Humpback Chub Research Articles

Nonnative Fish Control in the Colorado River in Grand Canyon, Arizona: An Effective Program or Serendipitous Timing?

AbstractThe federally endangered humpback chub Gila cypha in the Colorado River within Grand Canyon is currently the focus of a multiyear program of ecosystem‐level experimentation designed to improve native fish survival and promote population recovery as part of the Glen Canyon Dam Adaptive Management Program. A key element of this experiment was a 4‐year effort to remove nonnative fishes from critical humpback chub habitat, thereby reducing potentially negative interactions between native and nonnative fishes. Over 36,500 fish from 15 species were captured in the mechanical removal reach during 2003–2006. The majority (64%) of the catch consisted of nonnative fish, including rainbow trout Oncorhynchus mykiss (19,020), fathead minnow Pimephales promelas (2,569), common carp Cyprinus carpio (802), and brown trout Salmo trutta (479). Native fish (13,268) constituted 36% of the total catch and included flannelmouth suckers Catostomus latipinnis (7,347), humpback chub (2,606), bluehead suckers Catostomus discobolus (2,243), and speckled dace Rhinichthys osculus (1,072). The contribution of rainbow trout to the overall species composition fell steadily throughout the study period from a high of approximately 90% in January 2003 to less than 10% in August 2006. Overall, the catch of nonnative fish exceeded 95% in January 2003 and fell to less than 50% after July 2005. Our results suggest that removal efforts were successful in rapidly shifting the fish community from one dominated numerically by nonnative species to one dominated by native species. Additionally, increases in juvenile native fish abundance within the removal reach suggest that removal efforts may have promoted greater survival and recruitment. However, drought‐induced increases in river water temperature and a systemwide decrease in rainbow trout abundance concurrent with our experiment made it difficult to determine the cause of the apparent increase in juvenile native fish survival and recruitment. Experimental efforts continue and may be able to distinguish among these factors and to better inform future management actions.

Trout Piscivory in the Colorado River, Grand Canyon: Effects of Turbidity, Temperature, and Fish Prey Availability

AbstractIntroductions of nonnative salmonids, such as rainbow trout Oncorhynchus mykiss and brown trout Salmo trutta, have affected native fishes worldwide in unforeseen and undesirable ways. Predation and other interactions with nonnative rainbow trout and brown trout have been hypothesized as contributing to the decline of native fishes (including the endangered humpback chub Gila cypha) in the Colorado River, Grand Canyon. A multiyear study was conducted to remove nonnative fish from a 15‐km segment of the Colorado River near the Little Colorado River confluence. We evaluated how sediment, temperature, fish prey availability, and predator abundance influenced the incidence of piscivory (IP) by nonnative salmonids. Study objectives were addressed through spatial (upstream and downstream of the Little Colorado River confluence) and temporal (seasonal and annual) comparisons of prey availability and predator abundance. Data were then evaluated by modeling the quantity of fish prey ingested by trout during the first 2 years (2003–2004) of the mechanical removal period. Field effort resulted in the capture of 20,000 nonnative fish, of which 90% were salmonids. Results indicated that the brown trout IP was higher (8–70%) than the rainbow trout IP (0.5–3.3%); however, rainbow trout were 50 times more abundant than brown trout in the study area. We estimated that during the study period, over 30,000 fish (native and nonnative species combined) were consumed by rainbow trout (21,641 fish) and brown trout (11,797 fish). On average, rainbow trout and brown trout ingested 85% more native fish than nonnative fish in spite of the fact that native fish constituted less than 30% of the small fish available in the study area. Turbidity may mediate piscivory directly by reducing prey detection, but this effect was not apparent in our data, as rainbow trout IP was greater when suspended sediment levels (range = 5.9–20,000 mg/L) were higher.

