THE IMPACT OF MARINE PROTECTION ON THE SIZE DISTRIBUTION AND SURVIVAL OF THE LOBSTER POPULATION IN THE INNER OSLOFJORD.

Abstract

The elimination of fishing pressure through the establishment of marine protected areas (MPAs) have been positioned as one of the ways to conserve the declining lobster population in Norway. MPAs have been credited with improved survival, abundance, growth rate, rebuilding of size structure and the strengthening of sexual selection observed in protected lobster populations. There is however insufficient documentation on the efficacy of small-sized MPAs in restoring the depleted lobster stock. I performed a short-term replicated Before-After Control-Impact (BACI) study using capture-mark-recapture data obtained from the MPA in Drøbaksundet and two adjacent harvested sites two years before and two years after the MPA was established. Investigations were done to test the potential long-term effect of protection and the short-term effect of harvesting on population parameters such as catch-per-unit-effort (CPUE), sex ratio, body length, proportion of berried females and the probability of lobsters to be legal sized in both the MPA and fished sites. Lobster sampling sessions were conducted in September and December, i.e., before and after the lobster harvest season. Two years after the MPA was established, there was a rise in lobster CPUE in the MPA with more than a two-fold increase over the pre-MPA values, males responded more positively than females. A moderate rise in CPUE was also observed in the harvested sites. Mean length of lobsters in the MPA rose, with the increase in males doubling that of females. There was also an increase in the proportion of females in the MPA with the potential of a more balanced sex ratio after protection. Additionally, the proportion of legal sized lobsters in the MPA increased, accompanied by a non-linear relationship between lobster length and the probability to be berried, females attained a peak probability to be berried at 30 - 32.5 cm and a subsequent decline thereafter. The impact of harvest (October-November) was marked by a general decline in lobster CPUE from the pre- and post-harvesting sampling periods. There were also site-specific responses in mean length and an increased probability for lobsters to be female after harvest. The impact of harvest resulted in females being more likely to be berried at 27.5-30 cm with lobsters in areas open to harvest having a reduced probability to be legal sized compared to those in the MPA after harvest. I conclude that even a small sized MPA, like the one in Drøbaksundet, despite being in its early phase can be effective in restoring the depleted lobster population in the inner Oslofjord.

The elimination of fishing pressure through the establishment of marine protected areas (MPAs) have been positioned as one of the ways to conserve the declining lobster population in Norway. MPAs have been credited with improved survival, abundance, growth rate, rebuilding of size structure and the strengthening of sexual selection observed in protected lobster populations. There is however insufficient documentation on the efficacy of small-sized MPAs in restoring the depleted lobster stock. I performed a short-term replicated Before-After Control-Impact (BACI) study using capture-mark-recapture data obtained from the MPA in Drøbaksundet and two adjacent harvested sites two years before and two years after the MPA was established. Investigations were done to test the potential long-term effect of protection and the short-term effect of harvesting on population parameters such as catch-per-unit-effort (CPUE), sex ratio, body length, proportion of berried females and the probability of lobsters to be legal sized in both the MPA and fished sites. Lobster sampling sessions were conducted in September and December, i.e., before and after the lobster harvest season. Two years after the MPA was established, there was a rise in lobster CPUE in the MPA with more than a two-fold increase over the pre-MPA values, males responded more positively than females. A moderate rise in CPUE was also observed in the harvested sites. Mean length of lobsters in the MPA rose, with the increase in males doubling that of females. There was also an increase in the proportion of females in the MPA with the potential of a more balanced sex ratio after protection. Additionally, the proportion of legal sized lobsters in the MPA increased, accompanied by a non-linear relationship between lobster length and the probability to be berried, females attained a peak probability to be berried at 30 - 32.5 cm and a subsequent decline thereafter. The impact of harvest (October-November) was marked by a general decline in lobster CPUE from the pre- and post-harvesting sampling periods. There were also site-specific responses in mean length and an increased probability for lobsters to be female after harvest. The impact of harvest resulted in females being more likely to be berried at 27.5-30 cm with lobsters in areas open to harvest having a reduced probability to be legal sized compared to those in the MPA after harvest. I conclude that even a small sized MPA, like the one in Drøbaksundet, despite being in its early phase can be effective in restoring the depleted lobster population in the inner Oslofjord.