**7. Longlining**

In longline fishing, we distinguish between three different modes of operation: bottom longline, surface longline, and autoline. In Norway, autoliners in the high seas account for 70% of the total catch. This fleet consists of 20 vessels, and the total annual catch in 2019 from this segment was 57 428 t. Target species are demersal fish, such as cod and saithe.

A longline consists of a long rope (the mainline) attached to branch lines with hooks. The mainline is a multifilament line (polyester and polyamide) consisting of three or four cord sections twisted together to form a long rope. The length can be up to 180 kilometers. Depending on the fishery, the mainline generally ranges from 4 to 11 mm in diameter. The supply is made of polyester, but the hooks, swivels, and stoppers are steel; see **Figure 4a**.

#### **7.1 Usage and causes of microplastics**

Most of the wear and tear occur during hauling. Hauling is performed by a powered line hauler, where the rail roller guides the longline over the rail of the ship before the de-hooker and hook cleaner remove the fish and unused bait from the hooks. Furthermore, the twist remover takes out the twist in the line before the hook separator guides the hook into the storage rack, where they are held in magazines.

**Figure 4.** *Autoline with stoppers, swivels, and clamps (a) and a worn-out autoline with the cords split apart (b). Photo: SINTEF.*

On a traditional autoline vessel, the hauling equipment is located on the vessel's starboard side. However, there has been a shift toward hauling through moonpools in the center of the vessels. As the moonpool is placed with the lowest magnitude, it may reduce wear on the line. However, this is only an assumption as no such research exists in the area.

### **7.2 Calculated wear**

To calculate the annual mass loss due to wear and tear from longlines (L), we use the following formulae:

$$L = \mathbf{N}\_{\vee} \mathbf{N}\_{l} L\_{l} \mathbf{W}\_{m} P\_{L} / LT \tag{5}$$

NV is the number of vessels in the fleet, which we get from the Norwegian statistics for fisheries [25]. Nl is the average number of lines for each vessel, Ll is the average length of each line, Wm is the average weight per meter, PL is the average percentage loss when the line is worn-out, and LT is the average lifetime. All of these parameters were acquired through our interviews with fishers.

Additionally, we got one worn-out line, **Figure 4b**, which we measured to establish the mass loss. This line has been in operation for 1.5 years and has been hauled approximately 300 times. A new line is 180 m long and weighs 16 kg. Our worn-out line weighs 14.9 kg, thus reduced by 1.9 kg. This gives a percentage loss of 11.9%, which coincides with what our fishers told us (12%). Measuring only one line does not give a statistically sound result, but it underpins the statements from the fishers.

**Table 7** shows the calculated loss for longlines and the parameters used for the calculations Eq. (5). We split the lines into four categories as each category's parameters differ. The total loss from longlines sums to 37.2 t annually.

### **7.3 Longlining worldwide**

As for gillnets, we cannot provide well-founded numbers on the usage of longliners worldwide. Typically, longlines are used to catch tuna in Japan, Taiwan, Korea, Cuba, and Oceania. Other nations use longlines to catch halibut in the northern

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


#### **Table 7.**

*Calculated annual mass loss for different types of longlines.*

Pacific. Therefore, our best estimate is to use the method for gillnets, assuming the Norwegian share of worldwide longline use is about 3%. In that case, the total microplastics from longlining worldwide is 1 240 t.

### **8. Discussion**

#### **8.1 On the methodology**

The methodology is based on interviews with fishers to acquire essential parameters we need to calculate loss from gear. The annual loss percentage is a crucial parameter we have tried to check using different methods, such as measuring wornout ropes and using numbers from the literature [24]. However, the many uncertainties are a weakness of the study. To get better estimates, we believe performing comprehensive studies on different types of worn-out gear is the way forward.

For calculating the global wear and tear, we need access to the number of gears used globally for each gear type. This may be available for some countries but is very hard to find. In addition, statistical parameters, such as rope lengths, diameters, and average lifetime, are necessary to perform reliable calculations for different areas. We acknowledge that our methodology for finding the global wear and tear on fishing gear has many weaknesses, but on the other hand, it is the best we can do for now. Improving the methodology is an important area for future work.

#### **8.2 The results**

**Table 8** summarizes the worldwide microplastics generated from the wear and tear of the different fishing gear we have considered, together with the Norwegian numbers. We have used the Norwegian fishing fleet as the basis for our calculations. Although the Norwegian numbers are uncertain, we believe they, in general, are close to reality, considering that the uncertainties may equalize each other.

