Reducing bycatch of marine mammals

The most effective way of reducing bycatch of marine mammals is to avoid using set and trawl nets within their habitats. Set nets are a particular risk to coastal dolphins including Hector’s, Maui’s and common dolphins. While less of a risk than set nets to some species, trawl nets are a particular risk to Hector’s and Maui’s dolphins, common dolphins and New Zealand sea lions. Set net prohibitions have been put in place along areas of the coast to protect the Hector’s and Maui’s dolphins. 1918  There has also been a recent prohibition on set netting attached to the marine mammal sanctuary located off the Taranaki bight. 1919

The Deepwater Group represents the owners of quota for deep-water and middle depth fish species in New Zealand. The group has developed a set of voluntary operational procedures which relate to marine mammals. These are given to all skippers of vessels involved in the jack mackerel fishery (where there is a particular conflict with common dolphins) and annual briefings are carried out to reinforce the message. The recommended procedures include: 1920

  • The officer on watch confirms that the area is visibly clear of dolphins before the net is deployed
  • If the vessel turns during trawling, it must haul the doors to the surface so the end of the net is closed before making the turn
  • Nets are not to be launched or retrieved between 02.30 and 04.30 hours
  • A dolphin dissuasive device (acoustic pinger) is to be deployed on night tows

Experiments with acoustic alarms on nets overseas showed promising results initially, but have not yet proved to be practically applicable. Other measures that can help to reduce, but not eliminate, the risk of marine mammal bycatch include:

  • Increasing the depth of the trawl net, in the case of conflicts with common dolphins
  • Adopting new technologies to enable animals to escape from trawl nets, such as the Sea Lion Exclusion Device (SLED) – discussed further below
  • Avoiding dumping offal when the net is being shot (launched) or hauled 1921
  • Keeping watch for marine mammals, and if any are sighted, not deploying a net and leaving the area
  • Hauling the net in as quickly as possible
  • Recovering any accidentally lost fishing gear 1922

In response to sea lion bycatch in the squid trawl fishery, the fishing industry has developed a Sea Lion Exclusion Device (SLED), and this has been deployed in squid trawl nets over recent years. It consists of a grid that is placed across the net in front of the cod-end where the fish accumulate. The gaps in the grid are large enough for squid and other fish to swim through, but are too small for the larger sea lions. SLEDS are designed so that marine mammals, which reach the grid, are forced to swim up, forward and out of the exit hole on top of the net. Research work on SLED efficacy suggests that SLEDs can increase the likelihood of survival for sea lions that enter a trawl net. More information can be found in the Aquatic Environment and Biodiversity Annual Review 2016 1923  and in the Ministry for Primary Industries Fisheries Assessment Plenary May 2016. 1924  

Diagram of a SLED inside a trawl net (Source: Ministry for Primary industries, 2013b - Image courtesy of the Deepwater Group)


  2. Deepwater Group, undated





Last updated at 4:07PM on November 23, 2017