To Stir Up Turmoil On The Ocean Floor

Long gone are the days of simple fishing lines and traps, as commercial fishing evolved through the technological advancements and the mechanization of fishing methods [1,2]. No longer are their activities limited by depth as is evident from the commonly utilized bottom-trawling methods. However, the evolution of the fishing industry comes at a cost when bottom-trawling ends up triggering a cascade of sediment-based pollution issues on the ocean floor. 

Bottom Trawling 

Bottom-trawling is a fishing method that involves the use of a weighted meshed net that is kept open while it sinks to the seafloor and towed across the seabed [3]. Fish are trapped in the net as machinery onboard the fishing vessel hauls the net back up to the surface [3]. Depending on the target species of the haul, different features are attached to the nets but no matter the type of equipment, the bottom-trawling method is generally thought to be destructive regardless [3,4]. 

Its destructive nature lies in the fact that its large nets are indiscriminate with what they hit, collapse and tear out in their goal to capture as many fish as possible along the seafloor. In the process, it is inevitable that sediment on the seabed be stirred up and resuspended in the water. 

Resuspension of sediments in Eidangerfjord in Norway

This sediment disturbance phenomenon brought upon by bottom trawling was investigated specifically in a fjord in Norway by Bradshaw et al in a field investigation detailed as in Figure 1 below [5]. While not many fisheries operate there, a small prawn fishery can be found operating within the fjord that  utilizes a bottom-trawling method [5]. Although their scale of operations is limited to 4 fishing boats and the use of small trawls, the impact of their bottom-trawling activities were still significant [5]. 

Why does this spell trouble for the environment?

We must first consider the anthropogenic history of the fjord. In the 1950s, a nearby magnesium production plant had released toxic by-products such as dioxins and polycyclic aromatic hydrocarbons (PAHs) into a neighboring fjord which eventually leaked into the Eidangerfjord [5,6]. While the authorities have attempted to reduce the extent of contamination through restrictions and treatments, the contamination persists in the accumulated sediment on the seabed [5]. Secondly, we must also consider the role of sediments as highly effective sinks and sources of contaminants [7]. Studies have shown that sediments take up 

Their research also revealed that the lasting blanket of suspended sediment paired with the slow sediment transport rate out of the fjord indicates a “minimal net removal and/or degradation of contaminants” [5]. This means that whatever contaminants entered the fjord stayed within the fjord for a long period of time. The settling of contaminated sediments onto the seabed also creates conducive anaerobic conditions for the continued preservation of pollutants [7]. This would have led to the long-term accumulation of toxins within the fjord – a sink of contaminants that threatened to bring massive repercussions due to bottom-trawling [5,7]. Although Eidangerfjord may not have been directly contaminated by the magnesium plants and measured lower pollution levels than Frierfjord, the long residency period of the contaminants exacerbates the impact that the contaminants have on the ecosystem [6]. 

In this study, blue mussels Mytilus edulis were introduced into the fjord as field-exposed indicators of contaminant uptake by marine organisms in the fjord [5]. The concentration of contaminants such as PAHs and polychlorinated dibenzo-p-dioxins and -furans (PCDD/Fs) in the mussels were used to gauge the bioavailability of the contaminants [5]. The increasing concentrations within mussels nearer to the seabed revealed a definite uptake of contaminants by marine organisms, with sediments being the clear source of contaminants [5,8]. Studies have shown that the uptake of such contaminants are more prominent in filter feeders, but biomagnification through the food web guarantees the persistence of these contaminants in most other aquatic organisms in the ecosystem [5,8]. It is without a doubt that concentrations rise exponentially up the food chain, with humans bearing the highest concentrations [9]. Estimations also revealed that continued consumption of these marine organisms by humans could lead to potentially toxic repercussions [5,8]. 

Moving forward, environmental risk assessments of contamination in sediment beds should be carried out prior to bottom trawling to evaluate the extent of potential harm to the environment [7]. Factors such as contaminant types, species of marine organisms in the ecosystem, and other external stressors that affect the bioavailability of the contaminants should be monitored. Guidelines on bottom trawling operations need to take into account these assessments to decide on appropriate locations and post-trawling management efforts. The next blog post will evaluate how sediment resuspension by bottom trawling can be exacerbated by climate change and in turn, how it can contribute to it as well.

References:

[1] Brandt, A. R.F.T. von , Borgstrom, . Georg A. , Pike, . Dag , Purrington, . Philip F. and Sainsbury, . John C. (2022, February 1). commercial fishing. Encyclopedia Britannica. https://www.britannica.com/technology/commercial-fishing

[2] Commercial fishing methods. Sustainable Fisheries UW. (2019, August 8). Retrieved March 9, 2023, from https://sustainablefisheries-uw.org/seafood-101/commercial-fishing-methods/

[3] Fisheries, N. O. A. A. (2022, July 6). Fishing gear: Bottom trawls. NOAA. Retrieved March 9, 2023, from https://www.fisheries.noaa.gov/national/bycatch/fishing-gear-bottom-trawls#:~:text=Bycatch%20Reduction-,Bottom%20trawling%20is%20a%20fishing%20practice%20that%20herds%20and%20captures,Bottom%20trawl

[4] L Windom, H., & Stickney, R. R. (1976). Environmental aspects of dredging in the Coastal Zone. C R C Critical Reviews in Environmental Control, 6(2), 91–109. https://doi.org/10.1080/10643387609381635

[5] Bradshaw, C., Tjensvoll, I., Sköld, M., Allan, I. J., Molvaer, J., Magnusson, J., Naes, K., & Nilsson, H. C. (2012). Bottom trawling resuspends sediment and releases bioavailable contaminants in a polluted fjord. Environmental Pollution, 170, 232–241. https://doi.org/10.1016/j.envpol.2012.06.019

[6] Hylland, K., Ø. Aspholm, O., Knutsen, J. A., & Ruus, A. (2006). Biomarkers in fish from dioxin-contaminated fjords. Biomarkers, 11(2), 97–117. https://doi.org/10.1080/13547500600565602

[7] Chiaia-Hernández, A. C., Casado-Martinez, C., Lara-Martin, P., & Bucheli, T. D. (2022). Sediments: Sink, archive, and source of contaminants. Environmental Science and Pollution Research, 29(57), 85761–85765. https://doi.org/10.1007/s11356-022-24041-1

[8] Fletcher, C. L., & McKay, W. A. (1993). Polychlorinated dibenzo-p-dioxins (PCDDs) and dibenzofurans (pcdfs) in the aquatic environment — a literature review. Chemosphere, 26(6), 1041–1069. https://doi.org/10.1016/0045-6535(93)90194-a