A Dall's Porpoise caught in a fishing net

Bycatch are species caught in a fishery while it is intended to catch another species or reproductively immature juveniles of the target species.

Cetacean bycatch is the incidental capture of non-target cetacean species by fisheries. Species which are seriously affected by this include dolphins, porpoises, and whales. Bycatch can be caused by entanglement in fishing nets and lines, or direct capture by hooks or in trawl nets.

Cetaceans, such as dolphins, porpoises, and whales, can be seriously affected by entanglement in fishing nets and lines, or direct capture by hooks or in trawl nets. Cetacean bycatch is increasing in intensity and frequency. In some fisheries, cetaceans are captured as bycatch but then retained because of their value as food or bait. In this fashion, cetaceans can become a target of fisheries.

As dolphins are mammals and do not have gills they may drown while stuck in nets underwater. This bycatch issue has been one of the reasons of the growing ecolabelling industry, where fish producers mark their packagings with something like "Dolphin Friendly" to reassure buyers. However, "dolphin friendly" does not mean that dolphins were not killed in the production of a particular tin of tuna, but that the fleet which caught the tuna did not specifically target a feeding pod of dolphins, but relied on other methods to spot tuna schools

Cetacean bycatch is increasing in intensity and frequency. This is a trend that is likely to continue because of increasing human population growth and demand for marine food sources, as well as industrialization of fisheries which are expanding into new areas. These fisheries come into direct and indirect contact with cetaceans. An example of direct contact is the physical contact of cetaceans with fishing nets. Indirect contact is through marine trophic pathways where fisheries are severely reducing fish stocks that cetaceans rely on for food. In some fisheries, cetaceans are captured as bycatch but then retained because of their value as food or bait. In this fashion, cetaceans can become a target of fisheries.

Mitigating bycatch methods:

• Acoustic deterrent devices
• Fishing regulations and management
• Temporary closure of fisheries during the short period of the year when cetaceans are migrating through the area
• Observers on boats should be present on fishing vessels to spot cetaceans in the water so that they can be avoided

Source: Wikipedia (All text is available under the terms of the GNU Free Documentation License)

For more information (background, pictures, experiments and references): Bycatch, Cetacean Bycatch