Simulated trapping and trawling exert similar selection on fish morphology

Commercial fishery harvest can influence the evolution of wild fish populations. Our knowledge of selection on morphology is however limited, with most previous studies focusing on body size, age, and maturation. Within species, variation in morphology can influence locomotor ability, possibly making some individuals more vulnerable to capture by fishing gears. Additionally, selection on morphology has the potential to influence other foraging, behavioral, and life‐history related traits. Here we carried out simulated fishing using two types of gears: a trawl (an active gear) and a trap (a passive gear), to assess morphological trait‐based selection in relation to capture vulnerability. Using geometric morphometrics, we assessed differences in shape between high and low vulnerability fish, showing that high vulnerability individuals display shallower body shapes regardless of gear type. For trawling, low vulnerability fish displayed morphological characteristics that may be associated with higher burst‐swimming, including a larger caudal region and narrower head, similar to evolutionary responses seen in fish populations responding to natural predation. Taken together, these results suggest that divergent selection can lead to phenotypic differences in harvested fish populations.     

Characterizing morphological (co)variation using structural equation models: Body size,allometric relationships and evolvability in a house sparrow metapopulation

Body size plays a key role in the ecology and evolution of all organisms. Therefore, quantifying the sources of morphological (co)variation, dependent and independent of body size, is of key importance when trying to understand and predict responses to selection. We combine structural equation modeling with quantitative genetics analyses to study morphological (co)variation in a meta‐population of house sparrows (Passer domesticus). As expected, we found evidence of a latent variable “body size,” causing genetic and environmental covariation between morphological traits. Estimates of conditional evolvability show that allometric relationships constrain the independent evolution of house sparrow morphology. We also found spatial differences in general body size and its allometric relationships. On islands where birds are more dispersive and mobile, individuals were smaller and had proportionally longer wings for their body size. Although on islands where sparrows are more sedentary and nest in dense colonies, individuals were larger and had proportionally longer tarsi for their body size. We corroborated these results using simulations and show that our analyses produce unbiased allometric slope estimates. This study highlights that in the short term allometric relationships may constrain phenotypic evolution, but that in the long term selection pressures can also shape allometric relationships.     

Bony pelvic canal size and shape in relation to body proportionality in humans

Obstetric selection acts on the female pelvic canal to accommodate the human neonate and contributes to pelvic sexual dimorphism. There is a complex relationship between selection for obstetric sufficiency and for overall body size in humans. The relationship between selective pressures may differ among populations of different body sizes and proportions, as pelvic canal dimensions vary among populations. Size and shape of the pelvic canal in relation to body size and shape were examined using nine skeletal samples (total female n = 57; male n = 84) from diverse geographical regions. Pelvic, vertebral, and lower limb bone measurements were collected. Principal component analyses demonstrate pelvic canal size and shape differences among the samples. Male multivariate variance in pelvic shape is greater than female variance for North and South Africans. High‐latitude samples have larger and broader bodies, and pelvic canals of larger size and, among females, relatively broader medio‐lateral dimensions relative to low‐latitude samples, which tend to display relatively expanded inlet antero‐posterior (A‐P) and posterior canal dimensions. Differences in canal shape exist among samples that are not associated with latitude or body size, suggesting independence of some canal shape characteristics from body size and shape. The South Africans are distinctive with very narrow bodies and small pelvic inlets relative to an elongated lower canal in A‐P and posterior lengths. Variation in pelvic canal geometry among populations is consistent with a high degree of evolvability in the human pelvis. Am J Phys Anthropol 151:88–101, 2013. © 2013 Wiley Periodicals, Inc.     

