Aquatic mammals

All are relatively large and most are cryptic, in that a good portion of their time is spent below the surface of the water. Most marine mammals are too large for extensive trapping and are best surveyed with distance sampling, though trapping can be conducted on some dolphin, manatee, or otter species that inhabit coastal waters. Direct observations are limited to the animal's time above the surface of the water or using scuba equipment. Radio telemetry can provide important movement data, but difficulties include the loss of signals when the animal is below the surface of the water. Sonar can be used to track movements and sounds of larger whales.

Freshwater mammals are generally smaller and more readily trapped. These mammals also usually spend some portion of their time on land and leave obvious sign such as gnawing on trees or construction of homes, nests, and dams. Radio telemetry and direct observations are more effective with this group because of their time out of water. Small semi-aquatic insectivores are cryptic, difficult to capture, and leave no observable signs. With this subgroup, neither direct observations or radio telemetry have proven effective. They can best be sampled with pitfalls or some form of kill trap.

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