Size-selective harvesting fosters adaptations in mating behaviour and reproductive allocation, affecting sexual selection in fish

Evolutionary implications of size-selective mortality on the ontogenetic development of shoal cohesion: a neurochemical approach using a zebrafish, Danio rerio, harvest selection experiment

Abstract

Size-selective mortality may evolutionarily alter life-history as well as individual behavioral and physiological traits. Moreover, size-selective mortality can affect group behavioral traits, such as shoaling and collective properties (e.g., shoal cohesion), which are relevant for finding food and reducing risk of predation. Here, we present experimental evidence using selection lines of zebrafish (Danio rerio) that were exposed to positive (large-harvested), negative (small-harvested), and random (control) size-selective mortality for five generations, followed by eight generations during which harvesting was halted to remove maternal effects and to study evolutionarily fixed outcomes. We investigated changes in shoal cohesion and turnover in monoamines in zebrafish through ontogeny. To that end, we repeatedly measured inter-individual distance in groups of eight fish and the turnovers of dopamine and serotonin in brains of fish from juvenile to the adult stage at 40-day intervals. We, firstly, found that shoal cohesion was overall consistent through ontogeny at group levels suggesting the presence of collective personality. Secondly, we found a decrease in shoal cohesion through ontogeny in the small-harvested and control lines, while the large-harvested line did not show any ontogenetic change. Thirdly, the selection lines did not differ among each other in shoal cohesion at any ontogenetic stage. Fourthly, dopamine turnover increased through ontogeny in a similar way for all lines while the serotonin turnover decreased in the large-harvested and control lines, but not in the small-harvested line. The large-harvested line also had higher serotonin turnover than controls at specific time periods. In conclusion, intensive size-selective mortality left an evolutionary legacy of asymmetric selection responses in the ontogeny of shoal cohesion and the underlying physiological mechanisms in experimentally harvested zebrafish in the laboratory.

Significant statement

The evolution of animal behavior can be affected by human activities both at behavioral and physiological levels, but causal evidence is scarce and mostly focusing on single life-stages. We studied whether and to what extent size-selective harvesting, a common selection pattern in fisheries, can be an evolutionary driver of the development of shoal cohesion during ontogeny. We used a multi-generation experiment with zebrafish to study cause-and-effects of opposing size-selection patterns. We quantified shoal cohesion, and serotonin and dopamine turnover in the brain. We found that shoal cohesion emerged as a collective personality trait and that behavioral and physiological responses were asymmetrical with respect to the opposing selection patterns.

Size-selective mortality fosters ontogenetic changes in collective risk-taking behaviour in zebrafish, Danio rerio

Size-selective mortality is common in fish populations and can operate either in a positive size-selective fashion by harvesting larger-than-average fish or be negatively size-selective by harvesting smaller-than-average fish. Through various mechanisms (like genetic correlations among behaviour and life-history traits or direct selection on behaviour co-varying with growth rate or size-at-maturation), size-selection can result in evolutionary changes in behavioural traits. Theory suggests that both positive and negative size-selection without additional selection on behaviour favours boldness, while evolution of shyness is possible if the largest fish are harvested. Here we examined the impact of size-selective mortality on collective boldness across ontogeny using three experimental lines of zebrafish (Daniorerio) generated through positive (large-harvested), negative (small-harvested) and random (control line) size-selective mortality for five generations and then relaxed selection for 10 generations to examine evolutionarily fixed outcomes. We measured collective risk-taking during feeding (boldness) under simulated aerial predation threat, and across four contexts in presence/absence of a cichlid. Boldness decreased across ontogeny under aerial predation threat, and the small-harvested line was consistently bolder than controls. The large and small-harvested lines showed higher behavioural plasticity as larvae and developed personality earlier compared to the controls. The large-harvested line showed increased variability and plasticity in boldness throughout ontogeny. In the presence of a live predator, fish did not differ in boldness in three contexts compared to the controls, but the large-harvested line showed reduced behavioural plasticity across contexts than controls. Our results confirmed theory by demonstrating that size-selective harvesting evolutionarily alters collective boldness and its variability and plasticity.

