Trawling on seamounts: effects on corals

By Jennifer Dean,
Marine Conservation student

Have you ever wondered how your seafood is caught?  Many of the species commonly consumed by humans, such as cod, flounder, and shrimp, are caught by a method called bottom trawling (montereybayaquarium.org).  Bottom trawling is a large scale fishing method that has been used for many years, consisting of dragging a large net or nets along the seafloor to scoop up fish that live near the bottom.  Unfortunately, this method is not very ecosystem-friendly.  Most trawl nets include heavy wooden or metal frames to keep the nets open, and these frames drag along the bottom, creating troughs, re-suspending sediment, and damaging organisms.  Recently people have begun to realize just how devastating these impacts can be on seafloor habitats.  In fact, just this past September it was shown that bottom trawling severely alters the ocean floor through smoothing of the local topography (Puig et al. 2012).  One ocean habitat that has been heavily impacted by bottom trawling is that of seamounts.

A trawl, from OceanExplorer.NOAA.gov

Seamounts are essentially mountains in the ocean.   Soft sediments cover most of the deep sea floor, but seamounts provide rare areas of rock bottom. These rocky surfaces, combined with the hydrodynamics of seamounts, make them unique environments within the deep (Clark et al. 2012).  They are highly productive areas, often boasting large numbers of fishes, as well as a wide variety of invertebrates, including both mobile and non-mobile species.  Because of this, fishermen have been targeting seamounts for years, in order to increase their catches.  Some of the most important creatures living on seamounts are corals.  Corals are structure-building organisms, which often means that areas with healthy coral populations have high biodiversity, since other organisms can use them for protection.  However, bottom trawling poses a huge threat to coral communities.  Some attempts have been made at estimating trawling risk to corals, but the actual risk varies depending on the individual seamount (Clark & Tittensor 2010).  The fragility of corals means that trawling frequently leaves them demolished, as has been observed in deep-water corals off the coast of Florida (Reed et al. 2007).  Additionally, a recent study by Althaus et al. (2009) found that coral communities on seamounts take very long times to recover from this type of damage.

In their study, Althaus et al. compared the species and amounts of coral on several seamounts in the ocean off Tasmania, to evaluate the impacts of trawling.  They looked at a variety of seamounts, some of which were open to trawling, some that had formerly been trawled (but had been closed 5-10 years ago) and some that had never been trawled.

Researchers used video and still images to identify species of corals and other non-mobile species, as well as to calculate the approximate areas covered by those organisms.  They examined the diversity of species, particularly focusing on corals, and compared the abundance, diversity, and evenness of the coral cover between the three types of seamounts.

The study showed a natural decrease in coral abundance with increasing depth on the non-trawled seamounts.  This means that shallower undisturbed seamounts are likely to have more corals than deeper undisturbed seamounts.  There were less corals on trawled seamounts compared to non-trawled, and the organisms seen were very different in areas that had been trawled versus those that hadn’t.  Non-trawled mounts were mostly covered in dense thickets of the coral Solenosmilia variabilis, while trawled seamounts rarely showed it.   A few patches of S. variabilis were seen on trawled seamounts within areas difficult to reach by trawls (usually holes or dips in the surface of the seamount that the nets don’t reach).

Non-trawled seamounts had higher numbers and wider variety of organisms, likely due to the increased shelter and food provided by the corals.  In contrast, the trawled seamounts generally had low densities of both coral and other organisms. This is a strong indication that the removal of the corals through trawling is capable of changing the biological make-up of seamounts.

More importantly, the authors found that the species diversity and richness of seamounts that had formerly been trawled were very similar to those that were still actively being trawled.  They had shown very little recovery despite not having been trawled for 5-10 years.  This gives us an idea of the long periods of time required by these animals for growth, and subsequently, to recover from the damage we have caused.

Figure 4 from Althaus et al. (2009). “Representative images of seamounts (b) that were never trawled, (c) where trawling ceased 5 to 10 yr ago and (d) where trawling is active.”

Considering the natural decline in coral abundance with depth, the authors suggest that the most important areas to protect are the shallow seamounts that still harbor patches of branching corals.  The few changes that were seen on formerly trawled seamounts are a signal that eventually these areas will begin to recover from trawling.  The recovery will take a long time (decades, the authors say) but if we want to preserve these habitats, it is vital that the shallow seamounts be closed to trawling so the corals can begin rebuilding.  Clark et al. (2012) summarize the main points of focus in order to move forward with conservation and management of seamounts.  These include descriptions of biodiversity, population connectivity, fisheries impacts, and post-impact recovery.

Studies such as this one by Althaus et al. reveal the devastation caused by bottom trawl fisheries.  If this method of fishing is allowed to continue, there may eventually be few corals left on seamounts, and this study has shown that without corals, there are fewer organisms present.  If we don’t give these corals the time they need to recover, there may be few fish left on the seamounts to catch.

REFERENCES

Althaus F, William A, Schlacher TA, Kloser RJ, Green MA, Barker BA, Bax NJ, Brodie P, Schlacher-Hoenlinger MA (2009) Impacts of bottom trawling on deep-coral ecosystems of seamounts are long-lasting.  Mar Ecol Prog Ser 397:279-292 http://iiiprxy.library.miami.edu:3016/abstracts/meps/v397/p279-294/

Clark MR, Tittensor DP (2010) An index to assess the risk to stony corals from bottom trawling on seamounts.  Mar Ecol 31(Suppl. 1):200-211

Clark MR, Schlacher TA, Rowden AA, Stocks KI, Consalvey M (2012) Science Priorities for Seamounts: Research Links to Conservation and Management. PlosOne 7(1):e29232.

Monterey Bay Aquarium Seafood Watch. Accessed 2 November, 2012. http://www.montereybayaquarium.org/cr/cr_seafoodwatch/sfw_gear.aspx#trawling

Puig P, Canals M, Company JB, Martin J, Amblas D, Lastras G, Palanques A, Calafat AM (2012) Ploughing the deep sea floor.  Nature 489:286-289.

Reed JK, Koenig CC, Shepard AN (2007) Impacts of bottom trawling on a deep-water Oculina coral ecosystem off Florida. Bull Mar Sci 81(3):481-496.