There is increasing evidence that intense commercial fishing pressure is not only depleting fish stocks but also causing evolutionary changes to fish populations with serious consequences for the viability of marine fish communities. Overlooked within the context of fisheries-induced evolution (FIE) is the likelihood that, within a given species, variation in physiological traits among individuals – and especially those related to energy balance (e.g. metabolic rate) and swimming performance (e.g. aerobic scope) – could make some fish more catchable or more likely to suffer mortality after discard. Selection on these traits could produce major shifts in the fundamental structure and function of fish in response to fishing pressure that are yet to be considered but which could directly determine population resource requirements, resiliency, geographic distributions, and responses to environmental change. This pioneering project addressed this gap in knowledge with three main goals: (1) to examine whether physiological traits make some individuals more catchable by commercial fishing gears, and whether the environment modulates such effects; (2) to investigate the extent to which physiological traits influence recovery and survival after escape from fishing gear or discard; and (3) to determine whether selection on catchability generates changes in physiological traits that reduce population resiliency or erode the ability to cope with environmental change. Given that several fisheries have not recovered despite lengthy moratoriums, there is a pressing need to understand the long-term physiological effects of FIE on fish stocks and their capacity to rebound after fishing pressure is lifted.
We observed, using a combination of laboratory and field studies, that fish with specific metabolic traits and locomotor performance were more susceptible to capture by fishing. Importantly, the strength of selection was highly dependent on several factors. Firstly, the type of fishing gear is important: passive fishing gears such as traps or angling were less likely to select on physiological phenotypes while active gears such as trawling were more likely to capture fish with a lower swimming ability. Notably, vulnerability to capture is consistently repeatable among individual fish, indicating a strong potential for evolutionary effects. We also found that the current environmental conditions (temperature and hypoxia) strongly alter to degree of selection that can take place due to fishing by modulating the degree of phenotypic plasticity and variation within a population. Finally, we also showed that the social environment has a pronounced effect on the strength of selection, as the behaviour of individual fish with specific physiological traits becomes homogenised when they follow the behaviour of their schoolmates. This effect seems particularly strong for fish traps and is likely responsible for the decreased selection potential caused by these gears. We were also interested in whether physiological traits influence the likelihood of mortality of individual fish after release or discard. This aim was also addressed through a combination of laboratory studies, studies in the wild, and an unforeseen opportunity to use a large (20 m long, 7 m wide, 5 m deep) tank at a public aquarium. There was little evidence that the degree of stress or potential for recovery was related to the physiological phenotypes of individual fish. This includes the same metabolic and locomotor traits that can make the fish more susceptible to capture in the first place. Finally, using laboratory populations of zebrafish which were selectively bred based on their vulnerability to passive and active fishing gears, we found fish with a deeper profile and more muscular tails avoided capture more often. Heart rate measured in the offspring of fish subjected to generational selection using active (trawling) and passive (trapping) gears showed differences in the way populations responded. Low vulnerability trawl fish and high vulnerability trap fish both showed a lower resting heart rate compared to fish within the same gear type that differed in vulnerability. This is indicative of two key findings: (1) a divergence exists between the selective pressures exerted by the two gear types – trapping preferentially removes individuals from a population with a lower resting heart rate whilst trawling removes individuals with a higher resting heart rate; and (2) changes are either the result of a single mechanism influencing resting heart rate across both gear types, or multiple mechanisms acting in synergy (i.e. both driving resting heart rate down).
