Commercial seabed trawling and dredging

  • Image, Commercial seabed trawling and dredging.

    Seabed trawling and dredging (where fishing gear is towed near or along the ocean floor) can physically damage seabed (benthic) habitats and species. These fishing methods can also stir up sediment from the seabed, creating sediment plumes that can smother sensitive species. Recovery times for affected habitats and species depend on their sensitivity and the area affected by trawling or dredging. Bottom trawling is carried out on or near the seabed in both shallow and deep waters. Dredging is carried out on the seabed in shallow waters and targets marine creatures such as scallops. This measure focuses on deepwater areas (waters deeper than 200m).

    We classified Commercial seabed trawling and dredging as a case study.

    Key findings

    Pressure on seabed habitats and species from commercial trawling and dredging has been steadily reducing in New Zealand waters since the 1990s. This is due to reduced number of trawling and dredging tows.

    • Inshore and shelf seabed environments had 51 percent of their area trawled between 1990 and 2011. The inshore and shelf seabed environments are classes A to E identified in the Benthic-Optimised Marine Environment Classification (BOMEC).
    • Nearly half (44 percent) of the seabed shallower than 400 metres has been trawled at least once between 1990 and 2011.

    Figure 1

    Note: Commercial fishers use CELR, TCER, and TCEPR forms to record trawl fishing. CELR – catch effort landing return was used by vessels smaller than 28m fishing in shallow inshore waters up until 2008; TCER – trawl catch effort return replaced the CELR form in 2008. It was introduced for small trawl vessels (6–28m long), and allows individual trawl positions to be reported; TCEPR – trawl catch effort processing return is used primarily by vessels longer than 28m operating in waters deeper than 200m. TCEPR forms report individual trawl positions.

    Figure 2

    Note: BOMEC = Benthic-Optimised Marine Environment Classification. The trawling area for coastal classes A–E underestimates the full extent of trawling activity because data from CELR (catch effort landing return) forms are excluded (Ministry for Primary Industries, 2016). The total area for class K, lower-depth steep slopes of North Cape, is too small to display. See Marine environments for a map of where BOMEC classes occur.

    Figure 3

    Note: The footprint area is the area of seafloor swept at least once by trawling. 

    Definition and methodology

    We report on three aspects of bottom trawling:

    • the number of trawl tows over time, as an indication of changes in fishing effort and therefore fishing pressures on the seabed (figure 1)
    • the area of trawled seabed in BOMEC (Benthic-Optimised Marine Environment Classification) (Leathwick et al, 2005) benthic environments (figure 2)
    • the area of trawled seabed in four depth classes: 0 to 400 metres, 400 to 800 metres, 800 to 1,200 metres and deeper than 1,200 meters (figure 3).

    The data is derived by overlaying the TCEPR trawl footprint for the 1989–90 to 2010–11 fishing years on BOMEC and depth classes (Black & Tilney, 2014). The BOMEC has 11 trawled classes: five inshore and shelf (A, B, C, D, and E), three upper-slope (F, G, and H), two northern mid-depth (I and J), and one southern mid-slopes class (L). BOMEC classes are broad surrogates for the different habitats that are affected.

    See Marine environments for a map of where BOMEC classes occur.

    This indicator focuses largely on trawling for deepwater fisheries. There is some overlap with the information on Commercial coastal seabed trawling and dredging

    Due to differences in data grooming methods, the BOMEC and depth-class analyses are based on a different number of tows (31,994 tows in 2011) than the analysis on trawl tow numbers (32,246 tows in 2011). This amounts to a 0.8 percent difference in number of tows used for the various analyses.

    The commercial fishing year for most fish stocks goes from 1 October to 30 September, but some fish stocks have a fishing year of 1 April to 31 March.

    Data quality

     Topic Classification   Relevance Accuracy 
    Resource use and management, and other human activities  Case study



    See Data quality information for more detail.


    Black, J & Tilney, R (2015). Monitoring New Zealand’s trawl footprint for deepwater fisheries: 1989–1990 to 2010–2011. New Zealand Aquatic Environment and Biodiversity Report 142. Retrieved from

    Leathwick, JR, Rowden, A, Nodder, S, Gorman, R, Bardsley, S, Pinkerton, M, … Goh, A (2012). A Benthic-Optimised Marine Environment Classification (BOMEC) for New Zealand waters (PDF, 3MB). New Zealand Aquatic Environment and Biodiversity Report No. 88 (p54). Retrieved from

    Archived pages

    See Commercial seabed trawling and dredging (archived October 2016).

    Updated 27 October 2016

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