Bottom trawling

About bottom trawling fishing

Trawling is when a boat tows a fishing net. 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.

Fishers report to us what type of fishing gear they use, the depth of the gear, the depth of the seabed, and the start and end positions of each tow.

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.

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 used within 1 metre of the seafloor.

In the 2023–2024 fishing year, almost 216,000 tonnes, or 69% 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

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. Other unique areas such as hydrothermal vents support distinctive biological communities.

Deepwater corals can live for hundreds of years and are slow to recover from any disturbance. Most coral species in New Zealand waters are legally protected under the Wildlife Act 1953. In addition, some seafloor species are classified as "threatened" or "at risk". 

How fishing can impact the seafloor

Seafloor habitats and communities within the towpath 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
• 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 sedimentary habitats, such as muddy or sandy habitats.

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 by:

• 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.

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 within:

• 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 based on 4 key criteria. They:

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

Read more about benthic protection areas

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 (the aggregated swept area)
• estimate how much trawling is taking place in new areas
• monitor trends in the extent and intensity of contact over time.  

Trawl footprint and aggregate swept area measures help quantify the impact that trawling has 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.

AEBR 344 Extent and intensity of bottom contact by commercial trawling in New Zealand waters, 1990–2022 [PDF, 5.9 MB]

How we map trawl tows and estimate swept area

Before Electronic Reporting (ER) and Geospatial Position Reporting (GPR)

Tows in the years 1989–90 to 2017–18 are mapped by plotting the reported start and end positions of each tow and drawing a straight line 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.

Since Electronic Reporting (ER) and Geospatial Position Reporting (GPR)

Trawl footprint estimates now use Geospatial Position Reporting (GPR) data to define the area of contact with the seafloor more accurately. This allows estimates to be based on vessel path, rather than straight lines between start and end points. Since 2018–2019, estimation of the trawl footprint and aggregated swept area has incorporated GPR data.

Trawl footprint

The trawl footprint is the total area of the seabed that has or may have been contacted by trawl gear at least once. This measurement includes all trawl tows using either bottom trawl gear, or mid-water trawl gear within 1 metre of the seabed. 

The trawl footprint has been estimated since 1990 for deepwater fisheries and since 2008 for all trawl fisheries. The estimated trawl footprint from 1990 to 2024 is 465,102 square kilometres. This is about:

• 11% of the total area within the boundary of New Zealand’s EEZ and territorial seas
• 34% of the area that can be fished (which is down to depths of 1,600 metres).

The estimated trawl footprint for the 2023–24 fishing year was 68,048 square kilometres.

Aggregated swept area

The aggregated swept area is the sum of all individual tows trawled area and will exceed the footprint where tows overlap one another. If a particular area is contacted by multiple tows, the area will be counted once as part of the trawl footprint but will be counted multiple times in the calculation of the aggregated swept area. The mapped aggregated swept area provides a spatial representation of the intensity of trawling in areas. 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).

How much trawling is taking place in new areas

We measure how much trawling is taking place in areas which have not been contacted by trawl gear to understand if and how the footprint is spreading. In 2024, 3.2 square kilometres of the trawl footprint were in new area. Overall, the annual footprint in new areas has been decreasing over time as fishing activity tends to occur within previously fished areas.

Table: Trawl footprint from 2007–08 to 2023–24

This table provides annual trawl footprint, aggregated swept area, and new area estimates for all fisheries from 2007–08 onwards. The footprint estimates for the fishing years from 2018–19 onwards were determined using the mapped vessel path, and are more accurate indications of the extent of trawling than previous years, where estimates were based on straight lines. Note that inshore trawl effort could not be mapped prior to 2007–08, and the large new area trawled estimated for 2007–08 is mostly a consequence of no inshore effort having been mapped rather than a real expansion of trawling in this year.