Grande Silvery Minnow Hybognathus Amarus Research Articles

Hatchery supplementation increases potential spawning stock of Rio Grande Silvery Minnow after population bottlenecks

AbstractObjectiveSupplementation of imperiled wild fish stocks with captively raised fish is a commonly used conservation tool. Programs designed to maintain or improve fish populations through supplementation should be evaluated to determine whether they are meeting conservation objectives. The Rio Grande Silvery Minnow Hybognathus amarus is a small‐bodied, endangered minnow endemic to the Rio Grande basin of the southwestern United States. The wild population of Rio Grande Silvery Minnow has been supplemented with captive‐reared fish since 2002. Our objective was to determine whether supplementation measurably increases the number of spawning fish after years of population bottlenecks; this objective was simplified because nearly all hatchery‐released fish have been given identifying markings.MethodsWe leveraged a long‐term fish sampling data set (18 years) covering the species' contemporary range and a single‐season, high spatial coverage fish sampling data set to determine whether the number of potential spawners was increased by the addition of hatchery fish.ResultHatchery‐reared fish increased the catch rates of spawners by up to an order of magnitude in some years. We also observed that most hatchery‐reared fish were recaptured near the point of release. We were able to determine that supplementation with hatchery fish increased the abundance and distribution of Rio Grande Silvery Minnow and provided the desired demographic boost after severe population bottlenecks.ConclusionReleasing hatchery‐reared fish may be a useful tool for conservation of imperiled freshwater fishes. However, there is a need for spawning and rearing infrastructure that matches the life‐history of the species and sufficient postrelease monitoring to evaluate the effectiveness of hatchery supplementation. Use of hatchery‐reared fish has been beneficial for improving population resiliency of Rio Grande Silvery Minnow following recruitment bottlenecks; however, natural recruitment can greatly exceed hatchery supplementation and conditions promoting natural recruitment should be restored.

Fish Rescue during Streamflow Intermittency May Not Be Effective for Conservation of Rio Grande Silvery Minnow

Streamflow intermittency can reshape fish assemblages and present challenges to recovery of imperiled species. During streamflow intermittency, fish can be subjected to a variety of stressors, including exposure to crowding, high water temperatures, and low dissolved oxygen, resulting in sublethal effects or mortality. Rescue of fishes is often used as a conservation tool to mitigate the negative impacts of streamflow intermittency. The effectiveness of such actions is rarely evaluated. Here, we use multi-year water quality data collected from isolated pools during rescue of Rio Grande silvery minnow Hybognathus amarus, an endangered minnow. We examined seasonal and diel water quality patterns to determine if fishes are exposed to sublethal and critical water temperatures or dissolved oxygen concentrations during streamflow intermittency. Further, we determined survival of rescued Rio Grande silvery minnow for 3–5 weeks post-rescue. We found that isolated pool temperatures were much warmer (>40 °C in some pools) compared to upstream perennial flows, and had larger diel fluctuations, >10 °C compared to ~5 °C, and many pools had critically low dissolved oxygen concentrations. Survival of fish rescued from isolated pools during warmer months was <10%. Reactive conservation actions such as fish rescue are often costly, and in the case of Rio Grande silvery minnow, likely ineffective. Effective conservation of fishes threatened by streamflow intermittency should focus on restoring natural flow regimes that restore the natural processes under which fishes evolved.

