The bonytail (Gila elegans), federally listed as endangered, is considered functionally extirpated from its historical range, and its presence in the Colorado River Basin now relies entirely on stocking programs. Lake Havasu, Arizona, California, and Nevada, is one of the few release locations for hatchery fish and sites where stocked individuals are occasionally captured. Information regarding the basic ecology of this species is limited to past field observations and a small number of telemetry projects in the basin. The result is a general lack of knowledge regarding how to better inform managers of the post-stocking fate and habitat use of hatchery-reared bonytail and of practical measures to increase the survival of stocked fish.
A multi-year research project was implemented on Lake Havasu in which the post-stocking distribution, habitat use, and mortality of bonytail were documented. Six iterations of an acoustic telemetry study and five iterations of remote passive integrated transponder (PIT) scanning within Lake Havasu from spring 2013 to winter 2016 were completed. 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 and during spring in Regional Park Moabi and winter in Laughlin Lagoon. Fish were tracked intensively by active and passive efforts for at least 1 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, Marsh & Associates, LLC, 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 telemetry-tagged bonytail throughout the entirety of the 3-year study, 44 were determined mortalities, and 35 were permanently lost to the study (never contacted again). The majority of these 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 a result of the inability to detect a signal even though the tag was present and functioning properly.
Most remote PIT scanning contacts (at least 55%) occurred within the first 2 weeks post-stocking. Over the course of 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 constricted 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 through both telemetry and remote PIT scanning efforts. Optimal release locations may also include availability of cover in the form of bulrush, structure (e.g., culverts and riprap), and turbidity (to reduce the impact of piscivorous birds).