Shoal familiarity modulates effects of individual metabolism on vulnerability to capture by trawling

Abstract

Impacts of fisheries-induced evolution may extend beyond life history traits to more cryptic aspects of biology, such as behaviour and physiology. Understanding roles of physiological traits in determining individual susceptibility to capture in fishing gears and how these mechanisms change across contexts is essential to evaluate the capacity of commercial fisheries to elicit phenotypic change in exploited populations. Previous work has shown that metabolic traits related to anaerobic swimming may determine individual susceptibility to capture in trawls, with fish exhibiting higher anaerobic performance more likely to evade capture. However, high densities of fish aggregated ahead of a trawl net may exacerbate the role of social interactions in determining an individual fish's behaviour and likelihood of capture, yet the role of social environment in modulating relationships between individual physiological traits and vulnerability to capture in trawls remains unknown. By replicating the final moments of capture in a trawl using shoals of wild minnow (Phoxinus phoxinus), we investigated the role of individual metabolic traits in determining susceptibility to capture among shoals of both familiar and unfamiliar conspecifics. We expected that increased shoal cohesion and conformity of behaviour in shoals of familiar fish would lessen the role of individual metabolic traits in determining susceptibility to capture. However, the opposite pattern was observed, with individual fish exhibiting high anaerobic capacity less vulnerable to capture in the trawl net, but only when tested alongside familiar conspecifics. This pattern is likely due to stronger cohesion within familiar shoals, where maintaining a minimal distance from conspecifics, and thus staying ahead of the net, becomes limited by individual anaerobic swim performance. In contrast, lower shoal cohesion and synchronicity of behaviours within unfamiliar shoals may exacerbate the role of stochastic processes in determining susceptibility to capture, disrupting relationships between individual metabolic traits and vulnerability to capture.

Keywords: Ecophysiology; fishing; metabolism; social behaviour; trawl.

Figure 1

Layout of the miniature trawl net in the working section of the swim tunnel. Thick black lines indicate portions of the net that are flush with the walls of the swim tunnel, while portions of the net mouth bordered with small orange buoys indicate available escape routes.

Figure 2

Relationship between anaerobic metabolic capacity (EPOC) and time spent in the net in familiar (blue) and unfamiliar (orange) shoals. Lines represent linear regression between log (EPOC) and log (time in net), for each shoal composition, while the shaded area corresponds to 95% confidence intervals. Three replicates are shown for each fish (n = 40) in both familiar (blue) and unfamiliar treatments (orange).

Figure 3

Temporal synchronisation of the four behaviours within a 5 s window (escapes, re-entries, in net, out of net) among four fish in familiar (blue) and unfamiliar (orange) trawl trials, as shown by CD ratios. Top: boxplot of CD for each behaviour. Each overlaid dot represents the CD calculated for one trial. Values > 1 represent events clustered in time. Boxplot upper and lower hinges represent the 25th and 75th percentiles, respectively, while the horizontal line within the box represents the median. Length of the whiskers represents the range of data points between each hinge, and 1.5× the difference between 25th and 75th percentiles. Bottom: bar chart showing the proportions of trials where CD ratio for each behaviour was > 1 (ie the number of trials were clustering for that behaviour was identified).