Research Themes -
Human Impacts on Marine Mammals
Humans have very wide ranging impacts on marine mammals. The most obvious is the harvesting of marine mammals for commercial or subsistence purposes. Overharvesting has reduced some marine mammal populations to very low levels, resulting in concern for their continued existence eg St Lawrence beluga, blue whale and killer whales. Marine mammals are also taken as incidental catches during commercial fishing activities.
Human activities such as marine mammal-watching, petroleum exploration and commercial shipping may also impact on marine mammals. Considerable work remains to be completed to explore these impacts more fully. Marine mammal watching may impact on marine mammals in cases where too many boats approach the animals and interfere with normal resting, breeding or feeding activities. Petroleum activity such as seismic exploration may cause physical damage to marine mammal hearing or interfere with marine mammal feeding, migration or communication. This may have short term impacts on individual whales or longer term impacts 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 cumulative impacts. For example current areas off the Nova Scotia are currently known as one of the noisiest areas in the world for seismic exploration with constant, high sound levels recorded as far away as the mid-Atlantic during the summer months.
Coastal development also has an impact on marine mammals by increasing marine traffic or through the loss of habitat eg seal haulout sites, or feeding areas. Finally, the dumping of waste into the environment results in the transfer to marine mammals, of parasites normally associated with humans or terrestrial wildlife.
Scope of the small cetacean bycatch problem worldwide
Center for Marine Conservation, Duke University, Beaufort, North Carolina, 28516, U.S.A.
Fisheries bycatch poses a significant threat to many populations of small cetaceans, but there are few published estimates of the magnitude of these outside North America and Europe. It is possible to estimate 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. The mean annual bycatch of small cetaceans during this period was approximately 3,000 (Table 1). More than 80% of this bycatch occurred in gill net fisheries (Figure 1). Annual bycatches declined significantly over the decade, primarily due to a reduction in the number of harbour porpoises primarily in the Gulf of Maine, after the implementation of take reduction measures in a demersal gill net fishery. It is possible to derive a crude first estimate of small cetacean bycatch in the world's fisheries by expanding U.S. bycatch with data on fleet composition from the Food and Agriculture Organization. The annual global bycatch of small cetaceans is in the hundreds of thousands (Table 3); these removals are likely to have significant demographic effects on many populations. Better data are needed urgently to fully understand the impact of these interactions.
Why and how are small cetaceans caught in fishing gear?
Despite a considerable effort in the last 10 years or more towards developing solutions to the wide spread bycatch of harbour porpoises in bottom set gill nets, the reasons why porpoises become entangled in these nets are still obscure. A number of hypotheses have been put forward to explain the observations made, but there does not seem at the moment to be a consensus as to which hypotheses are the most plausible. The existing knowledge about harbour porpoises, in particular their target detection abilities, and the circumstances leading to bycatch has, however, improved considerably in recent years. We believe that this knowledge can be used to reduce the number of plausible hypotheses, and ultimately lead the way towards long-term solutions to the bycatch problem. In the presentation we will try, in a logical fashion, to confront the various hypotheses with the existing knowledge about harbour porpoises and bycatch, and hopefully stimulate a fruitful discussion about why harbour porpoises are caught in gill nets.
Net knowledge: a review of cetacean bycatch in trawl fisheries
University of New Hampshire, Ocean Process and Analysis Laboratory, Institute for the Study of Earth, Oceans, and Space,142 Morse Hall, Durham, New Hampshire, 03824, U.S.A.
In the past several decades, the expanded use of trawl nets globally has led to increased interactions between marine mammals and trawl gear, causing injury or death to animals and costing fishers time and money. Researchers must understand the behaviours and/or foraging patterns that play a role in cetacean bycatch for mitigation strategies to be successfully implemented. Past and current research has tested several gear modifications and acoustic devices to reduce cetacean bycatch in trawl fisheries. Exclusion devices have been successful at reducing cetacean bycatch in trawl fisheries; however, the results have been variable, and room for improvement exists. To date, acoustic pingers have not been successful at reducing cetacean bycatch in trawl gear, but research on alternative acoustic deterrent systems is underway. In formulating bycatch mitigation plans, scientists and managers must consider that strategies which successfully reduce bycatch may differ depending on area, species, and fishery. In addition, due to annual and seasonal variability, multiple mitigation methods may be more effective than relying on a single strategy.
- Date modified: