Íslenskar landbúnaðarrannsóknir - 01.09.1976, Qupperneq 10

Íslenskar landbúnaðarrannsóknir - 01.09.1976, Qupperneq 10
8 ÍSLENZKAR LANDBÚNAÐARRANNSÓKNIR are, however, very much lower than those found by Guðjónsson, and Carlin’s re- sults are intermediate between the two. As an example we can see that the return-rate for a 15 centimeter fish according to the 1971 experiment is ca. 4%. Carlin (1969) has a value of 7 % for this length group but Guðjónsson (1972) ca. 12%. These data are compared in graph 1. It has been taken into account that Carlin (1969) measured the total length of his smolts. It does not seem to make much difference for this pur- pose. More recent information from the 1973 experiment suggests very pronounced varia- tion between years. The 1972 Tagging Experiment. 1. Groups Tagged. Groups tagged and the major experiments conducted are shown in Table 4. As in 1971 there was a great deal of tagging of two-year-old smolts to get a better idea of the year-to-year variation in returns from these releases on a production basis. One group of two-year-old smolts was tagged with a modified Carlin tag using locally made polyethylene thread (group 5). In October of 1971 a group of prospective one-year-old smolts and a control group of two-year-olds were subjected to normal photo- period, using artificial light. The lights consi- sted of several 250 watt mercury light bulbs suspended ca. 1 meter above the water surface The amount of light at the water surface was measured in lux, and was found to be on the average 1300—1400 lux ranging from 200 lux to 2300 lux. The lights were turned on in the morning and off at night automatically, using an electrical clock. The clock was set every 3 days to correct sunrise and sunset times. At the beginning of the experiment the one-year-olds were only 10 cm long. It was therefore necessary to keep them in fairly warm water (9— 10°C) until the beginning of December. They were then kept in cold water (2—3°C) until lst of February 1972 when they were again kept in warmer water (9—10°C). As a result of the warming up they were up to 12,7 cm ave- rage length by mid-March. When they were released they had been about 30 weeks under artificial light. The two-year-old control group was in natural temperatures all through the winter. A group of relatively small two-year-old smolts from outdoor ponds was fin-clipped with a left-ventral mark. This was done to get an idea of the possible returns of un- tagged two-year smolts. 2. The Electric Fishing. During the first week of August the outlet creek from the Kollafjörður Fish Farm was fished with electricity to estimate the tend- ency of the smolts not to migrate to sea. In contrast to earlier years there were no marked or tagged fish in the outlet at all. This was not unexpected for the two-year-old smolts but very encouraging with respect to the one- year-olds. It seemed that all the fish in the photoperiod experiment had been ready to migrate to sea. The real validity of this as- sumption remained to be substantiated by the return-rate of these fish from the sea. 3. Adult Returns. The remrns of tagged fish to the Fish Farm are presented in Table 5. The remrns of the various groups of two- year-old smolts vary from 4,1—9,8% ave- raging about 6,0%, Of these, 96% returned after one year but 4% after two years. This remrn-rate is somewhat higher than the re- mrn-rate for the 1971 tagging experiment, but it must be considered that the smolts
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