Overriding Effects of Species‐Specific Turbidity Thresholds on Hoop‐Net Catch Rates of Native Fishes in the Little Colorado River, Arizona

AbstractI examined the effects of turbidity, discharge, and temperature on hoop‐net catch rates of native humpback chub Gila cypha, speckled dace Rhinichthys osculus, flannelmouth suckers Catostomus latipinnis, and bluehead suckers C. discobolus in the Little Colorado River, Arizona. Results indicated that native fish catch rates were primarily influenced by whether turbidity levels were below or above species‐specific thresholds of approximately 545 nephelometric turbidity units (NTU) for humpback chub, 221 NTU for speckled dace, 846 NTU for flannelmouth suckers, and 70 NTU for bluehead suckers. The effects of discharge were negligible, but discharge did dictate the turbidity level. Turbidity levels at hoop‐net deployment relative to the turbidity thresholds predetermined much of the catchability of fish. Catch rates were highest in a high‐catch zone consisting of the lowermost turbidities, which ranged up to the start of a transition zone for each species (humpback chub, ≤54 NTU; speckled dace, ≤29 NTU; flannelmouth sucker, ≤81 NTU; bluehead sucker, ≤26 NTU), and the secondary effects of temperature were only detectable in this zone. Catch rates within the transition zone decreased at higher turbidities up to the thresholds and thereafter remained consistently low at all higher turbidities within this low‐catch zone. The effects of the turbidity thresholds on catch rates decreased for larger fish of all species. Theoretically, the effects of turbidity reflect a behavioral switch by native fishes from relying primarily on structural cover (e.g., hoop nets) to using turbidity as cover to reduce the predation risk as turbidity increases from below to above the thresholds. Moreover, turbidities within the low‐catch zone could be totally incapacitating the fishes' visual capabilities such that fish perceive these turbidity levels as unlimited visual cover. The turbidity threshold concept probably applies to the hoop‐net catch rates of other fishes in other riverine systems.

Evidence of Young Humpback Chub Overwintering in the Mainstem Colorado River, Marble Canyon, Arizona, USA~!2009-10-07~!2010-01-25~!2010-04-30~!

Global climate change models predict that the southwestern United States will become warmer and drier, re- sulting in decreased reservoir volumes, increased water temperatures, and changes in fish distributions. Such conditions may also result from drought and water extraction. Our observations of humpback chub Gila cypha allowed us to test pre- dicted impacts of increased water temperatures released from Glen Canyon Dam on this federally listed endangered cyp- rinid endemic to the Colorado River Basin. We modeled the potential that the young humpback chub we captured in 2006 and 2007 were hatched and reared up to 50 km upstream from the Little Colorado River, the natal origin of the majority of the humpback chub population below Glen Canyon Dam. Larval and adult humpback chub have been observed by others in our study reach in the post-dam era. Humpback chub assumed to be juveniles have been observed in our study area, but their ages were not confirmed. Our observations of humpback chub were coincident with not only increased water tem- peratures as a result of regional drought, but also decreasing rainbow trout population numbers and some constraints on dam release fluctuations. One or all of these factors could have contributed to increased survivorship of young humpback chub. The most parsimonious explanation for our observations was that the largest young humpback chub captured at our study site took advantage of warmer water temperatures to grow to one and two years of age. Continued favorable habitat conditions in this reach would likely support recruitment to adulthood and reproduction. Managers wish to increase the likelihood that a self-sustaining mainstem Colorado River population of humpback chub is established, distinct from the larger Little Colorado River population. Our results indicate that a separate population may be successfully established.

Development of a Temperature-Dependent Growth Model for the Endangered Humpback Chub Using Capture-Recapture Data

Model derived predictions of fish growth are frequently required for detailed investigations of population dy- namics to inform management decisions. Simple growth models are typically fit to paired age and length data, but age data is often not available from endangered species because of restrictions on lethal or invasive sampling methods. Growth increment data from capture-recapture studies can be used to inform such models, but currently available methods to fit data to growth models may produce biased predictions when growth is variable among individual fish, or when growth rate varies non-linearly with fish size. This study used a recently proposed growth model derived from basic bio- energetic principals to estimate growth of humpback chub in Grand Canyon, Arizona. The modeling framework allows incorporation of temperature-dependent shifts in growth rate associated with both seasonal variability in water tempera- ture and ontogenetic migrations between the seasonally warm Little Colorado River and the constantly cold Colorado River. Results indicate that consideration of temperature-dependent shifts in growth rate are critical to accurately describe the growth of humpback chub, and that management actions aimed at increasing water temperature in the thermally modi- fied Colorado River could aid the recovery of this species by increasing growth rate and survivorship.

