Unequal Thermal Risks in a Socioeconomically Important Tropical Reef Fishery

Andrew R Villeneuve , Merrill Baker-Médard, Bemahafaly Randriamanantsoa, Easton R White 

Climate change poses an understudied threat to the coastal reef systems of the Western Indian Ocean (WIO), a biodiversity hotspot critical for the maintenance of subsistence fisheries. The southwestern coast of Madagascar is especially sensitive to disturbances due to the high level of socioecological dependence on functioning reefs.
Fishing in this region is spatially structured. For example, gleaning (pêche-à-pied) is undertaken predominantly by women and children walking in intertidal reef flat and crest habitats during low tide, while boat-based fishing occurs across a range of reef geomorphic zones. These zones may experience different microclimates due to differences in depth, water residence time, and oceanic exposure. To characterize the thermal microclimates of these zones, we deployed temperature loggers for over a year across reef geomorphic zones on Salary Reef, Madagascar. We further examined modeled tide elevation data over the deployment period and the 18.6-year tidal cycle. Subtidal loggers showed similar mean temperature but diverged up to 27% in cumulative hours above 30 °C. Compared with gridded temperature datasets, logger observations revealed higher extremes and lower minima, underscoring the added fidelity of in-situ measurements. The intertidal logger recorded extreme thermal stress (up to 48.8 °C) in gleaning habitats, with maximum heat events occurring in transitional “shoulder” seasons rather than peak summer. Tidal analyses showed that midday low tides in these transitional periods drive extreme intertidal warming. Fisheries in Madagascar will likely experience uneven and seasonally distinct thermal risks. Climate adaptation must consider how microclimate variability mediates socioecological vulnerability in coastal oceans.