The influence of environmental variables on the abundance and temporal distribution of Octopus cyanea around Mafia Island, Tanzania

Abstract

Abundance and distribution of short-lived species like Octopus cyanea depends on favorable environmental conditions, which are essential for growth and successful recruitment. However, the influence of environmental variables on the abundance and distribution of O. cyanea remains unknown, particularly in the Western Indian Ocean (WIO) region, despite its importance as a fishery resource. This study investigated the role of environmental variables (sea surface temperature [SST], chlorophyll-a [Chl-a], rainfall and wind) on the abundance and temporal distribution of O. cyanea around Mafia Island in Tanzania for a period of 60 months (2013–2017). Octopus catch data were obtained from TANPESCA fish processing factory, while environmental data were obtained from the National Aeronautics and Space Administration - NASA satellite (SST and Chl-a) and the Tanzania Meteorological Agency - TMA (rainfall and wind). The results for the O. cyanea CPUE showed two peaks, between March and October, suggesting successful recruitment and stable environmental conditions during these periods. The additive effect of wind speed and Chl-a with a 4-month lag, was the best model for predicting the abundance and temporal distribution of O. cyanea around Mafia Island followed by the additive effect of SST, wind and Chl-a. Temperature influences octopus growth, while strong wind facilitates the mixing of water, which brings the bottom nutrient rich water to the euphotic zone for increased photosynthesis. In conclusion, the abundance and temporal distribution of O. cyanea are influenced by the additive effect of wind, Chl-a and SST with time-lagged effect of four months. This study recommends seasonal closure of the octopus fishery between April and July and reopening in March and October for sustainable harvesting of this valuable resource.