Bottom trawling | NZ Government

Bottom trawling summary facts

Around 31% of New Zealand’s exclusive economic zone (EEZ) is closed to bottom trawling. Bottom trawling is also prohibited in about 21% of the territorial sea.
The trawl footprint (the total area trawled in a given year) has been declining over time. In 2021, it was 74,500 square kilometres.
The total area trawled from 1990 to 2021 is 462,643 square kilometres, or about 11% of New Zealand’s EEZ and territorial seas.
In the 2021/2022 fishing year, 70% of all fish caught commercially in New Zealand were caught using either bottom trawl gear or mid-water trawl gear within 1 metre of the seabed.

About bottom trawling fishing

Trawling is when a fishing net is towed by a boat. Nets can be towed either:

through the water column (mid-water trawl)
along the seafloor (bottom trawl).

Trawling is one of the most common methods of fishing. Large quantities of fish can be caught in a single tow. Many of New Zealand's popular fish live on or near the seafloor and are commonly caught by trawl fishing, like orange roughy, squid, and snapper.

Fishers report to us what type of fishing gear they use, the depth of the gear, and the depth of the seabed.

The gear used to trawl depends on the fish being targeted and the area being fished. Bottom trawling gear tends to be heavier than mid-water trawl gear and is designed to be used in contact with the seafloor when fishing. Mid-water trawl gear can be used near the seafloor and make contact with it, but heavier bottom trawling gear can cause more damage under normal operating conditions.

When measuring the extent of bottom trawling in New Zealand, we include:

all fishing using bottom trawl gear, and
fishing with mid-water trawl gear where this is within 1 metre of the seafloor.

In the 2021/2022 fishing year, almost 250,000 tonnes, or 70% of the volume of all fish caught commercially in New Zealand was caught using bottom trawl gear or mid-water trawl gear within 1 metre of the seabed.

Graph: Estimated catch totals and number of bottom impact trawl events

graph on the stimated catch totals an dnumber of bottom impact trawl events 2021 2022 — This graph shows the amount of estimated catch from bottom impact trawl events for each fishing year from 2007/08 to 2021/22, including the number of events that relate to the catch for each year. Overall, the estimated catch and bottom impact trawl events have decreased slightly.

Why the seafloor is important

New Zealand’s seafloor is diverse in both geological and biological terms. It’s not entirely flat – some parts are made up of features like trenches, sea mounts, ridges, and rises. Some areas are made up of soft sediment. The seafloor is an important habitat for a number of marine species across their life cycles. Species that form complex structures such as corals and sponges support high levels of biodiversity.

Deepwater corals can live for hundreds of years and are slow to recover from any disturbance. Some species are legally protected under the Wildlife Act 1953 are defined as "threatened" or "at risk". Other unique areas such as hydrothermal vents support biological communities.

How fishing can impact the seafloor

Seafloor habitats and communities within the tow path of bottom contacting trawl gear will be impacted to varying extents depending on a range of factors, including:

the type of gear used (design and weight) and the towing speed,
features of seafloor habitats, including their natural disturbance regimes,
the species present, and
the frequency and intensity of fishing.

Bottom contacting trawls can affect seafloor habitats and communities by damaging or removing structure forming species, reducing habitat complexity and altering the seafloor structure. The greatest impacts from bottom trawling are on hard complex grounds, while the effects are less on sandy bottoms.

Some seafloor species can withstand bottom trawling due to their size, shape, and where they live. Surface living species are affected more frequently and severely than burrowing species. Species most sensitive to trawl activity are animals that protrude from the seafloor and are structurally fragile, like:

stony, gorgonian, and black corals
sponges
hydrocorals
bryozoans.

Bottom trawling can damage or remove animals living on the surface of the seabed within the trawl path. Seafloor communities can be altered, with reduced abundances of long-lived species, and lower species richness and diversity. Recovery from trawl impacts can take from days to centuries, depending on the intensity of trawling, the habitat features and the species affected.

Areas protected from bottom trawling

Around 31% of New Zealand’s exclusive economic zone (EEZ) is closed to bottom trawling. Bottom trawling is also prohibited in about 21% of the territorial sea (the area within 12 nautical miles of the coast).

The effects of bottom trawling are managed through:

closing certain areas to trawling
limiting which sized vessels can fish in an area – for example, vessels over 46 metres in length are prohibited from trawling in the territorial sea.

The catch limits imposed under the Quota Management System also act to limit the amount of trawling that occurs in our waters.

Some closures to trawling were put in place to limit the effects of fishing on the seafloor environment, like seamount closures and benthic protection areas.

The seafloor is also protected from bottom-impacting fishing with:

marine reserves
marine mammal sanctuaries
recreational areas
submarine cable closure zones.

The benthic protection area (BPA) proposal was brought into effect in 2007. It sets aside a broadly representative sample of seafloor habitats, in essentially pristine condition. BPAs were selected on the basis of 4 key criteria. They:

are large areas
are relatively unfished
have simple boundaries
are broadly representative of the marine environment.

Monitoring the impacts of fishing on the seafloor

We collect a range of information about each individual fishing event which enables us to:

map the location of bottom contacting tows
estimate the seabed area contacted (the trawl footprint),
assess the intensity of contact in different regions by vessels targeting different species,
estimate how much trawling is taking place in new areas,
monitor trends in the extent and intensity of contact over time.

The trawl footprint is a fundamental part of assessments, currently in development, that seek to quantify the impact that trawling has had on seafloor species. The trawl footprint is also used alongside other information on the distribution of benthic species and important habitats to help inform planning processes for closures or protected areas.