Relationship of Effective Size to Hatchery Supplementation and Habitat Connectivity in a Simulated Population of Rio Grande Silvery Minnow

Simultaneous management actions often are used to improve the status of imperiled species, yet the effects of these actions can be difficult to determine. The endangered Rio Grande Silvery Minnow Hybognathus amarus is a short‐lived fish with a dispersive life history and thus requires—but does not occupy—unfragmented habitat for recruitment and survival. We used Rio Grande Silvery Minnow as a model system for an individual‐based simulation study to evaluate responses of genetic effective size to hatchery supplementation and fish passage in a managed population. Simulations were designed to test effects of fish passage on the relationship between estimates of inbreeding (NeI) and variance (NeV) effective sizes, which differ consistently (NeI >> NeV) in long‐term genetic‐monitoring data from hatchery‐supplemented Rio Grande Silvery Minnow populations. Values of different effective population size measures should be identical in a demographically stable and connected (admixed) population but can differ substantially under nonequilibrium conditions. When a barrier prevented upstream dispersal, NeI was associated negatively with rate of downstream dispersal and positively with supplementation rate, whereas NeV was associated negatively with both. This mirrored observations from Rio Grande Silvery Minnow. Individual effects of dispersal and supplementation, however, were difficult to discern due to significant interaction between these factors. When connectivity was restored, NeI and NeV depended on supplementation rate, with positive association between NeI and supplementation rate versus a negative association for NeV and interaction terms, and effects of dispersal were nonsignificant. Although fish passage did not alter the difference between NeI and NeV, our study suggests that for Rio Grande Silvery Minnow, and potentially other intensively managed species in regulated rivers, fish passage may help to distinguish effects of management actions, such as supplementation, from effects of other demographic influences. More generally, explicit analytical consideration of differences in effective population size estimates can provide important details of genetic responses to management.

Reproductive Potential of Captive Rio Grande Silvery Minnow

AbstractCaptive propagation and augmentation of the Rio Grande Silvery Minnow Hybognathus amarus is necessary for continued persistence of the species until habitat can be restored. Augmentation occurs using captive fish within the first year (age 0) through age 2; however, older year‐classes (ages 3–4) have been released into the wild. We quantified fecundity and compared egg quality across four reproductive age‐classes (1–4) of hatchery‐reared Rio Grande Silvery Minnow. Batch fecundity (total number of spawned eggs) ranged from 2,029 eggs in age‐1 fish to 10,588 eggs in age‐4 fish. Standing stock of vitellogenic (i.e., yolked) oocytes remaining in ovaries after spawning increased from 988 oocytes at age 1 to 4,924 oocytes at age 4. Total fecundity (i.e., batch fecundity plus standing stock of yolked oocytes) increased from 3,017 eggs and yolked oocytes in age‐1 fish to 15,522 eggs and yolked oocytes in age‐4 fish. Of note, batch fecundity ranged from 57% to 68% across the four age‐classes indicating that over half of the total fecundity occurs in the first spawn. Average percent fertilization of eggs (63–82%) was not detectably different across the four age‐classes, while average egg diameter (4.1–5.5 mm) increased as age increased. Fecundity increased across all four reproductive age‐classes of Rio Grande Silvery Minnow, with the larger portion of total fecundity occurring in the first spawn across all age‐classes. The timing of spawning with optimal conditions in the Rio Grande is critical for survival and ultimately recruitment into the wild fish population.

Age and Growth of the Rio Grande Silvery Minnow, an Endangered, Short‐Lived Cyprinid of the North American Southwest

AbstractThis study provides the first comprehensive evaluation of age and age–length relationships from an extant population of the federally endangered Rio Grande Silvery Minnow Hybognathus amarus. The surveyed population was from three fragmented reaches within about 290 km of the middle Rio Grande in New Mexico, the last protected population in the historical range of the species. A total of 158 Rio Grande Silvery Minnows were collected in autumn 2009 (n = 83) and spring 2010 (n = 75), with sampling designed to fill six 10‐mm size‐class bins that represent the size range characteristic of the juvenile and adult population. Age was then determined directly for each specimen independently by two investigators, each using otoliths and scales; growth rates were estimated via back‐calculation of length at age from otoliths. The results confirm that individuals in the contemporary population are short‐lived, with very few surviving to their second or third summers. For historical comparison, the ages of a small collection of Rio Grande Silvery Minnows taken in August 1874 were evaluated using scales. The specimens in this historical collection ranged from age 0 to age 2, similar to the age range for the autumn 2009 collection and incongruent with a previously published report of five age‐classes (ages 1–5) for 13 of these specimens. Our results corroborate the age‐structure derived from temporal tracking of length frequency histograms generated under the Rio Grande Silvery Minnow Population Monitoring Project and validate current management of the species as short‐lived. Our age range and age–length relationships are also similar to those of other well‐studied Hybognathus spp., and the results from our study may aid management of these closely related, short‐lived pelagophil minnows. While additional research is needed to clarify the areas of age–length overlap, the crucial findings of this study should facilitate implementation of an appropriate management policy for this short‐lived, federally endangered cyprinid.

