Brownification in lakes kills female perch appetites

Clear lake water is a delight to the eye. Clear water is a necessity for perch, since their method of feeding is based largely on their sense of sight.

Brownification in lakes kills female perch appetites

The water in many lakes has turned from transparent to the colour of turbid beer. This is thought to be because of the winter rains brought by climate change: mild and rainy winters have increased the amount of nutrients and organic substances in waterways.

A research group on sustainable fishing, headed by Professor Hannu Lehtonen from the Faculty of Biological and Environmental Sciences of the University of Helsinki, has already proven that perch lose the competition for food to, for example, roaches in dark waters.

Now, much to its surprise, the group has discovered that female perch suffer from darkening water more than male perch.

- The female perch we studied remained small in dark waters, because they did not eat as effectively as those in clear waters. The young of small females have weaker survival ability than the young of larger individuals, says Researcher Satu Estlander, who is a member of Professor Lehtonen’s research group.

In clear water, quite expectedly, female perch eat more voraciously than male perch. Adult female perch grow faster than males, for size determines their breeding success. The females need to grow as large as possible, so that they can produce plenty of offspring. The spawn of large mother fish contains more nutrition for the offspring.

According to Estlander, the eating efficiency of female perch decreased dramatically in dark water.

- The darker the water, the smaller the female perch. Water turbidity did not affect the growth and size of male perch. For now, the reason for the differences in feeding behaviour between the sexes remains a mystery.

Controlled fishing

Fishers hope to catch large female perch. The combined effect of fishing and climate change may strongly affect the evolutionary regulation of fish.

- The effects of climate change should be considered in fishing management, so that fishing can be controlled according to the principles of sustainable development of fish stocks, says Estlander.

Text: Kirsikka Mattila & Satu Estlander
Photo: Satu Estlander
11.8.2010
Translation: AAC Global
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