Develop and optimize passive fisheries techniques for the Belgian part of the North Sea
I did my professional practice at ILVO, which translated stands for 'institute of agriculture, fishery and food research'. They work for Flanders on a wide range of projects with an overall goal of progressing to a more sustainable agriculture and fishery, keeping the economic aspect in mind. As a new project certain areas in the Belgian North Sea are turned into intgrated sites, where mariculture of mussels and seagrass will be started up. These regions won't be accesable for the often damaging, normally beam-trawling Belgian fleet and makes them lose a big part of their fishing grounds. To make this project porfitable for them as well, ILVO has the task to come up with a passive alternative that can work within these integrated sites without harming its functioning. My remote task consited out of writing a scientific review of all existing passive fishing techniques, in order to have a good overview which will help to decide which ones will be tested. As the regular passive technique wont be enough, the interesting part of this project involved finding possible innovations that could be added to the regular gear – light, odour or sound – in order to optimize catch rates.
A short overview of my review.
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Nets
Gill nets are vertically, usually stationary stretched out nets with a low visibility, kept in place by buoyant and anchored corners or lines on each side. Fish entangle themselves by unknowingly swimming trough a mesh and getting stuck when they fail to swim back because of their gills that block the reverse movement.
For application within the integrated sites, these nets have just too many down sides. Ghost fishing and possible by-catch problems are a risk too high, together with the findings that this, usually not dammaging technique, when repeatedly anchored at one place, still causes a signifacant amount of harm to the seafloor.
Same problems applies to trammel nets, consisting of tree vertical nets placed parallel, in a row, each having a different mesh size. Compared to gill nets they would lead to less mortality in caught fish, but the technique is less size selective as also smaller fish get entangled .
Angling
Longlining is done by one main (long) line with several vertical lines attached to it, each holding a hook with bait, attracting fish. Depth is adaptable by adding floats or weights to the construction.
Jigging, is another angling technique where the line is attached to a ‘jigmachine’ that automatically moves the bait vertically up and down, known as a jigging motion, to lure the fish. The bait consist of a lead weight attached to a hook.
We predict that these techniques might be difficult to apply in the relatively small integrated areas, e.g. jigging involves encountering and following large group of fish, which would be impossible here, and their is also a large entangling risk with the surrounding, maricultural structures.
Pots
Pots and traps are rigid cages, that lure fish into structures that prevent them from escaping. This results in living and qualitative fish. The pots get set, are usually left for some time, before fishermen come to haul them, collecting the catch and refreshing the bait.
As the pictures already suggest, we consider this technique to be the most optimal application within the integrated sites. Its overall impact is minimal and it has been shown to be succesful for our target species, Crabs and Cuttlefish.
Little is known about the impact of the specific structural factors of pots on species specific catch rates. Further research is needed for optimizing crab (and possibly lobsters) and cuttlefish pots.
Crab can be attracted the whole year around, using baited pots. Cuttlefish can be cought during the spawning period in May and June, making use of their spawning behaviour. Initially by trying to lure in females with either a bottle of milk or a broom of branches, in order to make the males follow.
Innovations
Artificial light is a very promising enrichment technique, for both crab and cuttlefish. For crab, several studies have been conducted with succesful outcomes. They seem mainly attracted by white, blue and green light, yet their motivation to approach remains unclear.
For cuttlefish, information concerning their reaction towards lighted gear doesn’t exist, but another, related cephalopodan species is very well known for its positive response. Around 95% of the world fisheries uses artificial light to catch squid, again white, blue and green seem to have the biggest effect. These colour preferences are very interesting since both squid and cuttlefish are known to be colourblind. Some possible explanations for this are: (i) They make use of chromatic aberration using their non-axial, W-shaped pupils, which would actually make their eyes perceive a colour-spectrum after all; (ii) The use of some sorts of photoreceptors that can be found in their tissues, which shows they might be able to detect light with their skin. Further research is needed.
Site specific tests are in order since environmental factors seem to have an effect on how the light can be perceived. ILVO already purchased green Lindgren-Pitman lights for these experiments.
Odour playing an important role in baited gear already, can have a big influence. Unfortunuately I was not able to entirely finish my review . These innovations being the most promising and experimental part, we decided I will continue writing after the IMBRSea symposium. However, it is already clear more studies are needed concerning the exact stimulating aspects of odour and its dispersal patterns as well as research towards species specific features (preferences and thresholds), in order to come up with a long lasting, precise bait.
I coulnd't finish sound enrichement either, but this probably won't be applied. The noise pollution that might come with it makes the other two innovations the better options.
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The trials with these enriched pots - certainly equiped with green LED lights, possibly with some form of odour enrichment - will hopefully start this summer. If all goes well I may go volontueer during some of these lab and field experiments, to get a taste of the practical side as well.
