rocky or soft seafloor
The source saysSea otters inhabit temperate coastal waters with rocky or soft sediment ocean bottom. They live in offshore forests of giant kelpNote: "Shallow" is ADW's "they prefer coastal waters up to 30 meters deep".
The page saysa sea otter has no insulating fat at all
The source saysBecause sea otters do not have any insulating fat, the fur is responsible for heat maintenance.
The page saysSea otters are considered a keystone species: they play a major role in their community by controlling plant-eating invertebrates
The source saysThey are considered a keystone species and play a major role in the community by controlling herbivorous invertebrates.Note: "Plant-eating" glosses "herbivorous".
The page saysBy preying on sea urchins, otters stop the urchins from overgrazing kelp forests, which lets the kelp thrive and adds to the variety of sea life
The source saysSea otters prey on sea urchins, thereby preventing sea urchins from overgrazing the kelp forest. This allows the kelp forest to thrive and contributes to an increase in marine diversity.
The page saysIndividual otters tend to specialize; one may eat only urchins and crabs while another eats mostly fish, depending on its skills and the food nearby
The source saysIndividuals tend to be specialized in their choice of prey; one otter may consume only urchins and crabs while another may eat mostly fish, depending on the abilities of the individual and local food availability.
The page saysThe variety in the sea otter's diet also reduces competition among the seafloor grazers it eats and supports greater diversity in those species
The source saysThe variety in the sea otter diet reduces competition between benthic grazers and supports greater diversity in those species.Note: "Seafloor" glosses "benthic"; ADW lists these grazers (sea urchins, limpets, chitons) among the prey.
The page saysBy the turn of the 20th century, hunting had left between 1,000 and 2,000 sea otters worldwide
The source saysSea otters were hunted to near extinction (1000 to 2000 individuals worldwide) ... until the turn of the 20th century
The page saysThe 1911 International Fur Seal Treaty banned the hunt
The source saysthe United States, Russia, Japan, and Great Britain reached an agreement in 1911 called the International Fur Seal Treaty, banning the hunting of fur-bearing sea mammals
The page saysThe 1989 Exxon Valdez oil spill killed approximately 5,000 Alaskan sea otters
The source saysThe Exxon Valdez oil spill of 1989 had a dramatic effect on the Alaskan sea otter population, killing approximately 5,000 individuals.Note: A past event, so the ADW page's age does not date it; no current page in the pack gives a different toll.
The page saysPockets of loose skin under its forearms store a tool, usually a rock, so its paws are free while it eats, and the same pockets carry food during dives
The source saysSea otters have patches of loose skin under the forearms that they use to help store tools (usually a rock) so they can have free “hands” while eating, and to transport food during diving.
The page saysTo break a hard shell, some otters hold the rock on their chest and drive the prey into it, while others lay the prey on their chest and hit it with the rock
The source saysSea otters break open prey items with hard shells or exoskeletons with a rock. Some otters hold the rock on their chest and drive the prey into the rocks. Others leave the prey on their chests and hit the prey with the rocks.
The page saysAn otter keeps the same rock for many dives
The source saysThe same rock is kept for many dives.
The page saysSea otters prey on sea urchins, which stops the urchins from overgrazing the kelp forest
The source saysSea otters prey on sea urchins, thereby preventing sea urchins from overgrazing the kelp forest.
The page saysThat lets the kelp forest thrive and adds to the variety of sea life there
The source saysThis allows the kelp forest to thrive and contributes to an increase in marine diversity.
The page saysA keystone species is one whose loss from an area would eventually wipe out many more species there, and Animal Diversity Web names the sea otter as its example
The source saysa species whose presence or absence strongly affects populations of other species in that area such that the extirpation of the keystone species in an area will result in the ultimate extirpation of many more species in that area (Example: sea otter)Note: ADW's glossary entry for keystone species.
The page saysTheir fur, prized for its extreme density and insulating quality, was a major part of the fur trade from the mid 1700s to 1911
The source saysThe fur of sea otters was of great importance in the fur trade from the mid 1700s to 1911. Their fur was coveted due to its extreme density and insulating quality.
The page saysBy the turn of the 20th century, hunting had left between 1,000 and 2,000 sea otters worldwide
The source saysSea otters were hunted to near extinction (1000 to 2000 individuals worldwide) ... until the turn of the 20th centuryNote: Also covers the answer's opening "Very close." (ADW: "hunted to near extinction").
The page saysThe International Fur Seal Treaty banned the hunt in 1911
The source saysreached an agreement in 1911 called the International Fur Seal Treaty, banning the hunting of fur-bearing sea mammals

Source
04

Hughes et al. 2013

Hughes et al. (2013). Recovery of a top predator mediates negative eutrophic effects on seagrass. Proceedings of the National Academy of Sciences.

The page saysIn one estuary exposed to extreme nutrient pollution from farming, sea otters helped eelgrass, a seagrass, grow and spread, counteracting the harm those nutrients do
The source saysIn a study of seagrass beds in an estuarine ecosystem exposed to extreme nutrient loading, we use a combination of a 50-y time series analysis, spatial comparisons, and mesocosm and field experiments to demonstrate that sea otters (Enhydra lutris) promote the growth and expansion of eelgrass (Zostera marina) through a trophic cascade, counteracting the negative effects of agriculturally induced nutrient loading.

Source
05

Yeates, Williams & Fink 2007

Yeates, Williams & Fink (2007). Diving and foraging energetics of the smallest marine mammal, the sea otter (Enhydra lutris). Journal of Experimental Biology.

The page saysResearchers call the sea otter the smallest marine mammal
The source saysAs the smallest and one of the most recently evolved marine mammals, sea otters face physiological challengesNote: The authors measured the energetics of resting, grooming, diving and foraging; the title calls the species "the smallest marine mammal".
The page saysA foraging dive burns nearly twice the energy predicted for a seal, partly because of the otter's buoyancy and swimming style
The source saysThe cost of a foraging dive for sea otters was nearly twice that predicted for phocid seals, which was attributed in part to elevated locomotor costs associated with buoyancy and swimming style.

Source
06

Doroff, Burdin & Larson 2021

Doroff, Burdin & Larson (2021). Enhydra lutris. The IUCN Red List of Threatened Species 2021: e.T7750A219377647.

The page saysEndangered
The source saysEndangeredNote: stats.conservationStatus; category read through GBIF's copy of the Red List.
The page saysThe latest IUCN Red List assessment, published in 2021, lists the sea otter as Endangered
The source saysEndangeredNote: Category read through GBIF's copy of the Red List; the assessment (made in 2020) was published in the 2021-3 Red List update, as its DOI shows.
The page saystoday the IUCN Red List lists the sea otter as Endangered
The source saysEndangeredNote: Category read through GBIF's copy of the Red List (2021 assessment, the latest).

How the table was made: a writer drafted the profile from these sources and logged each claim with its quote, a reviewer with nothing but this table checked every sentence said no more than its quote, and a script confirmed each quote appears word for word in the source. The method is in the site'sAbout page.