With sub-surface gillnets the headline of the net is set below a specified depth, usually 2 m in research to date. This method of gear deployment is still in an experimental stage.
While sub-surface gillnets have been used successfully to reduce sea turtle, shark and cetacean bycatch in Pakistan's tuna gillnet fishery [1,2], it has been found that they are not effective in reducing bycatch of deep dwelling cetaceans . Species belonging to family Delphinidae (Risso’s dolphin), Family Kogiidae (dwarf sperm whales and pygmy sperm whales) and Family Ziphiidae (Longman’s beaked whale, Mesoplodon sp. and Cuvier's beaked whale) were reported to get entangled in both surface and sub-surface gillnets . These deep dwelling species dive to 300 to 500 m (or more) to feed on meso- and bathypelagic cephalopods, fish and crustaceans. It appears that while surfacing, these cetaceans cannot avoid either surface or sub-surface gillnets .
For seabirds, research is very limited and inconclusive and importantly, sub-surface nets can entangle birds during setting and hauling .
Notes from the Pakistani tuna gillnet fishery. Compared with surface gillnets, sub-surface gillnets reduce the chances of net fouling during deployment and retrieval, due to the arrangement of net floats. Sub-surface gillnet floats are placed in pairs attached to the headline by a 2m rope, while floats on surface gillnets are placed every 4.5 m on the headline . Also, because sub-surface gillnets result in fewer entanglements of endangered, threatened or protected (ETP) species, less time is spent handling/disentangling/ discarding ETP animals. These improvements in fishing efficiency provide incentive for fishers to switch to the new method .
While use of sub-surface gillnets can decrease catch of important target species, such as billfish and dolphinfish, these losses have been offset by increases in catch of other valuable target species, including yellowfin, longtail and skipjack tunas .
- Moazzam M (2022) Bycatch of deep dwelling cetacean in gillnet fisheries of Pakistan. In: IOTC - 18th Working Party on Ecosystems & Bycatch. IOTC-2022-WPEB18-24, Online
- Moazzam M, Khan MF (2019) Issues related to adoption of subsurface gillnetting to reduce bycatch in Pakistan. In: IOTC - 15th Working Party on Ecosystems and Bycatch. IOTC-2019-WPEB15-48, La Saline Les Bains, Reunion Island
- Parker G, Molloy J (2017) Stocktake of measures for mitigating the incidental capture of seabirds in New Zealand commercial fisheries. In: ACAP - Eighth Meeting of the Seabird Bycatch Working Group. SBWG8-Doc-INF20, Wellington, New Zealand