Fishing can have an adverse effect on other species and the wider marine environment. For example:
- some fishing methods can cause widespread harm to extensive areas of marine habitat, notably seabed (bottom) trawling and dredging
- most fishing methods result in the accidental death of non-target species (bycatch) including other fish species, sharks, rays, seabirds, fur seals, sea lions, and dolphins (see Our marine birds and mammals)
- removing too many target and non-target species can disrupt the natural balance of species within the wider marine ecosystem.
Our reporting programme’s data provide a partial picture of fishing pressures
Of the pressures listed above, we report in this chapter on commercial seabed trawling and dredging, the status of commercial fish stocks, and the marine trophic index for the Chatham Rise. These data provide a partial picture of the pressure from fishing on marine ecosystems. A fuller picture would include data and analysis on the impact of commercial, customary, and recreational fishing on a greater range of non-target species and wider marine ecosystems.
New Zealand’s most destructive commercial fishing methods have decreased
Seabed (bottom) trawling and dredging are fishing methods where large nets (bottom trawling) or heavy metal baskets (dredging) are towed near or along the ocean floor. Trawling is carried out in both shallow and deep waters and is used to catch a range of species, for example, hoki and squid. Dredging is carried out on the seabed in shallow waters and often targets shellfish species such as scallops and oysters.
Seabed trawling and dredging can have a major impact on seabed habitats and species, and are the two most destructive fishing methods used in New Zealand. When trawls and dredges are dragged along the seabed, they disturb sediment, damage corals, and scoop up seabed species such as crustaceans and brittle stars. The extent of damage will depend on a number of factors, including the type, weight, and size of equipment, its towing speed, and the nature of the seabed habitat.
Commercial seabed trawl and dredge tows have decreased
The number of commercial fishing trawl tows and dredge tows in New Zealand waters has fallen (Ministry for Primary Industries, 2016a; Black & Tilney, 2015), reducing pressure on seabed habitats and species. The number of trawl tows decreased 50 percent from a peak of over 145,000 tows in 1997 to over 72,000 in 2014 (see figure 10). The number of dredge tows reported in New Zealand waters decreased 83 percent from a peak of over 260,000 tows in 1996 to over 43,000 in 2014.
The extent of seabed trawled for the first time, where the potential for damage is greatest, has been decreasing each year since 2007. Similarly, the overall area trawled each year has generally decreased since 2003 (Black & Tilney, 2015).
This graph shows the number of fishing trawl tows between 1990 and 2014. Visit the MfE data service for the full breakdown of the data.
Trawling occurs mostly in shallower waters less than 1,500 metres. Table 2 shows the percentage of seabed in our EEZ and territorial sea that was trawled at least once between 1990 and 2011, at different depths. As shown, 44 percent of the seabed up to 400 metres in depth was trawled at least once.
Percentage of seabed in the EEZ and territorial sea trawled at least once from 1990 to 2011
Depth of seabed (metres)
Seabed trawled at least once (percent)
Less than 1 percent
Source: Ministry for Primary Industries
Recovery of seabed habitats
There is limited research in New Zealand on the recovery rates of seabed habitats that have been subject to trawl and dredge tows. Overseas studies indicate that recovery time can take from months to years depending on the nature of the seabed (Ministry for Primary Industries, 2016a).
While local fishing rules vary around the country, recreational fishers can use a dredge – usually to collect scallops. This causes damage to seabed habitats, although on a much smaller scale than a commercial operation. There are daily limits to the number and size of shellfish that can be taken per person.
We are not aware of any data on the extent of recreational dredging around the country and how this may be changing coastal marine habitats. Recreational dredging is prohibited in the Fiordland (Te Moana o Atawhenua) marine area, the internal waters of Fiordland, and Paterson’s Inlet in Stewart Island (Rakiura).
Our commercial fish stocks are managed with the aim of ensuring future harvests
Since 1986, New Zealand has managed its commercial fisheries under a quota management system (QMS). QMS gives quota holders a property right to harvest a fish stock up to a maximum level, known as the total allowable commercial catch. QMS controls the total allowable commercial catch of almost all the main fish stocks within New Zealand’s EEZ.
The total allowable commercial catch is established separately for each fish stock, with the aim of ensuring future harvests. It includes adjustments for recreational and customary fishing and other fish-related mortality.
Fisheries managers define a fish stock as overfished when the fish stock:
- is depleted and needs to be actively rebuilt
- has collapsed and closure should be considered to rebuild the stock as fast as possible (Ministry for Primary Industries, 2016c).
In 2015, 17 percent of New Zealand’s fish stocks were assessed as being overfished, requiring active management intervention to rebuild stock. This included stocks of southern bluefin tuna, Pacific bluefin tuna, orange roughy, snapper, John dory, and scallops.
The 17 percent of New Zealand’s fish stocks overfished in 2015 compares with an estimated 29 percent of fish stocks overfished worldwide (based on a 2011 assessment) (Food and Agriculture Organization of the United Nations, 2014).
In 2015, 78 percent of the total landings of fish by weight and value came from stock of known status (Ministry for Primary Industries, 2016).
Information on the status of the main commercial species has markedly improved over the last decade. In 2015, of 344 fish stocks which cover most of the main commercial species, we knew the status of 157 stocks for assessing stocks against the soft limit – the management limit when fish stocks are assessed as depleted and need to be actively rebuilt. The 2015 stock was an increase in known status of fish stock of 65 percent since 2009.
One of the roles of our reporting programme is to assess the ecological impacts of fishing on marine ecosystems. The fisheries management data we reported on above was not designed for this purpose – a limitation of this report.
For more detail see Environmental indicators Te taiao Aotearoa: State of fish stocks.
A net full of lookdown dory, ling, and hoki. Hoki are widely distributed throughout New Zealand waters and the species is an important commercial fishery. The fishery poses a particular risk of bycatch for Southern Buller’s albatross and Salvin’s albatross (Ministry for Primary Industries, 2016b). (Photo: Peter Marriot, NIWA)
Marine trophic index indicates no determinable impact of commercial fishing on demersal fish in the Chatham Rise
The trophic level is the position of an organism within a food web – for example, phytoplankton are at level 1 at the base of the food web, omnivores are generally at level 2 or 3, and predatory species have levels greater than 3. The marine trophic index (MTI) is the mean (average) trophic level of fisheries landings (fish species caught and landed). It is used internationally to measure changes in marine ecosystems associated with fishing and climate variability.
A gradual decline in MTI indicates the structure of fish populations is changing, with a relative increase in populations of smaller fish and invertebrates lower in the food web and fewer of the larger, predatory fish populations. This can indicate that predatory fish – the main commercial fish species – are being overexploited by fishing or are being affected by other pressures, such as climate variability.
Our reporting programme has used MTI to measure the abundance and diversity of demersal fish in the Chatham Rise, one of the most productive fishing grounds in our EEZ. Demersal fish populations include the main commercial species of hoki, hake, ling, oreo, and orange roughy.
Data from 1992 to 2014 showed no statistically significant long-term change in the demersal fish community, with a mean MTI of 3.9 indicating a predatory fish community for the area. This supports the view that fishing and other pressures, such as climate variability, have not caused a determinable change in populations of large demersal fish in the Chatham Rise over this period. MTI for the Chatham Rise is not representative of the wider EEZ.
For more detail see Environmental indicators Te taiao Aotearoa: Marine trophic index: Chatham Rise.