Behavioral observations of the endangered Rio Grande silvery minnow in a conservation aquaculture facility

Abstract A major reason why conservation aquaculture is needed to improve the success of aquaculture-assisted fisheries is that traditional production aquaculture produces fish with mal-adaptive behaviors. These behaviors can be produced via domestication and culture techniques, and preventing these mal-adaptive behaviors requires integrating improvements in genetic management and culture protocols. The genetic protocols needed to minimize hatchery-induced genetic changes have received considerable attention, but changing the way fish are raised has received less effort. Conservation aquaculture cultures fish in environments that resemble their native habitats so that when stocked, they behave like wild fish rather than hatchery fish. A purpose built-conservation aquaculture facility can also be used to learn about a species’ behavior and how it reacts to changes in the environment, something which can be difficult or expensive to study in the wild. These observations can then be used to help direct both propagation and recovery management. This paper provides the rationale for why genetic management, culture systems, and management practices need to be altered to produce fish that are behaviorally similar to wild fish for aquaculture-assisted fisheries programs. It then provides a description of some of the behaviors of the endangered Rio Grande silvery minnow Hybognathus amarus that were observed at the Los Lunas Silvery Minnow Refugium, a purpose-built conservation aquaculture facility, and explains how some of these behaviors can be used in culture and recovery management. Behaviors described are: schooling; predator avoidance; feeding behavior; use of vegetation for cover and predator avoidance; habitat use by bottom substrate; location in the water column; upstream movement via a fish ladder; movement upstream in a high-velocity channel; response to changes in water level; spawning behavior; seine avoidance; and Kaah-chee-nyee Srkaash, a behavior described for the first time.

Reduction in Spring Flow Threatens Rio Grande Silvery Minnow: Trends in Abundance during River Intermittency

AbstractHabitat fragmentation and changes in flow regime can structure fish assemblages, resulting in extirpations or invasions. A guild of freshwater stream fishes that spawn semi‐buoyant, nonadhesive eggs directly in the water column are particularly susceptible to extirpation in fragmented streams. The pelagic broadcast‐spawning Rio Grande Silvery Minnow Hybognathus amarus was listed as endangered in 1994 and has been intensely managed since. I used mean May flows and the number of times the channel dried within a year to predict numbers of Rio Grande Silvery Minnow captured in isolated pools in the Middle Rio Grande, New Mexico. Adult Rio Grande Silvery Minnow numbers increased as previous year's mean May discharge increased, and generally decreased with each subsequent drying event. Similarly, numbers of young‐of‐year Rio Grande Silvery Minnow increased with increasing mean May discharge in the current year. However, young‐of‐year minnow were either very abundant or nearly absent in isolated pools, depending on mean May discharge. Overall trends show a strong decrease in total yearly numbers of adult and young‐of‐year Rio Grande Silvery Minnow collected in isolated pools from 2009 to 2014, coupled with an increase in the proportion of hatchery‐reared adults. Overall numbers fell from >1,000/km in 2009 to <15/km in 2014. Hatchery‐reared fish increased from 0% in 2009 collections to 90% in 2014 collections. Managers should focus recovery efforts on providing spring flows that improve recruitment, and avoiding consecutive years of low spring flow because any population recovery will likely be negated by three consecutive years of recruitment failure. A self‐sustaining population of Rio Grande Silvery Minnow is unlikely without management options that mitigate the effects of multiyear droughts.Received August 3, 2015; accepted February 12, 2016 Published online June 21, 2016

