Giant trevally spawning aggregation highlights importance of community fisheries management no-take zone

Abstract

Location and timing of spawning aggregations for large teleost predators either remain elusive or constitute core-fishing sites in developing countries, limiting or confounding management actions for sustainable harvesting. In the Quirimbas Archipelago in northern Mozambique, limited artisanal fisheries catch data are available, and only emerging management plans are in place in scattered isolated locations. Artisanal fisheries, however, are the dominant fisheries sector in the region, employing a large proportion of the local population and contributing the main food source for consumption. Giant trevally (Caranx ignobilis) is a highly valued fish and consumed locally but data on giant trevally spawning locations is limited (Westernhagen 1974). This is a concern, given spawning aggregations are extremely susceptible to over fishing, but their functional role is imperative for maintaining ecosystem resilience. Within a community declared no-take zone on the eastern point of Vamizi island, a large aggregation of giant trevally were observed by scuba divers on December 15, 2013, two days prior to the full moon. The fish entered the shelf edge at ∼15–20 m depth from the adjacent deep-water reef channel during daylight hours (15:00) and two hours after high tide. The aggregation numbered >1,000 fish that were estimated to be approximately 80–100 cm total length and larger (Fig. 1a). Individuals were observed pair chasing and displaying color changes (silver to black; Fig. 1b). The fish remained in aggregation formation on the shelf for 1–2 min surrounding the divers before returning to the deep-water channel. Although the physical release of gametes was not observed, indirect evidence commonly used to infer spawning activity, indicate these fish were in the process of spawning