Survival, Condition, Habitat Use, and Predation on Stocked Bonytails (Gila elegans) in the Green River, Colorado and Utah

The endangered bonytail Gila elegans, a large-bodied, main-stem cyprinid endemic to the Colorado River Basin of the American Southwest, was once widespread and abundant in warm-water-stream reaches. Negative effects of altered flow and temperature regimes downstream of dams, other habitat changes, and establishment of nonnative fishes have reduced populations of native fish throughout the basin, and wild bonytails may be extirpated. Hatchery reared bonytails are stocked in formerly occupied habitat to rebuild depleted populations, but their ecology is poorly understood. In 2002–2007, sampling in the middle Green River from upstream and downstream of stocking locations in Dinosaur National Monument documented survival of bonytails for ≤4 months, but apparently none survived longer. Many fish at large ≤4 months had Lernea or fungal infections, weighed an average of 20% less than fish when released, and had relatively low rates of growth. Post-stocking rates of dispersal downstream (ca. 1 river km/day) were considerable, and potentially, biologically significant. Bonytails occupied pools, eddies, runs, backwaters, and riffles, and co-occurred in eddies with the roundtail chub Gila robusta and humpback chub Gila cypha in Whirlpool Canyon. We also documented two instances of predation by smallmouth bass Micropterus dolomieu on bonytails ≤225 mm in total length. Reduced predation by large-bodied, nonnative, piscivores and increased resistance to disease and other stressors that reduce body condition may increase survival of stocked bonytails in this portion of the Green River. Alternative stocking strategies, including use of alluvial and floodplain areas, and effects of size on survival, are being evaluated.

Removal and Quantification of Asian Tapeworm from Bonytail Chub Using Praziquantel

AbstractThe spread of the Asian tapeworm Bothriocephalus acheilognathi has been identified as a potential threat to endangered cyprinid fishes, such as bonytail Gila elegans and humpback chub G. cypha. Praziquantel is often used to remove Asian tapeworms from these species, but little information is available on the effectiveness or side effects of these treatments. I performed laboratory experiments using bonytails (110–457 mm total length) to evaluate the dosage and time needed to effectively remove 100% of Asian tapeworms using praziquantel. A low dose of 0.7 mg/L for 24 h was not effective at removing all Asian tapeworms, whereas a 24‐h exposure at 1.5 mg/L and higher was 100% effective. A bath treatment of 12‐h duration only appears effective at the highest dose of 36 mg/L, but not all Asian tapeworms are cleared from the host within this time frame. No short‐term mortality or side effects of praziquantel treatment were observed. Praziquantel bath treatments can effectively and safely remove 100% of Asian tapeworms from bonytail if dosage and treatment duration are adequate. In some cases, repeated treatments may be necessary to ensure that Asian tapeworms are not spread into new environments.

DISPERSAL OF NONNATIVE FISHES AND PARASITES IN THE INTERMITTENT LITTLE COLORADO RIVER, ARIZONA

Abstract We sampled nonnative fishes stranded in isolated pools near Grand Falls in the Little Colorado River (LCR), Arizona, after the river ceased flowing (21 June and 12 July 2005) to evaluate whether nonnative fishes can invade the perennial, lower 21 km of the LCR from upriver sources. The encroachment of nonnative fishes could jeopardize resident populations of endangered humpback chub, Gila cypha, and other native fishes in the lower LCR. We captured red shiner, Cyprinella lutrensis; common carp, Cyprinus carpio; fathead minnow, Pimephales promelas; black bullhead, Ameiurus melas; and plains killifish, Fundulus zebrinus, all of which have been captured >132 km downriver in the lower LCR and >127 km upriver in the closest perennial sources. Moreover, we detected Asian tapeworm, Bothriocephalus acheilognathi, in 9 of 30 common carp examined. Our findings suggest that nonnative fishes, including those hosting parasites, can invade the lower LCR from upriver sources >250 km away during freshets and pro...