Comparison of Fish Communities at Random and Nonrandom Locations in a Sand‐Bed River

AbstractSurveys based on nonrandom site selection, or convenience samples, are often a necessary part of large‐scale monitoring programs to help minimize costs. The reliability of convenience samples to inform managers about distributions or population status of imperiled species is questionable, however, because the samples may not be representative of the whole population. We compared fish community data from 20 nonrandom, long‐term monitoring sites for Rio Grande Silvery Minnow Hybognathus amarus with those from 20 randomly chosen samples collected during two surveys (one in summer, one in autumn) in the Rio Grande, New Mexico. We compared the species richness, community composition, and the catch per unit effort (CPUE). Fish species compositions, which were similar between both sets of summer and autumn surveys, were nearly identical in the autumn surveys. Similarly, we found consistent Rio Grande Silvery Minnow CPUE between surveys; summer random surveys estimated 0.32 fish/100 m2 sampled, whereas summer nonrandom surveys estimated 0.37 fish/100 m2 sampled. In autumn, both surveys showed a marked decline in Rio Grande Silvery Minnow; random surveys found 0.08 fish/100 m2 sampled (95% confidence interval 0.04–0.18), whereas the nonrandom surveys failed to collect any Rio Grande Silvery Minnow. Both surveys showed a reduction in species richness between summer and autumn with a corresponding increase in dominance by Red Shiner Cyprinella lutrensis and a decline in Rio Grande Silvery Minnow. We failed to find any meaningful differences in either fish community or Rio Grande Silvery Minnow CPUE between random and nonrandom sites, suggesting that the long‐term, nonrandom locations currently used to monitor the Rio Grande Silvery Minnow population are representative of the middle Rio Grande. We believe our results are applicable to many monitoring programs in systems with a homogeneous distribution of mesohabitats; nonetheless, we recommend that managers assess potential bias in monitoring programs based on convenience samples.

Effect of High Stocking Rates on Growth and Survival of the Endangered Rio Grande Silvery Minnow Hybognathus amarus

The endangered Rio Grande silvery minnow Hybognathus amarus was cultured in 2.44-m-diameter outdoor tanks in a 131-day yield trial to assess growth, survival, and percentage of taggable-sized fish (<TEX>${\geq}35mm$</TEX> total length) when stocked at 500/tank (1.07 million/ha), 1,000/tank (2.14 million/ha), and 1,500/tank (3.21 million/ha). At harvest, fish averaged 45.6 mm and 0.94 g in the 500/tank treatment, 42.6 mm and 0.74 g in the 1,000/tank treatment, and 38.4 mm and 0.55 g in the 1,500/tank treatment; the differences were significant (P = 0.05). Survival in the three treatments was 70%, 64%, and 52%, respectively, but the differences were not significant. Percent taggable-sized fish was 86%, 89%, and 65%, respectively, but the differences were not significant. Yield was 672.5 kg/ha, 1,026.6 kg/ha, and 887.8 kg/ha, respectively; yield in the 1,000/tank treatment was significantly greater than that in the 500/tank treatment, but was not significantly greater than that of the 1,500/tank treatment. This facility is a conservation facility and a major goal is to raise fish without formulated feed. Fertilization produced good growth for the first month, but little growth occurred during the second month so supplemental feed had to be used for the final 60 d of the yield trial.

