Human impacts

Biofouling Organisms Tell All About The Age Of Lost Fishing Gears

Reviewing: Enrichetti, F., Bavestrello, G., Betti, F., Rindi, F., Tregrosso, A., & Bo, M. (2021). Fate of lost fishing gears: Experimental evidence of biofouling colonization patterns from the northwestern Mediterranean Sea. Environmental Pollution, 268, 115746.

Fig. 1 Abandoned fishing gear polluting a tropical coral reef. Image from Richard Whitcombe

Abandoned, lost or otherwise discarded fishing gears (ALDFG) (Fig. 1) constitute a significant portion of marine litter, with about 640,000 tons lost in the oceans every year. This would require about 30,000 garbage trucks to hold! Lost and discarded fishing gears can have serious impacts on ecosystems and wildlife which often get caught or entangled in them. They can also smother organisms living on the seafloor. Additionally, fishing gears are generally made of nylon, an artificial polymer. Nylon is known for its stability, which means that it can persist in the marine environment for a long time (for as long as 600 years!). As they degrade through water moving, sediment abrasion and biological activity, ALDFG can release microplastics in the ocean, a major contributor to marine plastic pollution. With the expansion of fishing operations globally and the constant improvement of fishing gear durability, the impacts of ALDFG are likely to grow further. This growth prompts the need for further understanding of the fate of ALDFGs in the marine environment and how marine communities respond.

Biofouling organisms and ALDFG: a sticky relationship

Biofouling organisms are organisms that settle and grow on any free submerged surface such as ALDFG. Biofouling can be categorized as microfouling, which include bacteria and other microorganisms, or macrofouling, which include larger organisms like seaweeds, mussels, and barnacles. Biofouling organisms usually populate a surface in succession, that is, the process starts with microfouling organisms first then macrofouling organisms next. Given that biofouling has a time dependence, we can measure the age of lost fishing gear by studying the organisms growing on them over time. Information about the age of the lost gear would help in monitoring legal as well as illegal, unreported, and unregulated fishing activities.

The experiment: a 3-year observation of this sticky relationship between biofouling organisms and fishing gear

Fig. 2 The northwestern Mediterranean sea, with the black arrow indicating the site where this research was carried out. Image from Wikipedia.org

In this study, the researchers monitored the type of biofouling organisms that would grow on three different types of fishing gears. Two of them are nylon lines with different diameters, a thin one of 0.01 inches and a thick one of 0.04 inches. The third one is a fishing net made of multifilament nylon. A total of 108 gears (36 for each type) were deployed in the northwestern Mediterranean Sea (Fig. 2) by scuba divers at a depth of 30 m on an underwater cliff. They were then monitored over ~3 years. Samples were collected at irregular intervals (sometimes after days, sometimes after months) and analyzed for the type of biofouling organisms that grew on them.

What did biofouling organisms tell us about the age of lost fishing gears?

Fig. 3 Example of a (a) coralline algae and (b) filamentous algae growing on abandoned, lost or discarded fishing gear underwater. Images from Creative Commons and botanycompanion.wordpress.com.

The researchers found that as soon as the fishing gears were submerged, they were quickly covered by biofouling organisms and were completely covered within one year of immersion. Among all three fishing gears, nets had the highest diversity of biofouling organisms. Coralline algae (Fig. 3a) and filamentous algae (Fig. 3b) were the most recurrent organisms, being present on every gear. Overall, diversity, biomass (or the weight of the organisms), and surface area coverage increased throughout the experiment, meaning that the longer the gears are underwater, the greater the amount and type of organisms that would settle on them (Fig. 4). An interesting finding from this study was that the month and season in which the gear was submerged seemed to influence the composition of the microfouling organisms like the initial settlers. Different biofouling organisms were present in the summer compared to the winter, probably due to the different environmental conditions prevailing in each season. These conditions include food and light availability, temperature, and the amount of nutrients in the water.

Fig. 4 Underwater images showing different amount of fouling organisms during various months of observation. The numbers represent the number of months the gear was submerged and the highlighted months represent summer months. Image from Enrichetti et al. (2021)

The amount and type of organisms also varied with the type of fishing gear; the thick lines (0.04 inches) provided a larger surface for organisms to settle on compared to thin ones (0.01 inches). The fishing nets, with the woven filaments, provided an even larger surface for organisms to settle. While the nylon that makes up the fishing gears usually degrade over time when submerged due to exposure to solar radiation, wave action, and biological organisms themselves, this study showed no signs of deterioration on the fishing gears after 3 years. This was likely due to their depth as they received less solar radiation and the thick envelope of biofouling organisms around the gear shielded it to some degree.

To conclude their study, they proposed a four-stage colonization pattern (Fig. 5), identified from their study, which would help determine the age of the fishing gear:

  • The first stage, generally during the first month, consists of simple, microfouling organisms with a low diversity.
  • The second stage, called a transitional stage, occurs from two to four months of immersion in the water with a 50% increase in the number and diversity of organisms that settled.
  • The third stage occurs from five to eleven months of immersion, where the number and diversity of organisms increased between 50 and 80%. Coralline algae (Fig. 3a) is more dominant during this stage.
  • Finally, in the fourth stage between one and three years, the number of organisms reach between 80-100% and larger macrofouling organisms such as mussels are dominant.
Fig. 5 The four-stage colonization pattern identified in Enrichetti et al. (2021) to determine the age of abandoned, lost, or otherwise discarded fishing gear at sea.

Although this method seems promising for determining the age of lost fishing gears in other places, it is important to keep in mind that not all sites are the same. The position of the gear on the seafloor (partially in place, floating, or collapsed on the bottom) may influence the organisms’ colonization on them depending on water current or sedimentation levels. Still, the possibility of aging ALDFGs could provide useful information for ocean conservationists and could be expanded on a larger geographic scale to address the global problem of fishing debris.

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