The effect of gillnet twine thickness on capture pattern and efficiency in the cod fishery

The threat to marine ecosystems and society posed by abandoned, lost, or otherwise discarded fishing gear (ALDFG) is of global concern. The significance of the problem is illustrated by the establishment of a dedicated topic group on the issue by the ICES/FAO Working Group on Fishing Technology and Fish Behaviour.

Gillnets are a major contributor to ALDFG due to their widespread use. A potential way to reduce ALDFG from gillnets is to strengthen the gears using thicker twine. However, the viability of gillnet designs using thicker twine depends on maintaining catch efficiency as it is commonly assumed that twine thickness impacts catch efficiency.

This study investigated the mechanisms of capture and catch efficiency of thicker twine gears in the Norwegian North-East Atlantic (NEA) cod fishery compared to conventional types in order to understand their viability for the most profitable fishery in Norway. The gillnets tested used 0.8 mm thick commercially available twine and 0.7 mm thick conventionally used twine, in two designs, one with a 210 mm mesh and another with a 230 mm mesh.

The results of the tests showed no significant difference in the capture mechanisms or efficiency of gillnets of thicker twine. While the thicker 0.8 mm twine gillnets were 30% stronger and stiffer, they did not impact the capture mechanisms of fish of different sizes significantly, when compared with conventional 0.7 thick twine gillnets. Although the catch efficiency of 0.8 mm/210 mm gillnets for cod in the length class of 105 cm was found to be 14% lower than the catch efficiency of conventional 0.7mm /210mm gillnets, overall, the catch efficiency of gillnets in the two mesh sizes was not significantly affected.

As the thicker twine has higher breaking strength, it may reduce gear turnover and loss. Therefore, if the thicker 0.8mm gillnets tested were mainstreamed, this change could help reduce marine plastic pollution from fisheries. Also, these results suggest that thicker twine could be used for biodegradable gillnets to compensate for their inferior catch efficiency, potentially making them more viable in the future.