Fæða sjóbirtings og sjóbleikju í Dyrhólaósi

Ágrip

Fæða var athuguð hjá 72 sjóbirtingum (Salmo trutta) og 167 sjóbleikjum (Salvelinus alpinus) í Dyrhólaósi sem er ísalt sjávarlón í Vestur-Skaftafellssýslu. Fiski var safnað í júní, júlí, september og október árið 1990. Fæða tegundanna var breytileg eftir á söfnunartíma og svæðum í ósnum. Fæða sjóbirtings var mest agnir í júní (Mysidacea). Síðast í júlí og um miðjan september var uppistaða fæðunnar hins vegar sandsíli (Ammodytidae) en um miðjan október voru agnir aftur í mestum mæli. Marflær (Amphipoda) voru einnig áberandi í fæðunni einkum framan af sumri. Sandsíli voru mun algengari í fæðunni nálægt ós í sjó en fjær honum, en þar voru marflær algengari í fæðunni. Meðalþungi fæðudýra í mögum jókst með fisklengd, og var að jafnaði 33 sinnum meiri hjá stærstu (>40 cm) sjóbirtingunum en þeim minnstu (<20 cm). Fæða sjóbleikju var einhæf. Aðeins tvær fæðugerðir fundust mögum bleikjunnar, þ. e. agnir og marflær. Á svæðum þar sem selta var lág, voru marflær þýðingarmeiri fæða en agnir og virtist lítill breytileiki á milli söfnunardaga. Meiri breytileiki virtist í seltumeiri sjó nálægt sjó en agnir voru aðalfæðan í júní og september en marflær í júlí og október. Hjá sjóbleikju komu fram tiltölulega litlar breytingar í fæðusamsetningu eftir stærð fiska undir 40 cm, þar sem agnir og marflær voru í svipuðum mæli í fæðunni. Hins vegar virtust marflær mun meira étnar af stærri fiski (>40cm). Hjá sjóbleikju óx meðalþungi fæðudýra tiltölulega lítið með fisklengd.

Abstract

Food was observed in 72 anadroumus brown trout (Salmo trutta) and 167 sea-run Arctic charr (Salvelinus alpinus) in the brackish water lagoon Dyrhólaós, which is an sea lagoon in South Iceland. Fish were collected in June, July, September and October in 1990. The food of both species varied during the collection period and according to the areas of sampling in the lagoon. In June, the main food item for the trout was Mysids (Mysidacea). At the end of July and in mid September, however, the basic food was sandeels (Ammodytidae), but in the middle of October, Mysids were again the most abundant food. Amphipods (Amphipoda) were also prominent in the diet, especially in early summer. Sandeels were common in the diet in samples taken near the esturary to the sea but Amphipods were more frequent further inside the lagoon. Average prey weight increased with fish length, and was on average 33 times heavier in the largest (>40 cm) trout than in the smallest (<20 cm). The food of Arctic charr was monotonous. Only two types of food were found in their stomachs, i.e. e. Mysids and Amphipods. On sampling sites at the inner part of the lagoon, where salinity was low, Amphipods were more important food than Mysids and it seems to be little variation between sampling dates. A greater variation was seen in the more saline waternear the sea. Mysids were the dominant food in June and September but Amphipods in July and October. In the case of Arctic charr, relatively small changes in food were observed in fish under 40 cm in length, where Mysis and Amphipods were taken in similar proportions. However, Amphipods appeared to be much more eaten by larger fish (>40 cm). In Arctic charr, average prey weight increased relatively little with fish length. Results are discussed in light of other observations.