PROBLEMS OF FISH-LARVAE SURVIVAL IN NATURE AND THE REARING OF ECONOMICALLY IMPORTANT MIDDLE EUROPEAN FRESHWATER FISHES WILHELM EINSELE Federal Institute for Aquatic Research and Commercial Fisheries Scharfling, Austria INTRODUCTION In early times, when fish culturists began artificial brooding of fish eggs and consequent stocking of natural waters with fry, the value of these activities was unquestioned. It was naively taken for granted that if this practice were followed, one would more than make good the decrease of fish populations caused by commercial fishing. The complexity of the problems, as we now know them-or believe to know % of Fry feeding at lux Relative feeding index them-was not readily seen, but it seemed sound to No. copepodites > believe that since eggs and fry were much more pro- perliter 600 100 40 20 10 1 0.5 600 100 40 20 10 1 0.5 0.3 tected in the hatchery than in the wild, the per- centage surviving to adult fish should be much 2020 0000_________-_ 001.51.0 0 0 0 0 00 greater. As we all know, the experiences with most 12.. .. .....~ ..... 45 50 25 5 0 0 0 0 2.1 1.0 1.4 1.8 0 0 0 0 methods and concepts did not lead to the expected _____ _____-__--- 70 60 25 5 0 0 0 0 2.0 1.4 1.8 2.0 0 0 0 0 successes. Many people working in fisheries manage- - - _ - - - - - - - - - - - - - ment now think that the time is ripe for discarding 85 i5 80 15 10 0 0 0 2.1 1.8 2.0 2.3 2 0 0 0 _____-__-_-_---- the whole scheme. 330..........__.._. 95 90 80402510 5 0 2.62.52.82.42.22.02.0 0 Are they justified or would it be worthwhile to try __-- _ ________--- again? Naturally not in the way it was done before, 1000....__._......._ 100 95100603520 15 0 2.82.92.72.42.42.51.3 0 but with modern concepts based on special analyses. Legend to Table: Coregonid fry of the Mondsee-race were used in this experiment. In my opinion, the effort should be made. What The left half of the Table shows the percentage of fry that fed at the different light intensities and the varying food-densities to which they were exposed. The are the reasons for my optimistic views? At the be- right half of the Table gives the average relative amounts of food eaten by fry ginning let me put forward two principal statements under the dieerent conditions. Relative amounts are indicated by &none eaten: l+ne to three ropepodites eaten: 2-gut partially filled; 3-gut well filled. Four based on the results of studying fry of various sal- copepodites Der liter were chosen as a minimum exDerimenta1 number on the basis monid fishes, but especially white fish (Coregonus of average numbers of copepodites per liter found in the Mondsee in March. spp.) and brown trout (Salnao trutta). (1) It is now At a light intensity of 100 lux, one finds that there firmly established that in our alpine lakes only one is practically no change in the percentage of feed- to ten adults will result from 10,000 naturally- ing fry from that at 600 lux, although the relative spawned coregonid eggs. (2) On the other hand it is amount of food that a fry eats is much lower at not difficult to raise the percent larval survival in 100 lux than at 600 lux. the laboratory from 0.1 or 1 per thousand to prac- tically 100 percent. No special devices are needed. Other illuminating comparisons can also be made. One need only offer the fry enough light (over 100 In particular the one between feeding conditions (as lux) and a high enough density of zoo-plankters, pre- expressed by light intensity and food density) and ferably Diaptomus copepodites, and the mortality will individual feeding capability (as expressed by the approach zero. With a lowering of the food density filling of the gut). The bottom horizontal row of and/or the light, survival will diminish and finally figures in Table 1 shows that at a density of 1000 will reach zero. Therefore by varying these two nat- food organisms per liter, there is no significant dif- ural environmental conditions, you may have 100 erence in the percentage of fish feeding at light in- percent survival or none at all. tensities of 600 to 40 lux; the percentage feeding A series of experiments summarized in Table 1 then decreases rapidly. However, the amount that was which I performed 20 years ago show this double de- eaten by those individuals which did feed was almost pendency. The experiments were carried out in glass equal, down to a light intensity of one lux. These tanks. Light intensity at the surface was measured data axe illustrative of the problem and the existence with a Lange Photometer. The food concentration of individuality