COMPARATIVE GROWTH AND CONSUMPTION POTENTIAL OF RAINBOW TROUT AND HUMPBACK CHUB IN THE COLORADO RIVER, GRAND CANYON, ARIZONA, UNDER DIFFERENT TEMPERATURE SCENARIOS

We used bioenergetics models for humpback chub, Gila cypha, and rainbow trout, Oncorhynchus mykiss, to examine how warmer water temperatures in the Colorado River, Grand Canyon, Arizona, through a proposed selective withdrawal system (SWS) at Glen Canyon Dam, would affect growth, consumption, and predation rates. Consumption by the rainbow trout population was at least 10 times higher than by the smaller humpback chub population. Water temperature increases of 6°C during autumn increased growth of humpback chub and likely increased their survival by reducing the time vulnerable to predation. Water temperature increases caused by drought in 2005 did not alter humpback chub growth as much as the SWS. Increased temperatures might cause changes to the invertebrate community and the distribution and abundance of other warmwater nonnative fishes. The implications on the entire aquatic community need to be considered before any management action that includes increasing water temperatures is implemented.

Parasitism and Body Condition in Humpback Chub from the Colorado and Little Colorado Rivers, Grand Canyon, Arizona

AbstractGlen Canyon Dam has greatly altered the Colorado River in Grand Canyon. The Little Colorado River (LCR) provides a small refuge of seasonally warm and turbid water that is thought to be more suitable than the Colorado River for endangered humpback chub Gila cypha. However, the LCR has low productivity and contains nonnative fishes and parasites, which pose a threat to humpback chub. The Colorado River hosts a different suite of nonnative fishes and is cold and clear but more productive. We compared condition factor (K), abdominal fat index (AFI), and presence and number of two introduced pathogenic parasites (Lernaea cyprinacea and Bothriocephalus acheilognathi) between juvenile (<150 mm total length) humpback chub from the LCR and those from the Colorado River during 1996–1999. Both K and AFI were lower and L. cyprinacea prevalence and B. acheilognathi prevalence were higher in LCR fish than in Colorado River fish for all years. Mean K and AFI were 0.622 and 0.48, respectively, in the LCR and 0.735 and 2.02, respectively, in the Colorado River, indicating that fish in the Colorado River were more robust. Mean prevalence of L. cyprinacea was 23.9% and mean intensity was 1.73 L. cyprinacea/infected fish in the LCR, whereas prevalence was 3.2% and intensity was 1.0 L. cyprinacea/infected fish in the Colorado River. Mean prevalence of B. acheilognathi was 51.0% and mean intensity was 25.0 B. acheilognathi/infected fish in the LCR, whereas prevalence was 15.8% and intensity was 12.0 B. acheilognathi/infected fish in the Colorado River. Increased parasitism and poorer body condition in humpback chub from the LCR challenge the paradigm that warmer LCR waters are more suitable for humpback chub than the colder Colorado River and indicate the need to consider the importance and benefits of all available habitats, as well as biotic and abiotic factors, when managing endangered species and their environment.

Distribution and Movement of Humpback Chub in the Colorado River, Grand Canyon, Based on Recaptures

AbstractMark–recapture data from the federally endangered humpback chub Gila cypha in the Colorado River, Grand Canyon, were analyzed from 1989 to 2002 to determine large‐scale movement patterns and distribution. A total of 14,674 recaptures from 7,127 unique fish were documented; 87% of the recaptures occurred in the same main‐stem river reach or tributary as the original captures, suggesting restricted distribution by most fish. A total of 99% of all recaptures were from in and around the Little Colorado River (LCR), a tributary of the Colorado River and primary aggregation and spawning location of humpback chub in Grand Canyon. Time at liberty averaged 394 d, but some fish were recaptured near their main‐stem capture location over 10 years later. Proportionally fewer large (>300‐mm) humpback chub exhibited restricted distribution than small (<200‐mm) fish. However, several fish did move more than 154 km throughout Grand Canyon between capture and recapture, suggesting that limited movement occurs throughout Grand Canyon. The majority of the recaptured fish remained in or returned to the LCR or the Colorado River near the LCR. Although many large‐river fishes exhibit extensive migrations to fulfill their life history requirements, most of the humpback chub in Grand Canyon appear to remain in or come back to the LCR and LCR confluence across multiple sizes and time scales. Detecting trends in the overall abundance of this endangered fish in Grand Canyon can probably be accomplished by monitoring the area in and around the LCR.

