Derelict fishing gear threatens inland water bodies too

Abandoned, lost, or otherwise discarded fishing gears (ALDFG) also known as derelict nets or ghost nets, are fishing nets that have been abandoned or lost at sea or in other water bodies. These nets can continue to fish indiscriminately, entangling and killing marine or freshwater life for years or even decades.

One of the main reasons that ghost nets are so harmful is that they are made of synthetic materials such as nylon, which do not biodegrade quickly. Instead, they persist in the water for decades, continuing to fish and entangle different species. Ghost nets can trap and kill a wide range of animals, including fish, turtles, fish eating birds, and even dolphins, whales, and sharks in the seas.

They can also cause damage to the seabed, destroying important habitat for bottom-dwelling species. Ghost nets are a particularly significant problem in areas where fishing activity is high, such as near shorelines or in fishing grounds. They can also be more prevalent in areas where there are high winds and currents, which can cause fishing gear to become lost or abandoned.

The challenge of lost or abandoned gear is not restricted to marine waters

Efforts to address the problem of ghost nets have focused on prevention and removal. Preventative measures include implementing regulations to require the use of more durable and biodegradable fishing gear, increasing education and awareness among fishermen, and improving tracking and retrieval systems for lost gear. Removing derelict nets from water bodies is challenging and costly. It often requires specialised equipment and expertise, as well as significant funding. However, there are several initiatives underway to address the problem of derelict nets, including the Global Ghost Gear Initiative, which aims to tackle the problem through a combination of prevention, removal, and recycling. While ALDFG is usually associated with marine and coastal waters where much of the world’s fishing efforts are concentrated, inland water bodies suffer from the problem too, though it is less well documented. In Lake Ohrid, a transboundary lake in the south-western Balkans shared by North Macedonia and Albania, a pilot study was carried out in 2017 to evaluate the technical possibilities to detect and remove ALDFG and to analyse the catches made by the nets to gauge their impact on the ecosystem. The study1 was conducted by a team of researchers from institutes in North Macedonia and Germany as one of the activities for the trilateral Project Transboundary Conservation and Sustainable Use of Biodiversity at Lakes Prespa, Ohrid and Shqodra/Skadar.

Lake Ohrid, the deepest of the Balkans’ three freshwater seas is also known as the oldest lake in Europe, oligotrophic, a museum of “living fossils”, rich in biodiversity, with relatively low fish production but with high endemism, is inhabited among others by very attractive and commercially valuable fish as Lake Ohrid trout (Salmo letnica) and Ohrid belvica (S. ohridanus). Three hatcheries are involved in the artificial spawning and breeding of Lake Ohrid trout (two in North Macedonia and one in Albania) for restocking purposes. Since 1935 the main purpose of the North Macedonian Hydrobiological Institute—Ohrid hatchery is to replace what is taken from the lake by fishing. Commercial fishing in Lake Ohrid dates back 100 years. Nowadays, apart from the commercial fishing in the Albanian part of the lake, which is conducted by some 100 fishers organised into a fisheries management organisation there is also illegal fishing present on both sides of the border mainly targeting trout species but also carp, bleak and eel.

The most commonly used fishing gear both for commercial and illegal fishing is gillnets of different mesh sizes, but fishing with lines and seines is also practiced. Gillnets are a passive gear and therefore a fuel-efficient way of catching fish. A suspended like a curtain in the water column using anchors and buoys and does little damage to the benthic communities. It is also selective, catching fish of a certain size while allowing juveniles to escape. However, if a gillnet is lost, abandoned, or discarded it can continue to trap and kill fish and other species. In addition, as it gets worn down it contributes to the plastic pollution in the water body for many decades.

Different tactics used to detect and retrieve ghost gear

Depending on the bottom configuration and depth, three different methods were deployed by the researchers for locating, mapping, and removing ALDFG: echo sounder, divers, and dredging, of which the two latter were combined. Using previously known information about the areas where ALDFG had been recorded either by professional or recreational fisherman, local citizens or divers from the local diving clubs, and the experience of the Hydrobiological Institute in Ohrid, for the first time a systematic removal of ALDFG from Lake Ohrid was performed.