Seafloor species captured in trawl gear are recorded by observers and, where experts are needed to identify a particular animal, samples are returned for identification. This has led to the discovery of species new to New Zealand and new to science.

The extent of bottom contact by commercial trawling and dredging in NZ waters, 1989–90 to 2020–21 [PDF, 10.5 MB]

Trawl footprint

The trawl footprint is the total area of the seabed that has or may have been contacted by fishing gear. This measurement includes all fish caught commercially in New Zealand using either bottom trawl gear, or mid-water trawl gear within 1 metre of the seabed.

In 2021, the trawl footprint was 74,500 square kilometres (km²). The total area trawled from 1990 to 2021 is 462,643km². This is about:

11% of the total area within the boundary of New Zealand’s EEZ and territorial seas
33% of the area that can be fished (which is down to depths of 1,600 metres).

We have been estimating the trawl footprint since 1990 for deepwater fisheries and since 2008 for all trawl fisheries. How trawl effort is reported has changed over time. For example, in 1989 we began requiring some larger vessels (mostly those greater than 28 metres in length) to report start and end positions of trawl tows. Since 2007, smaller vessels have been required to report the start position of tows.

Table: Trawl footprint from 2007-08 to 2020-21

Fishing year	Estimated annual footprint (km²)	Estimated annual aggregated footprint (km²)	Newly trawled footprint (km²)
2007–08	92,240	154,337	906.8
2008–09	90,034	151,878	83
2009–10	94,568	162,887	58.1
2010–11	91,583	156,056	128.2
2011–12	87,925	152,329	30.5
2012–13	85,666	146,113	31.2
2013–14	90,434	151,980	40.3
2014–15	88,996	149,051	163
2015–16	86,332	146,402	75.8
2016–17	88,146	154,330	65
2017–18	88,810	158,505	28.4
2018–19	80,701	142,798	76.4
2019–20	70,302	129,594	57.2
2020–21	74,508	130,081	53.1

The extent of the trawl footprint will vary in each annual footprint project analysis because a full set of data is extracted for each project and includes the most recent fishing year of data, as well as any updates to the underlying previous years of data. There have also been refinements to grooming algorithms which can produce slight changes in the location and distance of trawls.

How much trawling is taking place in new areas

Since our records began in 1990, we measure how much trawling is taking place in new areas which have not been contacted by fishing gear. In 2021, 53.1km² of the trawl footprint were newly trawled areas. Overall, the newly trawled area per year has been decreasing over time.

Where and when trawling occurs

The introduction of electronic position and catch reporting in 2018/19 has improved the level of detail about where and when trawling is occurring. We now have a more accurate measurement of where trawling is occurring. As a result the way we measure the trawl footprint has changed over time.

Some of the area in the footprint will have been trawled repeatedly. Overall, the frequency of areas being trawled has been reasonably steady over the past 10 years. The intensity of bottom contact is greater in some areas, like the central east of the North Island (FMA2) and the south-east of the South Island (FMA3). This is a result of greater levels of fishing in these areas, as well as the localised area of some species targeted.

How we measure trawl footprint

To estimate the trawl footprint, the reported start and end positions of each tow are plotted, and straight lines are drawn between them. The lines are then buffered by the standard width of the trawl doors to create a rectangle for each tow which represents the area of seabed contacted by the trawl gear, known as the swept area.

Before electronic reporting, for some vessels, end positions were not required to be reported and the coordinates reported were accurate to only 1 nautical mile. In addition, with only start and end positions available, we can only plot straight lines, whereas we know trawl tows are often not straight. As a result, the position of these plotted tow tracks is an approximation.

We continue to explore ways to improve our methods. A key improvement will be the transition from straight lines being drawn between start and end points of trawling events to calculate the trawl footprint, using Geospatial Position Reporting (GPR) data, to define the area of contact with the seafloor more accurately.

Forum to manage the effects of bottom trawling in the EEZ

Fisheries New Zealand and the Department of Conservation have established a stakeholder forum on managing the effects of trawling in the EEZ. The forum included members from:

environmental non-governmental organisations
the fishing industry
the National Iwi Chairs Forum.

Forum members provided recommendations in late 2022, which will inform public consultation on measures to reduce the impacts of bottom trawling.

Managing the effects of bottom trawling in the EEZ forum – Terms of reference [PDF, 200 KB]

Managing bottom trawl and Danish seine in the Hauraki Gulf

As part of the Revitalising the Gulf strategy, Fisheries New Zealand is further limiting Danish seining and bottom trawling activity in the Hauraki Gulf to carefully selected access areas or "trawl corridors". This will help protect the seafloor and its ecosystems.

These measures will help address the impacts of bottom trawling in the Hauraki Gulf in a way that sustains and supports the ecosystem. With input from mana whenua and stakeholders, Fisheries New Zealand is designing, selecting, and designating areas or "corridors" within the Gulf where bottom trawling and Danish seining can continue to occur but with less impact on biodiversity.

A research project published in 2023 to support this process and a Hauraki Gulf specific spatial planning tool is in place. Fisheries New Zealand is working to finalise options for trawl corridors in 2023, using the spatial planning tool that has been developed and with input from mana whenua, key stakeholders, and the Hauraki Gulf Fisheries Plan Advisory Group.

Find out more about the Revitalising the Gulf strategy

Who to contact

If you have questions about bottom trawling, email info@mpi.govt.nz