Abundance Trends and Status of the Little Colorado River Population of Humpback Chub

AbstractThe abundance of the Little Colorado River population of federally listed humpback chub Gila cypha in Grand Canyon has been monitored since the late 1980s by means of catch rate indices and capture–recapture‐based abundance estimators. Analyses of data from all sources using various methods are consistent and indicate that the adult population has declined since monitoring began. Intensive tagging led to a high proportion (>80%) of the adult population being marked by the mid‐1990s. Analysis of these data using both closed and open abundance estimation models yields results that agree with catch rate indices about the extent of the decline. Survival rates for age‐2 and older fish are age dependent but apparently not time dependent. Back‐calculation of recruitment using the apparent 1990s population age structure implies periods of higher recruitment in the late 1970s to early 1980s than is now the case. Our analyses indicate that the U.S. Fish and Wildlife Service recovery criterion of stable abundance is not being met for this population. Also, there is a critical need to develop new abundance indexing and tagging methods so that early, reliable, and rapid estimates of humpback chub recruitment can be obtained to evaluate population responses to management actions designed to facilitate the restoration of Colorado River native fish communities.

Ontogenesis of Endangered Humpback Chub (Gila cypha) in the Little Colorado River, Arizona

The largest population of endangered humpback chub Gila cypha inhabits the Colorado River below Glen Canyon Dam and the lower 14 km of the Little Colorado River (LCR), Arizona. Currently, adults from both rivers spawn and their progenies grow and recruit to adulthood primarily within the LCR, where we studied G. cypha's life history using hoop net capture data. Humpback chub undergo an ontogenesis from diurnally active, vulnerable, nearshore-reliant young-of-the-year (YOY; 30–90 mm total length) into nocturnally active, large-bodied adults (≥180 mm TL). During the day, adults primarily resided in deep midchannel pools; however, at night they dispersed inshore amongst the higher densities of YOY conspecifics. Many YOY G. cypha shifted to nocturnal habitats that provided greater cover, possibly, to avoid inshore invading adults. These findings mirror predator-prey scenarios described in other freshwater assemblages, but do not refute other plausible hypotheses. Gila cypha piscivorous activity may escalate in hoop nets, which can confine fish of disparate sizes together; adults were significantly associated with YOY conspecifics and small dead fish in hoop nets at night and eight G. cypha (156–372 mm TL) regurgitated and/or defecated other fish body parts during handling following capture. Gila cypha can definitely be piscivorous given the opportunity, but the magnitude of their piscivorous activity in the wild is debatable.

Experimental infection of the endangered bonytail chub (Gila elegans) with the Asian fish tapeworm (Bothriocephalus acheilognathi): impacts on survival, growth, and condition

Bothriocephalus acheilognathi Yamaguti, 1934, a tapeworm known to be pathogenic to some fish species, has become established in the endangered humpback chub ( Gila cypha Miller, 1964) in Grand Canyon, USA, following the tapeworm’s introduction into the Colorado River system. The potential impact of this tapeworm on humpback chub was studied by exposing the closely related bonytail chub ( Gila elegans Baird and Girard, 1853) to the parasite under a range of conditions that included potential stressors of humpback chub in their natal waters, such as abrupt temperature change and a limited food base. Survival of infected fish under low food rations was considerably lower than that of control fish, and mortality of infected fish began 20 days earlier. Growth of infected fish was significantly reduced, and negative changes in health condition indices were found. No significant negative impacts were revealed from the synergistic effects between temperature shock and infection. Bothriocephalus acheilognathi does present a potential threat to humpback chub in Grand Canyon and should be considered, along with conventional concerns involving altered flow regimes and predation, when management decisions are made concerning conservation of this endangered species.

