Portions of the lower Colorado River between Parker and Laguna dams were surveyed during the period January 2006 to April 2008 as part of a broad program to assess efficacy of the stocking program for razorback sucker Xyrauchen texanus. Our findings document that short-term mortality is high and long-term survival is nil because of predation losses to nonnative fishes and to fish-eating birds. The program has not resulted in establishment of persistent populations, and continued stocking is not recommended under current practices. Instead, stocking should be only into habitats that are depleted of nonnative fishes and where control of avian predators can be developed. Natural feed delivery during fish rearing, pre-release training, and temperature acclimation at the time of stocking also are recommended to enhance post-release survival.
The study area included the main river channel and selected confluent, watercraft-accessible backwaters and side channels in La Paz and Yuma counties in Arizona, and San Bernardino, Riverside, and Imperial counties in California. Primary methods were boat electrofishing and trammel netting that resulted in contact with a total of 24,602 fish representing at least 21 species, including 2,433 razorback sucker contacts (9.9% of the total catch) and 182 bonytail Gila elegans contacts (0.7% of total catch).
All razorback sucker are thought to have been stocked and many (1,075) contained PIT tags at capture. Of these 1,075 fish, 959 had been tagged at the time of stocking. Time-at-large ranged from 2 – 913 days, but most fish were encountered within a few months of release. Sex ratios of characterized fish were 468 female, 777 male, 836 juvenile, and 200 unknown. Mean total length (TL) for females was 45.4 cm (range 34.2 – 65.0), for males was 38.8 cm (range 29.0 – 58.5), and for juveniles was 34.9 cm (range 28.3 – 39.9).
All bonytail are thought to have been stocked and 93.9% contained hatchery-implanted wire tags. Only 27, however, contained PIT tags; all of these tags had been implanted at the time of stocking. Sex ratios were almost uniformly “unknown.” One fish was recorded as female, and one as juvenile; however, no secondary sexual characteristics were present to verify these assessments. Mean TL for bonytail was 33.4 cm (range 27.0 – 41.0).
Twelve adult razorback sucker (>50 cm) were affixed with radio tags in an effort to track dispersion of mature fish and locate spawning sites. Tagged fish were released into A-7 (Arizona) and C-7 (California) backwaters between December 2006 and February 2008, and were subsequently contacted only in A-7 Upper before tags expired and tracking was discontinued in April 2008.
Multifaceted investigations into factors contributing to apparent loss of stocked fish were performed as part of this study. Environmental conditions and fish health are generally of little concern because most fish are captured in good condition, and both spawning and successful reproduction have been observed. Water physico-chemistry in lower Colorado River backwaters appears to be sufficient for sustaining fish communities, at least in most places and at most times. Parasite infection is variable but not of concern.
Primary mortality factors were piscine and avian predation on released fish. Observations indicated that piscine predation was ubiquitous in all stocked sites. Estimated predator abundances, though low in certain localities, are sufficient to impact stocked populations. Overwhelming evidence of avian predation on stocked fish indicates it is a significant threat to survival. Fish were captured with wounds from failed avian predation as early as 2 days post-release. Quantification of fish depth at capture indicated a shift from pelagic to demersal swimming within 100 days post-release, a behavioral change that may reduce avian predation. Smaller size classes of two experimental stocks of razorback sucker were under-represented in the catch, suggesting that vulnerability to avian predation may be related to fish size.
Regardless of release size, estimated annual survivorship was less than 30% for stocked razorback sucker. Evidence of long-term survival was rare, indicating that population augmentation through stocking has failed to replace lost wild populations or establish new ones. Continued stocking into the lower Colorado River is not recommended under current practices because mortality threats are substantial and conditions are not conducive to long-term survival of stocked fish. Instead, population augmentation should occur in habitats depleted or devoid of predators. Present and proposed predator free habitats do not mitigate avian predation, and appropriate control measures thus are advocated. If continued stocking to the lower Colorado River is mandated, then recommendations are provided for enhancement of survival of stocked fish, including pre-release training, natural feed delivery, and temperature acclimation.