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Researching human impacts on marine mammals

Learn how we're researching human impacts on marine mammals, including the issue of porpoise bycatch.

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Human impacts

Human activities impact marine mammals, such as:

  • commercial shipping
  • petroleum exploration
  • marine mammal watching, which may interfere with animal:
    • resting
    • feeding
    • breeding
  • coastal development, which:
    • increases marine traffic
    • decreases habitat like seal haul-out (resting on land) sites and feeding areas
  • waste dumping, which spreads parasites from humans or terrestrial wildlife
  • harvesting for commercial or subsistence purposes, leading to overharvesting and incidental catches
    • an incidental catch happens when a species is accidentally caught instead of the one that was targeted (also called bycatch)


Petroleum activity, such as seismic exploration, may impact marine mammals by:

  • interfering with:
    • feeding
    • migration
    • communication
  • causing physical hearing damage

Impacts may be:

  • short term on individual whales
  • longer term on survival, if access to critical feeding zones is limited by high sound levels

Industrial impacts need to be examined on a project-by-project basis, as well as total impacts over time. For example, areas of Nova Scotia are currently known as the noisiest in the world because of seismic exploration. They have constant, high sound levels recorded as far away as the mid-Atlantic during the summer months.

Worldwide small cetacean bycatch problem

Fisheries bycatch poses a significant threat to many populations of small cetaceans, such as porpoises. A cetacean is any aquatic mammal that eats meat, is finned and has a blowhole.

However, there are few published estimates of the size of bycatches outside North America and Europe. We can estimate the total small cetacean bycatch in U.S. fisheries from data contained in the stock assessment reports required by the U.S. Marine Mammal Protection Act.

From these reports, the average annual bycatch of small cetaceans was around 3,000. More than 80% of this bycatch occurred in gill net fisheries.

After the implementation of take reduction measures in a demersal gill net fishery, annual bycatches declined significantly from 1990 to 1999. This was because the measures reduced the number of harbour porpoises, primarily in the Gulf of Maine.

To estimate small cetacean bycatch in the world's fisheries, we can expand the U.S. bycatch data with that on fleet composition from the Food and Agriculture Organization. 

The annual global bycatch of small cetaceans is in the hundreds of thousands. These removals are likely to have significant demographic effects on many populations. We need better data to fully understand the impact of these interactions.

This topic is further explored in Andrew Read's presentation Global Scope of Small Cetacean By-Catches.

Why and how small cetaceans are caught in fishing gear

Despite our efforts to prevent bycatch of harbour porpoises, we still don't know the reasons why porpoises become entangled in bottom set gill nets.

The existing knowledge about harbour porpoises has improved considerably in recent years, including information on:

  • their target detection abilities
  • circumstances leading to bycatch

We can use this knowledge to find long-term solutions to the bycatch problem.

This topic is further explored in the Larson and Tougaard presentation Why are Porpoises Caught in Gill Nets?

Cetacean bycatch in trawl fisheries

The expanded use of trawl nets globally has increased interactions between marine mammals and trawl gear. This causes injury or death to animals while costing fishers time and money.

To successfully implement strategies that will lessen bycatch, we must understand behaviours and foraging patterns.

Modifications to gear and acoustic devices have been successful in reducing cetacean bycatch in trawl fisheries. Exclusion devices have had varied results, leaving room for improvement.

To date, acoustic pingers haven't been successful at reducing cetacean bycatch in trawl gear. Research on alternative acoustic deterrent systems is underway.

In formulating plans to reduce bycatch, scientists and managers must consider that strategy success may differ depending on:

  • area
  • fishery
  • species

In addition, due to annual and seasonal variability, multiple mitigation methods may be more effective than relying on a single strategy.

The topic is further explored in Erika Zollett's presentation Net Knowledge: A Review of Cetacean Bycatch in Trawl Fisheries.

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