Trap catch of green sea urchins Strongylocentrotus droebachiensis in kelp beds and barren grounds on the Norwegian coast

Summary

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.