Big Pikes Makes Water Trouble

By fish out large cannibal pikes can provide cleaner water and fewer environmental toxins in both small pikes, roach(Rutilus rutilus)og perch. That shows results of a doctoral thesis at the University of Life Sciences. Large pikes = trouble. By fishing out large pikes can change the dynamics completely in a fish community, and both contribute to cleaner water and fewer environmental toxins in 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,Norway. Large pikes in the lake was the victim of hard pike fishing, and the effects of the water ecosystem did not take long to see.

Changed population structure -Small pikes had a much higher survival when the cannibal pikes was removed” said Chhatra. -We set nets throughout the year, and fishing out the pikes over 26 inches. The number of small pikes increased significantly. -This suggests that the population recruitment in the lake is strongly controlled by the cannibalism, says the Nepali student. At the same time, with he increased catch of small pikes the number of roach went down, while the frequency of large perch went up in the nets catch.

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

-Fish affects both directly and indirectly the amount of algae-eating plankton in the water, said Chhatra. The results showed that the diet of roach and perch changed in step with the out fishing of cannibal pike. Roach and perch had significantly increased consumption of zoo plankton in the diet. Large perch eat roach, and roach are often regarded as a slightly problematic fish. Roachs eat zoo plankton, and zoo plankton eats phytoplankton -Roach also freeing phosphate by eating the organic bottom sediment. A reduction in the amount of roach will help provide better water quality, underlines the doctoral student.

Environment toxic content of the fish were 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. The results of Chhatra showed that mercury levels in both pike,roach and perch decreased significantly after removing the cannibal pikes. -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 pike-cakes of our catch,and it taste really good. We need not to worry about hazardous substances in the pike that year, smiles the student.

Long term work is crucial. -The top predator has a significant effect on the entire ecosystem in Aarungen, with both cleaner water, better water quality and lower environmental toxins in the fish as consequence of good governance,concludes Chhatra. But fishing in Aarungen has not continued after the Ph.D. programme, so the big cannibal pikes are on the way back.He mentioned several places in both Sweden and Denmark where the successful management has led to the restoration of contaminated lakes.

Harald Hoel, expert of pike fishing.
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