Abstract
Improving the selectivity of a fishing gear is one technical management measure to significantly reduce by-catch of non-commercial species or undersized individuals. The efficiency of selective device is mainly estimated by comparing species composition, the biomass and length spectrum of caught individuals and escapees while the functional traits of species are rarely accounted for. Using an innovative technical device to reduce catches of undersized individuals in a multispecific bottom trawl fishery in the Bay of Biscay, namely a T90 mesh cylinder, we measured functional traits on both caught and escaped individuals of 18 species. Using a Principal Component Analysis and K-means partitioning, we clustered species into 6 groups illustrating 6 different locomotion strategies. We identified functional traits related to body size, visual ability and locomotion, differing between caught individuals and escapees using Linear Mixed-effects Models. As expected, escapees were smaller on average but also tended to be more streamlined, with a high position of the eyes and fin features characteristic of manoeuvrability and propulsion. Here, we present how a trait-based approach can shed light on the biological characteristics influencing the efficiency of selective devices.
Highlights
Multispecific fisheries using trawl gears generate discards through the capture of unwanted species, individuals below the minimum conservation reference size (MCRS)[1] or because of the poor state of caught individuals[2]
To let unwanted species escape from the gear, specific designs of the net, mesh or grids are to be adapted to their morphology[4,5,6]
When selectivity relies on body size, the mesh size may be determined by the MCRS10, fitting the selection curve is often challenging[11], especially due to variability of fish condition[12], fish behaviour[13] or fish contact probability with the selective device[9]
Summary
Defined as individual characteristics (e.g. morphological, physiological, behavioural) “which impact fitness indirectly via their effects on growth, reproduction and survival, the three components of individual performance” (definition from[19], see references therein). We propose to test whether functional traits of fish and cephalopod individuals caught during the sea trials of a T90 cylinder inserted in the extension of the trawl in the Bay of Biscay, can explain their response to the selective device. The T90 mesh based techniques, i.e. a diamond mesh turned 90° and remaining wide open throughout the fishing process, was first introduced in the early 1990s in the Baltic Sea[27]. It was later tested in the Bay of Biscay[28] and in other European ecoregions[29,30]. We applied a trait-based approach to understand the efficiency of a T90 cylinder in a two-step process: i) identifying if escapees and caught individuals have different functional profiles (i.e. combinations of trait values) and ii) identifying which traits significantly differ between the two groups of individuals ( called fractions)
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