Writing in its US patent, the team outlined the potential to develop an aquaculture growth-promoting supplement in light of surprise findings that yellow perch, when indirectly exposed to a walleye (Sander vitreus) predator attack, doubled in growth. The scientists were now focused on identifying what substance caused the growth spur, its source and establishing whether a synthetic version could be used in an aquaculture setting.
Terence Barry, senior scientist in animal sciences and aquaculture research lab director at the University of Wisconsin-Madison, said there was significant potential for the yellow perch industry.
“Perch has a lot of attributes that make it a really good fish for aquaculture. ...But there aren't many people raising it. There are a lot of good things about this fish but what's really bad about this fish is it grows really slowly. The market size is only 150g, compared to salmon which can be as much as 4kg, and it still takes two years in a pond to get a market fish. If we don't fix that, I don't think we'll get an industry,” Barry told FeedNavigator.
The finding that yellow perch growth doubled when the fish were exposed to a predator attack was “an interesting discovery” for the sector, he said, because it meant that, ultimately, yellow perch may be able to reach market size within a year. “It's a big deal. If the fish can grow a lot faster, it could become the basis for a pretty big industry.”
Far off a feed find?
Barry said the team's current focuses were twofold: identifying the substance causing the growth spur and its source – a “super hard” task – and identifying the mechanism of action in the responding prey fish.
“We're cloning all of the growth genes of perch and we're going to work out the expression. We're going to look at how they change and respond to predation and what the growth-promoting pheromone is. ...We're going to figure out how it works and whether we can apply it in an aquaculture setting,” he said.
A synthetic version of the substance to pump into perch ponds would be the “most advantageous” option for industry, incorporated into water, water-soluble solvents, lactose or sugar in liquid or solid form, although that was “probably a way down the road”, he said.
The team didn't yet know where the substance is derived from – prey, predator or both – so that was a key focus in current experiments, Barry said.
The predatory effect...
The early hypothesis that growth was simply triggered by the release of a pheromone similar to Chondroitin-sulphate – the alarm substance excreted by zebra fish – was proven false. The chemical did elicit an 'alarm response' in the yellow perch but did not spur growth.
The team's current hypothesis was now more complex – looking at a potential combination of substances, perhaps pheromones from the perch along with substances released by the predator through feces or urine, post-attack. They were also investigating the concept that these substances were delivered through the water as an odor, with growth response in the yellow perch triggered through the olfactory system.
Writing in the patent filing, the team said: “It is well known that 'alarm substances' released from the skin of prey fish can alert conspecifics to danger. There is also limited evidence that such alarm factors can induce changes in body shape in prey fish so that they are less susceptible to predation. There are no reports, however, of odors that can increase the overall growth rate of cultured fish.”
He said the team had set up walleye predators in tanks above the yellow perch with water pumped down into the prey tanks after the walleye were fed and after the predator defecated. The prey tanks were split into receiving either: water post-attack; water post-defecation or both.
“We're trying to compare the alarm substance to the feces and the effect of both,” Barry said.
Another hypothesis the team was looking into, he said, was whether the predation effect only worked at certain life stages of the yellow perch, for example just in young fish. “It kind of makes sense, as it evolves to help the fish escape predation. It might only work in the small guys but that's still huge for industry,” he said.
The team hoped to have a better idea about the substance and its source within a month, Barry said, as experiments were already well underway. Eventually, he said it would also be worth looking into whether this predation effect existed in other fish species, once more was understood in yellow perch.