Population Dynamics of the Planktivorous Arctic Charr Salvelinus alpinus ("murta") in Thingvallavatn

The annual yield of the commercial fishery for planktivorous charr ("murta") in Thingvallavatn has varied between zero and 73 tonnes since 1958, with two periods of high catches: 1963-1965 and 1976-1984. The commercial fishery is performed with 22 mm bar mesh gill nets on the spawning grounds. The nets selectively catch mature fish over 17.5 cm in length. From 1983 to 1987 the catches decreased from 63 tonnes to zero. During this period mean size of murta spawners decreased from 18 to 13 cm for males, and from 19 to 15 cm for females, without any reduction in age at maturity. This indicates that the reduction in fish size renders the commercial nets ineffective. There were no indications that the reduction in commercial catches was caused by overfishing or recruitment failure. Conversely, echo-sounder stock estimates indicated an increase in murta abundances from 1985 to 1987. No year-classes present in the catches appeared to be unusually weak, whereas several year classes (e.g. 1981, 1983, 1984) appeared relatively strong.