Sexual Dimorphism in Aggression: Sex-Specific Fighting Strategies Across Species

Aggressive behavior is thought to have evolved as a strategy for gaining access to resources such as territory, food, and potential mates. Across species, secondary sexual characteristics such as competitive aggression and territoriality are considered male-specific behaviors. However, although female–female aggression is often a behavior that is displayed almost exclusively to protect the offspring, multiple examples of female–female competitive aggression have been reported in both invertebrate and vertebrate species. Moreover, cases of intersexual aggression have been observed in a variety of species. Genetically tractable model systems such as mice, zebrafish, and fruit flies have proven extremely valuable for studying the underlying neuronal circuitry and the genetic architecture of aggressive behavior under laboratory conditions. However, most studies lack ethological or ecological perspectives and the behavioral patterns available are limited. The goal of this review is to discuss each of these forms of aggression, male intrasexual aggression, intersexual aggression and female intrasexual aggression in the context of the most common genetic animal models and discuss examples of these behaviors in other species.

Does parental angling selection affect the behavior or metabolism of brown trout parr?

The behavior of organisms can be subject to human-induced selection such as that arising from fishing. Angling is expected to induce mortality on fish with bold and explorative behavior, which are behaviors commonly linked to a high standard metabolic rate. We studied the transgenerational response of brown trout (Salmo trutta) to angling-induced selection by examining the behavior and metabolism of 1-year-old parr between parents that were or were not captured by experimental fly fishing. We performed the angling selection experiment on both a wild and a captive population, and compared the offspring for standard metabolic rate and behavior under predation risk in common garden conditions. Angling had population-specific effects on risk taking and exploration tendency, but no effects on standard metabolic rate. Our study adds to the evidence that angling can induce transgenerational responses on fish personality. However, understanding the mechanisms of divergent responses between the populations requires further study on the selectivity of angling in various conditions.

Size Selective Harvesting Does Not Result in Reproductive Isolation among Experimental Lines of Zebrafish, Danio rerio: Implications for Managing Harvest-Induced Evolution

Simple Summary

Mortality in fish populations is commonly size-selective. In fisheries, larger fish are preferentially caught while natural predators preferentially consume smaller fish. Removal of certain sized fish from populations and elevated fishing mortality constitute a selection pressure which may change life-history, behaviour and reduce adult body-size. Because behaviour and body-size are related and influence mating preferences and reproductive output, size-selective mortality may favour subpopulations that less readily mate with each other. Our aim is to test this possibility using three experimental lines of zebrafish (Danio rerio) generated in laboratory by removing large-sized, small-sized and random-sized fish for five generations. We tested mating preferences among males and females and tested if they spawned together. We found males and females of all subpopulations to reproduce among themselves. Females generally preferred large-sized males. Females of all lines spawned with males, and males of all lines fertilised eggs of females independent of the subpopulation origin. Our study shows that size-selective mortality typical of fisheries or in populations facing heavy predation does not result in evolution of reproductive barriers. Thus, when populations adapted to fishing pressure come in contact with populations unexposed to such pressures, interbreeding may happen thereby helping exploited populations recover from harvest-induced evolution.

Abstract

Size-selective mortality is common in fish stocks. Positive size-selection happens in fisheries where larger size classes are preferentially targeted while gape-limited natural predation may cause negative size-selection for smaller size classes. As body size and correlated behavioural traits are sexually selected, harvest-induced trait changes may promote prezygotic reproductive barriers among selection lines experiencing differential size-selective mortality. To investigate this, we used three experimental lines of zebrafish (Danio rerio) exposed to positive (large-harvested), negative (small-harvested) and random (control line) size-selective mortality for five generations. We tested prezygotic preferences through choice tests and spawning trials. In the preference tests without controlling for body size, we found that females of all lines preferred males of the generally larger small-harvested line. When the body size of stimulus fish was statistically controlled, this preference disappeared and a weak evidence of line-assortative preference emerged, but only among large-harvested line fish. In subsequent spawning trials, we did not find evidence for line-assortative reproductive allocation in any of the lines. Our study suggests that size-selection due to fisheries or natural predation does not result in reproductive isolation. Gene flow between wild-populations and populations adapted to size-selected mortality may happen during secondary contact which can speed up trait recovery.