Consistent Selection towards Low Activity Phenotypes When Catchability Depends on Encounters among Human Predators and Fish

Together with life-history and underlying physiology, the behavioural variability among fish is one of the three main trait axes that determines the vulnerability to fishing. However, there are only a few studies that have systematically investigated the strength and direction of selection acting on behavioural traits. Using in situ fish behaviour revealed by telemetry techniques as input, we developed an individual-based model (IBM) that simulated the Lagrangian trajectory of prey (fish) moving within a confined home range (HR). Fishers exhibiting various prototypical fishing styles targeted these fish in the model. We initially hypothesised that more active and more explorative individuals would be systematically removed under all fished conditions, in turn creating negative selection differentials on low activity phenotypes and maybe on small HR. Our results partly supported these general predictions. Standardised selection differentials were, on average, more negative on HR than on activity. However, in many simulation runs, positive selection pressures on HR were also identified, which resulted from the stochastic properties of the fishes’ movement and its interaction with the human predator. In contrast, there was a consistent negative selection on activity under all types of fishing styles. Therefore, in situations where catchability depends on spatial encounters between human predators and fish, we would predict a consistent selection towards low activity phenotypes and have less faith in the direction of the selection on HR size. Our study is the first theoretical investigation on the direction of fishery-induced selection of behaviour using passive fishing gears. The few empirical studies where catchability of fish was measured in relation to passive fishing techniques, such as gill-nets, traps or recreational fishing, support our predictions that fish in highly exploited situations are, on average, characterised by low swimming activity, stemming, in part, from negative selection on swimming activity.     

Genomic architecture of habitat‐related divergence and signature of directional selection in the body shapes of Gnathopogon fishes

Evolution of ecomorphologically relevant traits such as body shapes is important to colonize and persist in a novel environment. Habitat‐related adaptive divergence of these traits is therefore common among animals. We studied the genomic architecture of habitat‐related divergence in the body shape of Gnathopogon fishes, a novel example of lake–stream ecomorphological divergence, and tested for the action of directional selection on body shape differentiation. Compared to stream‐dwelling Gnathopogon elongatus, the sister species Gnathopogon caerulescens, exclusively inhabiting a large ancient lake, had an elongated body, increased proportion of the caudal region and small head, which would be advantageous in the limnetic environment. Using an F₂ interspecific cross between the two Gnathopogon species (195 individuals), quantitative trait locus (QTL) analysis with geometric morphometric quantification of body shape and restriction‐site associated DNA sequencing‐derived markers (1622 loci) identified 26 significant QTLs associated with the interspecific differences of body shape‐related traits. These QTLs had small to moderate effects, supporting polygenic inheritance of the body shape‐related traits. Each QTL was mostly located on different genomic regions, while colocalized QTLs were detected for some ecomorphologically relevant traits that are proxy of body and caudal peduncle depths, suggesting different degree of modularity among traits. The directions of the body shape QTLs were mostly consistent with the interspecific difference, and QTL sign test suggested a genetic signature of directional selection in the body shape divergence. Thus, we successfully elucidated the genomic architecture underlying the adaptive changes of the quantitative and complex morphological trait in a novel system.     

Evolution of body size in Galapagos marine iguanas

Body size is one of the most important traits of organisms and allows predictions of an individual's morphology, physiology, behaviour and life history. However, explaining the evolution of complex traits such as body size is difficult because a plethora of other traits influence body size. Here I review what we know about the evolution of body size in a group of island reptiles and try to generalize about the mechanisms that shape body size. Galapagos marine iguanas occupy all 13 larger islands in this Pacific archipelago and have maximum island body weights between 900 and 12 000g. The distribution of body sizes does not match mitochondrial clades, indicating that body size evolves independently of genetic relatedness. Marine iguanas lack intra- and inter-specific food competition and predators are not size-specific, discounting these factors as selective agents influencing body size. Instead I hypothesize that body size reflects the trade-offs between sexual and natural selection. We found that sexual selection continuously favours larger body sizes. Large males establish display territories and some gain over-proportional reproductive success in the iguanas' mating aggregations. Females select males based on size and activity and are thus responsible for the observed mating skew. However, large individuals are strongly selected against during El Ni?o-related famines when dietary algae disappear from the intertidal foraging areas. We showed that differences in algae sward ('pasture') heights and thermal constraints on large size are causally responsible for differences in maximum body size among populations. I hypothesize that body size in many animal species reflects a trade-off between foraging constraints and sexual selection and suggest that future research could focus on physiological and genetic mechanisms determining body size in wild animals. Furthermore, evolutionary stable body size distributions within populations should be analysed to better understand selection pressures on individual body size.     

