**2. Methodology**

#### **2.1 Causes of wear on fishing gear**

The leading causes of fishing gear wear are abrasion with the sea bottom and hauling equipment. Specifically, ropes and nets dragged along the seafloor create heavy wear on trawls and Danish Seine ropes, and for these gear types, it is common to change the parts that are in contact with the sea bottom after 1–2 years and in some cases even more often. The extent of wear depends on the seafloor condition, meaning that a rocky bottom creates much more wear than a sandy bottom.

Another significant cause of abrasion is the onboard hauling equipment. Hydraulic net haulers, as shown in **Figure 4**, or net drums are the two most common haulers in use, and for both, the heavy stress caused by the ropes pressing toward the equipment causes abrasion that gradually tears down the ropes. For the net hauler, the rope is squeezed between two plates, creating even more stress on the rope.

In addition to the gradual abrasion caused by contact, plastic ropes also get degraded by other causes. The most common is UV radiation, leading to the fragmentation of the plastic fibers. Fishing gear is constantly exposed to UV radiation, and proper storing of the gear is essential to prevent degradation. In addition, fragmented plastic ropes have accelerated wear when in contact with the hauler or the seafloor. According to a study from the United Kingdom, plastic ropes lying in the sea at 10 m depth lose an average of 0.39% (PP), 1.02% (PA), and 0.45% (PE) per month caused by abrasion due to UV radiation [24].

Also, gear dragged in the water wears due to the friction force between water and gear. This effect is much less than bottom contact but still significant, combined with the fragmentation effect caused by UV radiation. However, dragging occurs mainly during hauling, and the hauling equipment is considered a much more significant source of depletion.

#### **2.2 Methodological approach**

The research and reports on the wear and tear of fishing gear are scarce. Thus, we need to establish a methodology we can use to approach the solution. Due to the complexity, it requires an enormous effort to get exact numbers of microplastics generated from fishing gear. Instead, our goal is to get a rough estimate of the number of microplastics in the sea to get a feeling of how severe the problem is and determine which fishing gear causes the most pollution.

*Microplastics Derived from Commercial Fishing Activities DOI: http://dx.doi.org/10.5772/intechopen.108475*

For each fishing gear considered, we interviewed five fishers to get basic information on their use of the gear. This information includes the average number of gear in use, the length of the ropes, the average lifetime, and their estimate of the wear and tear when the gear is replaced. In addition, we checked some of these figures against the sales figures from leading gear manufacturers. Furthermore, the Norwegian statistics for fisheries [25] provide the number of vessels for each fishing gear and their total catch.

Additionally, we collected samples from dispatched gear, mainly seine ropes and longlines, and measured the diameter and weight of the ropes to calculate the depletion. These calculations were then compared with the information from the fishers, further providing a better estimate of the wear and tear. The result gives the average percentage of wear for each type of fishing gear. We then calculate the total wear and tear by finding the total number of gear used.

Expanding the scope to include all fishing gear worldwide is indeed a challenging exercise. Unfortunately, an overview of all fishing vessels and their gear is not readily available, nor are the conditions for their fishing. Therefore, we must settle for rough estimations. We have statistical data for the global catch produced by FAO since 1950 [26], and our first approach is to use this data and assume the same conditions apply to other nations than Norway. In this way, we assume the amount of microplastics generated per ton of fish is equal for every nation. Using the statistical data from FAO [26], we find that Norway accounts for about 3.0% of all catches worldwide, which is a starting point for our calculations.

### **3. Danish seine**

In a recent report, we have described the plastic pollution caused by the Danish seine fishery [27]. Therefore, we do not go into detail but briefly describe this fishing gear and the main findings. For details, we refer to [27].

#### **3.1 Usage and causes of wear**

The Danish seine comprises a conical net with rope arms at each side. The ropes usually have a steel wire core since they must withstand heavy forces when dragged along the seabed. Several variants of this fishing method exist, including the original Danish seine method, called Anchor seining. Other methods are fly-dragging, also called Scottish seine, and tow-dragging, also called the Japanese method. For all of these variants, the rope arms are dragged along the seabed while the net wraps around the catch. The rope arms may be several kilometers in length. The sea bottom contact tears heavily on the ropes, and usually, the fishers replace them after 18 months. This replacement rate may be specific for Norway and possibly differ for other countries' fisheries.

#### **3.2 Calculated wear**

By comparing the weight of new and used seine ropes, we estimate the loss due wear and tear. Furthermore, based on interviews with the fishers, we get statistics on the ropes used. We calculate the annual wear on the Danish seine fishery in Norway to be 77–97 t plastic. These figures depend on several factors, such as the average lifetime, sea bottom conditions, stretching of the ropes, and the average rope arm length and number of seine in use. In other words, there are many possible sources of error, but we believe we are close to the actual value. Then, by finding the number of vessels in use for other countries and assuming the same wear is also valid, we estimate the annual worldwide plastic pollution from seine fishery to be about 311 t [27].
