David L. Ward

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

Abstract The 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 abu...

Differences in Swimming Ability and Behavior in Response to High Water Velocities among Native and Nonnative Fishes

We conducted swimming performance tests on native and nonnative fishes commonly found in Arizona streams to evaluate the extent of differences in swimming ability among species. Fishes with similar mean lengths were subjected to stepwise increases in water velocity in a laboratory swim tunnel until fish could no longer maintain position. Nonnative fathead minnows Pimephales promelas and red shiners Cyprinellalutrensis exhibited swimming abilities similar to native longfin dace Agosia chrysogaster, speckled dace Rhinichthys osculus and spikedace Meda fulgida. Nonnative mosquitofish Gambusia affinis exhibited swimming ability similar to native Gila topminnows Poeciliopsis occidentalis. Desert suckers Catostomus clarki, bluehead suckers Catostomus discobolus and speckled dace exhibited behavioral responses to high water velocities that may confer energetic advantages in swift water. Differences in swimming ability do not appear to adequately explain the disproportionate removal of nonnative fishes via flooding. Behavioral responses to high flows are more likely the mechanism that allows native fish to persist in streams during flood events.

Effects of Intraspecific Density and Environmental Variables on Electrofishing Catchability of Brown and Rainbow Trout in the Colorado River

Abstract We investigated electrofishing catchability (q) for brown trout Salmo trutta and rainbow trout Oncorhynchus mykiss in the Colorado River, Grand Canyon National Park, Arizona, over a range of fish densities, water temperatures, turbidities, conductivities, shoreline types, and seasons. The covariance of q with rainbow trout density strongly resembled random distributions, thereby suggesting no relationship between q and rainbow trout density. The catchability of rainbow trout was greater in turbid water (≥480 nephelometric turbidity units (NTU)) than in clear water (≤10 NTU), although lower water temperature may have contributed to this effect. The catchability of rainbow trout was greatest over sand–silt shorelines. The catchability of brown trout increased sharply to levels above those predicted from random chance up to about 0.025 fish/m2 and then assumed an asymptotic or declining relationship with intraspecific fish density. In contrast to the situation with rainbow trout, the catchability of...

Effects of Holding Environment and Exercise Conditioning on Swimming Performance of Southwestern Native Fishes

Abstract Rare native fish are often reared in ponds or tanks at hatcheries for later stocking into streams with depleted populations. Fish reared in standing water may experience increased stress, downstream displacement, or high predation mortality when released into lotic environments. We compared the swimming performance of captive fish held in standing water, captive fish exercised in flowing water, and wild fish captured from a stream to evaluate the effects of exercise conditioning and holding environment on swimming performance. The swimming performance of flannelmouth sucker Catostomus latipinnis, bonytail Gila elegans, razorback sucker Xyrauchen texanus, and spikedace Meda fulgida held in standing water increased by 10, 15, 26, and 40%, respectively, after exercise conditioning in flowing water (10–100 cm/s) for as little as 10 d. Exercising fish reared in standing water may improve swimming performance and increase survival.

Removal and quantification of asian tapeworm from bonytail chub using praziquantel

Abstract The 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...

Effects of Repeated Hoopnetting and Handling on Bonytail Chub

ABSTRACT We evaluated the effects of repeated hoopnet sampling and subsequent handling on the endangered bonytail chub (Gila elegans) to determine if these sampling techniques cause increased mortality or reduce fish growth. A total of 327 bonytail chub was tagged with passive integrated transponder (PIT) tags and placed in a 0.04 ha pond on July 2003. These fish were sampled with hoop nets during four, three-day sampling events, two in fall (September-October 2003) and two in spring (June 2004). Fish were measured and weighed with the time required to complete all steps for processing an individual fish being recorded. Individual fish were handled from one to eight times during the study, with >95% recaptured five or fewer times. Fish that were recaptured multiple times grew less in length (P<0.001) and weight (P<0.001) than fish that were not recaptured. Fish recaptured up to five times grew only 12.8% of their initial weight compared to fish not recaptured which grew 29.7% of their initial weight.

Effects of turbidity on predation vulnerability of juvenile humpback chub to rainbow and brown trout

Abstract Predation on juvenile native fish by introduced rainbow trout Oncorhynchus mykiss and brown trout Salmo trutta is considered a significant threat to the persistence of endangered humpback chub Gila cypha in the Colorado River in Grand Canyon. Diet studies of rainbow trout and brown trout in Glen and Grand canyons indicate that these species eat native fish, but impacts are difficult to assess because predation vulnerability is highly variable depending on the physical conditions under which the predation interactions take place. We conducted laboratory experiments to evaluate how short-term predation vulnerability of juvenile humpback chub changes in response to changes in turbidity. In overnight laboratory trials, we exposed hatchery-reared juvenile humpback chub and bonytail Gila elegans (a surrogate for humpback chub) to adult rainbow trout and brown trout at turbidities ranging from 0 to 1,000 formazin nephlometric units. We found that turbidity as low as 25 formazin nephlometric units signif...

Interactions between Desert Pupfish and Gila Topminnow Can Affect Reintroduction Success

Abstract In fisheries, recovery actions for species of concern sometimes involve reintroductions of multiple species. As an example, desert pupfish Cyprinodon macularius and Gila topminnow Poeciliopsis occidentalis historically had sympatric distributions and inhabited similar environments, and they are often reintroduced into the same waters. Success of these stockings has varied greatly, and a hypothesis developed among conservationists that desert pupfish populations were more likely to establish and persist if they were stocked first and allowed to establish before Gila topminnow were stocked. We examined historical records of desert pupfish reintroductions and conducted laboratory experiments to evaluate this hypothesis. For the reintroductions, we evaluated 29 ponds where four general stocking strategies (treatments) were implemented: (1) only desert pupfish were stocked, (2) desert pupfish were stocked first and allowed to establish and then Gila topminnow were stocked, (3) both desert pupfish and ...

A Variable-Speed Swim Tunnel for Testing the Swimming Ability of Age-0 Fish

Abstract Laboratory swim tunnels are a valuable tool for studying fish physiology and the responses of fish to controlled environmental conditions. We combined aspects of large and small swim tunnels to construct an apparatus that can confine age-0 fish to a specific area while producing a wide range of velocities. We used low-cost plumbing supplies and a centrifugal pump to create a swim tunnel capable of producing velocities from 1 to 66 cm/s. A DC electric barrier and screened knife gate valves prevented fish from leaving the test section. A flowmeter incorporated into the swim tunnel measured water velocity within the apparatus.

Selective Removal of Nonnative Fishes Using Supaverm®: Toxicity Screening for a Candidate Species-Specific Piscicide

ABSTRACT I evaluated the chemical Supaverm® for potential use as a selective piscicide by testing its toxicity to native and nonnative fishes commonly found in Arizona. In repeated laboratory tests, Supaverm® dosed at 13 μg/L killed nonnative fathead minnow (Pimephales promelas), red shiner (Cyprinella lutrensis), yellow bullhead (Ameiurus natalis), largemouth bass (Micropterus salmoides) and smallmouth bass (Micropterus dolomieu) without harming native longfin dace (Agosia chrysogaster), Gila chub (Gila intermedia), or Gila topminnow (Poeciliopsis occidentalis). The apparent selectivity of this chemical makes it a good candidate for further evaluation as a species-specific fish toxin.