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Fast not food for a fussy fish

April 04, 2002

Most fish eat heartily during the summer in preparation for the cold and hungry winter months. But the burbot or Lota lota – a freshwater cousin of cod - does things differently, going on a summer diet when fish food is at its peak.

Scientists at the Leibniz Institute of Freshwater Ecology and Inland Fisheries in Berlin have been studying the unusual Lota lota fish in the hope of understanding why it chooses to fast in periods when food is plenty. To their surprise, the research group lead by Dr Iris Hardewig has observed that the species still keeps growing in length and weight during summer despite an apparent energy deficit.

“We were very surprised to realise that Lota lota is able to grow despite an energy deficit,” says Dr Hardewig. “This is very unusual. Normally fish lose weight when energy consumption is not sufficient to fuel the energetic needs of the organism, just like we do when we go on a diet. The phenomenon we observed in Lota lota is as if we go on a diet and still gain weight.”

At the Society for Experimental Biology conference on Tuesday 9 April, Dr Hardewig will present her group’s findings on how the Lota lota keeps growing. It appears that the energy necessary for growth is derived from liver reserves, which are degraded during the summer and refilled during winter. Effectively the Lota lota goes into energy debt during summer, repaying the debt over the winter period.

The reduction in feeding rates during summer is thought to be related to stressfully high water temperatures. Lota lota is the only freshwater species of the gadoid family. All other gadids (such as cod Gadus morhua) are marine fish and adapted to cold waters. Lota lota has maintained this trait from its marine relatives, but has colonised temperate freshwater habitats with high water temperatures during summer. High temperatures (up to 25°C) can be endured by Lota lota, but cause a state of reduced activity and, therefore, reduced feeding rate. Thanks to the debt management strategy fasting is not disadvantageous in terms of growth maximisation.

According to Dr Hardewig, “Lota lota is the first species showing this survival strategy in temperate waters. Further investigations may show whether this phenomenon is also observed in other cold-stenothermal fish that colonise temperate habitats. But,” she warns, “our data also suggest that Lota lota may be extremely vulnerable towards increases in summer temperatures and, thus, to global warming.

Society for Experimental Biology

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