Nathan R. Franssen

Anthropogenic habitat alteration induces rapid morphological divergence in a native stream fish

Anthropogenic habitat alteration creates novel environments that can alter selection pressures. Construction of reservoirs worldwide has disturbed riverine ecosystems by altering biotic and abiotic environments of impounded streams. Changes to fish communities in impoundments are well documented, but effects of those changes on native species persisting in reservoirs, which are presumably subjected to novel selective pressures, are largely unexplored. I assessed body shape variation of a native stream fish in reservoir habitats and streams from seven reservoir basins in the Central Plains of the USA. Body shape significantly and consistently diverged in reservoirs compared with stream habitats within reservoir basins; individuals from reservoir populations were deeper‐bodied and had smaller heads compared with stream populations. Individuals from reservoir habitats also exhibited lower overall shape variation compared with stream individuals. I assessed the contribution of genotypic divergence and predator‐induced phenotypic plasticity on body shape variation by rearing offspring from a reservoir and a stream population with or without a piscivorous fish. Significant population‐level differences in body shape persisted in offspring, and both populations demonstrated similar predator‐induced phenotypic plasticity. My results suggest that, although components of body shape are plastic, anthropogenic habitat modification may drive trait divergence in native fish populations in reservoir‐altered habitats.

Shared and unique morphological responses of stream fishes to anthropogenic habitat alteration

Understanding population-level responses to novel selective pressures can elucidate evolutionary consequences of human-altered habitats. Stream impoundments (reservoirs) alter riverine ecosystems worldwide, exposing stream fishes to uncommon selective pressures. Assessing phenotypic trait divergence in reservoir habitats will be a first step in identifying the potential evolutionary and ecological consequences of stream impoundments. We tested for body shape divergence in four stream-adapted fishes found in both habitats within three separate basins. Shape variation among fishes was partitioned into shared (exhibited by all species) and unique (species-specific) responses to reservoir habitats. All fishes demonstrated consistent significant shared and unique morphological responses to reservoir habitats. Shared responses were linked to fin positioning, decreased body depths and larger caudal areas; traits likely related to locomotion. Unique responses were linked to head shape, suggesting species-specific responses to abiotic conditions or changes to their trophic ecology in reservoirs. Our results highlight how human-altered habitats can simultaneously drive similar and unique trait divergence in native populations.

Morphological divergence and flow-induced phenotypic plasticity in a native fish from anthropogenically altered stream habitats

Understanding population-level responses to human-induced changes to habitats can elucidate the evolutionary consequences of rapid habitat alteration. Reservoirs constructed on streams expose stream fishes to novel selective pressures in these habitats. Assessing the drivers of trait divergence facilitated by these habitats will help identify evolutionary and ecological consequences of reservoir habitats. We tested for morphological divergence in a stream fish that occupies both stream and reservoir habitats. To assess contributions of genetic-level differences and phenotypic plasticity induced by flow variation, we spawned and reared individuals from both habitats types in flow and no flow conditions. Body shape significantly and consistently diverged in reservoir habitats compared with streams; individuals from reservoirs were shallower bodied with smaller heads compared with individuals from streams. Significant population-level differences in morphology persisted in offspring but morphological variation compared with field-collected individuals was limited to the head region. Populations demonstrated dissimilar flow-induced phenotypic plasticity when reared under flow, but phenotypic plasticity in response to flow variation was an unlikely explanation for observed phenotypic divergence in the field. Our results, together with previous investigations, suggest the environmental conditions currently thought to drive morphological change in reservoirs (i.e., predation and flow regimes) may not be the sole drivers of phenotypic change.

Use of Stable Isotopes to Test Literature-based Trophic Classifications of Small-bodied Stream Fishes

Abstract Defining the trophic position of stream organisms is a first step in understanding the ecology of lotic systems. Whereas trophic positions of stream fishes have been traditionally assigned based on dietary analysis, stable isotope ratios may provide additional information on the validity of this approach and may be used to verify energy acquisition assumed from dietary studies. In this study, we assessed the concordance of literature-based trophic classifications and isotopic δ15N signatures for small-bodied fishes from four streams in Kansas, Oklahoma and New Mexico. ANOVA results revealed no significant difference (F2,27 = 1.71, P = 0.201) in trophic position based on δ15N values among three broad trophic classifications derived from literature sources (algivores/detritivores, omnivores and invertivores). Both the prevalence of omnivory in stream fishes and potential biases associated with isotope fractionation at different trophic levels poses potential problems when classifying lotic fish into trophic positions.

Fish Community Responses to Mechanical Removal of Nonnative Fishes in a Large Southwestern River

ABSTRACTEstablishment of nonnative fishes has contributed to the decline of native fishes worldwide. Efficacy of mechanical removal of nonnative fishes in large streams has been difficult to ascertain, and responses by native fishes after removal is equivocal. We summarize results of efforts on the San Juan River, New Mexico, Colorado, and Utah, to suppress nonnative Channel Catfish and Common Carp densities through removal via electrofishing. We assessed spatial and temporal trends in the densities of abundant fishes in relation to removal of nonnative fishes. Common Carp densities declined river-wide after removal but Channel Catfish densities only decreased in upper reaches. Sources of Channel Catfish juveniles and barriers to nonnative fish movement likely influenced the effectiveness of removal. Responses of native fishes to removal were not evident in most species and size classes. Results show that nonnative removal can be partly successful, but the complexity of large river systems limited the abi...

