Many drops make a river
The cumulative effect originates from the repetition or aggregation of the same pressure in number, time and space – one hunter versus 1000 hunters, one hunting day versus 365 hunting days, and by the combination of different pressures – catch + by-catch + ship strike + reproductive failure associated with pollution for example.
Independently, individual stressors or stress events might have a minimal effect on the individuals or the population, but they do not act in isolation.When aggregated and/or combined with each other, the resultant effect might be cumulative – the final effect is the sum of the individual effects, but may also be synergistic – the final effect is greater than the sum of the individual effects.
Many drops make a river, little things combined form a larger result. A single automobile using gasoline or even diesel is unlikely to have a measurable effect on air quality, but millions of cars will. Likewise, one encounter between a whale group and one whale watching boat is unlikely to create a long-term disturbance, but daily encounters with 10s of vessels might.
A flu is usually a harmless disease. Being old only become lethal at a certain point. But an older person catching a flu might rapidly become a lethal adventure. Likewise, direct catches might well be regulated to sustainable level by the implementation of catch quota, but if by-catch or ship strikes also remove individuals from the population, the overall cumulative mortality may become unsustainable. In the Canadian Arctic, a risk assessment has predicted that lethal ship collision of narwhals associated with the exploitation of an iron ore mine would be equivalent to the hunting quota allowed to the nearby community.
Hunting removals are the human impact most focused on by the public, but in this chiefly post whaling and sealing age, they are only the tip of the iceberg when it comes to anthropogenic pressures and threats on marine mammals.