Hydrobiologia 260/261: 215-221, 1993. A. R. O. Chapman, M. T. Brown & M. Lahaye (eds), Fourteenth International Seaweed Symposium. © 1993 Kluwer Academic Publishers. Printed in Belgium. 215 Population studies of Laminaria hyperborea from its northern range of distribution in Norway Kjersti Sjotun, Stein Fredriksen', Tor Eiliv Lein, Jan Rueness & Knut Sivertsen 2 Department of Fisheries and Marine Biology, University of Bergen, Bergen High Technology Centre, N-5020 Bergen, Norway; 'Department of Biology, University of Oslo, P.O. Box 1069 Blindern, N-0316 Oslo 3, Norway; 2 NFH, University of Tromso, Dramsvn, 201B, N-9000 Troms, Norway Key words: Laminaria hyperborea, northern limit, biomass, age distribution, production Abstract Eight populations of Laminaria hyperborea from wave-exposed localities in Finnmark (70-71 °N) were compared with two populations in Vega (65 N). Standing stock in shallow water (3-5 m) was within the same range in the two areas (6-16 kg fr. wt. m-2). Both the highest and lowest value of standing stock were registered in Finnmark. Degree of wave-exposure was determined by the numbers of sec- tors exposed to open sea at each locality, with each sector given a relative wind force value. The most wave-exposed locality in Vega had a slightly higher annual biomass production as a function of plant age compared with the populations from Finnmark within a similar range of wave exposure. Of the two localities in Vega the most wave-exposed one had a higher annual biomass production per plant than the more sheltered one. The most wave-exposed locality examined was situated in Finnmark and ex- hibited the lowest annual biomass production per plant. Recruits (one-year-old plants) were found at all the localities except at one locality in Finnmark. The plants reached a higher age in Finnmark (13-18 years) than in Vega (8-9 years). Introduction Laminaria hyperborea grows in the northeastern Atlantic, from Iceland, the North Cape of Nor- way and the Kola Penninsula to Portugal (Kain, 1971a). It is not clear why L. hyperborea does not penetrate further north into the Arctic. The south- ern limit of distribution is probably due to a growth and reproductive boundary (Ltining, 1986), and possible competition with L. ochroleu- ca and Saccorhiza polyschides (Kain, 1979). Laminaria hyperborea plants may reach their maximum size between 55 ° N and 65 ° N (Kain, 1971a), although local conditions are generally demonstrated to have a greater influence on plant growth than latitude (Kain, 1967). Under shel- tered conditions the plants develop a special mor- phology, L. hyperborea f. cucullata (Svendsen & Kain, 1971). During the 1950s the standing crop of Lami- naria hyperborea in northeastern Norway was es- timated to be 7.4 kg fresh weight m- 2 at 2-6 m depth (Grenager, 1956). However, these results were obtained by using a spring grab, which un- derestimates the standing crop (Kain, 1971 la). Re- liable biomass estimates of L. hyperborea in northern Norway have not been available. This paper presents a study of Laminaria hy- perborea populations from the northernmost part of its geographical range. The populations are