not only among the members of a was chosen to approximate that found in the Mondsee single species, but also of a single race. in March. Two major questions seem to remain outstanding at At a light intensity of 600 lux, as seen in Table 1, this point. (1) What is the relation between the ex- the density of food organisms plays an important perimentally established data, illustrated in Table 1, Translated by George 0. Schumann. and the natural ecological conditions of lakes contain- REPORTS VOLUME X, 1 JULY 1962 TO 30 JUNE 1963 "5 ing coregonid fishes! and (2) In what way can the achieved by manipulating temperature daring devel- data provided by Table 1 be used to advantage in opment. I have been investigating the duration of improving stocks of harvestable fish? development of fish eggs in relation to temperature I shall discuss first the question of the relation be- on a broad scale. Ny results may be summarized as tween experiment and nature. Coregonid fishes spawn follo~s: in cold water-some species and races on the falling In general, European freshwater fish eggs develop water temperatures in the Fall, others during the within a temperature range of 0-27°C. Different Winter, and some upon rising temperatures in the fishes have different levels in this range. Coregonids Spring. The spawning habits and seasons of the same and brown trout may be cultured at temperatures species may vary considerably in different lakes. In from 0-12°C with about equally good results. Char all cases, however, the spawned eggs sink to the lake eggs produce exceptionally vigorous fry at O"C, and bottom and, in most of them, spend all their de- losses begin to occur at 8-9°C caused by cracking of velopmental time at a temperature of 4" C. Approxi- the egg-shells sometime before hatching. Northern mately 75 to 80 days are required for development to pike has its safe interval between 6 and 18°C and hatching at this temperature, and fifteen days for carp between 15 and 27°C. the absorption of the yolk sa? to a stage when If the temperature is lowered by 10°C, develop- the yolk-sac fry first begins to feed. The total de- mental time increases by 5 times. This means that velopmental time from newly spawned egg to free- the time from fertilization to the feeding stage of swimming fry is about three months. At this stage coregonid eggs can be as short as 35 days or as the fry are very small and live in open waters, or the long as six months. Therefore we axe able to release pelagial zone of lakes. At the time of first feeding, a feeding coregonid fry at a time when the chances for critical stage in the life history of the fry makes its survival are maximal. appearance. Formerly well water was used in many hatcheries In most Austrian, South-German, and Swiss lakes became it is free of turbidity, relatively free of coregonid fry are faced with the critical stage during bacteria and not in danger of freezing. In our region February and March. The lakes may then still be well water has a fairly constant temperature (ea. covered with ice and snow and light intensities may 8°C). Coregonids take about 50 days to develop at reach very low levels at the water surface. this temperature which is at the upper range of their In the case of very deep lakes which do not freeze temperature tolerance. Because most coregonid races over, an unstratified condition exists and strong winds spawn between the second half of November and the will cause a complete overturn of the water mass. first half of January, the fry from the hatcheries Some deeper alpine lakes have an ice cover only using well water had to be released in January or once in three to seven years. Even though some deeper February. In performing this, holes had to be made lakes may not freeze over-low temperatures reduce through the ice and snow-covered lakes frequently. the amount of plankton to very low levels-in par- Stocking in this manner had little or no effect on ticular, copepodites are rare and nauplii are just the fish population ! beginning to appear. There may be as few as 30,000 Eggs of the European brown trout need 1.6 times to 50,000 copepodites below one m2 of surface. Even the time for development needed by coregonid eggs these few organisms may be scattered into the depths at any temperature ; however, considering only the by winds causing circulation of the lake water, result- interval from hatching to ready-to-f eed size, trout ing in densities of a few or only one "edible" crusta- fry need about 3 times as many days as the coregonid cean per liter.