Observations of Hatchery‐Reared Rio Grande Silvery Minnow Using a Fishway

AbstractWe designed a study to determine if Rio Grande silvery minnow Hybognathus amarus, an endangered species, would use an instream, rock channel fishway on the Rio Grande near Albuquerque, New Mexico. We implanted 6,657 Rio Grande silvery minnow with passive integrated transponders and used a passive scanning station with two antennas located within the fishway to document movements from seven release locations, up to 19.7 km upstream and 13.5 km downstream of the diversion dam. Between 11 March and 21 August 2011, 243 individuals were detected (3.7% of all fish released), with 74.1% fish being detected from upstream release sites and 25.9% from downstream release sites. We determined 14 Rio Grande silvery minnow successfully ascended the fishway, and an additional 20 fish from downstream release locations were detected on the upstream antenna only, inferring passage. We conclude Rio Grande silvery minnow can use appropriately constructed fishways.Received October 17, 2011; accepted March 21, 2012

Using Fyke‐Net Capture Data to Assess Daily Trends in Abundance of Spawning Rio Grande Silvery Minnow

AbstractAssessments of the assumptions underlying catch‐per‐unit‐effort (CPUE) data are necessary to determine whether catch data provide a reliable relative abundance metric. Rio Grande silvery minnow Hybognathus amarus were collected with fyke nets from seasonally flooded habitats of the Rio Grande in New Mexico to determine habitat use by the species during spring runoff in May and June of 2008 and 2009. The CPUE was calculated as fish/h to assess relative abundance among sites and sampling dates due to differences between the durations of the samples in 2008 and 2009. The assumption that the number of fish captured with fyke nets is proportional to effort was assessed by using a multiple‐regression model. Catch per unit effort was also compared over the course of a day to determine whether diel differences in catch rate exist. Catch of Rio Grande silvery minnow increased with fyke‐net soak time during 2008 and 2009; however, the proportion of the variability explained by the model was greater for 2009 than for 2008. The CPUE did not differ among 24‐h collections at high density (mean = 8.24 fish/h) or low density (mean = 0.12 fish/h) sites. These results indicate that fyke‐net catches of Rio Grande silvery minnow are proportional to soak time, and calculation of CPUE as fish/h may be used to compare the relative abundance of the species among and between fyke‐net samples collected from seasonally flooded habitats.Received June 20, 2011; accepted February 1, 2012

Rapid species replacements between fishes of the North American plains: a case history from the Pecos River

A non-native population of plains minnow Hybognathus placitus in the Pecos River, New Mexico, USA, replaced the endemic, ecologically similar Rio Grande silvery minnow Hybognathus amarus in less than 10 years. Competitive exclusion is hypothesized as a mechanism of replacement. The following evidence was examined for potential support: (1) the historical population trend of H. amarus versus H. placitus; (2) relative suitability of the modified flow regime for H. amarus versus H. placitus; (3) potential for habitat overlap; and (4) spawning periodicity and body length in the non-native population of H. placitus. Historical data indicate H. amarus did not decline until H. placitus was present, after which it disappeared rapidly and H. placitus proliferated. The natural flow regime of the Pecos River was changed via base-flow reductions and capture of springsnowmelt runoff, making it similar to flow regimes associated with likely source populations of non-native H. placitus. Spring runoff is an important spawning cue for H. amarus, but not H. placitus and H. placitus appears to be naturally more tolerant of diminished streamflow. Extant H. placitus were associated with a relatively pristine river reach that was most likely the stronghold for the replaced H. amarus population. Given high ecological similarity, habitat overlap was likely high during the period of replacement. Hybognathus placitus in the Pecos River had a protracted spawning season (April through September), whereas extant H. amarus in the Rio Grande had a short spawning season (a few days in May or June during snowmelt runoff). Most H. amarus die after their first spawning season and few exceed 60 mm standard length (SL), whereas quite a few H. placitus survive at least until their second spawning season and exceed 60 mm SL. Co-occurrence of multiple spawning cohorts may stabilize annual reproductive output of H. placitus and larger individuals may be more fecund and produce larger eggs that survive better. Spawning flexibility, greater size, and higher environmental tolerance likely gave H. placitus a reproductive and survival advantage over H. amarus, consistent with a deterministic pattern of rapid species replacements in the plains, in which tolerant, competitive species from the Red River drainage or Gulf of Mexico coast rapidly replace more sensitive, endemic congeners in disturbed, remnant habitats.

