Knowing how fish react to different types of fishing gear is critical to understanding:
- how they are captured
- how this process can be modified to allow undersized and unwanted fish to escape
What We Already Know
Fish are herded by the noise and sight of trawl doors moving over the seabed and by the sand clouds they create. Some of these fish move out of the path of the net, but those that fall within will try to swim ahead of the advancing ground gear or avoid it by swimming to the side or underneath. As fish tire they turn, or drift back into the net. Once inside the net some may try to swim and escape through the meshes, but many will avoid the netting and swim down the 'clear' path towards the cod-end. Inside the cod-end there are further opportunities to escape through the open meshes or through square mesh panels. Different species behave in subtly different ways, and this affects their probability of being caught. Fish behaviours in relation to fishing gears have been the subject of research at Marine Scotland Science (MSS) (formerly the Fisheries Research Services) for many years.
In the early years observations were made by scuba divers. A towed vehicle allowed divers to observe and film fish in and around trawl nets. Nowadays a remotely operated vehicle fitted with highly sensitive cameras and sonars is used down to depths of 120 metres. As well as hours of videos tape recorded in the field, MSS scientists have conducted many experiments to improve our understanding of the behavioural principles and physiology behind fish reactions. For example, the minimum light levels at which fish can see and avoid objects and the effect of using netting of different colours has been investigated. Extensive studies of the swimming endurance of different species have been possible using the unique gantry tank in the Fish Behaviour Unit at MSS Marine Laboratory.
Colour and Contrast of Netting - Does it Affect the Escape Behaviour of Fish?
Escape behaviour of fish in cod-ends is influenced by a number of gear and environmental factors, including the visual impact of the netting. When seen from inside the net, against down-welling light, pale green or white netting can present a lower contrast compared with darker coloured twine, and therefore appear to be a clearer escape route to fish inside the net. Using a combination of video observations and twin trawl selectivity experiments. MSS staff are exploring the possibility that colour and contrast in netting could enhance the efficacy of escape panels.
Quantifying Survey Trawl Selectivity
An on-going MSS research and development project aims to quantify the efficiency of the demersal survey trawl net at catching target species (e.g. haddock, whiting and cod) using a suite of different approaches, including twin trawling methods, video and acoustic observations. By collecting simultaneous acoustic and video data at
the net mouth and further forward between the wings we can build up a picture of where particular fish species are escaping. The multi-beam sonar mounted on the RCTV allows us to view the entire volume of the net mouth and count individual fish entering the net or escaping above and around it. The sonar cannot tell us what species these fish are, but low light cameras mounted on the net enable identification at certain points.