Frustrated fish get feisty

Rainbow trout fight more aggressively when they don’t get their usual treat

DON’T TEASE  Withholding a treat that a rainbow trout has come to expect can prompt what looks like a fish version of frustration. 

Ken Hammond/USDA

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Failing to get an expected snack rouses fish to fight with extra ferocity. The change in behavior is especially marked in undersized fish facing a big, scary opponent.

The fish version of frustration can drive little fish to posture at and fight big fish that they would normally flee from, says Marco Vindas of the University of Oslo. Two of 11 of these overheated David-and-Goliath matchups ended with the little fish unexpectedly trouncing the big one.

To induce this extra aggression, all Vindas and his colleagues had to do was unpredictably withhold tidbits of shrimp that the fish had come to expect, the researchers report April 23 in the Proceedings of the Royal Society B.

Fish fights can reveal a lot about aggression and how brain biology relates to behavior among vertebrates, Vindas says.

Other research has shown that withholding rewards can intensify aggression among birds and mammals. But Vindas says various earlier experiments, some observing solitary fish for signs of frustration, had raised questions about whether fish have the mental capacity for unsatisfied expectations. Previously, he and his colleagues essentially teased captive groups of Atlantic salmon with treats that sometimes showed up when expected and sometimes didn’t. The unrewarded groups showed extra agitation.

For a more detailed look at the possibility of fish frustration, Vindas and colleagues have now focused on captive, year-old rainbow trout (Oncorhynchus mykiss). The researchers fed trout full rations of their regular meal of fish pellets but later flashed a light and offered shrimp bits as a snack. For trout, the shrimp is “basically like candy,” Vindas says.

After eight days of light flashes followed by snacks, Vindas matched each small fish with an opponent roughly 40 percent heavier, knowing that the fish readily face off with others nearby. Such a big difference usually means smaller fish immediately flee from a confrontation or surrender quickly if they do fight.

Immediate or swift surrender was exactly what Vindas saw when both individuals had had their snacks. Only one of the 11 undersized fish in the match-ups even bothered to try an attack.

When researchers frustrated 11 pairs of the fish, flashing the light but delivering no shrimp, the confrontations lasted on average more than three times as long. In each pair, Vindas says, “The little fish was acting like it’s more dangerous,” flaring its fins, gaping its mouth or making other provocative displays of a real contender. Their big opponents postured in turn and assessed the situation, resulting in a first phase of confrontation that took seven times as long as usual.

Fights lasted longer as the fish charged at and nipped each other. Trout occasionally kill each other in fights, though usually one combatant just gives up and swims away. In the 11 fights between frustrated fish, two of the big fish ended up backing down, leaving the little fish victorious.

What’s especially interesting about the fish reactions is their brain chemistry, says Ryan Earley of the University of Alabamain Tuscaloosa, who also studies fish fights. The study raises the possibility that the rush of aggression might be driven by changes in concentrations of the neurochemical serotonin.

Serotonin was once suggested as reducing aggression, Vindas says. More recent studies, however, link aggression with short-term rises in serotonin in certain brain areas. The new trout study finds a rise in serotonin in a bit of the fish brain called the dorsomedial pallium. That uptick supports a notion that the structure may work in fish much as the amygdala does in mammals, dealing with emotional reactions.

FISH FIGHT  Two young rainbow trout face off to determine dominance. First they posture and assess each other. After the first few attacks, the video jumps to a point of escalated fighting and then to the end, when the established winner makes one-sided aggressive moves against the loser. Credit: M. Vindas  

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