The UK study [20] concludes with 326 million to 17 billion fragments of microplastics from the UK fishery. It would be great to compare this to our calculations, but unfortunately, the size and weight of a fragment are undefinable. However, their measurements show the mass loss from haulers ranges from 12 μg to 1050 μg per meter hauled for new and 10-year-old ropes, respectively [20], indicating that older


**Table 8.**

*Summary of the Norwegian and global microplastics generated from wear and tear on fishing gear.*

ropes wear more quickly than new ones. Therefore, we calculated the mass loss per meter rope hauled based on our calculated mass loss in Norway for some gear components worn primarily due to the hauler. The results are shown in **Table 9**, indicating that for surface longlines our results are comparable to [20], whereas, for gillnets and crab pot ropes, we are possibly underestimating the wear. Another explanation is that the haulers used for crab pots and gillnets are of the drum types that are more gentle to the ropes than the ones tested by [20].

For Danish seine, we have a reasonable understanding of the number of vessels involved worldwide; hence, the total numbers are well founded. Also, the wear and tear from the Norwegian fleet are based on interviews with fishers backed up by measurements on worn-out ropes. Thus, the numbers presented for the Danish seine are probably the ones best founded.

For trawls, we only consider sea bottom trawls as these are the ones contributing the most to the pollution. We do not consider Shrimp trawls as the demanded information is more challenging to get, and these trawls do not have plastic parts in contact with the seafloor. Yet, dolly ropes and other protective ropes are dragged along the seafloor, contributing considerably to the total plastic pollution. Hence, we believe that the numbers we present for trawl pollution are highly underestimated. The microplastic pollution originating from dolly ropes and other protective ropes is undoubtedly a topic that needs further investigation. Also, we do not have numbers for the worldwide use of trawlers; this is also a field in which more research is needed.

For gillnets, we use average numbers for the number of nets per fisher. These are based on interviews with fishers but are still questionable as the fishing fleet is large, and there may be significant differences between fishermen. Further differences are introduced when we consider gillnet fishing worldwide. Fishers in many countries probably use other types of ropes than Norway, and the average number of nets is


**Table 9.**

*Calculated mass loss per meter hauling for some gear components.*

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

tough to estimate. Hence, the amount of microplastics from global gillnet fishing is a very rough estimate, and better statistics on using gillnets globally are needed.

For pots, we split between snow crab and other types of pots due to the different operating modes. As for the other gears considered, there are many uncertainties in the parameters used for the calculations, with the percentage loss being the most dubious. However, the numbers for Norway are considered well founded. For the global loss, there are probably significant differences in how the fishery is performed, and the average amount of pots and ropes used varies from country to country. Therefore, our estimates for the global loss are highly uncertain, and more research is needed to establish more accurate numbers.

Also, the main uncertainty for longlining is establishing a correct number for the annual percentage loss. Our fishers have estimated this to be 8%, which may be correct but difficult to verify. We don't have information on the global number of vessels and the parameters for each type of line. Hence, our numbers for the global loss are based on loose assumptions. More research must be performed on the global use of longlines to get more accurate results.

### **9. Conclusions**

Our results show that the number of microplastics originating from fisheries worldwide is 4 622 t, which is probably a conservative estimate. We omitted some gears, like ordinary seine, and we only considered the rockhopper gear for the trawl. Hence, dolly ropes and trawl mats are not included, and we believe they contribute significantly to the total amount. The number of microplastics from lost and abandoned fishing gear is estimated to be 45000 tons [23]. This number is almost 10 times higher than our calculation but can make sense since lost and abandoned gear are complete, not only fragments. If correct, this suggests that the effort should be put into avoiding such lost and abandoned gear. However, we believe that finding ways to reduce microplastic wear and tear from commercial fishing activities is an important task too.

Despite many uncertainties, our calculations and results can provide helpful information and are essential to highlight the topic of microplastics originating from ordinary fishing activities. Then, we believe future research will lead to more accurate numbers. Specifically, more research on the global use of longlines, gillnets, and crab pots is needed, and a better understanding of dolly ropes and trawl mats is essential since these components contribute significantly to the total pollution.

### **Acknowledgements**

Jørgen Vollstad at SINTEF Ocean has contributed to the work by sharing knowledge of the fisheries in Norway and helped to find the underlying numbers for the calculations. Bård Johan Hanssen at SINTEF Nord has helped to find statistics for the number of fishers and catches in Norway.

The Norwegian Directorate of Fisheries has partly supported this work.

*Advances and Challenges in Microplastics*
