**5. Gillnets**

#### **5.1 Usage and causes of microplastics**

Gillnets are the most important commercial fishing gear for the Norwegian coastal fleet, where cod (*Gadus morhua*) and saithe (*Pollachius virens*) are the most important target species. In 2019, 74 864 t cod and 32 032 t saithe were caught using this gear.

A gillnet consists of the float line, the lead line, and the netting. The float and lead lines usually consist of polypropylene and polyethene, but the lead line also has lead inside the core to make it sink. In addition, a drop line connects to a buoy at the sea surface. The drop line consists of polypropylene and polyethene at the lower part and polyester at the upper part. The netting consists of single or multistrand monofilament nylon.

The lifetime of a gillnet is highly variable and based on geographical location, vessel size, and hauling frequency, which in turn depends on the type of fishing. For example, some vessels fish all year round and replace a thousand nets yearly. Others, who only operate during the cod season, may replace the net each 4–5 years, or even after10 years. Also, as for the trawl, there are significant variations in the lifetime depending on the seabed quality.

Drop and float lines are rarely in contact with the seafloor, and abrasion mainly occurs during hauling and setting. Although it is challenging to calculate wear and tear solely from the hauling equipment, gillnet fishing at great depth causes more significant stretches and squeezes during hauling than fishing in relatively shallow waters. Blue halibut is one of the species caught at such great depth. Lead lines, on the other hand, are more prominent to wear and tear due to their contact with the sea bottom. **Figure 2** shows an example of a worn-out lead line (a) and a typical gillnet hauler (b). The ropes are squeezed between two plates at the hauler, creating

**Figure 2.** *Worn-out lead line (a) and a typical gillnet hauler (b). Photo: SINTEF.*

significant abrasion. Another common type of hauler is the drum winch, which also creates abrasion of the ropes but is much more gentle.

#### **5.2 Calculated wear and tear**

To estimate the wear and tear on gillnets, we need knowledge of several parameters, such as the average number of gillnets per vessel, the thickness of the ropes, the average lifetime, and the average percentage mass loss. We got these numbers from our interviews with fishers, confirmed by information from gear suppliers.

On average, a fisher has 200 gillnets consisting of netting, lead, and float lines. They are tied in strings of different lengths, where the length of each component is 27.5 m. The total length (LG) of all 200 gillnets is 5500 m. In addition, a fisher has 2000 m of drop lines on average. For the netting, the weight is 2.2 kg for one net, i.e., the complete 27.5-meter length, giving a weight per meter (Wm) of 80 g. For the drop line, 14 mm is the typical diameter, weighing 88 g per meter. For the float line, a 15 mm rope is used with a weight of 100 g per meter, and for the lead line, a 12 mm diameter is used with a weight of 73 g per meter, excluding the lead.

Next, we have to estimate the annual loss percentage. The fishers estimate a percentage loss, PL, of about 8% when the ropes are worn out, and for the average lifetime (LT), they estimate 15 years for lead lines, 20 years for float lines, and 25 years for drop lines. The numbers are based on an average of 40–60 trips per year. The netting is replaced every 4 years on average. The netting often gets stuck in rocks at the sea bottom, creating holes and small pieces that are torn apart. Due to this, it must be replaced more often. We estimate a loss percentage of 3%, giving an annual loss percentage of 0.75%. To calculate the annual loss (L) for each component, we then use the following formulae:

$$L = \mathbf{N}\_{\vee} L\_{G} \mathbf{W}\_{m} \mathbf{P}\_{L} / LT \tag{2}$$

**Table 2** shows the calculated loss per component and vessel and the total loss per fleet. According to the Norwegian statistics for fisheries [25], the number of vessels using gillnets is NV = 1472.

### **5.3 Gillnets worldwide**

Gillnets are perhaps the most used fishing gear worldwide. However, in many areas, there are differences in use from the Norwegian method described above.


**Table 2.** *Calculated annual microplastics loss from gillnets per vessel and total per fleet.* Smaller boats are typical, with other types of equipment and ropes with other dimensions. Therefore, estimating the global amount of microplastics based on Norwegian numbers is very challenging. Our approach is to use the Norwegian numbers and divide them by the estimated Norwegian share of the total catch. Unfortunately, the statistics are grouped on species, not gear type, meaning we cannot know the Norwegian catch share for a specific gear type. To overcome this problem, we assume the catch share for gillnets is the same as the total catch share for all gears in use. This approach is a gross simplification, which we have to keep in mind.

