Evaluating various methods for assessing population density and ecology of the common octopus (Octopus vulgaris) in Corsica (Western Mediterranean Sea).
Supervisor: Sylvie Gobert (Univ. Liege)
The common octopus, Octopus vulgaris, is the most commonly fished octopus species in the Atlantic and the Mediterranean Sea. However, despite its extensive exploitation, knowledge on its distribution, population dynamics and ecology are still widely lacking in many regions, hindering effective management. This preliminary study aimed to determine the suitability of three methodologies for specific research aims, estimate the population density at two locations in the North of Corsica, and enhance the knowledge of the O. vulgaris population. The study was the first to investigate this species in the Bay of Calvi and the Natural Marine Park of Cap Corse and Agriate (PNMCCA). Day- and nighttime visual censuses were undertaken by scuba diving, recording octopuses and their dens. Fishing traps were deployed for 24 hours, and subsequent body measurements taken of the octopus caught. Furthermore, clay pots were placed underwater, and octopus presence was recorded by scuba diving after one to three months. The catch per human effort (CPHE = number of octopuses/ hour of human effort) was calculated for each method and each study site. Results indicated that densities at the study sites were lower than those reported in other regions, suggesting a possible naturally lower octopus density in the North of Corsica. Environmental factors, such as substrate type, time of day and season were identified to influence octopus distribution, and the sex and adult-juvenile ratio. Findings suggested that abundance also varied with site and method, with nighttime visual censuses yielding the highest mean octopus count with the least effort. Octopuses showed preferences for different pot sizes, which could provide useful in establishing protected areas where pots are used as enrichment. Each methodology demonstrated useful to achieve various research objectives and was recommended for use in different studies. This preliminary research underscored the importance of more extensive sampling and long-term population assessments for the establishment of baseline densities to better understand the health of O. vulgaris stocks. The study provided first insights into the O. vulgaris population in Northern Corsica and may serve as a first step towards future conservation efforts for this species in the Mediterranean and Atlantic Ocean.