The fieldwork resulted in the detection of 243 nets of which 36 were completely and 163 partially recovered. Lengths of the nets varied from 5 to 500 m with mesh sizes from 12 to 140 mm and hight ranged from 1.7 m to 6 m. The total length of nets retrieved was close to 13 km. All the nets, except one, were single layered gillnets and most were made of synthetic materials, but also threaded nets more than five decades old were recorded entangled with nylon nets. They were typically found stranded on the rocky bottom of the lake or on other underwater obstacles, with a few free-floating nets as well. The team’s experience was that echo sounding was effective up to a depth of 60 m and when the net was free floating. At greater depths the echo sounder did not give a reliable signal. Lake Ohrid is a deep lake with maximum depth of 298 m and mean depth of 151 m, so only part of the lake’s volume was explored (Photo 2). The divers detected 173 nets at two different sites, one along the shore in the area of Lake Ohrid trout spawning grounds and the other at 40 m depth in the immediate vicinity of the drinking water pump system of the City of Ohrid, entangled to form a wall of nets 6 m high, totalling over 10 km in length.

The creeping devices that were towed along the lake bottom retrieved over 1,100 m of nets at depths ranging from 15 to 80 m. This was achieved by engaging nine fishermen or fish guardians from both countries that share the lake, as transboundary activity paired in five fishing boats. In addition to nets, other ghost fishing gear was recovered included fishing lines with spinners and jigs, polyamide ropes, monofilament, hooks and net anchors (Photo 3).

The retrieved nets provided insights into the damage ALDFG does as they revealed different species that had been trapped. Both living and dead animals in various stages of decomposition were found totalling 116 fish and four birds. Barbell, Ohrid belvica, and roach were the most frequently present species accounting for 97 of all the fish in the removed nets. The researchers also established that more fish were found in the floating nets than in those that were entangled on the ground or wrapped around other underwater obstacles. The nets trapped not only fish but also crustaceans and birds. In addition, they can be dragged along the ground by currents and are likely to negatively affect benthic communities. Moreover, the abrasion of nets as they move along the bottom adds to plastic litter which can ultimately enter food chains as it degrades into tiny particles (microplastic). The presence of decomposing fish in nets located in fish spawning areas may cause spawning fish to avoid these areas, especially the two trout species.

Prevention is cheaper and better than cure

The researchers also concluded that reliable detection of the nets was critical to efforts to remove them. While echo sounders could detect floating nets, they were not as efficient when the nets had sunk to the bottom. More sophisticated technology such as remote underwater vehicles equipped with cameras may prove better at detecting nets on the lake bottom. Using creeping devices was time consuming but effective at detecting ghost nets on the bottom as well as removing them if they were in good condition and not entangled on the ground. These devices could be used to clear larger areas of the lake bottom although dragging the devices along the ground can potentially damage benthic features thereby reducing the quality of the bottom habitat. Diving on the other hand was useful if it was targeted at areas that were known to accumulate abandoned nets, but it was also dangerous as divers often got entangled in the nets themselves. Recovery of the ghost nets required the services of a suitable vessel equipped with winches to pull up the nets. The vessel also supported the work of the divers to lift the nets from the bottom with the help of airlifts. The research showed that ALDFG in freshwater environments is a source of issues similar to those found in marine habitats and deserves closer study and more action to remove it. The scientists recommended that joint activities by North Macedonian and Albanian authorities to identify and remove ghost nets should be carried out annually, the reporting of lost nets by fishers should be made mandatory, and the synthetic material used for nets should be replaced with biodegradable compounds.

Following on from the project, this year two activities regarding ALDFG detection and removal are planned in North Macedonia with governmental support, one at Lake Ohrid and the other at Lake Prespa.