The biology and fishery of common octopus (octopus Vulgaris, cuvier 1797) in the Kenyan south coast
Although common octopus catches are increasing globally, lack of information on the species biology and fishery has been a major concern in its management particularly in Kenya. The present study aimed at investigating the fishery, reproductive and feeding biology of common octopus from Shimoni and Vanga which are some of the major fish landing sites in the Kenyan South coast. Sampling was done monthly from November 2010 through November 2012 using traditional fishing spear ‘mkuki’ or ‘shomo’. For each specimen, body weight (BW), total length (TL), dorsal mantle length (DML), ventral mantle length (VML) and gonad weights were recorded. Maturity stages and Gonadosomatic Index (GSI) were determined using standard methods. Stomach content analysis was determined using both the frequency of occurrence method and the dominant (numerical) method. For the processing and markets of common octopus, information was collected from logbook records, and landings from fishermen and fish industries. A total of 1,599 specimens were collected, 746 males and 853 females. The size distribution range of the females varied between 5.40 cm and 24.50 cm DML and from 0.07 kg to 5.50 kg BW, while the DML of males ranged from 5.80 to 18.50 cm and the BW from 0.08 kg to 3.95 kg. The monthly mean DML values ranged between 9.52 ± 1.81cm and 13.53 ± 2.51cm at Shimoni and 9.14 ± 1.56 cm to 13.00 ± 2.36 cm at Vanga. The monthly mean DML and BW values showed a decreasing trend over the two-year period, an indication that octopus population was under high exploitation pressure. The sex ratio was 1:1.1 (males: females) for both Shimoni and Vanga during the study period. However, there was no seasonal or annual significant of females. The female length at first maturity was 10.8 cm (DML 50%), while for the male was 10.5 cm (DML 50%), respectively. There was no significant difference, statistically between female and male length at first maturity. The common octopus preferred crustaceans (mainly crabs) to other food items and its diet preference was not influenced by seasons. The processing of octopus was done using both traditional techniques and modern facilities that are fully compliant with the food safety standards for export markets. The value-added octopus final products ranged from traditional dried octopus to chilled and frozen packed octopus. The species was marketed fresh, frozen and dried, mostly for human consumption. A small local market was found to exist for common octopus of which most of it was sold in local seafood tourist restaurants. International markets existed in various countries including Italy, Spain, France and Portugal. The scientific information gathered during this study on the fishery, reproductive and feeding biology of common octopus will go a long way in informing policy directions and the management of octopus in Kenya. Key words: common octopus, fishery, reproductive, feeding, Shimoni, Vanga.