Gila trout eggs
Mora National Fish Hatchery staff prepare eggs for shipment to the Abernathy Fish Technology Center for fatty acid analysis.
Gila trout are a threatened species in the southwest United States. But now the results of a study recently conducted at the US Fish and Wildlife Service’s Mora National Fish Hatchery could help improve on broodstock health and egg viability, by evaluating genetic lineage, and using nutritional variables, thermal conditioning and hormone treatments.
As part of the study at the Fish and Wildlife Service (FSW) fish hatchery in Mora, Arizona, Main Diamond Gila Trout (Oncorhynchus gilae) broodstock were exposed to different thermal cycles and hormonal injection to improve reproductive success before spawning.
“The study was undertaken to increase our understanding of the effects of hormone treatment (luteinizing hormone releasing hormone analog, LHRHa) and thermal conditioning on Gila trout egg fatty acid profile and subsequent progeny survival,” Ron Twibell, US Fish and Wildlife Service, Abernathy Fish Technology Center, explained to Hatchery International.
Twibell and his colleagues were interested in determining whether any specific changes in egg fatty acids from Gila trout of different lineages or between Gila trout subjected to different hatchery practices (thermal conditioning and hormone treatment) were associated with differences in reproductive success.
Hatchery-reared Gila trout broodstock (Main and South Diamond lineages; approximately three years old) were randomly placed into two separate recirculating aquaculture systems (one for each lineage) in November–December of the year prior to spawning.
After the fish were moved into the recirc systems, males and females were segregated into separate tanks. Water was recirculated through each tank and make-up water, supplied from a larger recirculation system at Mora fish hatchery, was added. Both systems contained a parabolic screen filter, ultraviolet filtration, air-stirred bead filters, and a cooling unit.
Overhead fluorescent fixtures were controlled to provide a natural photoperiod combined with supplemental light from windows. All fish were fed a commercially available 5.5-mm-pellet, low-phosphorus feed intended for trout.
Within a year, feeds and feeding practices (i.e., transition through lots of feed, duration of time on each feed, and feeding regime) were identical for Main and South Diamond lineages.
Broodstock, eggs and hormones
Hatchery-reared Gila trout broodstock from the Main Diamond lineage were randomly separated into two groups. Again, special care was taken to not grade fish, to avoid artificial selection of broodstock group (THERMAL + LHRH). Animal husbandry, spawning, and egg collection methods were exactly the same as described for the first objective.
The other group, a control, was placed in another recirculation system consisting of raceways without thermal manipulation. The raceway system was also supplied with filtered water from a larger recirculation reuse system utilized at Mora National Fish Hatchery.
A sample of eggs from each female was collected for fatty acid analysis, and the remaining eggs were fertilized by Main Diamond males. All eggs were incubated by family in Heath trays. Water temperature ranged from 10.6°C to 12.2°C with eggs hatching in approximately 30 days. Eggs and alevins were also hand-counted to determine fecundity and hatch rates for this objective.
Lipid and fatty acid analyses
Before fertilization, approximately 10 eggs were collected from each female for fatty acid analysis. All eggs were collected from the female into a Ziplock bag. Ten eggs were randomly removed from each bag and placed into glass test tubes, covered in nitrogen gas, capped, frozen and shipped to Abernathy Fish Technology Center (AFTC, Longview, Washington) on dry ice for fatty acid analysis.
Thermal conditioning of broodstock accompanied by injections of exogenous hormone prior to spawning improved progeny survival over historic hatchery means as well as for broodstock reared in raceways at constant temperatures without hormones. Better egg fatty acid content appears to be associated with this improvement.
Eggs from Main and South Diamond broodstock fed the same feeds had similar hatch rates but could be differentiated on the basis of fatty acid profile. Future trials should further evaluate the influence of dietary fatty acids on egg deposition and hatch rate, while acknowledging that changes in reproductive performance may differ across lineages, according to the researchers.
Broodstock should continue to be conditioned with thermal cycling and use of hormones to induce ovulation, as these techniques resulted in significantly better hatch rates in the current study than use of only photoperiod manipulation to stimulate reproductive development.
Twibell noted that the results will help Gila trout hatchery managers in the Southwest, US.
“First, it’s clear that thermal conditioning and hormone treatment of females improves reproductive success of Gila trout,” he explained. “Second, results of the egg fatty acid analysis provide a possible mechanism for the higher reproductive success in Gila trout subjected to thermal conditioning and hormone treatment.”
“The higher percentages of... fatty acids measured in Gila trout subjected to thermal conditioning and hormone treatment may have contributed to higher survival because they were available for future use by the developing embryo. These fatty acids are precursors of eicosanoids which support embryonic development and survival.”
Future trials should further evaluate the influence of dietary fatty acids on egg deposition and hatch rate, while acknowledging that changes in reproductive performance may differ across lineages, he explained.
- Erich Luening