This probably should have gone under the annual Southern Fried Science Week of Ocean Pseudoscience, but last week kept me busy enough to effectively prevent me from doing any writing here. For some reason, I’ve been hearing from multiple fishermen, people who have worked with my labmates and are generally pretty smart people, that trawling is “good for the seafloor.” This was covered back in December over at Deep Sea News, regarding a study on mud bottom habitats that aimed to see whether this was the case. As marine biologists, conservationists, or fisheries scientists, we likely assume that trawling isn’t so great for the environment, since by its nature it pretty much obliterates everything in its path. However, at least a few rather intelligent commercial fishermen (and according one of them, at least one college professor) have really latched onto the idea that trawling actually improves the environment by stirring up relatively homogenous environments and creating structure where there wasn’t any. If we resist the instinct to yell “holy crap that’s wrong,” it almost makes a little sense: like plowing a field, it stirs up nutrients, which in agricultural fields allows for increased growth. So plowing the seafloor is like plowing a cornfield, which is good for the corn, so plowing must work like that underwater, right?
Where the “trawling=plowing” analogy breaks down is that an agricultural field is an artificial environment created and modified by people for the benefit of harvesting, selling, and eating the products of that environment. A field is outside and has plants growing in it, but it’s about as natural as the potted plants in your house. Without near-constant human intervention, a farm field will become overgrown with non-crop plants and eventually become a forest. Plowing, like fertilizing and pesticide use, is a way of artificially increasing the productivity of that field environment and is necessary because the actual environment that would be there otherwise works totally differently.
Keeping with the seafloor=field analogy, an unkempt field transitioning into a forest produces a lot of larger, slower-growing plants like bushes and trees, which are analogous to the corals and macroalgae in the ocean in that they provide complex habitat. Complex habitat is important for the nursery and foraging areas of exactly those species we’re trying to catch and eat. Though complex habitats have the most fish and would therefore be desirable places to trawl, repeated trawling destroys the complexity of that habitat (and also increases the risk of gear damage) eventually creating a far less productive patch of seafloor that will no longer be contributing to the stocks fishermen are after.
Of course there is science to back all this up, so I’m not totally relying on comparing complex bottom habitat to forests and fields. In hard-bottom cobble habitats off of Alaska, much of the habitat structure comes from relatively fragile sponges and other habitat-forming invertebrates. Freese et al. (1999) found that these habitat-forming invertebrates were more damaged and at lower densities in areas that had been trawled. These sponges and anthozoans were associated with fish aggregations, so what reduces the inverts also effectively reduces the fish living among them. Some of the critters associated with the complex habitats created by sessile invertebrates are exactly the species that are so valuable to Alaskan fisheries. Among them are the world-famous Alaskan red king crab, which depends on complex habitat for larval settlement and nursery habitat (Loher and Armstrong 2000). Less nursery habitat equals less crabs. In this way trawl fisheries may be hurting the crab fisheries.
Add to this that it can take decades for these habitat-forming invertebrates to recover (or centuries for deepwater corals), and it becomes clear that trawling isn’t plowing fields, it’s plowing old-growth forest.
To be fair, the “trawling is good” argument isn’t assuming that trawling is occurring in complex coral and sponge-based habitat, it’s assuming that the trawling is turning over seemingly featureless mud and sand bottoms. To the naked eye these relatively flat environments may seem like they could use a little disturbance, but in reality mud and sand environments are filled with invertebrates crawling along the bottom and within the sediments themselves. In both mud (Hinz et al. 2009) and sandy (Prena et al. 1999) bottoms, infaunal (the creatures living within the sediment) and epibenthic (creatures crawling along the bottom) communities were significantly impacted by trawling, with effects including lowered diversity and increased damage to certain especially vulnerable species. In areas that had been frequently trawled, the entire benthic community could be changed (Hinz et al. 2009).
So what are the effects of trawl disturbance in sandy and mud habitats? After all, these habitats typically lack the habitat-forming sessile inverts like sponges and corals, and most of the epibenthic life is fairly mobile. However, those polychaete worms, bivalves, and other creatures living within the sediment are important links in the food web, so factors affecting them eventually make their way up to the fish we like to eat. In a survey on the effects of bottom trawling in the Irish Sea, Hiddink et al. (2011) determined that the condition of plaice, a commercially-important flatfish, was lower in heavily-trawled areas. Their findings suggest that industrial-level trawling disrupts the local food web by reducing the diversity of those seemingly “unimportant” sand-dwelling critters, leading to malnourishment among plaice and a lower-quality product for fishermen.
Even in habitats that are, at first glance, featureless and seemingly in need of some restructuring, the consensus is that trawling creates degraded habitat. Consistent trawling in the same areas actually does more long-term harm than good for fisheries. This is not to demonize trawling, because every fishing method has the potential to harm the environment. The claim that trawling somehow benefits the environment is demonstrably false, and claiming that it does only harms the credibility of the fisherman’s case. Trawling isn’t going away as a fishing method anytime soon, and there are ways to make it less environmentally destructive, but mowing down an entire area to capture fish will always be a major problem for sustainability.
Freese, L., Auster, P., Heifetz, J., & Wing, B. (1999). Effects of trawling on seafloor habitat and associated invertebrate taxa in the Gulf of Alaska Marine Ecology Progress Series, 182, 119-126 DOI: 10.3354/meps182119
Hinz, H., Prieto, V., & Kaiser, M. (2009). Trawl disturbance on benthic communities: chronic effects and experimental predictions Ecological Applications, 19 (3), 761-773 DOI: 10.1890/08-0351.1
Hiddink, J.G., Johnson, A.F., Kingham, R., & Hinz, H. (2011). Could our fisheries be more productive? Indirect negative effects of bottom trawl fisheries on fish condition Journal of Applied Ecology : 10.1111/j.1365-2664.2011.02036.x 2011
Loher, T., & Armstrong, D.A. (2000). Effects of habitat complexity and relative larval supply on the establishment of early benthic phase red king crab (Paralithodes camtschaticus Tilesius, 1815) populations in Auke Bay, Alaska Journal of Experimental Marine Biology and Ecology, 245 (1), 83-109 DOI: 10.1016/S0022-0981(99)00157-4
Prena, J., Schwinghamer, P., Rowell, T.W., Gordon, Jr., D.C., Gilkinson, K.D., Vass, W.P., & McKeown, D. (1999). Experimental otter trawling on a sandy bottom ecosystem of the Grand Banks of Newfoundland:analysis of trawl bycatch and effects on epifauna Marine Ecology Progress Series, 181, 107-124 DOI: 10.3354/meps181107