Development of a Bioenergetics Model for Humpback Chub and Evaluation of Water Temperature Changes in the Grand Canyon, Colorado River

AbstractThe construction of Glen Canyon Dam above the Grand Canyon (Arizona) has reduced the water temperature in the Colorado River and altered the growth rate and feeding patterns of the federally endangered humpback chub Gila cypha. A bioenergetics model for humpback chub was developed and used to examine how warmer water temperatures in the lower Colorado River (achieved through a temperature control device [TCD] at Glen Canyon Dam) might influence their growth rate and food requirements. Parameter values for humpback chub were developed by Monte Carlo filtering and fitting to laboratory growth. Parameter bounds were established from the literature for Gila species, random parameter sets were selected within these bounds, and the growth of modeled humpback chub was compared with criteria from a laboratory growth experiment at 24°C. This method of parameter estimation could be applied to other imperiled fishes where physiological studies are impractical. Final parameter values were corroborated by comparison with the growth rates of humpback chub from independent field and laboratory studies. Simulations indicated that increasing water temperatures from approximately 9°C to 16°C during summer and fall, the change expected from the TCD, may have a minimal effect on humpback chub growth rate unless food availability also increases with temperature. To evaluate the effects of increased temperatures on humpback chub in the lower Colorado River, it will be essential to monitor their growth rate, the invertebrate community, and the predators of humpback chub, which are also influenced by temperature changes. Bioenergetics models for humpback chub and their predators should be helpful tools for identifying potential scenarios and evaluating the complex interactions resulting from a TCD.

Effect of Baiting on Hoop Net Catch Rates of Endangered Humpback Chub

AbstractThe effects of unbaited, scented (odor emitted by inaccessible hatchery feed), and baited (accessible feed) single‐throated hoop nets on the catch rates of the federally endangered humpback chub Gila cypha (≥100 mm total length) were assessed in the Little Colorado River, Arizona, to determine how baiting affected the catch of these fish. Of a total of 1,297 unique humpback chub captured, 262 (20%) were caught in 179 unbaited nets, 330 (25%) in 179 scented nets, and 705 (54%) in 178 baited nets. Humpback chub retention in unbaited nets did not differ from a 50:50 chance event (P = 0.075) but probably peaked in baited nets where these fish were preoccupied with feeding (i.e., gorging themselves), which would explain the higher catch rates in baited than in scented or unbaited nets (P ≤ 0.001). Moreover, humpback chub catch rates were not significantly higher in scented than in unbaited nets (P = 0.779), which suggests that elevating both “fish attraction” and “fish retention” are essential to substantially increase their captures in these nets. However, scenting may still be a viable option when used in conjunction with passive gears possessing higher fish retention capabilities or to target species less prone to escape. The findings of this study should provide valuable insight to anyone wishing to optimize catch rates by baiting or scenting passive entrapment gear.

Comparison of electrofishing and trammel netting variability for sampling native fishes

The variability in size structure and relative abundance (CPUE; number of fish ≥200 mm total length, LT, collected per hour of electrofishing or trammel netting) of three native Colorado River fishes, the endangered humpback chub Gila cypha, flannelmouth sucker Catostomus latipinnus and bluehead sucker Catostomus discobolus, collected from electrofishing and trammel nets was assessed to determine which gear was most appropriate to detect trends in relative abundance of adult fishes. Coefficient of variation (CV) of CPUE ranged from 210 to 566 for electrofishing and 128 to 575 for trammel netting, depending on season, diel period and species. Mean CV was lowest for trammel nets for humpback chub (P = 0·004) and tended to be lower for flannelmouth sucker (P = 0·12), regardless of season or diel period. Only one bluehead sucker >200 mm was collected with electrofishing. Electrofishing and trammel netting CPUE were not related for humpback chub (r = −0·32, P = 0·43) or flannelmouth sucker (r = −0·27, P = 0·46) in samples from the same date, location and hour set. Electrofishing collected a higher proportion of smaller (<200 mm LT) humpback chub (P < 0·001), flannelmouth suckers (P < 0·001) and bluehead suckers (P < 0·001) than trammel netting, suggesting that conclusions derived from one gear may not be the same as from the other gear. This is probably because these gears fished different habitats, which are occupied by different fish life stages. To detect a 25% change in CPUE at a power of 0·9, at least 473 trammel net sets or 1918 electrofishing samples would be needed in this 8 km reach. This unattainable amount of samples for both trammel netting and electrofishing indicates that detecting annual changes in CPUE may not be practical and analysis of long‐term data or stock assessment models using mark‐recapture methods may be needed to assess trends in abundance of Colorado River native fishes, and probably other rare fishes as well.

Fish humps in two Colorado River fishes: a morphological response to cyprinid predation?