BOTH MALE AND FEMALE SEXUAL ORNAMENTS REFLECT OFFSPRING PERFORMANCE IN A FISH

Secondary sexual characters are often expressed in both sexes (mutual ornamentation), but are less often studied simultaneously. We studied the adaptive signaling function of male and female ornamentation in a mutually ornamented fish, the whitefish Coregonus lavaretus. In an experimental design in which nongenetic environmental effects were minimized, we found that highly ornamented females, males, and their parental combinations had offspring with better swimming performance and predator‐avoidance ability than less ornamented individuals or combinations. Furthermore, highly ornamented females had larger offspring that also had higher yolk volume than less ornamented individuals. Offspring swimming performance was not dependent on offspring size and was only weakly affected by yolk volume, which suggest that swimming performance and measured morphological traits are independent fitness measures. In conclusion, mutual ornamentation of whitefish may signal the quality of individuals in both sexes, which may indicate ongoing directional selection for these ornamental traits. However, offspring fitness traits were also dependent on parental combination, which suggests that genetic compatibility effects may weaken the directional selection and the indicator value of the ornamentation.     

Morphological adaptations to marine life in snakes

We investigated morphological adaptations to aquatic life within animals that exhibit a structurally simple, elongate body form, i.e., snakes. This linear body plan should impose different biomechanical constraints than the classical streamlined body shape associated with propulsion by fins, feet, or wings. Our measurements of general body shape of terrestrial, amphibious, and marine snakes (all from the same phylogenetic lineage, the Elapidae) show that seasnakes display specialized morphological attributes for life in water. Most notably, the cross‐sectional body shape is circular in terrestrial snakes but dorso‐ventrally elongated in seasnakes (due to a prominent ventral keel); amphibious species (sea kraits) exhibit an intermediate shape. The tail of amphibious and marine species (a major propulsive structure during swimming) is higher and thinner than in terrestrial snakes (i.e., paddle‐shaped) but shorter relative to body length. The evolution of a laterally compressed shape has been achieved by an increase in body height rather than a decrease in body width, possibly reflecting selection for more effective propulsive thrust, and for an ability to maintain hydrodynamic efficiency despite the minor bodily distension inevitably caused by prey items and developing offspring. J. Morphol., 2011. © 2011 Wiley‐Liss, Inc     

Assessing evolutionary consequences of size-selective recreational fishing on multiple life-history traits,with an application to northern pike (<Emphasis Type="Italic">Esox lucius</Emphasis>)

Despite mounting recognition of the importance of fishing-induced evolution, methods for quantifying selection pressures on multiple adaptive traits affected by size-selective harvesting are still scarce. We study selection differentials on three life-history traits—reproductive investment, size at maturation, and growth capacity—under size-selective exploitation of northern pike (Esox lucius L.) with recreational-fishing gear. An age-structured population model is presented that accounts for the eco-evolutionary feedback arising from density-dependent and frequency-dependent selection. By introducing minimum-length limits, maximum-length limits, and combinations of such limits (resulting in harvestable-slot length limits) into the model, we examine the potential of simple management tools for mitigating selection pressures induced by recreational fishing. With regard to annual reproductive investment, we find that size-selective fishing mortality exerts relatively small positive selection differentials. By contrast, selection differentials on size at maturation are large and consistently negative. Selection differentials on growth capacity are often large and positive, but become negative when a certain range of minimum-length limits are applied. In general, the strength of selection is reduced by implementing more stringent management policies, but each life-history trait responds differently to the introduction of specific harvest regulations. Based on a simple genetic inheritance model, we examine mid- and long-term evolutionary changes of the three life-history traits and their impacts on the size spectrum and yield of pike. Fishing-induced evolution often reduces sizes and yields, but details depend on a variety of factors such as the specific regulation in place. We find no regulation that is successful in reducing to zero all selection pressures on life-history traits induced by recreational fishing. Accordingly, we must expect that inducing some degree of evolution through recreational fishing is inevitable.     

