The perilous life of the young fishes

Small fish can be eaten by fish which is about double as large. To reduce the risk of being eaten, it’s therefore important to grow and to secure resources for more difficult periods. The ultimate goal is to be sexually mature, mate and thus bring their genes further to next generation…

What decides whether an individual survives after hatching? Usually the mortality is largest during the initial life-phase after hatching and it’s therefore important to determine the factors which determine the survival of an organism. For fish fry, central elements to determine survival are density of potential predators (for instance larger sized fish fry) and environmental conditions (water temperature, water level and food abundance), especially when salmonids fry emerges from the gravel after hatching to have their first meal.  Because of the winter conditions, it has been extra difficult to investigate survival of fish fry in freshwater during winter. For land living creatures we often believe that winter has extreme conditions with respect to temperatures etc.  In water, however, the surface is covered with ice, which isolates from cold conditions and temperatures in water are extremely stable compared to the temperatures man experiences in our terrestrial habitat during wintertime.

By measurements of fat content of fish fry to different periods we have been able to uncover some of the secrets of the winter in freshwater.  The fish fry use fat stores during periods with scarce food resources as a mean to survive. For the river living salmonids Atlantic salmon and brown trout, about half of the energy need is covered by feeding on invertebrates in the river and half comes from fat stores laid down in summertime.

There are large differences between different rivers in Norway which secures the ability of the young fish to survive in its specific area.  Differences between rivers may therefore be brought about. In the world of salmon parr you can really talk about the fat one from northern Norway and the lean one from southwest Norway. Salmonid parr do not lie dormant in wintertime, but they can save energy by hiding down in the gravel and reduce metabolism to a minimum. The winter is too long for a fry to survive on its fat resources only, and the parr must therefore up and seek food during winter. This is dangerous, as many predators are present which can have a liking of a meatfull fish fry, for instance larger fish individuals or the dipper (“fossekall”) (Cinclus cinclus). This is the reason for the fat ones from northern Norway. Already in their first, hectic dangerous summer, the fry must find food in a river full of competitors and predators. The food should preferably be used to build body, so fewer could it you, but in order to survive the long, meagre winter period must also energy stores be available. Over 20% of the available summer energy is used for storing-purposes for a salmon fry from Finnmark. This energy should preferably be used for growth, but stores are needed in order to survive winter.  The fish living in south-eastern Norway has almost no winter period, and can build body without any big winter concern. The salmon fry in south-eastern Norway has also  better temperature conditions during summer and can therefore go to sea and  smoltify already after two winters compared to 4-5 winters in Finnmark.  Fatness costs also for fish fry!