One of the most important processes in the hatchery life of salmon is the transition from freshwater to saltwater, and what managers decide to feed those fish during the switch is crucial for their health and survival.
A recent study by researchers from the Abernathy Fish Technology Center in the US State of Washington compared physiological and nutritional values of standard hatchery feed and a commercially available transition diet for the first six weeks of hatchery rearing to see whether the transition diet altered the Chinook salmon smolt condition.
"Makah National Fish Hatchery is very close to the ocean, and releases fish which have had low return rates in the past," said the author of the study, Kyle Hanson. "The hatchery manager wondered whether the use of a transition diet (which included increased salt) might aid in acclimation to the marine environment, and thus might result in increased survival and return rates for the stock."
But at the same time, the manager was hesitant to place all production on a new feed that was untested at the facility.
"This study was designed as a small-scale, controlled experiment to start to understand what effects a transition diet would have on fish from this population," explained Hanson.
Transition diets, often supplemented with salt and other additives, have been used to prepare Pacific salmon being moved directly from freshwater hatcheries to saltwater net pens for aquaculture production. However, the use of transition diets to prepare hatchery-reared salmon for out-migration to the ocean has shown mixed results.
Hanson's team, made up of Ronald Twibell, Richard Glen, James Barron, and Ann Gannam, compared the physiological and nutritional condition of juvenile Chinook salmon.
At release, NKA activity of fish fed the transition diet was higher than that of fish fed the control diet, though survival during a saltwater challenge was similar between the groups.
"Smoltification is a very complex process in salmon, and documenting changes based on diet is a difficult task," Hanson said. "We measured multiple indicators of smoltification including activity of enzymes in the gill that indicate saltwater tolerance, nutritional indicators of energy use, and mortality during a saltwater challenge."
He said the goal was to have multiple biological measurements related to different components of the smoltification process to increase the chances of measuring any effects of the diet on fish biology.
Differences were seen in the proximate composition of the fish, according to the study, with increased total body moisture and decreased lipid in the fish fed the transition diet relative to fish fed the control diet.
Whole-body fatty acid composition of studied fish was similar to the fatty acid composition of the feeds in both groups, though some key whole-body fatty acids did not follow this trend, according to the research. Palmitic acid, linoleic acid, eicosapentaenoic acid, and docosahexaenoic acid, varied between pre-feeding trial samples and fish fed the two diets.
Hanson said he'd like to get more data before making any broad conclusions.
"Our results were mixed on indicating whether the transition diet was having an effect on smoltification in the salmon. On one hand, gill enzyme activity was higher and lipid content was lower in fish fed the transition diet when compared to fish fed the control diet - both potential indicators of increased smoltification," he explained. "However, key fatty acids that are associated with smoltification and mortality during the seawater challenge were similar between the two groups, potentially indicating that there was no effect of diet."
Further research using different diet formulations with more salt or measuring different response variables in the fish is needed to tease apart any effects of the diet. He's also interested in looking at other species.
"It would be interesting to do similar studies with other species that are reared in facilities close to the ocean," Hanson added. "There is quite a bit of variation in salmon related to smoltification and migration to the ocean, so studies of other species could produce quite different results."
— Erich Luening