**4. Bottom trawl**

#### **4.1 Usage and causes of microplastics**

Bottom-trawl fishing, i.e., beam trawls, otter trawls, and dredges, is used worldwide and provides about a quarter of the marine catch [28]. A rich body of literature assesses the impacts of sea bottom trawling on benthic invertebrate disturbances and seabed alterations [29, 30]. However, research on the wear and tear of sea bottom trawl remains relatively understudied.

In Norway, the rockhopper ground gear has been commonly used in the sea bottom trawl fisheries for the last 30 years. In contrast to the previously used bobbin ground gear, the rockhopper has shown improved catch efficiency for Haddock (*Melanogrammus aeglefinus*) and Atlantic Cod (*Gadus morhua*) due to its increased contact with the seabed [31].

The trawl comprises trawl doors, bridles, sweeps, and ground gear (rockhopper). The fish is caught by dragging the trawl net along the seabed. The water is released through the net mesh, and the fish remains in the trawl bag. The ground gear consists of steel and rubber, while the wipers and doors consist of steel. Dolly ropes or other protective ropes used to protect the net consist of bundles of plastic (PE) threads. The ground gear discs are made from old dump truck tires and threaded onto heavy wires or chains. The most common are discs of 21 inches in diameter, but on the rough seafloor or when fishing for halibut, the discs can be 24 inches. The material composition of a standard dump truck tire is approximately one-third natural rubber, while the rest is a mixture of synthetic rubber and filler.

The lifespan of a sea bottom trawl depends on several factors, such as seabed composition, trawl type, traction speed, and local hydrodynamic forces (i.e., current). However, the seabed condition, like the roughness, is the decisive factor in how quickly the trawl components wear. Areas with fungus, followed by stone or rocky bottoms, are abrasive, while clay is the most gentle seafloor type. It is difficult to estimate the wear and tear on the trawl net itself. The line is rarely so worn that it peels off. Before this occurs, sections are exchanged or the line repaired. Fishers replace the protecting net regularly, but exact replacement rates are challenging to obtain from fishers and manufacturers. However, it is possible to estimate wear on the ground gear, which the fishers replace when worn out.

#### **4.2 Calculated wear**

To calculate the annual mass loss from the rockhopper gear, Lrh, we use the following equation:

$$L\_{rh} = N\_{dice} W\_{dice} N\_{tw} N\_v P\_w \frac{12}{LT} \tag{1}$$

Here, Ndisc is the number of discs at the rockhopper, Wdisc is the weight of each disc, Ntw is the number of trawls in use, Nv is the number of vessels, Pw is the average percentage wear when replaced, and LT is the average lifetime in months (**Figure 1**).

**Figure 1.** *Worn-out rockhopper gear (a) and dolly rope used to protect the trawl (b). Photo: SINTEF.*

In conversations with Norwegian shipping companies and gear manufacturers, Norwegian vessels mainly use 21-inch discs weighing between 20 and 23 kg, depending on the manufacturer. Thus, in the calculations, we used a weight per disc Wdisc = 21 kg. Gear has five sections, two starboards, two ports, and one center gear. Each section consists of 21 discs, giving a total of Ndisc = 105.

A rockhopper gear is touching the seafloor in all its length and thus causing massive depletion of the gear, see **Figure 1a**. The manufacturers state that the gear is replaced every 6–10 months, at which they are approximately 20% worn. However, the fishers in our study claim the gear last longer, approximately 12–18 months, but with more significant wear, usually 30–40%. In our calculations, we make a sober estimate of the lifetime to 10 months for a standard gear with a percentage weight loss of 20%. The wear is vessel-specific, so these numbers must be considered average for the fleet.

We consider only vessels that delivered more than 100 t catch during 2019, in which case the number of vessels is 63. Half of them use twin trawl. **Table 1** summarizes the calculations on wear and tear from rockhopper gear, which is close to 50 t annually.

The protecting ropes, like dolly ropes, are also heavily exposed to wear and tear. A Dutch research consortium, DollyRopeFree, has reported weight loss of 10–25% after 2 weeks of use [32]. Unfortunately, they do not mention from which type of trawl the samples were taken. We have not been able to calculate the annual plastic fragmentation from dolly ropes due to a lack of information. However, there is no doubt that dolly ropes and other protective ropes contribute to plastic debris in the ocean on a large scale. A worn dolly rope is shown in **Figure 1b**.


#### **Table 1.** *Calculated annual mass loss from rockhopper gear.*

### **4.3 Trawling worldwide**

Unfortunately, there are no registrations on trawlers worldwide, making it difficult to estimate the global share of microplastics from this gear. Also, the catch statistics are difficult to use since they are sorted by species, not gear type. Therefore, acknowledging that the Norwegian catch share is about 3% is the only way, we can now estimate the global loss. Hence, using the Norwegian numbers for trawl and dividing by the Norwegian share, we end up with a total global microplastic loss due to trawls of 1 656 t.