Morphological correlates of river velocity and reproductive development in an ornamented stream fish

Adaptive phenotypic divergence can arise when environments vary in ways favoring alternative phenotypic optima. In aquatic habitats, the costs of locomotion are expected to increase with water velocity, generally favoring a more streamlined body and the reduction of traits that produce drag. However, because streamlining in fish may come at the cost of maneuverability, the net benefits of drag reduction can differ not only among habitats, but also among individuals (or classes of individuals) that rely on locomotion for different uses (e.g., males vs. females or adults vs. juveniles). We tested these predictions by exploring relationships among river velocity, body streamlining, ornamental fin size, and male reproductive condition in the steelcolor shiner (Cyprinella whipplei), a small-bodied North American cyprinid. Overall, males in peak reproductive condition (defined by the development of sexually dimorphic tubercles) had less streamlined bodies and larger ornamental fins than males in lower reproductive condition or individuals lacking these secondary sexual characters (females and immature males). There was a relationship between river velocity and body streamlining only for males in peak reproductive condition, but it was in the opposite direction of our predictions: these males were less streamlined in faster rivers. We found only weak support for the prediction that ornamental fin size would be negatively associated with river velocity. Overall, these results suggest either that drag is not an important selective pressure in these habitats, or that the sexual selection advantages of a deep body and large fin compensate any natural selection costs for C. whipplei males. This study highlights the often overlooked diversity of selective pressures acting on streamlining in fishes, and can offer novel insights and predictions allowing a more nuanced understanding of fish ecomorphology.     

Parallel and non‐parallel morphological divergence among foraging specialists in European whitefish (Coregonus lavaretus)

Parallel phenotypic evolution occurs when independent populations evolve similar traits in response to similar selective regimes. However, populations inhabiting similar environments also frequently show some phenotypic differences that result from non‐parallel evolution. In this study, we quantified the relative importance of parallel evolution to similar foraging regimes and non‐parallel lake‐specific effects on morphological variation in European whitefish (Coregonus lavaretus). We found evidence for both lake‐specific morphological characteristics and parallel morphological divergence between whitefish specializing in feeding on profundal and littoral resources in three separate lakes. Foraging specialists expressed similar phenotypes in different lakes in both overall body shape and selected measured morphological traits. The morphology of the two whitefish specialists resembled that predicted from other fish species, supporting the conclusion of an adaptive significance of the observed morphological characteristics. Our results indicate that divergent natural selection resulting from foraging specialization is driving and/or maintaining the observed parallel morphological divergence. Whitefish in this study may represent an early stage of divergence towards the evolution of specialized morphs.     

