Contemporary evolution caused by fisheries: Contributions from experimental studies

Abstract

Evolutionary change is occurring within tens of generations or fewer in nature. This contemporary evolution is commonly caused by human activities, as they alter the selective pressures that the populations experience. Human harvesting activities (plant gathering, hunting and fishing) are imposing particularly high selective pressure on natural populations and thus, inducing great changes in the populations. In the case of fishing, the selection is commonly imposed on size, as large fish are more valuable commercially. However, fishing can also be selective on other traits, such as behaviour, morphology, sex, etc. Thus, theoretically fishing can cause evolutionary change in the exploited populations, which may not only affect population viability, but also productivity for fisheries. Exploited stocks are experiencing phenotypic change in life history traits, mainly age and size at maturation, growth, and fecundity. These observed changes occurred in the expected direction if fishing would be causing evolutionary change, referred to as fisheries-induced evolution (FIE). Evidence for FIE is accumulating from three different research areas, theoretical modelling, empirical evidence from the field, and experimental studies. Each one of these areas of research has contributed to establishing the current knowledge on FIE. However, it is still not clear whether the changes observed have a genetic basis, whether fishing selectivity is the main driver, and whether the changes are occurring at a fast enough pace to be considered in fisheries management plans. This thesis contributes to clarifying some of these questions using an experimental approach. Most evidence for FIE comes from analysing field data using the Probabilistic Maturation Reaction Norm (PMRN) approach. This method infers genetic change from phenotypic data, but its approach has been questioned. Chapter I is an experimental evaluation of the PMRN. We estimated PMRN from male guppies differing in growth rate and the social environment they were reared in. We found that the PMRN could not completely account for these environmental effects, but the PMRN method performed better when a measurement of maturation closer to the maturation decision (initiation of maturation, rather than completion) was considered. Moreover, the analyses of empirical data have focused on studying the effect of size-selectivity on maturation schedules, as this is the data readily available. Thus, the assessment of other effects of fishing selectivity has been scarce. Chapter II shows that fishing has potential to cause selection on traits other than size and such selectivity has broader consequences than changes in the time of maturation. In Chapter II, we studied the selectivity of passive and active fishing gears on fish personality (shy-bold axis). Shy individuals were caught less by the passive trap, while they were caught more often by the trawl. Shy individuals seem to grow faster than bold ones. We discussed that such selectivity may alter the population structure, but also the fishery productivity, as personality can be associated with productivity traits (e.g., growth). Evidence that fishing selectivity can cause genetic change comes from experimental studies. However, the applicability of such results has been questioned, due to the experimental conditions not being comparable to natural populations (reviewed in Chapter IV). In Chapter V, we aimed to study the effect of size-selective fishing in experimental guppy populations. Our populations were created intending to be more comparable to natural populations. Fishing pressure mimicked that of exploited populations in the oceans and in our experimental conditions ecological feedbacks and natural selection were allowed in selfrenewing and age- and size-structured experimental populations. We compared three different harvesting regimes (removing large individuals, removing small ones and sizeindependent harvest), which resulted in different growth rates, size at maturation and fecundity. Density-dependent processes heavily influenced these changes, but size-selective fishing also played a role. Unfortunately, the experiment described in Chapter V is in a too early phase to conclude whether the changes observed are genetic or phenotypic. I believe dissemination of scientific goals and results, particularly to the general public, is a very important aspect of research. Chapter III describes a savoury approach on how to present the ecological and evolutionary consequences of fishing to schools or undergraduate students. In this experiment, we used fish-shaped candies as a common resource that was exploited by university employees. Even with such a simple experimental setting, we observed the processes commonly present in a real fishery, overexploitation, the tragedy of the commons vs. close access, and evolutionary effects. This thesis aimed at contributing to the knowledge on the evolutionary effects of fishing. Particularly, it addressed the potential that the experimental approach has on studying contemporary evolution in a broad range of traits, caused by selective fishing. Additionally, it focused on several aspects of FIE that are currently on debate, as it intended to fill up some of the gaps that still remain in the study of FIE.