Knut Sivertsen

Overgrazing of kelp beds along the coast of Norway

The aim of this study was to better understand the down-grazing of kelp beds by sea urchins (Strongylocentrotus droebachiensis) along the coast of Norway. Barren grounds were first observed in sheltered areas along the coast of the counties of Trø ndelag, Nordland and Troms in 1974. In the 1980s, the barren grounds spread to areas more heavily exposed to waves. In the 1990s, the kelp beds were re-established in some localities in southern Trø ndelag, initially in wave-exposed areas. In the northernmost parts of Norway, i.e. the counties of Troms and Finnmark, the barren ground areas may still be increasing. Crabs (Cancer pagurus) and common eiders are the most common predators on urchins. Predation on sea urchins in kelp beds is probably not among the factors that limit the sea urchin populations. Along the coast of Nordland and further north, sea urchins are infected by nematodes, resulting in a low, but significant increase in their mortality. No re-growth of kelp beds has been found in the most infected areas. In the late 1960s and the early 1970s, a high occurrence of echinoderm larvae was observed in deeper waters. This was a period with cold water, which may have caused high recruitment of sea urchins. The bet-hedging life strategy of sea urchins may account for the sudden increase in the size of the populations. In the present paper I propose the hypothesis that higher individual growth rates and higher mortality rates in the south than in the north may explain the decrease in the populations, which may in turn account for the re-growth of kelp in the southern areas.

Population studies of Laminaria hyperborea from its northern range of distribution in Norway

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 sectors 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 exhibited 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).

On the brink of the Arctic: Unusual intertidal sub-Arctic kelp associations in the Porsangerfjord, North Norway

Abstract The kelps Laminaria digitata and Saccharina latissima are typically sub-littoral species. It is therefore surprising to find large intertidal kelp beds dominated by these two species in the inner Porsangerfjord, North Norway. The areas of the nine intertidal kelp beds ranged from 0.01 to 3 km2 and covered a total area of 8.3 km2. Both species had a prostrate growth form with short stipes, usually less than 10 cm long, independent of the length of the lamina. The laminae of L. digitata and S. latissima measured 77.0 ± 35.2 cm and 99.4 ± 41.0 cm (mean ± 1 SD), respectively. The associations occurred in sheltered areas with a muddy bottom on wide, flat areas of moraine. The areas are completely drained at low tide. The holdfasts were attached to small pebbles sunk in the mud. The mean biomass density of the two dominant kelp species was 4.1 ± 1.8 kg m−2. The mean biomass of all the algal species in the association was 6.1 ± 2.2 kg m−2. A total of 32 species were identified in the association, of which 16 species were annuals and 16 were perennials. All the species found in the kelp associations are also found in the Arctic. The prostrate growth form of L. digitata and S. latissima, the topography and substrate of the intertidal zone, low summer temperatures, low desiccation and protective sea ice during winter are possible factors contributing to the formation of these unusual intertidal kelp associations.

Trap Catch of Green Sea Urchins Strongylocentrotus droebachiensis in Kelp Beds and Barren Grounds on the Norwegian Coast

Abstract Field investigations were performed to find out if trap catch could be an alternative to diving to harvest green sea urchins (Strongylocentrotus droebachiensis). Laboratory experiments in y-maze tubes showed that starved sea urchins responded on tree different baits in about one minute, but they did not discriminate between offal of fish (saithe), sea urchin fodder, and lamina of kelp. The trap catch investigations were performed at 0–2.5 m depth in Ytre Kårvika in Troms County and 2-m depth at Hjelmsøya in Finnmark County in autumn 2002 and summer 2003, respectively. Repeated catches 12 times at the same area in Ytre Kårvika showed higher catches at the last six drags than the first six during one month of fishing. In the Hjelmsøya area highest catches were found in barren grounds with high densities (50–80 individuals m−2) of sea urchins, whereas in kelp beds the sea urchin catches did not exceed catches if baits were not used. Offal of catfish and cod heads caught higher catches than sea urchin feed and lamina of kelp. Ring traps (diameter 45 cm) caught better than dropnets, and dropnets caught better than box traps. The 13 drags with the best catches out of 41 drags caught in mean 1.43 kg trap−1 day−1. Probably during the highest catches, the ring traps were saturated. These traps caught 1.23% of the sea urchin resources in the area per day. Estimates show that an area can be harvested for 41–165 days until it is cleaned, depending on the densities of the resources and the harvesting strategy chosen in the estimates. The traps caught sea urchins with low gonad indices, and the sea urchin industry has to feed them to increase the gonad content before they can be equipped.

Benthic algal vegetation in Isfjorden, Svalbard

Benthic algal vegetation was investigated at 10 sites in Isfjorden, Svalbard. Five sites were visited during summer 2010 and five during summer 2012. Both the littoral and sublittoral vegetation were sampled, the littoral by hand-picking and use of a throwable rake and the sublittoral using a triangular dredge. A total of 88 different taxa were registered, comprising 17 Chlorophyta, 40 Ochrophyta, 30 Rhodophyta and the Xantophyceae Vaucheria sp. The green algae Ulvaria splendens (Ruprecht) Vinogradova was recorded in Svalbard for the first time. Most of the sites consisted of hard bottom substrate, but one site, Kapp Wijk, consisted of loose-lying calcareous red algae (rhodoliths) and had species not recorded elsewhere. The sublittoral at the other sites was dominated by kelp. Molecular analysis confirmed the presence of the red alga Ceramium virgatum and a dwarf form of the brown alga Fucus vesiculosus. This study provides a baseline for future studies investigating changes in the vegetation due to environmental changes.