Nuptial coloration of red shiners (Cyprinella lutrensis) is more intense in turbid habitats

Communication is shaped and constrained by the signaling environment. In aquatic habitats, turbidity can reduce both the quantity and quality of ambient light and has been implicated in the breakdown of visual signaling. Here, we examined the relationship between turbidity (quantified with long-term data) and the expression of carotenoid-based nuptial coloration in the red shiner (Cyprinella lutrensis), a small-bodied cyprinid. Males in more turbid habitats displayed redder fins, and an experimental manipulation of adult diet suggested that carotenoid intake alone did not explain among-population color differences. These results run counter to similar studies where signal expression decreased in turbid conditions, and may be explained by the non-territorial red shiner mating system, interactions between the mechanism of coloration and the signaling environment, or reduced cost of color expression in turbid habitats (e.g., reduced predation risk). Our results highlight how the behavioral and ecological contexts in which signals function can shape evolutionary responses to the environment.

Can a highly invasive species re-invade its native community? The paradox of the red shiner

Red shiners (Cyprinella lutrensis) are among the most widespread, ecologically general, and environmentally tolerant fish species in North America, and are highly invasive where they have been introduced outside their native range. However, long-term data on fish assemblages showed that red shiners gradually (1980s to 2006) disappeared from creeks that are direct tributaries of Lake Texoma (Oklahoma, USA) where they are native and historically had been numerically dominant. Following a major flood in 2007, red shiners were detected anew in some of these creeks, but repeatedly disappeared and re-appeared through November 2009. Given their invasive abilities where they are not native, their failure to become re-established prompted us to examine factors that affect their apparent inability to re-invade their native habitat. We established assemblages of five common fish taxa native to Brier Creek in 12 large, outdoor mesocosm stream units. Subsequently, we introduced red shiners at two densities of 10 or 30 per unit, six replicates each, to examine potential effects of propagule pressure on establishment success. Approximately six months later, we ended the experiment and recovered all fish. Red shiners failed to become established in the experimental units, regardless of initial stocking density. They also exhibited much lower survival than other species in the native community, which not only survived well but exhibited some recruitment. Red shiner survival was significantly negatively related to the number of sunfish (Lepomis spp.) that grew to adult size in experimental units, suggesting that predation can inhibit early stages of invasion by red shiners.

Consumption of native and nonnative fishes by introduced largemouth bass (Micropterus salmoides) in the San Juan River, New Mexico.

Abstract Intense predation on larval and juvenile fishes by introduced piscivores can be detrimental to recruitment of threatened and endangered native fishes. Introduced largemouth bass (Micropterus salmoides) in the San Juan River, New Mexico, Colorado, and Utah, rarely are collected as adults; however, juveniles (<200 mm total length) often occur in the same habitats as young-of-the-year native fishes. To evaluate the prevalence of native fishes in the diet of juvenile largemouth bass, stomach samples from juveniles were collected from the San Juan River in New Mexico during July and August 2005, when young-of-the-year native and nonnative fishes were present. Stomach contents of largemouth bass were identified as native catostomids, native cyprinids, or nonnative cyprinids based on pharyngeal teeth. Although nonnative fishes comprised >80% of the potential prey base, significantly more native fishes were identified in stomachs than nonnative fishes. The disproportional abundance of native fishes in the diet of juvenile largemouth bass suggests greater susceptibility of young-of-the-year natives to predation by largemouth bass in the San Juan River.

Rapid recovery of a fish assemblage following an ecosystem disruptive algal bloom

Abstract: Disturbance of freshwater ecosystems through cultural eutrophication has resulted in an increased global occurrence of harmful algal blooms (HABs). Ecosystem disrupting algal blooms (EDABs) are a subset of HABs that produce extensive disturbances across entire ecosystems. Prymnesium parvum is an EDAB species that has invaded freshwater systems worldwide, causing massive fish kills and other negative effects. Fish kills frequently occur during HABs and EDABs, but few studies exist of the long-term implications of these fish kills and the resilience and recovery of fish assemblages following kills. We sampled fish near- and offshore over an annual cycle encompassing a P. parvum EDAB in 2 coves (i.e., a bloom site and a reference site) of a southern Great Plains reservoir, Lake Texoma, Oklahoma—Texas (USA). Our objective was to document the extirpation and recovery of a fish assemblage in response to the disturbance of an EDAB event. Prymnesium parvum bloomed in 1 cove from mid-December 2008 until May 2009 and eliminated all fish during this period. Fish toxicity bioassays indicated no substantial differences in susceptibility among fish species to P. parvum toxins. Fish recolonized the bloom site rapidly in May 2009 after the bloom diminished. Fish assemblages were resilient to the P. parvum EDAB, and recovered to previous abundance, richness, and composition within 6 mo. Our results suggest that the reservoir-wide fish meta-assemblage enabled a rapid recovery of local fish assemblages after a spatially heterogeneous EDAB.

Upstream effects of a reservoir on fish assemblages 45 years following impoundment

Fish assemblage structure, rarefied species richness, species diversity and evenness of assemblages upstream of a reservoir in Oklahoma, U.S.A., were compared pre and post-impoundment as well as in contemporary collections from streams above and below the reservoir. There were significant shifts in assemblage structure between historical and contemporary collections above the reservoir but not between contemporary assemblages above and below the impoundment. Indicator species analysis revealed that the sand shiner Notropis stramineus and fathead minnow Pimephales promelas have declined, whereas largemouth bass Micropterus salmoides and western mosquitofish Gambusia affinis have increased in relative abundance in assemblages upstream of the impoundment. Species richness was lower in contemporary assemblages compared with historical assemblages. Furthermore, contemporary assemblages below the dam had lower species richness, diversity and evenness compared with contemporary collections above the dam. These results highlight the spatial and temporal extent of reservoirs altering fish assemblages upstream of impoundments.