Swimming Performance and Fishway Model Passage Success of Rio Grande Silvery Minnow

AbstractWe used a swim chamber, flume, and large‐scale fishway models to assess the swimming performance, behavior, and passage success of endangered Rio Grande silvery minnow Hybognathus amarus. Field‐captured silvery minnow (53–88 mm total length) swam 114–118 cm/s (i.e., up to 20.9 body lengths/s) in a swim chamber in water temperatures of 15, 19, and 23°C. The relationship between time to fatigue and water velocity showed that endurance declined sharply at velocities above 60 cm/s, a threshold that is consistent with critical swimming speed estimates and may represent a transition from aerobic to anaerobic metabolism. Water temperature and fish length were positively correlated with swimming performance. At water velocities of 60 cm/s and less, silvery minnow routinely swam the equivalent of 50 km (125 km maximum) in a swim chamber in less than 72 h. The proportions of silvery minnow that successfully ascended a flume over sand, gravel, or cobble substrate declined as water velocity increased to 53 cm/s. Passage times increased at higher water velocities and at a faster rate over sand substrate because fish were stationary for longer periods over the lower‐velocity boundary layers created by gravel and cobble at all velocities. Dual‐vertical‐slot fishway passage was 52% in a less turbulent flow of 78 cm/s; passage was 8% at a faster, more turbulent flow of 87 cm/s. Conversely, 75% of silvery minnow ascended a rock channel fishway with a holding pool present and a 1% hydraulic gradient in low (58‐cm/s) and high (83‐cm/s) mean flow velocities. Differences in willingness to swim, longer test duration, and the mosaic of water velocities created by the bed roughness elements may explain the higher silvery minnow passage success in the rock channel. Predictive swimming fatigue relationships, together with fish length and water temperature, may guide decisions regarding fishway lengths and velocities so as to permit passage of Rio Grande silvery minnow.

Diet Optimization of Juvenile Rio Grande Silvery Minnow

AbstractCaptive propagation of the federally endangered Rio Grande silvery minnow Hybognathus amarus has been identified as essential for the recovery of the species. A starter diet was initially developed for this cyprinid to support rapid growth through the larval stage. The diet, however, was too costly to support the grower phase of the species. A series of trials was conducted to determine whether a grower diet could be developed that would be cost effective while maintaining fish performance. Quadruplicate lots of Rio Grande silvery minnow (approximately 90 d posthatch, 0.39–0.42 g) were fed a reference diet or one of five experimental diets until just before gonadal maturation at two facilities in New Mexico (New Mexico State University [NMSU] and Dexter National Fish Hatchery and Technology Center [Dexter]). The diets varied in percent composition of ingredients to account for a range in feed costs. Cyclop‐eeze (Argent, Redmond, Washington), krill, liver, and fish meals were present in the reference diet and sequentially altered in the remaining test diets to maintain high protein levels in the diets (57.9–64.9% dry matter). Total weight gains at the end of the 16‐week trials were lower than expected (140–200%), with no differences among diets at NMSU. Although total weight gain was less at Dexter (105–150%), there were differences among diets. Specific growth rate was greater at both facilities in fish fed the reference diet and a diet lowest in fish meal (1.82% per day). The highest growth rate occurred across all diets within the first 30 d of both feed trials and then diminished by the end of the trials. There were no detectable differences in critical swimming speed or fat scores in fish fed diets with and without expensive feed ingredients or increasing fish meal. The most consistent growth was observed when fish were fed a diet low in fish meal, and our results clearly identified a less‐expensive grower formulation for the production of this imperiled cyprinid.

