Fishing traps on a harbor quay -
© Shutterstock (via The Conversation)
Researchers are developing a more efficient trap adapted to environmental issues and obligations, according to our partner The Conversation.
Today, 60% of fish stocks caught in France come from sustainably exploited populations (compared to 15% 20 years ago).
The analysis of this phenomenon was carried out by Sonia Mehault, engineer in biology and fisheries technology at the French Research Institute for the Exploitation of the Sea, with the collaboration of Fabien Morandeau and Dorothée Kopp (Ifremer).
Depending on the species caught, but also the seasons, fishermen must adapt their techniques.
The diversity of fish and shellfish targeted implies the use of different fishing gear: the line for the bass, the trap for the shellfish, the net for the red mullet for example.
We must also take into account the different actors and uses of coastal areas - whether for the development of renewable marine energies or respect for protected areas.
Fishermen must live together and develop their profession while taking these realities into account.
Another major concern is the need to carry out fishing that respects the resources and seabed.
As such, fishermen are asking for technological developments that would meet these sustainability issues to enable them to diversify their practices.
This is to alleviate the pressure on ecosystems to ensure fishing activity and also greater resilience in the event of a crisis.
Today, following the implementation of management measures, 60% of fish stocks caught in France come from sustainably exploited populations, compared to 15% 20 years ago.
Our trap project is a continuation of this effort.
Design a reliable and low impact machine
In this context, we have carried out work and experiments in the bay of Quiberon to develop a well-known fishing technique: the fish trap.
Our goal was to design an efficient machine adapted to current issues.
Used by fishermen for centuries, widespread in developing countries, the net is enjoying renewed interest, even if few fishermen in the field currently use it in mainland France.
We have observed this practice in particular in the Bay of Biscay, where traps effectively capture conger and pout;
but these species, little valued on the market, are of little interest to professional fishermen.
Fish trap dating from the Roman period © Paul van Krimpen / Wikimedia, CC BY-SA 4.0
The design of this “new” trap was subjected to multiple constraints: to develop a device that would not have a physical impact on the habitats of the seabed;
which would retain market-sized fish while letting juveniles escape;
capable of catching fish of commercial interest, but not crustaceans, as regulations prohibit it.
Finally, a trap capable of coping with strong currents when deployed in the water, while being easily maneuverable and occupying little space when on board a ship.
Meeting all of these goals and ensuring the ability of this gear to catch the target species in sufficient quantity is key for fishermen to adopt it voluntarily.
Grayets and baits
Our work began in the summer of 2018 with a phase of investigation and consultation with fishermen.
It was about understanding their constraints and expectations towards the trap;
and also to select together the most relevant fish species to target.
Gray sea bream (Spondyliosoma cantharus) © Shutterstock (via The Conversation)
It is the griset, or gray sea bream
(Spondyliosoma cantharus)
, which was chosen.
It is a species accessible to coastal fishermen and does not present any problem of stock management.
The next step was to list potential baits on the basis of the knowledge available on the diet of the griset and the practices of hook fishermen.
The attractiveness of each bait was evaluated using an ethogram, which allows to describe and quantitatively translate the behavior of the fish, while taking into account the environmental conditions - sea current, turbidity, time elapsed between laying the bait and approaching individuals.
After identifying the bait in which the griset shows the greatest interest, the design of the trap began by gradually testing several hypotheses, corresponding to different levels of design of the craft.
Each design level has been tested four times to confirm the repeatability of the behaviors.
A trap adapted to the target species
It is first of all a simple metallic structure, equipped with the best bait, which has been placed and compared in two configurations: one weighted, the other floated.
The advantage of the floated configuration lies in the fact that the bait is not accessible to crustaceans, making it possible to avoid their capture.
The best fish approach configuration was chosen to test the following hypotheses in the same way.
Does the fish have better access to the bait if: the trap is covered with a black net or a transparent net?
Does the trap have one or two entrances?
Is the chute simple or complex to avoid the exhaust?
The responses obtained on the basis of underwater video observations led to the design of a prototype trap designed to match the behavior of the griset.
This prototype will be tested in summer 2021 on board 2 professional fishing vessels operating in southern Brittany.
Gray birds approaching the experimental trap in Breton waters © Sonia Meuhault / Ifremer, CC BY-NC-ND
A prototype in the test phase
This step will make it possible to validate that the developed gear is capable of capturing sufficient griset under commercial fishing conditions, to verify the measurement profile of the individuals caught and to adjust the meshes accordingly to only capture individuals of marketable size.
It will also be a question of validating the ergonomics of the machine during its handling by the sailors, as well as the economic valuation of the fish during its sale thanks to surveys among crews and fishermen.
At the same time, the development of a fishing gear requires ensuring its least impact on the seabed.
This impact was measured using a video system which aimed to measure the movement and penetration of a trap initially placed on the bottom and subjected to tidal currents.
These measurements were supplemented by those of a turbidimeter making it possible to evaluate the resuspension of the sediments linked to the spinning and the turning of the trap, as well as to its possible movement during the fishing operation.
Almost zero impacts on the seabed
The bait selection process showed that among the series tested, the cockle (
Cerastoderma edule
) is one of the most significant attractants of the gray.
The observation of environmental variables in parallel with the behavior of the fish shows that the latter mainly tends to approach the bait facing the current.
This observation pleads in favor of a floating structure, free to orient itself in the water, favoring the diffusion of the bait in the axis of the chute.
The following stages in the development of the trap helped define the type of cover net and chute that allows the fish to enter.
Regarding the impacts on the bottom, the video recordings showed almost zero movement and trap penetration on the bottom (the movements recorded are less than the measurement error).
Our "Fishing" file
Future trials on professional vessels will indicate whether this fishing technique is transferable and viable for fishermen.
Indeed, the performance of this gear remains to be confirmed in other fishing sectors, with environmental conditions which may be different.
Any other non-target captured species will be observed and the mesh selectivity can then be adapted.
Important point: the capture by this method of attraction makes it possible to preserve the organoleptic qualities of the fish whose endurance is not tested and which undergoes less stress during the fishing operation.
Knowing that once the griset has been captured, it will be up to the consumer to decide whether or not he is ready to pay, or not, to consume the fish resulting from this new fishing technique.
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This analysis was written by Sonia Mehault, engineer in biology and fisheries technology, with the collaboration of Fabien Morandeau and Dorothée Kopp (all three members of the French Research Institute for the Exploitation of the Sea).
The original article was published on The Conversation website.
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