Bonytail Study
Lower Colorado River, AZ

Bonytail (Gila elegans) is one of the “big river” fishes that once was common and widely distributed throughout the Colorado River basin of western North American including parts of Arizona, California, Colorado, Nevada, Utah, and Wyoming in the United States, and onto the Colorado River delta in Baja California Norte and Sonora, Mexico. Water development and the introduction and establishment of non-native species resulted in widespread extirpation and declines in the distribution and abundance of Bonytail and other native species beginning early in the 20th century. Bonytail was federally listed as endangered in 1980 (U.S. Fish & Wildlife Service [FWS] 1980), and was functionally extirpated from the wild by 1990. The only Bonytail remaining today are in hatcheries or released from hatcheries into the Colorado River. These stocked populations are so small that reliable abundance estimates cannot be developed.

Stocking of Bonytail throughout the upper and lower basins has thus far failed to establish new populations despite significant efforts over many years by a suite of federal and state entities. Stocking in the lower Colorado River, mostly in Lake Havasu and Lake Mohave, was initiated more than 25-years ago, but recaptures are few and long-term survival has been immeasurable. Most stocked Bonytail were small, less than 12 inches in total length, and were likely lost to predation, but long-term survival for larger fish stocked in recent years was also below measurable within months of stocking. Stocked Bonytail in the upper Colorado River basin share a similar fate with little to no long-term survival.

Marsh and Associates (M&A) developed and implemented a 36-month native fish research project that specifically targeted Bonytail post-stocking distribution, habitat use, and mortality. Six iterations of acoustic telemetry and five iterations of remote passive integrated transponder (PIT) scanning were conducted downstream of Davis Dam starting in spring of 2013 through winter of 2016. Six to 24 Bonytail were surgically implanted with acoustic or radio tags and released during spring and autumn in the Bill Williams River and Blankenship Bend, during spring in Regional Park Moabi, and winter in Laughlin Lagoon. Fish were tracked intensively by active and passive efforts for at least one month. Additionally, remote PIT scanners were deployed to monitor PIT-tagged Bonytail released during a stocking event at each study site. In February 2014 and 2015, M&A participated in the week-long, multi-agency Native Fish Routine Monitoring “Roundup” at Lake Havasu. During this event, fish sampling was conducted predominantly through trammel netting efforts.

Out of a total of 85 Bonytail implanted with a telemetry tag during the three-year study, 44 were determined mortalities, and 35 were permanently lost to the study (never contacted again). The majority of lost fish (83%) were last located within the study area and were not contacted by submersible ultrasonic receivers specifically placed in locations to detect fish leaving the study area. Loss of contact with these tags may have been due to removal from the system (for example, by an avian predator), tag failure, or the result of an undetected signal, even when a tag was present and functioning properly.

Most remote PIT scanning contacts (at least 55%) of released Bonytail occurred within the first two weeks post-stocking. Throughout all PIT scanning iterations, 27% of Bonytail from the corresponding stocking event were contacted during winter 2014 in Blankenship Bend, 5% during both autumn 2014 and winter 2014–15 scanning at the Bill Williams River, 68% in spring 2015 at Regional Park Moabi, and 23% during winter 2015–16 scanning at Laughlin Lagoon.

Trammel netting efforts during the multi-agency Native Fish Routine Monitoring “Roundup” resulted in the capture of eight Bonytail in February 2014, one of which was inside the digestive tract of a largemouth bass, and no Bonytail in February 2015 or 2016.

Predation was a major threat to Bonytail survival at all study areas, and the data suggest that piscivorous birds accounted for a large proportion of mortality in telemetry-tagged fish. Tags recovered on land and under roosting sites and observed capture events provided direct evidence of bird predation. Increased total length of Bonytail at release may benefit their survival. Few fish survived long enough after release to determine habitat selectivity, although fish were documented to utilize bulrush (Scirpus sp.). Data do not suggest that fish disperse far from release sites. Off-channel locations with an obstructed connection to the lower Colorado River, where the potential for fish to leave the study area is minimized, such as Regional Park Moabi and Laughlin Lagoon, are ideal sites to track survival of Bonytail. Optimal release locations may also include the availability of cover in the form of bulrush, structure (e.g., culverts and riprap), and turbidity (to reduce the impact of piscivorous birds).