Body size, performance and fitness in galapagos marine iguanas

Complex organismal traits such as body size are influenced byinnumerable selective pressures, making the prediction of evolutionarytrajectories for those traits difficult. A potentially powerfulway to predict fitness in natural systems is to study the compositeresponse of individuals in terms of performance measures, suchas foraging or reproductive performance. Once key performancemeasures are identified in this top-down approach, we can determinethe underlying physiological mechanisms and gain predictivepower over long-term evolutionary processes. Here we use marineiguanas as a model system where body size differs by more thanone order of magnitude between island populations. We identifiedforaging efficiency as the main performance measure that constrainsbody size. Mechanistically, foraging performance is determinedby food pasture height and the thermal environment, influencingintake and digestion. Stress hormones may be a flexible wayof influencing an individual's response to low-food situationsthat may be caused by high population density, famines, or anthropogenicdisturbances like oil spills. Reproductive performance, on theother hand, increases with body size and is mediated by highersurvival of larger hatchlings from larger females and increasedmating success of larger males. Reproductive performance ofmales may be adjusted via plastic hormonal feedback mechanismsthat allow individuals to assess their social rank annuallywithin the current population size structure. When integrated,these data suggest that reproductive performance favors increasedbody size (influenced by reproductive hormones), with an overalllimit imposed by foraging performance (influenced by stresshormones). Based on our mechanistic understanding of individualperformances we predicted an evolutionary increase in maximumbody size caused by global warming trends. We support this predictionusing specimens collected during 1905. We also show in a common-gardenexperiment that body size may have a genetic component in iguanids.This ‘performance paradigm’ allows predictions aboutadaptive evolution in natural populations.     

Allometric relationships in morphological traits associated with foraging,swimming ability,and predator defense reveal adaptations toward brackish and freshwater environments in the threespine stickleback

Freshwater colonization by threespine stickleback has led to divergence in morphology between ancestral marine and derived freshwater populations, making them ideal for studying natural selection on phenotypes. In an open brackish–freshwater system, we previously discovered two genetically distinct stickleback populations that also differ in geometric shape: one mainly found in the brackish water lagoon and one throughout the freshwater system. As shape and size are not perfectly correlated, the aim of this study was to identify the morphological trait(s) that separated the populations in geometric shape. We measured 23 phenotypes likely to be important for foraging, swimming capacity, and defense against predation. The lateral plate morphs in freshwater displayed few significant changes in trait sizes, but the low plated expressed feeding traits more associated with benthic habitats. When comparing the completely plated genetically assigned populations, the freshwater, the hybrids, the migrants and the lagoon fish, many of the linear traits had different slopes and intercepts in trait‐size regressions, precluding our ability to directly compare all traits simultaneously, which most likely results from low variation in body length for the lagoon and migrant population. We found the lagoon stickleback population to be more specialized toward the littoral zone, displaying benthic traits such as large, deep bodies with smaller eyes compared to the freshwater completely plated morph. Further, the lagoon and migrant fish had an overall higher body coverage of lateral plates compared to freshwater fish, and the dorsal and pelvic spines were longer. Evolutionary constraints due to allometric scaling relationships could explain the observed, overall restricted, differences in morphology between the sticklebacks in this study, as most traits have diversified in common allometric trajectories. The observed differences in foraging and antipredation traits between the fish with a lagoon and freshwater genetic signature are likely a result of genetic or plastic adaptations toward brackish and freshwater environments.     

Function‐related Drivers of Skull Morphometric Variation and Sexual Size Dimorphism in a Subterranean Rodent,Plateau Zokor (Eospalax baileyi)

Sexual dimorphism is prevalent in most living organisms. The difference in size between sexes of a given species is generally known as sexual size dimorphism (SSD). The magnitude of the SSD is determined by Rensch's rule where size dimorphism increases with increasing body size when the male is the larger sex and decreases with increasing average body size when the female is the larger sex. The unique underground environment that zokors (Eospalax baileyi) live under in the severe habitat of the Qinghai‐Tibetan Plateau (QTP) could create SSD selection pressures that may or may not be supported by Rensch's rule, making this scientific question worthy of investigation. In this study, we investigated the individual variation between sexes in body size and SSD of plateau zokors using measurements of 19 morphological traits. We also investigated the evolutionary mechanisms underlying SSD in plateau zokors. Moreover, we applied Rensch's rule to all extant zokor species. Our results showed male‐biased SSD in plateau zokors: The body‐ and head‐related measurements were greater in males than in females. Linear regression analysis between body length, body weight, and carcass weight showed significant relationships with some traits such as skull length, lower incisor length, and tympanic bulla width, which might support our prediction that males have faster growth rates than females. Further, the SSD pattern corroborated the assumption of Rensch's rule in plateau zokors but not in the other zokor species. Our findings suggest that the natural underground habitat and behavioral differences between sexes can generate selection pressures on male traits and contribute to the evolution of SSD in plateau zokors.     

