Electronic monitoring in fisheries: Lessons from global experiences and future opportunities

Since the beginning of the 21st century, electronic monitoring (EM) has emerged as a cost‐efficient supplement to existing catch monitoring programmes in fisheries. An EM system consists of various activity sensors and cameras positioned on vessels to remotely record fishing activity and catches. The first objective of this review was to describe the state of play of EM in fisheries worldwide and to present the insights gained on this technology based on 100 EM trials and 12 fully implemented programmes. Despite its advantages, and its global use for monitoring, progresses in implementation in some important fishing regions are slow. Within this context, the second objective was to discuss more specifically the European experiences gained through 16 trials. Findings show that the three major benefits of EM were as follows: (a) cost‐efficiency, (b) the potential to provide more representative coverage of the fleet than any observer programme and (c) the enhanced registration of fishing activity and location. Electronic monitoring can incentivize better compliance and discard reduction, but the fishing managers and industry are often reluctant to its uptake. Improved understanding of the fisher's concerns, for example intrusion of privacy, liability and costs, and better exploration of EM benefits, for example increased traceability, sustainability claims and market access, may enhance implementation on a larger scale. In conclusion, EM as a monitoring tool embodies various solid strengths that are not diminished by its weaknesses. Electronic monitoring has the opportunity to be a powerful tool in the future monitoring of fisheries, particularly when integrated within existing monitoring programmes.     

Voluntary responses and limits of tolerance to pressure reduction and swimbladder expansion in farmed Atlantic cod

Atlantic cod and other teleosts with closed swimbladders have slow gas resorption rates and are therefore vulnerable to sudden pressure reductions that lead to swimbladder expansion and possible loss of behavioural control. This problem is of particular relevance to fish farmers, who require safe protocols for lifting of cages containing cod that account for swimbladder expansion. To recommend a limit for pressure reduction, we experimentally determined the maximum pressure reduction, relative to neutral buoyancy pressure, free-swimming farmed cod would expose themselves to. Classical reward conditioning was used to motivate cod that were neutrally buoyant at 2?C3.3 ATM (10?C23 m depth) to respond to a sound signal by swimming rapidly upwards to a feeding station positioned at 1.5 ATM (5 m depth). Sixteen cod were implanted with data storage tags (DSTs) that registered their ambient pressure throughout the experiment. The DST data showed that cod voluntarily stopped rapid ascents before they reached the feeding station at an average maximum pressure reduction of about 41% relative to their neutral buoyancy pressure. This was equivalent to a 70% swimbladder expansion relative to neutral volume. During the experiment, cod seldom remained above their neutral buoyancy depths, except when feeding, indicating a behavioural aversion to positive buoyancy. To avoid the loss of buoyancy control in cod, we conclude that single lifting events of sea-cages, which forces swimbladder expansion, should never exceed a 70% expansion of the swimbladder above the pre-existing neutral buoyancy volume.