Cannibalism is rare among wild fish

Mosquito fish and guppies are sometimes cannibalistic when held in captivity, but a new study shows that this is not common behavior in the wild. According to the study authors, rare instances of cannibalism among fish are likely due to intense competition for food. The research has implications not only for aquarists but also for our understanding of evolution.

To investigate how common cannibalism is, Professor Brian Langerhans at North Carolina State University analyzed years of data covering almost 12,000 fish from 17 species in the wild.

“These are data accumulated from several different projects over the years,” said Professor Langerhans, the study’s senior author. “To identify the mechanisms responsible for this sort of phenomenon in the wild, we needed really large sample sizes. So, we accumulated the data for this work while also doing other projects.”

The scientists looked at the stomach contents of 11,946 fishes, either by x-ray or dissection. Of all the examinations, they found only 35 cannibalism cases, accounting for less than 0.3 percent. Furthermore, the cases of cannibalism were found in only three species of mosquito fish.  

Study co-author Rüdiger Riesch of the Royal Holloway University of London began the project while a postdoctoral researcher in Langerhans’ lab between 2010 and 2012.

“In captivity, mosquitofish and guppies will practice cannibalism commonly enough that there are protocols in place in research labs and aquaculture to quickly separate offspring from the larger fish,” said Riesch. “But when you look at the diets of fish in the wild, you really don’t find much evidence of it. We wanted to find out whether and why cannibalism occurs in nature.”

The scientists found that the rare cases of cannibalism were not just opportunistic but caused by extreme food competition. Cannibalism only happened in cases where there were few predators on the fish, which caused their populations to grow too high.  

“Resource competition seems to be the main predictor of cannibalism,” said Professor Langerhans. “We also saw that a lack of predation has an indirect effect on cannibalism: Release from predation allows population density to skyrocket, which decreases resources. This same driving factor may be responsible for many cases of cannibalism across the animal kingdom in natural settings.”

The study is published in the journal Ecology & Evolution. 

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By Zach Fitzner, Earth.com Staff Writer