Sexually selected traits are larger and more variable in male than female beach-spawning capelin,Mallotus villosus

We evaluated whether morphological traits in capelin, Mallotus villosus, that appear to be sexually selected (pectoral fin, pelvic fin, anal fin, lateral ridge) were larger and more variable in males than females compared with naturally selected morphological traits (eyes, dorsal fin). Photographs were obtained of 136 capelin captured at two spawning sites and standardised measurements were taken of six morphological traits. Males had larger traits than females for a given body size and this was most pronounced in the traits thought to be sexually selected. Body size explained much of the variation in female traits but less variation in male traits, suggesting alternative selection pressures are involved. We suggest that larger male body size aids in endurance rivalry and sexually dimorphic traits help males to remain in physical contact with females while spawning on the beach.     

Can morphological and behavioral traits predict the foraging and feeding dynamics of social arachnids?

Complex social insect species exhibit task specialization mediated by morphological and behavioral traits. However, evidence of such traits is scarce for other social arthropods. We investigated whether the social pseudoscorpion Paratemnoides nidificator exhibits morphologically and behaviorally specialized individuals in prey capture. We measured body and chela sizes of adult pseudoscorpions and analyzed predation processes. Larger individuals spent more time moving through the colony and foraging than smaller pseudoscorpions. Individuals that captured prey had increased body and absolute chelae sizes. Although larger individuals had relatively small chelae size, they showed a higher probability of prey capture. Larger individuals manipulated prey often, although they fed less than smaller pseudoscorpions. Individuals that initiated captures fed more frequently and for more time than the others. Natural selection might be favoring individuals specialized in foraging and colony protection, allowing smaller and less efficient adults to avoid contact with dangerous prey. To our knowledge, there is incipient information regarding specialized individuals in arachnids, and our results might indicate the emergence of a morphologically specialized group in this species.     

Integrating environmental variation, predation pressure, phenotypic plasticity and locomotor performance

The Wujiang River, a tributary of the Three Gorges Reservoir, has many dams along its length. These dams alter the river's natural habitat and produce various flow regimes and degrees of predator stress. To test whether the swimming performance and external body shape of pale chub (Zacco platypus) have changed as a result of alterations in the flow regime and predator conditions, we measured the steady (U _crit) and unsteady (fast-start) swimming performances and morphological characteristics of fish collected from different sites along the Wujiang River. We also calculated the maximum respiratory capacity and cost of transport (COT). We demonstrated significant differences in swimming performance and morphological traits among the sampling sites. Steady swimming performance was positively correlated with water velocity and negatively correlated with the abundance of predators, whereas unsteady swimming performance was negatively correlated with water velocity. The body shape was significantly correlated with both swimming performance and ecological parameters. These findings suggested that selection pressure on swimming performance results in a higher U _crit and a more streamlined body shape in fast-flow and (or) in habitats with low predator stress and subsequently results in a lower COT. These characteristics were accompanied by a poorer fast-start performance than that of the fish from the slow-flow and (or) high-predator habitats. The divergence in U _crit may also be due in part to variation in respiratory capacity.     

