Short-term effects of a marine sanctuary on the local lobster population in the Oslo fjord, Norway

The European lobster (Homarus gammarus) population in Norway is over-fished and endangered, and marine protected areas (MPAs) are an increasingly utilized measure for protection of this species. Previous studies in Norway have shown that even small MPAs can benefit the local lobster population, and that protection responses can be detected after a short period of time, such as increased survival, population abundance and mean body size. Still, there are different demographic responses in different MPAs, which means there are regional effects that call for more investigations. This is the first study conducted in the Oslo fjord, presenting an analysis of the short-term demographic responses to protection of a new, small MPA. I have studied survival, population density and sex ratio in a fjord system, the Oslo fjord, with high pressure from fisheries, pollution, climate change and other human impacts, using mark-recapture methodology with a “before-after control-impact" design (BACI) applying robust design models. The sampling effort consisted of 3 sessions (September 2020, September 2021, and December 2021) with 20 traps x 5 days and ~24 hours soaking time. A total of 304 lobsters were captured and tagged, and 49 recaptures were registered. Within the MPA, male survival was shown to increase with body size and was substantially higher for large body sizes relative to male survival in the control area, proving that male lobsters benefit from sex-selective harvesting protection already after just one effective protection season. Simultaneously, a high and stable survival for females in the control area showed that females benefit from fishing regulations that protect egg-bearing females. Confidence intervals were large, especially for males in the control area and females in the MPA, due to small sample sets and few recaptures. The abundance and density of male lobsters in the MPA was much higher than in the control area after the first effective protection season, proving that fishing mortality is high in the harvested population and gives room for rapid demographic responses in a recently protected lobster population. The body size distribution in both MPA and control area are further testimony to a high and sizespecific fishing mortality in harvested areas of the Oslo fjord, showing low fractions of large males and of legal-sized females. A high fraction of small males in the MPA points to a high potential for rapid growth and increased phenotypic diversity in the protected population following protection. The results show that mean body size did not change notably for neither females, nor males, but strongly suggests that catchability for female individuals increased from September to December. Sex ratio investigations show that there is a relatively higher share of males in the MPA after the first effective protection season, relative to the control area. Combined, the results show that marine protected areas have short-term effects on local lobster demography, indicating that the fishing pressure in high and gives room for rapid responses in the lobster population.M-N