Sea Predators Reduce Fish Reproduction

Predatory animals can do a lot more than just kill their prey. Through intimidation alone, they can cause enough stress in their victims to reduce reproduction rates, shorten life spans, stunt growth and even alter gene patterns.

That message is being carried not by such mighty predators as tigers, grizzly bears or wolves, but by insignificant little fish.

The egg production of a killifish from the Caribbean island of Trinidad, for example, was cut in half only two weeks after the small fish was confronted by a menacing pike-like fish, according to a new study by two biologists.

“The findings may have broad implications for lots of things” both on land and in water, said Douglas F. Fraser of Siena College in Loudonville, N.Y., who conducted the research with James F. Gilliam of North Carolina State University.

“For instance, putting a predator and prey in sight of each other in a zoo might conceivably have a bearing on the reproductive output of a species that you’re trying to propagate.”

Reporting in the journal Ecology, the scientists noted:

“The suppression of reproductive output that we found had not been reported previous to this study, but reproductive suppression by predation threat may exist” in other predator-prey relationships.

“To my knowledge, it’s the first time such a short-term effect on the number of eggs produced by a prey has been shown,” said David N. Reznick, a biologist at UC Riverside.

New studies of predator-prey relationships have burgeoned in the past few years. “It’s still a very hot and trendy topic, and so far most of the work has been done with fish,” said zoologist Gary G. Mittelbach of the Kellogg Biological Station in Hickory Corners, Mich.

Mittelbach and his colleagues have shown that a predator such as a bass can stunt the growth of its bluegill prey by forcing the small sunfish into shallower, less desirable waters.

Predation also might age some animals prematurely by causing them to mature and reproduce earlier. A study of guppies harassed by a type of pickerel in Trinidad’s streams showed that both male and female guppies acquired the genetic ability to mature more quickly.

The stress affected the male guppies more than the females. “An extra genetic variation seems to be triggered that enables them to respond more quickly to the threat of predation,” said Reznick.

“In predation, your chances of dying are high, so you want to mature earlier. Male guppies were maturing in 10 weeks, where normally it would take 12.”

But earlier maturity didn’t give the males a longer life span. Both sexes died much earlier than their counterparts living in trouble-free waters. In low-predation sites, Reznick saw “guppies a year and a half old with the vigor and ability to produce eggs, whereas in dangerous areas maybe one in a thousand guppies lives to be that old.”

John A. Endler, a biologist at the UC Santa Barbara, who has done his own guppy research in Trinidad, observes that over many generations the fish changed colors and body shape to avoid predators.

“Their bodies became elongated and streamlined, and their color patterns--the blues, silvers, yellows, oranges--were reduced in size and frequency,” he said. “They become less conspicuous so they could avoid being seen.”