The destructiveness of bottom trawling and scallop dredging
Bottom trawling and scallop dredging are mobile bottom contact fishing methods that drag fishing gear across the seafloor, destroying valuable marine ecosystems and reef structures in their wake. Their effects are damaging and long-lasting. It takes at least 7 years for a trawled or dredged seafloor area to recover.
These fishing methods are also indiscriminate in what it catches. When dragging the large, weighted nets or cages across the seafloor, everything that happens to be in the way gets swept up in the net too. This means a large amount of unwanted, untargeted sealife, or bycatch, is caught in the net and brought to the surface, with little chance of survival.
In the Hauraki Gulf Marine Park, bottom trawling and scallop dredging has been happening for over 100 years, contributing to the sorry state of the Gulf now.
Under the new Hauraki Gulf Fisheries Plan, currently in development, bottom trawling and scallop dredging will be allowed to continue. This is despite massive public opposition to the fishing methods.
7 reasons why bottom trawling needs to be banned in the Hauraki Gulf Marine Park
1
Wastage
It is an indiscriminate fishing method that captures non-target species while scraping marine growth off the seabed.
2
Biodiversity loss
It causes long term damage to the seabed and reduces the density and diversity of species that live there.
3
Climate change
It releases as much carbon dioxide into the atmosphere as pre-Covid worldwide airline travel. Warming waters are becoming more acidic, unable to support normal ecosystem function.
4
Unnecessary wastage
Trawl tows can be 3 or 4 hours long, covering many kilometres. Some of the catch is damaged and dead by the time the trawl net is retrieved.
5
Detrimental to regional economies
It depletes local fish stocks impacting on regional, small-scale family fishing enterprises, making them unreliable and unprofitable for current and future generations of fishers.
6
Stifles innovation
It relies on bulk harvesting large quantities of poor quality fish with no incentives to develop innovative lower impact fishing techniques that deliver high quality, high value fish products. Alternative methods include long lining and trapping.
7
Widespread public opposition
There is overwhelming public support for bottom trawling and dredging to be banned from inshore waters, within 12 nautical miles of the coastline.
7 reasons why scallop dredging needs to be banned in the Hauraki Gulf Marine Park
1
Wastage
It is an indiscriminate fishing method scraping up seafloor flora and fauna while ploughing the seabed.
2
Biodiversity loss
It causes long term damage to the seabed and reduces the density and diversity of species that live there.
3
Dredge Inefficiency
There are a variety of dredge types for different seafloor types. All dredges have high scallop mortality rates associated, and all damage other species on the seafloor. For example, a New Zealand Box Dredge has around ~40% efficiency in harvesting scallops.
4
Catch inefficiency
For almost a decade in New Zealand, commercial catches are less than half of the allotted commercial catch amount.
5
Shell damage
Scallops caught by dredge often end up with chipped or broken shells, making them both poor quality for market and also damaging their main form of protection.
6
Stifles innovation
It relies on bulk harvesting large quantities of poor quality scallops with no incentives to develop innovative lower impact fishing techniques that deliver high quality, high value fish products. Alternative methods include hand-gathering and diving for scallops.
7
Unviable
Dredging plows through the seafloor, causing a loss of habitat structure and complexity. This affects juvenile scallop survival rates, which in turn skews numbers that survive to adults. Large numbers of adult scallops are needed for a sustainable fishery, which cannot occur with constant scallop dredging.
8
Widespread public opposition
There is overwhelming public support for bottom trawling and dredging to be banned from inshore waters, within 12 nautical miles of the coastline.
9
Habitat structure
Dredging essentially flattens the seabed – Repeated dredging causes a loss in topography and epibenthos, turning what may be a 3D structure into a less lively 2D one. 3D habitat structures are important for protecting a variety of sea creatures from predators, especially more vulnerable juveniles. Bottom trawling and dredging can both resuspend and bury biologically recyclable organic material, changing the flow of nutrients through the food web (Mayer et al., 1991).
10
Juvenile scallop survival
Loss of habitat structure means less chance for baby scallops to grow, as well as other baby fish who use 3D structures as nurseries. Predation rates of scallops are lower in places with higher habitat complexity.
