Big Pike Makes Water Trouble

By catching large cannibal pike, it helps to provide cleaner water and fewer environmental toxins for smaller pike, roach (Rutilus rutilus) and perch.

This article shows the results of a doctoral thesis at the University of Life Sciences.

Large pike = trouble

By removing large pike it can change the dynamics completely in a fish community and contributes to cleaner water and fewer environmental toxins in other fish.
Chhatra Mani Sharma has in the past four years been studying the significance of top predators in fish communities, by manipulating stocks in the lake Aarungen, in Norway.
Large pike in the lake were the victims of hard fishing and it did not take long to see the effects on the water ecosystem


Changed population structure

From the study, Chhatra found the following information:

  • Small pike had a much higher survival when the cannibal pike were removed
  • The number of small pike increased significantly when nets were set throughout the year catching pike over 26 inches in length
  • With the increased catch of small pike, the number of roach went down while the frequency of large perch caught in the nets went up

The Nepali student suggested that the population recruitment in the lake is strongly controlled by cannibalism.

Diet determines visibility

Aarungen is a lake with large supply of nutrients which creates good growing conditions for algae. Algae contributes to low visible depths in the water as well as poor water quality.

Chhatra also demonstrated that:

  • Fish affects both directly and indirectly the amount of algae-eating plankton in the water
  • The results showed that the diet of roach and perch changed in tandem with the out-fishing of cannibal pikes and had significantly increased their consumption of zoo plankton
  • Large perch eat roach, which are often regarded as a slightly problematic fish, then roach eat zoo plankton, and zoo plankton eats phytoplankton
  • Roach free phosphate by eating the organic bottom sediment so a reduction in the amount of roach will help provide better water quality

Environmental toxic content of the fish was lower

The content of hazardous substances in large fish-eating fish are often high because hazardous substances pile up at the top of the food chain.
Chhatra”s results showed that mercury levels in pike, roach and perch decreased significantly after removing the cannibal pike.
“Increased individual growth rates among the three investigated fish species have probably led to a “bio dilute” of mercury content.” said Chhatra. The level of chlorine organic hazardous substances in the fish went down sharply.
“In 2006 we made minced fish of our catch,and it tasted really good. We did not

worry about hazardous substances in the pikes that year.” smiles the student.

Long term work is crucial
Chhatra concluded that the top predator had a significant effect on the entire ecosystem in Lake Aarungen, with cleaner water, better water quality and lower environmental toxins in the fish as consequence of good governance.
Fishing in Lake Aarungen has not continued after the Ph.D. program ,so the number big cannibal pike are on the rise. .He mentioned several places in both Sweden and Denmark where the successful management has led to the restoration of contaminated lakes.