Reasons for Strandings

Marine mammals strand for a variety of reasons. Strandings can involve a single animal or multiple animals. Stranded marine mammals are animals that are:

• In need of apparent medical attention

• Dead on the beach, shore or in U.S. waters

• Alive and on the beach or shore but unable to return to the water

• Alive in U.S. waters where the water is so shallow it is unable to return to its natural habitat

Common reasons for strandings include injury, illness/disease and malnutrition. Human interaction incidents can also cause marine mammals to strand.

Injuries can result from interactions with natural predators like sharks and even breeding adults of the same species.

Marine mammal illness/disease includes parasite infestation such as tapeworm or lungworm, harmful algae blooms such as domoic acid toxicosis, viral infections such as herpes, bacterial infections such as leptospirosis and cancer.

Malnutrition may be the result of many things including young animals that have become prematurely separated from their mothers, animals that were recently weaned and are not thriving in finding food on their own, environmental and oceanographic events like El Niño that cause changes in the distribution and abundance of marine mammal prey and increased competition for prey.

Human interaction incidents can be:

• Direct – Deliberate: These are acts of harassment/violence by citizens such as purposeful shootings (gunshots, bow and arrows, crossbows, etc.) and striking with objects (gaffs, pipes, rocks, bricks, etc.)

• Direct – Well-Meaning: These are well-intentioned actions by citizens that intervene on an animal’s behalf. They include picking up pups believed to be abandoned and picking up animals believed to be in need of help. These actions are illegal as marine mammals are federally protected by the Marine Mammal Protection Act. It is illegal for unauthorized individuals to touch, disturb, feed or otherwise harass these animals. Illegal actions could also jeopardize the health of the animal.

• Direct – Non-Deliberate: This happens when marine mammals are affected by humans unintentionally. Injuries, illness and death can be caused as a result of fisheries bycatch, entanglements and boat/ship strikes.

• Indirect: These are a result of changes to habitat or prey. Examples include pollution (point source and non-point source), marine debris, coastal development, wetland loss, over fishing, climate change, ocean acidification and invasive species and pathogens.

At times marine mammals are found dead on beaches and in these circumstances with pinnipeds (seals and sea lions), CIMWI collects the latitude and longitude of the stranding location, morphometric data, carcass condition and sex of the animal. Animals are also photographed and examined for identification tags, external signs of injury and signs of human interaction. CIMWI reports these findings in the Marine Mammal Health and Stranding Response Program’s National Stranding Database.