Physiological Stress Responses of Rio Grande Silvery Minnow: Effects of Individual and Multiple Physical Stressors of Handling, Confinement, and Transport

AbstractIn an attempt to reestablish populations of Rio Grande silvery minnow Hybognathus amarus within its former range, initiatives were set forth to propagate the species in hatcheries, augment wild populations through repatriation, and rescue the species from isolated and receding pools. Physical stressors associated with these recovery efforts, however, result in stress that increases the vulnerability of the minnow to opportunistic pathogens and predation, possibly decreasing poststocking survival. We assessed the physiological stress response to standard management practices by characterizing the changes in plasma cortisol, glucose, and osmolality. When subjected to the individual stressors of 30 s of handling, 3 h of confinement (density, 100 kg/m3), or 3 h of transport (density, 40 kg/m3), moderate changes in plasma glucose and plasma osmolality were observed during recovery whereas changes in plasma cortisol were not detectable. When fish were subjected to consecutive stressors (30 s of handling only, 30 s of handling plus 3 h of confinement, or 30 s handling plus 3 h of confinement and 3 h of transport), plasma glucose exhibited a cumulative increase that was not observed for plasma cortisol. This increase in plasma glucose was observed within 3 h poststress when fish were subjected to a single stressor (22 mg/dL), two consecutive stressors (28 mg/dL), and three consecutive stressors (63 mg/dL). Plasma osmolality decreased from 282 to 269 mosmol/kg (compared with unstressed levels of 279 mosmol/kg) when minnows were subjected to three consecutive stressors, indicating moderate osmoregulatory dysfunction. Plasma glucose and osmolality returned to unstressed levels within 48 h, indicating that the species can regain its physiological homeostasis within a relatively short time as long as the stressors are reasonable in duration and intensity.

Comparison of Two Methods for Implanting Passive Integrated Transponders in Rio Grande Silvery Minnow

AbstractUniquely tagged animals provide an opportunity to study changes in population demographics and movement. Passive integrated transponder (PIT) tags are used to monitor growth, movement, and survival of fishes. Laboratory studies of tag retention and mortality in small‐bodied, warmwater cyprinids after insertion of PIT tags are rare. Mortality induced by two PIT tag implantation methods was compared, and tag retention after implantation was assessed. Relations between standard length and tag retention and between standard length and survival in Rio Grande silvery minnow Hybognathus amarus were examined. Fish were randomly selected and PIT‐tagged with 12.5‐mm tags via surgical incision or needle injection and were held in laboratory aquaria for 32 d. Mean (±SE) survival on day 32 was 99 ± 1% for control fish, 87 ± 6% for fish that received tags by incision, and 50 ± 5% for fish that received tags by injection. We also tagged 280 fish by incision and held them in aquaria for 49 d. On day 49, survival and tag retention were both 90%. Longer fish had higher survival, but tag retention was not related to fish length. Rio Grande silvery minnow can be reliably tagged with PIT tags if the fish are greater than 60 mm standard length. Treatment with antibiotics and holding of fish in captivity for a minimum of 6 d before release are recommended.

First Record of Bothriocephalus acheilognathi in the Rio Grande with Comparative Analysis of ITS2 and V4‐18S rRNA Gene Sequences

Bothriocephalus acheilognathi is an introduced tapeworm in North America often reported as a serious ecological threat to native fishes. In this paper, we report the first record of B. acheilognathi in the Big Bend region of the Rio Grande in Texas (known as the Río Bravo del Norte in Mexico). Identification of B. acheilognathi was confirmed by morphologic and genetic techniques (sequences of ITS2 and V4-18S rRNA genes). Its prevalence was 27% and its intensity ranged from 1 to 5 individuals in a January 2006 collection of 115 red shiners Cyprinella lutrensis. In addition, it was found in the Tamaulipas shiner Notropis braytoni, a Rio Grande endemic and a new host record. The occurrence of B. acheilognathi might have negative ecological impacts on endemic fishes in the Rio Grande. Several of the fishes that could serve as definitive hosts are of conservation concern. Its occurrence also might affect the success of reintroducing the Rio Grande silvery minnow Hybognathus amarus, which is federally listed as endangered, in this portion of the Rio Grande.