As the daughter of a former groundfisherman, my family’s income
depended on otter trawling when I was growing up, so perhaps that has influenced my perspective on
trawling. I think the issue of benthic impacts from trawling is much more
complex than it is often portrayed. As several studies have demonstrated (Kostylev
et al. 2005, Lindholm et al. 2004, Lokkeborg 2005), dragging influences different
habitat types in different ways and to different degrees. While dragging might
have detrimental effects on complex habitat with rocky substrate, the effects
may be negligible when compared with natural disturbance on dynamic, unstable
sandy bottom.
Think about it in terms of the terrestrial environment. If a
big net was swept across the Sahara Desert and the old growth forests of
northern California, do you think it would have the same effect on these
ecosystems? Of course not. The vertical structure and the ecological resilience
of these habitat types is completely different and therefore you would expect
the effects of trawling these areas to be quite different. The old growth
forest is a relatively stable environment that has taken centuries to establish
and so the effects of trawling would be significant. However, the desert
represents an unstable substrate subject to high levels of natural disturbance
(wind and heat), and the impact would likely be impacted relatively low. This
diversity of terrestrial habitat impact is not unlike the marine environment.
Furthermore, it is extremely difficult to quantify the
effect of trawling on benthic communities. Successful studies have only been
able to do this in benign coastal environments, since the task
demands either divers or cameras both of which require ideal conditions. We attempted to quantify incidental mortality of scallops and dredge impacts on sandy bottom in Southern New England using a ROV and an AUV in 2014. Despite the shallow water (~30 fm), calm sea conditions, and low currents it was nearly impossible to 1.) tow over the area that was surveyed pre-dredging, 2.) to locate the dredge path after towing and 3.) to accurately quantify the difference let alone yield consistent results. Further complicating the matter was the question of whether the pre-tow survey was representative of untowed conditions? Unlikely, since this area is regularly fished. What then do you quantify as "unimpacted" seafloor? It is not as straightforward of a study as one may think when reading the literature.
So the next time that you hear someone declare that dragging is inherently detrimental or even hear scientists confidently state dredge impact statistics, think twice about the issue and be aware that it's more complicated than some let on.