According to FIGIS [26], Norway accounts for 3.0% of the total catch of marine and diadromous species. Thus, the global amount of microplastics generated from gillnets estimates to 613.1 t.

### **6. Pots**

#### **6.1 Usage and causes of microplastics**

Crab pots are tied in strings, but the number of pots in each string varies on the type of fishery. They come in many different shapes, but the basic idea is to trap the crabs inside the pots. King crab pots are collapsible to ease storage, and Snow crab pots are conical so that they can be stacked.

In Norway, we have identified 426 vessels catching crabs based on the criteria that they have landed more than 400 kg per year. On average, we assume 15 strings with 15 pots per string, based on the interviews with fishers. The pots usually stand at 30–40 m depth with 20 m spacing. In addition, we have 772 vessels fishing for King crab in the northern part of Norway. For King crab, the depth is about 200 m, which causes more stress and more wear on the ropes during hauling. The typical rope diameter is 10 mm. There are different rope qualities, mostly polypropylene, polyethene, or nylon.

Fishing for snow crab takes place in the Barents Sea (Norwegian and Russian fishers) and the Northwest Atlantic and North Pacific, usually at depths of 220–300 m. A large number of pots in the string is typical for snow crab fishery, usually 200 on average. The ropes typically have a diameter of 22–24 mm due to the heavy stress they are exposed to. Thus, the rope thickness and number of pots are unique for this type of fishery. The pots have a conical shape, as seen in **Figure 3a**. A vessel fishing for snow crab may have 35 strings and thus a total of 7500 pots.

The hauling equipment is the most significant cause of wear and tear on the drop line and the connecting ropes between the pots. Ropes are hauled quickly from a depth of 220–300 m, causing significant abrasion. The winch also contributes to wear and tear on the ropes, pulling the ropes backward and through the boat to the bins behind.

Also, the plastic coating around the steel cracks during use, but there are significant differences in the quality and how much it cracks. Some of the cheaper pots are of low quality and tend to rust. When the iron rusts, the plastic coating explodes, as seen in **Figure 3b**, releasing large plastic flakes into the sea. We do not include the plastic originating from this coating in our calculations.

#### **6.2 Calculated wear**

To estimate the wear on the ropes, we distinguish between Snow crabs and other crabs and lobsters. Then we find the number of vessels involved in both categories and estimate the length of the ropes based on the average number of strings and pots

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

**Figure 3.** *Snow crab pot (a), with cracked coating (b). Photo: SINTEF.*

in each string. Finally, we estimate the average wear percentage based on fishers' and manufacturers' information.

For crabs and lobsters, the total number of vessels involved in Norway is 1198, half of them catching King crab. The average number of pots at each vessel depends on the type of crab, but we use 100 pots divided into 10 strings with 10 pots each. The distance between each pot is 20 m, and the drop line length is 50 m on average.

For snow crabs, there are only nine active vessels. After dialogue with the fishermen, we estimate an average of 7000 pots, spread over 35 strings with 200 pots in each string. The typical distance between each pot is 30 meters. The drop line at each end of the string is usually three coils of 110 meters.

To calculate the total length of ropes (LR) for the whole fleet, we use the following formula:

$$L\_R = N\_V N\_s \left[ L\_d + \left( N\_p - \mathbf{1} \right) L\_t \right] \tag{3}$$

Here, NV is the number of vessels, Ns is the average number of strings, Np is the number of pots in a string, Ld is the dropline length, and Li is the rope length between the pots in the string. The above information is summarized in **Table 3**, where the length calculations are based on Eq. (3).

Finally, we calculate the annual mass loss (L) due to wear and tear from Eq. (4):

$$L = \mathcal{W}\_{\text{su}} L\_R P\_L \tag{4}$$

Here, Wm is the rope weight per meter, and PL is the annual percentage loss. Finally, WT in **Table 4** is the total rope weight. In particular, the annual loss percentage is challenging to estimate. However, the results from [24] can be used as a starting point. The ropes are mainly PP/PE, and [24] suggests a monthly mass loss of 0.4% for


**Table 3.**

*Calculation of the total rope lengths in meters for crab fishery in Norway.*


#### **Table 4.**

*Calculation of the annual amount of microplastics due to crab fishing in Norway.*

such ropes just by lying in the sea. Additionally, the haulers contribute significantly to this loss. Since the ropes are not constantly in the sea, and their lifetime often is 10 years or more, we estimate an annual loss percentage of 1.5% for crab and lobster pots. The loss is set to 2.5% annually for snow crab ropes since the stress is much higher. The results are then summarized in **Table 4**.
