Perseverance of bonytail Gila elegans in the Colorado River basin relies entirely on stocking programs and Lake Havasu is one of few locations where individuals are occasionally captured. Most information regarding the basic ecology of this critically endangered species is limited to past field observation acquired from the now extirpated wild population and to studies conducted on isolated stocks reared in hatchery or backwater ponds. Results from an earlier telemetry study in Lake Havasu were inconclusive due to possible transmitter loss, premature mortality, and loss of contact with tagged fish, which prevented conclusions from being drawn about the habitat and biology of adult bonytail in that system. As a result, little information exists that could better inform managers of the post-stocking survival and habitat use of hatchery-reared bonytail.
We completed the second of a three-year comprehensive study using acoustic telemetry to describe and characterize inhabitance and dispersal of hatchery-reared bonytail by monitoring their movement and survival after release into Lake Havasu. Results from our initial April 2010 investigation established up to 95% of bonytail implanted with three-month acoustic transmitters and stocked at Bill Williams River National Wildlife Refuge (BWRNWR) were still being actively tracked at the conclusion of that study. Those fish predominantly utilized the low water clarity habitat found in and near BWRNWR. We also performed a transmitter retention study at Dexter National Fish Hatchery & Technology Center (DNFH&TC) that demonstrated bonytail implanted with three and six-month acoustic transmitters remained healthy and active.
Based on results from these first two investigations, 20 bonytail were implanted with six-month acoustic transmitters during December 2010 and released with 2,060 fish at BWRNWR to further document survival and habitat use. Two bonytail captured in BWRNWR during February 2011 netting efforts (see below) also were implanted with previously recovered acoustic tags, then re-released at their site of capture. Bonytail were tracked actively by boat and passively with a fixed array of submersible ultrasonic receivers through May 2011. Half of bonytail released in December 2010 were active at the end of three months, which was considerably fewer than the number of active fish remaining at the end of the April 2010 study. However, by the end of six months, up to 40% of the bonytail stocked in December 2010 and both fish released in February 2011 still remained active. All immobile tags were recovered using SCUBA within 5 km of the stocking site. No fish remains were located near the sites of recovery. Dispersal and habitat use largely resembled patterns displayed by tagged bonytail stocked during April 2010. Over the course of the study, 20 of 22 tagged bonytail (91%) dispersed between 2.6 km upriver into the Bill Williams River and 4.5 km uplake (toward Lake Havasu City) and two individuals dispersed up to 24 km uplake of the stocking site. Bonytail spent significantly more time in BWRNWR than elsewhere in Lake Havasu, where a majority (97%) of all contacts were made over the course of the study.
During February 2011, Marsh & Associates (M&A) participated in the multi-agency Native Fish Roundup on Lake Havasu. Nine fixed reaches of the reservoir were sampled using trammel nets and electrofishing, of which M&A assisted on three different reaches (trammel netting only). The collective group netting efforts of all participants involved resulted in the capture of 68 bonytail (67 of which were captured within BWRNWR). Two individuals were recaptures from the April and December 2010 telemetry studies.
Inhabitance data from the April and December 2010 telemetry studies and capture data from 2011 Lake Havasu Native Fish Roundup indicated bonytail showed strong preference for habitats found in BWRNWR. To test whether dispersal and survival were related to stocking location or habitat availability, a dual stocking was implemented simultaneously at BWRNWR and Cattail Cove State Park. During November 2011, 15 acoustic-tagged bonytail (ten implanted with six-month battery life transmitters, and five with 45-day battery life depth-sensing transmitters) were released with about 2,000 PIT-tagged bonytail at each location. Remote PIT-scanning antennas deployed throughout BWRNWR for a two-day period after stocking contacted 51 unique PIT-tagged bonytail, 50 of which contained a stocking history in the Lower Colorado River Native Fishes Database. Acoustic tagged fish were tracked weekly and associated habitat parameters (including turbidity) were measured through the end of December 2011. Preliminary active-tracking data indicate depth-tagged bonytail were contacted on average at 80% of the depth of the reservoir water column, though mean fish depth was greater during both crepuscular and nighttime hours than during the day. Fish location relative to the shoreline followed a similar trend; bonytail were contacted further from shore during crepuscular and nighttime hours than during the day. Turbidity readings for depth-tagged fish stocked at Cattail Cove were approximately one-third of those associated with fish stocked in BWRNWR. Continuous inhabitance of bonytail stocked at BWRNWR indicated those fish nearly exclusively utilized habitat found within the refuge. Continuous inhabitance of bonytail stocked at Cattail Cove was higher in Lake Havasu than BWRNWR, however, 20% of those individuals utilized habitats found in BWRNWR longer than those found Lake Havasu. Following the expiration of the acoustic depth tags in January 2012, a bi-monthly sampling routine was implemented to track the remaining fish. Data acquisition is planned through June 2012, and results and analysis will be presented in the 2012 Annual Report.