An interspecific comparison between morphology and swimming performance in cyprinids

Flow regimes are believed to be of major evolutionary significance in fish. The flow regimes inhabited by cyprinids vary extensively from still flow regimes to riptide flow regimes. To test (i) whether flow‐driven swimming performance and relevant morphological differentiation are present among fish species and (ii) whether evolutionary shifts between high‐flow and low‐flow habitats in cyprinids are associated with evolutionary trade‐offs in locomotor performance, we obtained data on both steady and unsteady swimming performance and external body shape for 19 species of cyprinids that typically occur in different flow regimes (still, intermediate and riptide). We also measured the routine energy expenditure (RMR) and maximum metabolic rate (MMR) and calculated the optimal swimming speed. Our results showed that fish species from riptide groups tend to have a higher critical swimming speed (U_crit), maximum linear velocity (V_max) and fineness ratio (FR) than fish from the other two groups. However, there was no correlation between the reconstructed changes in the steady and unsteady swimming performance of the 19 species. According to the phylogenetically independent contrast (PIC) method, the U_crit was actively correlated with the MMR. These results indicated that selection will favour both higher steady and unsteady swimming performance and a more streamlined body shape in environments with high water velocities. The results suggested that steady swimming performance was more sensitive to the flow regime and that for this reason, changes in body shape resulted more from selective pressure on steady swimming performance than on unsteady swimming performance. No evolutionary trade‐off was observed between steady and unsteady swimming performance, although U_crit and MMR were found to have coevolved. However, a further analysis within each typically occurring habitat group suggested that the trade‐off that may exist between steady and unsteady swimming performance may be concealed by the effect of habitat.     

Foraging mode affects the evolution of egg size in generalist predators embedded in complex food webs

Ecological networks incorporate myriad biotic interactions that determine the selection pressures experienced by the embedded populations. We argue that within food webs, the negative scaling of abundance with body mass and foraging theory predict that the selective advantages of larger egg size should be smaller for sit‐and‐wait than active‐hunting generalist predators, leading to the evolution of a difference in egg size between them. Because body mass usually scales negatively with predator abundance and constrains predation rate, slightly increasing egg mass should simultaneously allow offspring to feed on more prey and escape from more predators. However, the benefits of larger offspring would be relatively smaller for sit‐and‐wait predators because (i) due to their lower mobility, encounters with other predators are less common, and (ii) they usually employ a set of alternative hunting strategies that help to subdue relatively larger prey. On the other hand, for active predators, which need to confront prey as they find them, body‐size differences may be more important in subduing prey. This difference in benefits should lead to the evolution of larger egg sizes in active‐hunting relative to sit‐and‐wait predators. This prediction was confirmed by a phylogenetically controlled analysis of 268 spider species, supporting the view that the structure of ecological networks may serve to predict relevant selective pressures acting on key life history traits.     

Size‐selective fishing affects sex ratios and the opportunity for sexual selection in Alaskan sockeye salmon Oncorhynchus nerka

Selective exploitation can cause adverse ecological and evolutionary changes in wild populations and also affect sex ratios but few studies have empirically documented skewed sex ratios in exploited fishes (other than species with extreme sexual size dimorphism, SSD). To investigate the possibility of sex‐selective fishing on Alaskan sockeye salmon Oncorhynchus nerka, we assessed sex ratios in fish at two spatial scales: within each of five fishing districts and among 13 breeding populations in one of these districts. We predicted that populations’ sex ratios would vary based on the average size of fish and SSD because size affects vulnerability to fishing. At the larger scale, we found a small but significant bias in fish returning to four of the five fishing districts (average = 52% females), and in four of the five districts males were caught at significantly higher rates than females. At the finer scale there was marked variation in sex ratio on the breeding grounds, ranging from 36% to 47% males. Populations with fish of intermediate sizes experienced the greatest sex ratio biases; the greater vulnerability of males than females to fishing resulted from a combination of larger SSD and different harvest rates between the sexes associated with the fishery size‐selectivity curve shape. Skewed sex ratios may change competition and behavior on the breeding grounds, relaxing selection on male traits associated with mate choice by females or intra‐sexual competition and altering demographic and evolutionary pressures on the fish. Assessment of the size selectivity of fishing gear and the population's SSD can help to illuminate if and how exploitation can affect sex ratios. Future studies examining size‐selective fishing should also evaluate the consequences for sex ratios, as this might help explain changes in harvested population structure and sustainability.