Developing and testing a computer vision method to quantify 3D movements of bottom-set gillnets on the seabed

Abstract

Gillnets are one of the most widely used fishing gears, but there is limited knowledge about their habitat effects, partly due to the lack of methodology to quantify such effects. A stereo imaging method was identified and adapted to quantify the dynamic behaviour of gillnets in-situ. Two cameras took synchronized images of the gear from slightly different perspectives, allowing to estimate the distance from the observation unit to the gear such as in the human 3D vision. The sweeping motion on the seabed and the penetration into the sediment of the leadline of light and heavy commercial bottom gillnets deployed in sandy habitats in the Danish coastal plaice fishery were assessed. The direct physical disruption of the seabed was minimal as the leadline was not penetrating into the seabed. Direct damage to the benthos could however originate from the sweeping movements of the nets, which were found to be higher than usually estimated by experts, up to about 2 m. The sweeping movements were for the most part in the order of magnitude of 10 cm, and resulted in a total swept area per fishing operation lower than any of the hourly swept area estimated for active fishing gears. Whereas the general perception is that heavy gears are more destructive to the habitat, light nets were moving significantly more than heavy ones. The established methodology could be further applied to assess gear dynamic behaviour in situ of other static gears.

The low impact of fish traps on the seabed makes it an eco-friendly fishing technique

“…The swept area for an encircling trammel net [32], was found to be between 0.180 and 0.654 and km 2 [33]. Concerning static fishing gear, a swept area of about 0.04 km 2 for lightweight gillnets and 0.01 km 2 for heavy gillnets deployed in the Kattegat [25]. It is important to compare these swept areas in light of the catches' weight.…”

“…To the best of our knowledge, this study is one of the first attempt to characterize the impacts on the seafloor of two distinct fish traps rigged in lines. The used methodology proves to be an effective tool to evaluate the potential of static fishing gear on the seafloor [25]. These results are specific to the fishing conditions we experienced while taking the measurements in terms of currents, tidal coefficient, sediment type and depth and could thus be subject to variations if the traps were set in different environments.…”

“…The trap line was soaked in waters no deeper than 12 m in an area of muddy sand seabed for an 8-hour period including a change of tide. Each configuration was soaked twice for both high (between 72 and 104) and low (between 39 and 54) tidal coefficients (hereafter [25], with an observation device composed of two GoPro Hero 7 cameras mounted on a metal frame weighted with two ballasts of…”

“…The sweeping movements of static gear and modifications that this could cause to the substrate (e.g., sediment resuspension or modification of seabed topography) are rarely considered, although a recent study on bottom set gillnets revealed that sweeping movements could be up to 2 m [25]. One study reported trap movement according to the wind during the storm season in the Caribbean [22] and showed that single, buoyed traps could move a mean distance of 3.6 m, affecting an area of 4.6 m 2 when the trap was set in low depths.…”

Monitoring-System Development for a Bottom-Set Gillnet through Time-Domain Dynamic Simulations

“…The monitoring system may also be applied to protect fishing gear in case of deep-water turbulences. Even though there have been experiments [17,18] and numerical simulations [19,20] investigating the dynamic behaviors of the gillnet, these have only focused on small sections of the gillnet. This means that there has been no publication in the open literature which has attempted to investigate not only the global behaviors of the bottom-set gillnet but also the simulations of transient responses with various scenarios of line failures, which could be applied to the development of a sensor-based monitoring system.…”