Oceanography | Vol.28, No.4
COMMENTARY
Bathymetric Extent of Recent Trawl Damage to the
Seabed Captured by an ROV Transect in the Alboran Sea
By Michael L. Brennan, Miquel Canals, Dwight F. Coleman, James A. Austin Jr., and David Amblas
Bottom trawl fishing is among the most destructive anthropogenic pressures acting
on benthic ecosystems, but the full extent of the damage is undocumented because
of the limited number of deep-sea observations of impacted regions (e.g., Brennan
et al., 2012, 2016). As part of its continuing ocean exploration mission, in 2011,
E/V Nautilus conducted a remotely operated vehicle (ROV) survey along a tran-
sect in a submarine canyon in the Mediterranean’s Alboran Sea off southern Spain
at depths ranging from 1,200 m to <300 m (Coleman et al., 2012). This exploration
along the South Alboran Ridge offered the opportunity to directly observe with
video the bathymetric extent and intensity of recent trawling damage to the seafloor
in this area. This dive revealed large furrows running in multiple directions caused
by trawl doors scraping across the seabed. Little biological activity was evident in
the depth ranges where these scars were observed. The destructive nature of bot-
tom trawl fishing should be viewed with the same public affront as subaerial clear-
cutting of forests and strip-mining. The only difference is that the ocean hides trawl
damage from the public eye. The more we explore the deep sea, repeatedly map the
seafloor with sonar, and observe the seabed and its ecosystems with video captured
by ROVs, the greater we can understand the full impacts of trawling.
The deleterious and nonselective damage that trawling operations cause to the
seabed has been a subject of concern and debate among ecologists and fisheries man-
agers for decades (e.g., Caddy, 1973; Jones, 1992; DeAlteris et al., 1999; Demestre
et al., 2015). Bottom trawls have a long-lasting impact beyond their removal of
large quantities of fish from the ecosystem, including bycatch. Trawling destroys
benthic habitats and hard ground for invertebrates, smooths over seabed morphol-
ogy, and resuspends sediments (e.g., Watling and Norse, 1998; Ivanović et al., 2011;
De Juan and Demestre, 2012; Lucchetti and Sala, 2012; Norse et al., 2012; Martín
et al., 2014a). In the Mediterranean, the trawl fleet works along both the conti-
nental shelf and the continental slope. Trawls catch many species, although only
some of them are targeted, including blue whiting (Micromesistius poutassou), hake
(Merluccius merluccius), red mullet (Mullus spp.), octopus (Octopus vulgaris and
FIGURE 1. The ROV transect began at
nearly 1,200 m on flat, muddy seabed, with
small mounded burrows and clear bioturba-
tion. Below 850 m, no trawl marks are evi-
dent. Macrofauna, including rattail fish, sea
urchins, crabs, and blackmouth catshark, were
observed in the area.
FIGURE 2. At 0530 GMT, Nautilus crossed
paths with a trawling vessel and caused the
team to slow the ROV transect. When the vehi-
cles reached the area the where the fishermen
were operating, fresh trawl marks were visi-
ble on the seabed. New trawl marks are criss-
crossed with older scars, although all appear
recent, with rectangular-shaped edges rather
than the U-shaped scars that develop once
they become partially filled in with sediment.
FIGURE 4. This picture of fresh, deep trawl fur-
rows in the sediment shows larger clumps of
sediment that have settled next to the scar.
Smaller particles are resuspended into the
water column and can be transported further
downslope as a sediment cloud caused by the
turbulence of the weighted net and gear pass-
ing by (Jones, 1992; Puig et al., 2012).
FIGURE 5. Isolated bedrock outcrops com-
monly found on flatter slopes create habitat for
a variety of fauna that live on and around them.
Here, an outcrop is inhabited by corals as well
as a siphonophore and a visiting Conger eel
(Conger conger). Both of the latter were com-
monly seen during this transect.
FIGURE 3. Many ridges in the sediment were
observed during the ROV transect when mov-
ing upslope; steeper terrain indicates slope fail-
ures. Trawl operations can smooth over such
sedimentary features and also trigger slope
failures, as has been noted in the Black Sea
(Brennan et al., 2013).
1,187 m
823 m
814 m
781 m
692 m