Abstract
and Plectropomus, are important in local/international trade, and are particularly susceptible to overfishing due to their economic value, longevity, late maturation and, for some species, aggregation-spawning. Three species, P. areolatus, E. polyphekadion, E. fuscoguttatus, are threatened (IUCN Red List) and, when exploited on their aggregations, typically undergo declines unless managed. To effectively assess spawning aggregation status and identify changes over time following fishing or management, a robust sampling protocol is essential. This was developed and tested at a protected, but previously depleted, spawning site shared by these three species in Palau, western Pacific. Underwater visual census (UVC) tracked changes in fish abundance (numbers) across their aggregation site between 2009 and 2019. Census data on abundance and density were complemented by additional technologies to generate a more complete picture of this aggregation site and the three species, including stationary cameras to monitor fish with divers absent, stereo-video to measure fish lengths, and oceanographic instruments to measure variability in currents and water temperature. Results show that protection outcomes depend on biology and on active enforcement and that UVC survey design must adequately address temporal/spatial variability to effectively document changes in fish abundance. Over the decade-long study, P. areolatus, the fastest-maturing species, showed a near 4-fold increase in peak annual abundance (increasing from annual peak numbers of c.450 to 1800 fish), followed by a more modest increase in E. polyphekadion (increase from c.500 to at least 600 fish and a 2-fold density increase) and relative stability in the slowest maturing, longest-lived species, E. fuscoguttatus (stable between approximately 300 and 450 fish). The study highlighted need for caution when fish density is used as a proxy for abundance in studies when entire aggregations cannot be surveyed, because the two measures may become non-linear at higher abundances. The results clearly show the need for robust sampling design and that effective protection contributes to recovery of depleted spawning aggregations.
Highlights
At least 100 coral reef fishes from 20 families form spawning aggregations as an important means of reproduction (Sadovy de Mitcheson and Colin, 2012)
This study focused on three commonly co-aggregating groupers P. areolatus, E. polyphekadion, and E. fuscoguttatus
The highest number of fish recorded on a single day from each year, considered to be the annual peak abundance, showed a clear increasing trend for the fastest growing species, P. areolatus, with a near fourfold increase of 459–1,809 fish between 2009 and 2019 (Figure 2A)
Summary
At least 100 coral reef fishes from 20 families form spawning aggregations as an important means of reproduction (Sadovy de Mitcheson and Colin, 2012). Spawning aggregations (SA) are centers for mate selection and reproduction, being vitally important for the persistence of the populations of many fish species. Aggregations are highly dynamic, varying markedly in the number of fish gathering, spatial extent, duration, seasonal timing, and frequency of formation each year; they can build and disperse over short time periods (Sadovy de Mitcheson and Colin, 2012). Aggregations provide a window of opportunity for scientific surveys of the population to monitor changes over time once the species’ spatial and temporal dynamics and sources of variability have been incorporated into survey design (Colin, 2012)
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