Shoal size as a key determinant of vulnerability to capture under a simulated fishery scenario

Abstract

Group living is widespread among animals and has a range of positive effects on individual foraging and predator avoidance. For fishes, capture by humans constitutes a major source of mortality, and the ecological effects of group living could carry-over to harvest scenarios if fish are more likely to interact with fishing gears when in social groups. Furthermore, individual metabolic rate can affect both foraging requirements and social behaviors, and could, therefore, have an additional influence on which fish are most vulnerable to capture by fishing. Here, we studied whether social environment (i.e., social group size) and metabolic rate exert independent or interactive effects on the vulnerability of wild zebrafish (Danio rerio) to capture by a baited passive trap gear. Using video analysis, we observed the tendency for individual fish to enter a deployed trap when in different shoal sizes. Fish in larger groups were more vulnerable to capture than fish tested individually or at smaller group sizes. Specifically, focal fish in larger groups entered traps sooner, spent more total time within the trap, and were more likely to re-enter the trap after an escape. Contrary to expectations, there was evidence that fish with a higher SMR took longer to enter traps, possibly due to a reduced tendency to follow groupmates or attraction to conspecifics already within the trap. Overall, however, social influences appeared to largely overwhelm any link between vulnerability and metabolic rate. The results suggest that group behavior, which in a natural predation setting is beneficial for avoiding predators, could be maladaptive under a trap harvest scenario and be an important mediator of which traits are under harvest associated selection.

Keywords: collective behavior; evolutionary trap; fisheries‐induced evolution; metabolic rate; sociality.

Figure 1

Experimental set‐up used in the trapping vulnerability tests. Here, (i) is the acclimation tube, (ii) is the trap with mounted camera, (iii) denotes the plants, and (iv) is the overhead camera

Figure 2

The effects of shoal size on individual vulnerability to capture by trap, as estimated by: (a) time until first trap entry; (b) total time spent within the trap during a 1,200 s deployment; and (c) total number of trap entries. Each data point overlaid on Tukey‐style boxplots is data for one fish. Boxplot lower and upper hinges represent the 25th and 75th percentiles, respectively; the horizontal line within the box represents the median; the length of whiskers represents the range data points between each hinge and 1.5 ×  the difference between the 25th and 75th percentiles. Data beyond these limits are outliers

Figure 3

Kaplan–Meier curves of the survivorship of focal zebrafish (Danio rerio) trialled at different shoal sizes (IND n = 41; IND ₊₂ n = 40; IND ₊₄ n = 40; IND ₊₆ n = 38)

Figure 4

Relationship between residual log standard metabolic rate (mg O₂ per hour) and time to trap entry (s) at various shoal sizes (IND n = 34; IND ₊₂ n = 37; IND ₊₄ n = 38; IND ₊₆ n = 35). Lines represent least squares regression for each treatment (IND 0.01490 * SMR + 6.0377; IND ₊₂ 0.27353 * SMR + 5.38353; IND ₊₄ 0.06348 * SMR + 5.03955; IND ₊₆ 0.40622 * SMR + 4.82809)