Lag Effects in the Impacts of Mass Coral Bleaching on Coral Reef Fish, Fisheries, and Ecosystems

Abstract

The increased frequency and intensity of bleaching episodes has led to wide-scale loss of reef corals and raised concerns over the effectiveness of existing conservation and management efforts. The 1998 bleaching event was most severe in the western Indian Ocean where coral declined by up to 90% in some locations. Using fisheries independent data, we assess the long-term impacts of this event in the Seychelles on fishery target species, the overall size structure of the fish assemblage and assess the effectiveness of two marine protected areas (MPAs) in offering resilience. Fishery target species above size at first capture showed little change in biomass between 1994 and 2005, corroborating studies that suggest fisheries yields are currently not affected. Biomass remained higher in protected areas, indicating they are still effective in protecting fish stocks. However, the size structure of the fish communities, as described with size-spectra, changed in both fished and protected areas, with a decline in smaller fish (<30 cm) and an increase in larger fish (>45 cm). This is likely to represent a time lag response; with the larger fish that are lost to natural mortality and fishing no longer being replaced by juveniles. This effect is expected to be even greater in terms of productivity, affect fisheries and, as congruent patterns are observed for herbivores, suggests no long term resilience in the MPAs.Corallivores and planktivores demonstrate striking declines in numerical abundance which are greatest in MPAs, and associated with a similar pattern of decline in their preferred corals. There is an urgent need for climate mediated disturbance to be at the fore of conservation and management planning for coral reefs, which should include MPAs placed in areas of resistance and resilience to bleaching, and a greater emphasis on reducing other stressors to the system as a whole.3

Perceptions of trends in Seychelles artisanal trap fisheries: comparing catch monitoring, underwater visual census and fishers' knowledge

“…Kasye peze fishers tend to fish shallower than the reef slopes targeted by UVC, while kasye dormi fishers can expand beyond the shallow fringing reefs targeted by UVC onto the expansive Mahé plateau. Graham et al (2007) estimated that only 50-60% of trap fishing grounds overlapped with habitats and depths sampled by these UVC data.…”

“…The abundance and size of 134 species of diurnally-active reef-associated fish were recorded at each count (Jennings et al 1995;Graham et al 2006). Length calibration was achieved by estimating the lengths of a random assortment of lengths of PVC pipe before each day's sampling, until the observer was within an error range of 1cm; mean errors for 1994 and 2005 were 3.1% and 2.2%, respectively (Graham et al 2007). Abundance-length data were converted to biomass using published length-weight relationships (Letourneur et al 1998;Froese & Pauly 2006).…”

“…Total UVC-measured fish biomass, and biomass of target species (Table S1 in Appendix, see supplementary material at Journals.cambridge.org/ENC) that were large enough (> 6 cm body depth) to be caught by the inshore trap fishery (Graham et al 2007), were analysed for differences between 1995 and 2005. Linear models with year, site (see Fig.…”

Transitional states in marine fisheries: adapting to predicted global change

“…The severity of losses among small-and medium-sized fishes was clear from pre-and post-bleaching community size spectra (figure 3c), with higher post-bleaching size-spectral slopes reflecting widespread losses of small fish, even when large herbivorous fishes increased in abundance (figure 3d; Graham et al 2007). These losses were substantial, affecting both small species and small size classes of larger fishery target species .…”

Revisiting the Cassandra syndrome; the changing climate of coral reef research

“…The overall abundance and diversity of reef Wshes are certainly sensitive to declines in 'reef rugosity', which may occur several years after extensive coral mortality (e.g. Graham et al 2007). However, declines in reef rugosity do not always occur, and depend on relative contributions of contemporary coral growth versus erosion of the underlying reef framework ).…”