Náttúrufræðingurinn - 1976, Síða 74

176
NÁTTÚ RUF RÆÐIN G U R1N N
ment. The changed fishing methods may have reducecl spawning sites, as the
previously used technique of seining prevented the spawning sites from being
covered witli mud.
Oidy femrde sticklebacks were caught, probably because the males do not
reach tlie size taken by tlie nets. Comparison of the number of sticklebacks
with the number of chars caught, and taking into consideration, tliat only the
largest (oldes) females were caught, leads to the conclusion that the popula-
tion of sticklebacks in Mývatn is very nurnerous.
More food is found, both in abundance and in the number of species, in
the South-basin (Syðri-flói) than in the North-basin (Ytri-flói).
In the S-basin the stomach content of the char was found to correspond
with the peaks of the food organisms, this being midges (mostly pupal stage) in
the spring, benthic crustaceans (Lcpdurus arcticus and Eurycercus lamella-
tus) during summer and plankton (Daplmia longispina) in autumn. Stickle-
backs and Lymnaea are only taken during periods of low abundance of the
above mentioned organisms, as in the period between the emergence of midges
and the peak of the henthic crustaceans.
The abundance of food, important in the S-basin, was very low in the N-
basin, where sticklebacks, followed by Lymnaea, were the most important food
items of the char. This is considered to be a case of constrained feeding be-
havior. The choice of food varied little with size of char smaller or larger
than 20 cm.
The sticklebacks were found to take their food overwhelmingly i'rom the
benthos. This was mainly midge (larve), Eurycercus, Lepidurus and Cyclops.
For sticklebacks the main difference between the lake basins was, tliat Lymnaea
and their own eggs were more common food items in the N-basin than in the
S-basin.
An examination was made of food of char in 1933—1934, 1969 and 1972—73.
This permits the following generalization íor the seasonal variation of the
main food of char in Mývatn (S-basin).
midge (mostly pupal stage + adults)
benthic Crustacea
plankton
midge (larvae), sticklebacks
spring
summer
autumn
winter
HFIMILDARIT
Aas, Per, 1970: The Winter migrations of cliar (Salvelinus alpinus L.) in the
hydroelectric reservoirs Tunhovdfjord and Pálsbufjord, Norway. Rep. Inst.
Freshw. Res. Drottningholm 49:183-201.
Friðriksson, Á., 1939: Um murtuna í Þingvallavatni, með hliðsjón af öðrum
fiski í vatninu. Náttúrufr. 9:1-36.
Jensen, K.W., 1971: Fisk og Fiskestell nr. 5, Diift av fiskevann (fjölrit).
Kristjánsson, ]., 1972: Veiðni silunganeta. Veiðimálastofnun (fjölrit).