Extant fishes endemic to the upper Colorado River of the American southwest include only cyprinids and catostomids. A curious attribute in species of both groups is the presence of a large nuchal hump. Largest cyprinid humps occur in humpback chub, Gila cypha, and largest catostomid humps occur in razorback sucker, Xyrauchen texanus. Several authors have suggested the humps confer a hydrodynamic advantage to life in fast flow, but this premise has not been confirmed with experimental work. To test the role of humps in Colorado River fishes, we subjected whole-body casts of preserved specimens with humps and with humps removed to controlled flows in an experimental tank. These tests confirmed that humps increased drag coefficients for X. texanus and G. cypha with no additional lift component. High energetic costs of locomotion and position-holding with a large hump, and the additional metabolic expense of forming large humps, suggest that the humps are not relict structures. Instead, we argue that these large humps represent convergent evolution prompted by predation from a cyprinid piscivore. Colorado pikeminnow, Ptychocheilus lucius, top piscivore in the Colorado River system, is the only native fish capable of consuming large X. texanus and G. cypha, and it also is sympatric with them. However, lack of jaw teeth and a relatively small jaw gape limit the maximum prey size that P. lucius can consume. Based on gape size, about 55% of X. texanus and 71% of G. cypha could be consumed by even the largest P. lucius. However, vulnerability would increase to 73 and 83% respectively if these species did not have humps. Coevolution tends to favor predator defense mechanisms in prey most vulnerable to such a voracious predator. Development of a large nuchal hump provides a deep body that is difficult or impossible for P. lucius to ingest.

Conservation genetics in aquatic species: General approaches and case studies in fishes and springsnails of arid lands

Many aquatic species have become endangered because of the elimination and fragmentation of their habitat, human alteration of the environment, and introduced species. New applications and insights from genetic studies of endangered species are being used to formulate management plans to prevent extinction of endangered taxa. Here we discuss three applications of genetic techniques to the conservation of endangered aquatic species in the desert Southwest, USA. First, we discuss estimation of long-term effective population size in three endangered big-river fishes, bonytail chub (Gila elegans), humpback chub (G. cypha) and razorback suckers (Xyrauchen texanus), of the lower Colorado River. For all three species, the current census number is much smaller than the estimated effective population size in which these species evolved. Second, we discuss the determination of species, ESUs, and MUs in the endangered Sonoran topminnow. Molecular genetic data show that the Gila and Yaqui topminnows are different species. Experimental examination of pre-mating and post-mating reproductive isolation demonstrates that some reproductive isolation has already developed between these two species. Finally, springsnails (Pyrgulopsis) of the lower Colorado River basin exist only in remote and isolated springs. Examination of molecular variation generally supports the morphological designation of 16 different species, although a few species have several ESUs. There does not appear to be a correlation of genetic and geographic distance between species, suggesting that they are quite old. These genetic studies provide insight into the conservation of these rare aquatic species. Although mitigating ecological factors may be most significant in preventing extinction, genetic studies can provide the evolutionary context for endangered species and identify what units are most in need of conservation.

Standard Weight (Ws) Equations for Four Rare Desert Fishes

AbstractStandard weight (Ws) equations have been used extensively to examine body condition in sport fishes. However, development of these equations for nongame fishes has only recently been emphasized. We used the regression‐line‐percentile technique to develop standard weight equations for four rare desert fishes: flannelmouth sucker Catostomus latipinnis, razorback sucker Xyrauchen texanus, roundtail chub Gila robusta, and humpback chub G. cypha. The Ws equation for flannelmouth suckers of 100–690 mm total length (TL) was developed from 17 populations: log10 Ws = −5.180 + 3.068 log10TL. The Ws equation for razorback suckers of 110–885 mm TL was developed from 12 populations: log10 Ws = −4.886 + 2.985 log10TL. The Ws equation for roundtail chub of 100–525 mm TL was developed from 20 populations: log10 Ws = −5.065 + 3.015 log10TL. The Ws equation for humpback chub of 120– 495 mm TL was developed from 9 populations: log10 Ws = −5.278 + 3.096 log10TL. These equations meet criteria for acceptable standard weight indexes and can be used to calculate relative weight, an index of body condition.