Specific Fishing Methods

Each fishing method discussed previously can have very different impacts on the marine environment and these are described below. There are a number of methods which can be used to mitigate many of these impacts which are outlined in the section on elements of best practice.

Hand collection

This has minimal impacts other than on the targeted species. It enables only target species to be harvested and undersized specimens to be left undisturbed. Mechanical methods of collection are more indiscriminate and can impact on habitats and other organisms living on the sea floor.

Jigging

This is a relatively low impact fishing method that only catches the targeted species. If lines or jigs are lost at sea, they can become a hazard for other marine species, which may become entangled in the fishing equipment.

Potting

This is also a relatively low-impact fishing method, although fragile species located on the seafloor such as soft corals, sponges and bryozoans can be damaged if heavy pots are dropped on top of them. Undersized and non-target specimens can be returned to the sea relatively unharmed. Some seals and shags can drown as a result of being caught in rock lobster pots. Marine mammals can become entangled in pot lines.

Trolling

This is a low-impact and selective method of fishing. Few non-target species are caught and it has no impact on benthic habitats. Occasionally fur seals and seabirds can become hooked on the lures but this is a rare occurrence.

Long lining

This method results in little damage to the fish caught, which sometimes can then be sold for higher prices, but can incur higher costs to catch. Long lines generally have little impact on sea floor habitats, although if lines and weights are dragged across the seafloor they can damage sensitive benthic organisms. The size and shape of the hook and type of bait can be tailored to attract specific species, thereby reducing bycatch. Because this method of fishing uses a large amount of bait, it can place pressure on stocks of bait fish if these are not managed sustainably. Long lining can result in the death of seabirds, turtles and marine mammals which drown as a result of becoming caught on the hooks or entangled in the lines.

Gill netting

This has little impact on benthic habitats. However, compared to other methods, there is a higher risk of catching non-target species with gillnets including dolphins, penguins, shags, shearwaters, seals, sharks and fish. Such nets are a particular problem for the endangered Hector’s and critically endangered Maui’s dolphins which live in the shallow coastal waters where nets are commonly set.

Seining

This has different impacts on the marine environment, depending on the type of seining adopted. Because purse seining targets schooling fish, it usually results in little bycatch, although dolphins can be caught. It has little impact on benthic habitats as the net does not normally come in contact with the seafloor. Beach and Danish seining can be less selective than purse seining, depending on the extent to which schools of fish are targeted. Danish seining can damage sensitive benthic habitats as a result of the weighted net and ropes being dragged across the sea floor.

Trawling

This is a relatively indiscriminate method of fishing and can result in a large amount of bycatch. Fur seals, sea lions, dolphins and seabirds can become caught in the trawling equipment and drown. Fur seals, sea lions and common dolphins often deliberately swim into trawl nets to feed on the fish which have been caught.

Bottom trawling disturbs the seabed, and can damage or destroy the organisms and communities that live there. Heavy chains or rollers dragged along the seafloor crush many benthic organisms including sponges, bryozoans, seaweeds and soft corals. In addition, such equipment can flatten rocky and biogenic outcrops which provide refuge for juvenile species of fish. Trawling can also suspend sediment in the water column which can disrupt filter feeders.

Repeated trawling of an area can reduce its overall diversity and productivity. Recent research comparing trawled and untrawled areas around Separation Point, between Tasman and Golden Bays, identified significant differences. The trawled area showed a reduction in the size, biomass and productivity of marine life in the area, with the original shell-gravel substrate transformed to silt-mud, and larger long-lived species replaced by smaller opportunistic species. ¹⁸⁴⁴

A risk assessment exercise looking at activities within the EEZ and Extended Continental Shelf (ECS) was completed for the Ministry for the Environment by NIWA in 2012. This identified bottom trawling as creating the third most significant risk to New Zealand’s marine environment after ocean acidification and increasing sea temperatures from climate change. ¹⁸⁴⁵

Dredging

Dredging can also be harmful to benthic habitats. The heavy dredge scrapes the seafloor, crushing benthic organisms and flattening the seabed profile. It also suspends sediment in the water column, which reduces light levels, smothers remaining benthic organisms, and clogs up the apparatus of filter feeders. The dredge can also damage a significant proportion of the target species, such as scallops.

Summary

The following table summarises potential environmental impacts of commercial fishing methods (adapted from Forest & Bird, 2012):

Last updated at 3:05PM on November 23, 2017