Passive visual devices to facilitate dolphin escape from surface fishing nets

Passive visual devices to facilitate dolphin escape from surface fishing nets Open proposal – freely disseminated Author: Vincent DomenechDissemination: free and open – no patent – no exclusivity 1. Abstract Each year, several hundred thousand dolphins and small cetaceans die accidentally after becoming trapped in fishing nets, particularly surface gillnets. Despite the existence of deterrent devices (acoustic, visual, and regulatory measures), a significant proportion of bycatch persists, especially when animals are already inside areas delimited by nets. This document proposes a complementary, passive, and non-intrusive solution based on a simple perceptual principle: making crossing points above the upper edge of nets visually and cognitively identifiable, in order to allow dolphins to escape more easily. The proposed solution consists of integrating vertically emerging shapes (open arches or flexible blades) at the top of surface fishing nets. These devices are designed to remain permanently attached to the net and to require no additional handling by fishers. 2. Context and problem statement Accidental capture (bycatch) currently represents the leading cause of non-natural mortality among dolphins and other small cetaceans worldwide. Surface gillnets and drifting nets are particularly involved. In many documented cases, dolphins: - enter the area delimited by the net, - fail to locate an exit, - become exhausted or drown, even though escaping over the net would be physically possible. This suggests that the problem is not purely mechanical, but also perceptual and cognitive in nature. 3. Working hypothesis The central hypothesis of this project is the following: The upper edge of surface fishing nets may be perceived by dolphins as a continuous and impassable boundary, due to the absence of visual cues explicitly indicating the possibility of crossing over the net. In the absence of a clear visual discontinuity, the net forms a psychological barrier, even when a physical escape route exists. 4. Principle of the proposed solution The solution is based on a simple principle: Visually breaking the horizontal continuity of the upper edge of the net in order to signal the existence of potential escape routes. To achieve this, it is proposed to integrate passive visual devices at the top of surface fishing nets, with the following characteristics: - vertically emerging shapes (open arches or flexible blades), - visible above the water surface, - creating a clear disruption in the net’s horizontal line, - without creating a physical obstacle. These devices are not intended to frighten, repel, or constrain dolphins, but rather to provide missing perceptual information. 5. Description of the proposed devices 5.1. Shapes Two main geometries are considered: - vertically oriented open arches (half-circles or very wide U-shaped forms), - flexible vertical blades (wide bands, semi-rigid flag-like elements). Closed shapes are avoided in order to prevent obstacle effects or entanglement risks. 5.2. Orientation - strictly vertical orientation - function: breaking the horizontal line of the net Horizontal shapes are excluded, as they would reinforce the perception of a barrier. 5.3. Color - matte white or slightly off-white - optionally very light yellow as an alternative This choice is based on: - the dichromatic vision of dolphins, - their high sensitivity to contrast and silhouettes, - the need to avoid reflections and glare. 5.4. Materials - flexible or semi-rigid materials (marine-grade plastics, polyurethane, flexible composites), - resistant to saltwater, UV exposure, and repeated bending, - free of metal parts and sharp edges. 6. Integration into nets and operational constraints One principle is considered non-negotiable: The device must remain permanently attached to the net and require no additional handling by fishers. Possible integration methods include: - direct attachment to the headline, - integration into selected existing buoys (“augmented” buoys), - fixation via sleeves or stitching, with no dangling elements. Hauling and storage - the devices are designed to fold or lie flat under tension, - they do not introduce more constraints than conventional buoys, - no dismantling operations are required. 7. Compatibility with existing mitigation measures This solution is complementary to existing technologies, including: - acoustic pingers (deterrence), - LED lighting, - regulatory adaptations. It targets a phase that is currently insufficiently addressed: - the moment when the dolphin is already inside the netted area and searching for an exit. 8. Potential impact (prudent estimation) At the global scale, accidental captures of small cetaceans are estimated at between 300,000 and 400,000 individuals per year. Under a conservative scenario: - if this device were to improve escape rates by 5 to 15% on the nets concerned, - and if it were progressively combined with existing mitigation measures, - then it could contribute to saving several thousand to several tens of thousands of dolphins per year, without imposing additional constraints on fishers. These figures must be confirmed through field trials. 9. Validation strategy Validation of the concept relies on simple and progressive testing: - pilot projects on volunteer fishing nets, - comparative observations with control nets, - behavioral analysis (video recordings, direct observation), - feedback from fishers, - adjustment of shapes and spacing. 10. Project philosophy Open and freely disseminated solution No patent, no exclusivity Primary objective: effective reduction of dolphin mortality The project seeks solutions compatible with: - fishing activity, - economic constraints, - protection of living marine species. 11. Conclusion This proposal does not claim to solve the problem of dolphin bycatch on its own. It aims to provide a simple, passive, and testable improvement based on a perceptual approach that has so far been little explored. By integrating easily into existing practices, without noise, electronics, or operational constraints, it presents strong potential for wide adoption and large-scale acceptance. Contact / authorVincent Domenech26 bis Avenue Émile Zola13850 Gréasque FranceEmail : domenech2vincent71@gmail.com (open dissemination – collaboration welcome)