A key component of the marine ecosystem, birds and mammals are often the apex predators at the top of the food chain. As such they are sensitive to changes in the distribution and productivity of species at lower trophic levels of the food web. Therefore, they can be indirectly impacted by environmental changes as well as directly affected.
Marine mammals with limited geographical distributions can be highly vunerable to climate change as the effects of climate change are rapidly altering their habitat and many species require specific temperature ranges in which they must live.
By altering the abundance and structure of target populations, fishing could have secondary effects on the wider food-web, for instance, poor breeding success in seabirds has been linked with a low availability of sand eels. A wide range of scavengers including seabirds, fishes and benthic invertebrates are known to feed on discards. In the North Sea, it is estimated that seabirds consume 50% of discarded material. Between 2.5 and 3.5 million seabirds are potentially supported by fishery waste. There has been much discussion regarding the potential ecological implications of halting fishery discarding (a major aspiration of the ‘revised Common Fisheries Policy’ in 2012).
Bottom trawling is the fishing method with the largest by-catch of non-target fish species. The gillnet fishery is a potential threat to diving seabirds and marine mammals, especially harbour porpoises.
Some promising improvements have been seen for birds, such as the white-tailed eagle, since the decrease in PCBs and DDTs, following restrictions to their use in the 1970s. Marine mammals, on the other hand, continue to exhibit reproductive disorders, indicating that the levels of hazardous substances such as PCBs and dioxin continue to cause ecological harm. Studies on fish, sea birds, harbour porpoises and blue mussels document that TBT is still present in considerable concentrations in the Baltic Sea, despite restrictions in its use.
Some viruses and bacteria are transferred to marine mammals from human sewage, potentially pathogenic bacteria have been isolated in samples taken from harbour porpoise with so far unknown effects on the population.
Many marine animals use sound for important life functions (communication, navigation, avoiding predators, among others) and there is a growing scientific literature that shows that human noise interferes with marine mammals’ biological and social functions.
The establishment of artificial structures at sea for harvesting wind energy could cause changes in the structure and abundance of demersal fish species, as well as sea mammals and seabirds occurrence patterns.
Plastic causes entanglement and ingestion threatening mammals, fish, birds and turtles. During 2003-2007, 95% of 1295 fulmars sampled in the North Sea had plastic in their stomach and the critical level of 0.1 g of plastic was exceeded by 58% of birds, with regional variations ranging from 48 to 78%.