Imperial Ponds Final Report 2012

Imperial Ponds is a 19 hectare pond complex designed to provide habitat for two endangered native fish, bonytail (Gila elegans) and razorback sucker (Xyrauchen texanus). The ponds are part of the 50 year Lower Colorado River Multi-Species Conservation Program (LCR MSCP) goal to develop 146 hectares of backwater habitat along the Colorado River. Monitoring and research at Imperial Ponds is also funded by the LCR MSCP program under work task C-25 This is the final report of native fish monitoring and research efforts conducted at Imperial Ponds from October 2008 to September 2011. Monitoring during this period focused on bonytail and razorback sucker abundance, growth, reproduction, recruitment, and habitat association of resident razorback sucker. Over the study period, 1,105 Passive Integrated Transponder (PIT) scanner deployments resulted in 1,673 bonytail and 98,829 razorback sucker contacts. Few PIT tagged bonytail survived into this reporting period, and none were stocked during the study. However, untagged bonytail from natural recruitment in Pond 2 persisted throughout the period and 109 individuals were captured and PIT tagged in 2011. These fish account for 1,547 of the total bonytail contacts. Totals of 59, 272, and 198 razorback sucker were stocked into ponds 2, 4, and 6 respectively, and populations persisted throughout the study period. Mortality was low except for acute die-offs in ponds 4 and 6; monthly survival was estimated at 98.5, 96.6 and 97.4% for ponds 2, 4, and 6, respectively, in months without acute die-offs. There was a summer mortality event in Pond 4 between August and October, 2009, during which an estimated 74.0% of the population perished. A die-off occurred in the first two months post-release in Pond 6, with an estimated loss of 64.5% of the population within the time period. Totals of 49, 26, and 49 razorback sucker were moved from ponds 2, 4, and 6 to Pond 1 respectively. Two razorback sucker from each source pond were captured without PIT tags, and therefore could not have been included in the last population estimates for these ponds, which were 47, 30, and 52 for ponds 2, 4, and 6 respectively. Razorback sucker habitat association shifted significantly across seasons. Populations in different ponds had different seasonal associations except in summer and during the spawning season. During summer when water temperatures exceed 32 °C, deep open water areas were preferred and little activity was detected. Razorback sucker spawning activity appeared to peak in late winter/spring on the gravel boat ramps of ponds 2, 4, and 6, and the artificial spawning beds in ponds 1 and 6, with nearly all members of the population visiting these areas during the period January through March (in 2010 for ponds 2, 4, and 6, and 2011 for Pond 1). Radio telemetry conducted in ponds 2 and 4 during the summer months in 2010 and acoustic telemetry conducted in Pond 1 in 2011 provided additional support to the hypothesis that razorback sucker spend their summer days in deeper (>3 m), open water locations. Individual growth and recruitment success for native species was evaluated through sampling activities (nets, traps, and electrofishing) in the autumn of each year. A total effort of 10,892.27 hours of netting and trapping resulted in 305 bonytail and 168 razorback sucker captures and a total catch of 6,529 nonnative fish. Individual growth for razorback sucker was comparable to fish from Lake Mohave, AZ/NV and females grew faster than males. All but one of the bonytail captured were young recruited fish from Pond 2. In 2009 and 2010 a total of 733, 417, 519 minutes of active (lighted) larval netting effort was conducted in ponds 2, 4, and 6 respectively to detect spawning success of native fishes. This effort resulted in the capture of 11, 1, and 0 razorback sucker larvae for ponds 2, 4, and 6 respectively. One bonytail larva was also collected from Pond 2 in 2010. In 2011, 917 minutes of larval collection effort in Pond 1 resulted in the capture of 0 bonytail and 60 razorback sucker larvae. Threadfin shad (Dorosoma petenense), common carp (Cyprinus carpio), bluegill (Lepomis macrochirus), redear sunfish (Lepomis microlophus), warmouth (Lepomis gulosus), and western mosquitofish (Gambusia affinis), persisted throughout the study period in most ponds. In addition to these previously documented species, black crappie (Pomoxis nigromaculatus) was captured in ponds 3, 4, and 6 during autumn sampling in 2009 and one striped bass (Morone saxatilis), 430 mm total length, was captured on April 12, 2010 in Pond 2. Renovations were implemented in ponds 1 and 3 in 2009 and 2010. Attempts to eliminate non-native fishes from Pond 1 were not successful in removing western mosquitofish, but the renovation appeared completely successful in Pond 3. Water physico-chemistry parameters in all ponds have generally remained within acceptable limits established by the Imperial Ponds work group, i.e., pH <9.0, DO > 4 mg/l, and temperature < 33.3° C. During summer months (June – September) maximum values of pH and temperature and minimum values of DO exceeded limits, and mean values exceeded limits for ponds without active water management.

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