Factors relating to catch per unit effort in the exploratory Octopus vulgaris pot fishery of South Africa

Abstract

The initial phase of the South African fishery for Octopus vulgaris saw great interest from fishers to invest. This initial interest however, was not carried over into actual fishing as many applied for the right, but never started fishing. A lack of knowledge on how to best harvest octopus in South Africa, proved to be the downfall of the initial phase from 2003 to 2009. After efforts made by the then Department of Environmental Affairs and Tourism, as well as individuals in the fishery, a greater knowledge on octopus fishing practices was garnered from Australian octopus stake holders. This knowledge was applied to the South African fishery and saw one right holder start consistent fishing in the False Bay area, although four rights were allocated. Consistent data collection from 2014 to 2019 allowed for the completion of this study after a lack of data in initial phases. The Australian trigger trap was the gear deployed during this period, which was very different from the PVC pots used from 2003 to 2009. The change in gear came about because of the belief that the Australian-made gear was more stable in South African waters i.e. it provided a den site for octopus that would not be moving around in the strong currents. On-board and Factory weight data from False Bay was collated into various databases for analyses and verification. Since databases shared similar factors, gaps in data were filled in order to calculate the CPUE (catch per unit effort). Statistical analyses and standardisation of affecting factors and CPUE was conducted in R studio. CPUE is an important factor related to the management and monitoring of exploited species. CPUE was then plotted against various factors in order to determine their effects. The greatest influence on CPUE was found to be month, season and area, found to have statistically significant differences within years. These factors were tested using an ANOVA test. In order to determine which months, seasons or areas were statistically significant, the Tukey’s Honest Significant Difference test was used. Influences of these factors skewed the relationship between CPUE and inferred abundance, requiring standardisation. Month and Area were standardised to remove their effect on nominal CPUE to view the index of abundance trends of O. vulgaris in the False Bay Area based on standardised CPUE. A Generalised additive model was used to standardise data for analyses of CPUE, independent of factors that would cause an effect. Standardised CPUE trends revealed peaks in the population abundance in summer and spring, size distribution followed the same trend with medium and small animals but large animals were more prevalent in the autumn and winter months, and accounted for the greatest volume by weight, being caught in autumn. The fishing vessel (Albatross) harvested 178.7 tons between 2014 and 2019. The on-board database revealed a total of 77500 octopuses caught from 2016 to 2019, accounting for 77% of the total weight at 138.6 tons. The estimated total number of octopus caught was 101088 with an average catch percentage for False Bay of 43.5% per opening, with a mean mass of 1.8 kg per octopus caught and mean number of 0.99 octopus per pot. The west of False Bay had more pots checked (46301), number of octopus caught (43159) and number of days fished (135). However mean CPUE was higher in the east of False Bay (1.08 octopus/pot/line) compared to the west (0.94) and were found to be statistically significantly different between the two management zones. The findings in this study provide much needed scientific knowledge regarding the O. vulgaris fishery in False Bay, South Africa. The Department of Fisheries Forestry and Environment may make informed management decisions regarding the sustainable utilisation of the target species and continue to monitor the progress thereof. Based on results found in this study, it is recommended that the effort applied to the fishery be reduced from two right holders per fishing zone to one. Currently operations are commencing in this way, due to only one right holder fishing in the False Bay area. The restriction on gear currently applied, should be retained as the current number of 6000 pots shows no indication of adversely affecting the population of O. vulgaris.