Status and trajectory of an animal population depends on its demographic rates, and endangered species management, in particular, relies on such quantifiable population descriptors to guide the recovery process. Recovery goals for federally endangered razorback sucker Xyrauchen texanus (Abbott), family Catostomidae, require that two ―genetically and demographically viable, self-sustaining‖ adult populations, each exceeding 5,800 individuals, exist in the Upper Colorado River Basin before downlisting or delisting can occur. Current wild populations are so depleted that the first management action to achieve recovery is to reestablish populations with hatchery-produced fish. An integrated stocking plan was implemented in 2003 and stocking goals initiated in 2004 call for 9,930 age-2 (≥ 300 mm TL) individuals to be stocked in each of the middle Green River and upper Colorado River subbasins for each of six consecutive years, thus creating the presumptive recovery populations which must become self-sustaining to meet recovery goals. The stocking plan assumes annual survival rates of 50% for age-2 fish, 60% for age-3 fish, and 70% for adult (≥ age-4) fish.
We used tag recapture data to estimate apparent survival, φ, and capture probability, p, for 96,448 hatchery-reared razorback suckers stocked into Upper Colorado River Basin streams, 2004–2007. Annual recapture data included 1,511 recapture events of 1,470 individuals from 2005–2008. We investigated the following effects: rearing method, reach, year, and season of stocking, fish total length (TL) at time of stocking, and first year in the river after stocking versus subsequent years. Mean first-year survival rates for razorback suckers of average TL (301.5 mm) stocked from 2005–2007 were low for fish reared by all methods: 0.03, 0.05, and 0.08 for tank-, pond-, and intensively (combination of indoor tank and outdoor pond)-reared fish, respectively. Rates were higher in the 2004–2005 interval for pond-reared (0.20) and intensively-reared (0.27) razorback suckers; no tank-reared fish were stocked that year. Total length at stocking and 1st-year survival were positively correlated; survival of fish (reared by any method) smaller than 200 mm TL approached zero but increased to an average of 0.83 for the few fish larger than 500 mm TL. Mean 1st-year survival of razorback suckers of average total length at stocking (301.5 mm) was 0.09. Season of stocking had a large effect on razorback sucker 1st-year survival rate estimates. Mean rates for razorback suckers of average TL (301.5 mm) stocked during summer were 0.03, 0.03, and 0.04 for tank-, pond-, and intensively-reared fish, respectively. Stocking during spring produced the highest mean estimates: 0.20, and 0.29 for pond-, and intensively-reared fish, respectively. Only five tank-reared razorback suckers were stocked during spring, none of which were recaptured during the study period. Effects of tank-rearing and summer-stocking could not be distinguished, since nearly all tank-reared fish (94%) were stocked during summer months. However, the adverse effect of stocking during summer was demonstrated by similar, low 1st-interval survival of fish reared by other methods. Furthermore, proportions of summer-stocked individuals subsequently recaptured were 0.5% or lower for pond- and intensively-reared razorback suckers. Tank- and pond-reared razorback suckers had higher predicted 1st-interval survival when stocked into the lower Green River reach (GR1) than Colorado or Gunnison River reaches. Survival of intensively-reared fish did not differ among the Green River reaches. Comparisons among all rearing methods within reach GR1 were not possible due to imbalance of numbers stocked there per method and year. Survival rates for razorback suckers after their first intervals in the river were high: 0.75–0.94, depending on interval and rearing method. Capture probabilities were all low, ranging from an average of 0.10 for a fish (reared by any method) on its first occasion in the river after stocking down to 0.02 on subsequent occasions. Pond-reared razorback suckers were captured in the highest proportion (68% of all recaptures) and had the highest predicted recapture probabilities (0.11–0.31), when averaging across reaches and years of stocking.
Maintenance of self-sustaining populations is the underlying goal of all razorback sucker recovery efforts. Our results suggest that stocking fish larger than the currently recommended 300 mm TL in seasons other than summer would aid in accomplishing that goal. A cost-benefit analysis of hatchery production and stocking strategies is necessary to determine the trade-offs of raising fewer, larger razorback suckers with higher survival rates versus many, smaller fish with lower survival. Techniques to improve 1st-interval survival, including exercise conditioning and predator-avoidance training, should be investigated further. A comprehensive razorback sucker monitoring program, which includes early life stages as well as adults, would assist with increasing recapture probabilities. Employing a standardized stocking protocol would allow better estimation of the effects of certain important variables, such as rearing methods. Implementation of management strategies derived from these post-stocking survival rate estimates will immediately enhance recovery prospects for razorback sucker in the Upper Colorado River Basin.