Size-selective mortality induces evolutionary changes in group risk-taking behaviour and the circadian system in a fish

Intensive and trait-selective mortality of fish and wildlife can cause evolutionary changes in a range of life-history and behavioural traits. These changes might in turn alter the circadian system due to co-evolutionary mechanisms or correlated selection responses both at behavioural and molecular levels, with knock-on effects on daily physiological processes and behavioural outputs. We examined the evolutionary impact of size-selective mortality on group risk-taking behaviour and the circadian system in a model fish species. We exposed zebrafish Danio rerio to either large or small size-selective harvesting relative to a control over five generations, followed by eight generations during which harvesting was halted to remove maternal effects. Size-selective mortality affected fine-scale timing of behaviours. In particular, small size-selective mortality, typical of specialized fisheries and gape-limited predators targeting smaller size classes, increased group risk-taking behaviuor during feeding and after simulated predator attacks. Moreover, small size-selective mortality increased early peaks of daily activity as well as extended self-feeding daily activity to the photophase compared to controls. By contrast large size-selective mortality, typical of most wild capture fisheries, only showed an almost significant effect of decreasing group risk-taking behaviour during the habituation phase and no clear changes in fine-scale timing of daily behavioural rhythms compared to controls. We also found changes in the molecular circadian core clockwork in response to both size-selective mortality treatments. These changes disappeared in the clock output pathway because both size-selected lines showed similar transcription profiles. This switch downstream to the molecular circadian core clockwork also resulted in similar overall behavioural rhythms (diurnal swimming and self-feeding in the last hours of darkness) independent of the underlying molecular clock. To conclude, our experimental harvest left an asymmetrical evolutionary legacy in group risk-taking behaviour and in fine-scale daily behavioural rhythms. Yet, the overall timing of activity showed evolutionary resistance probably maintained by a molecular switch. Our experimental findings suggest that size-selective mortality can have consequences for behaviour and physiological processes.

Fisheries-induced selection against schooling behaviour in marine fishes

Group living is a common strategy used by fishes to improve their fitness. While sociality is associated with many benefits in natural environments, including predator avoidance, this behaviour may be maladaptive in the Anthropocene. Humans have become the dominant predator in many marine systems, with modern fishing gear developed to specifically target groups of schooling species. Therefore, ironically, behavioural strategies which evolved to avoid non-human predators may now actually make certain fish more vulnerable to predation by humans. Here, we use an individual-based model to explore the evolution of fish schooling behaviour in a range of environments, including natural and human-dominated predation conditions. In our model, individual fish may leave or join groups depending on their group-size preferences, but their experienced group size is also a function of the preferences of others in the population. Our model predicts that industrial fishing selects against individual-level behaviours that produce large groups. However, the relationship between fishing pressure and sociality is nonlinear, and we observe discontinuities and hysteresis as fishing pressure is increased or decreased. Our results suggest that industrial fishing practices could be altering fishes' tendency to school, and that social behaviour should be added to the list of traits subject to fishery-induced evolution.

Implications of size-selective fisheries on sexual selection

Fisheries often combine high mortality with intensive size selectivity and can, thus, be expected to reduce body size and size variability in exploited populations. In many fish species, body size is a sexually selected trait and plays an important role in mate choice and mate competition. Large individuals are often preferred as mates due to the high fecundity and resources they can provide to developing offspring. Large fish are also successful in competition for mates. Fisheries‐induced reductions in size and size variability can potentially disrupt mating systems and lower average reproductive success by decreasing opportunities for sexual selection. By reducing population sizes, fisheries can also lead to an increased level of inbreeding. Some fish species avoid reproducing with kin, and a high level of relatedness in a population can further disrupt mating systems. Reduced body size and size variability can force fish to change their mate preferences or reduce their choosiness. If mate preference is genetically determined, the adaptive response to fisheries‐induced changes in size and size variability might not occur rapidly. However, much evidence exists for plastic adjustments of mate choice, suggesting that fish might respond flexibly to changes in their social environment. Here, I first discuss how reduced average body size and size variability in exploited populations might affect mate choice and mate competition. I then consider the effects of sex‐biased fisheries on mating systems. Finally, I contemplate the possible effects of inbreeding on mate choice and reproductive success and discuss how mate choice might evolve in exploited populations. Currently, little is known about the mating systems of nonmodel species and about the interplay between size‐selective fisheries and sexual selection. Future studies should focus on how reduced size and size variability and increased inbreeding affect fish mating systems, how persistent these effects are, and how this might in turn affect population demography.