Hooded seals' migration from the moulting area in Tasiilaq district to the breeding grounds around Newfoundland and back. (Source Dept. of Fisheries and Oceans, Canada and Greenland Institute of Natural Resources)

Movements of hooded seals

Marine mammals are long-lived animals, with a record of 200-plus years for bowhead whales. They also move widely around, some migrating over very large distances. Records are held by gray whales (between Russia and Mexico) and northern elephant seals (between the Pribilof Islands and California), with migration of over 21,000 km a year.

During their lifetime, marine mammals experience a wide variety of temporal and geographical conditions. Over time and space, they face a large palette of stressors, influences which are potentially detrimental to the individual. Some are natural, but others are directly or indirectly generated by human activities, or so-called anthropogenic.

Independently, individual stressors might have a minimal effect on the individuals or the population, but they do not act in isolation. The cumulative impacts of the repetitive and combined past, current and future pressure of different stressors may severely affect individuals and populations. Also, the effects may not be simply additive, but may have synergistic (re-enforcing) impacts, the combined effect exceeding the sum of the individual effects.

Adopting an ecosystem approach to management is essential for understanding population dynamics and encompassing the interactive and cumulative impacts of the different anthropogenic stressors. Determining or predicting cumulative risk to individuals and populations is critical but particularly challenging, since some of the direct and less direct impacts are note easily quantifiable. Detecting impacts at population level can also be very difficult.

The impacts of some stressors such as persistent pollutants or changing ocean climate cannot be reduced readily. However, other stressors like noise, fisheries, shipping, by-catch and hunting can be managed in time and space to reduce their cumulative impact. Management decisions will require consideration of, and trade-offs between, all ecosystem services involving marine mammals to identify the best combination of stressors to reduce or limit, so populations are kept or brought back to a favourable state.

You can read more here on the different Stressors affecting Marine mammals and on how Many drops make a river, or how several stressors may combine in a significant cumulative impact.

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