EXEBUS: Ecological and Economic Impacts of the intensification of extreme events in the Benguela Upwelling System

Informations

Funding country

Norway

Acronym

-

URL

-

Start date

1/1/2020

End date

12/31/2024

Budget

355,347 EUR

Fundings

Abstract

Coastal ecosystems around the globe are subjected to an increasing impact of marine heatwaves (MHWs) as the ocean becomes warmer. Migratory aquatic species avoid extreme conditions by shifting their distributional range from the affected regions towards more favourable habitats. This natural process in species can cause grave consequences to coastal economies whose livelihood depends on fisheries. Angola is an example of such a dependence. Its fisheries are dominated by un-motorized artisanal fleets that rely on resource availability close to the coast. The main resource they target are sardinellas, coastal fishes characterised by strong distributional responses to climatic pressures. Developing scientific basis to understand the MHW impacts on the availability of sardinella to artisanal fishers in Angola has been the main activity of IMR in EXEBUS through 2022. The analyses were focused on three tasks: (1) identifying climatic drivers of variability in the seasonal migrations of sardinella, (2) describing the evolution of MHWs observed during the last three decades, (3) identifying patterns of correspondence between MHWs and distributional shifts of sardinella during the same period, and (4) engaging with stakeholders to share information and gather feedback on MHW impacts on fish availability and fishing effort. Task 1: The data on Angola’s sardinella population dynamics are scarce. To investigate climate-induced population changes, we have adopted a “habitat-based” conceptual framework. It rests on the assumption that observed population changes in small pelagic fish result from their adaptive strategies to climatic pressures within their habitat. In line with this hypothesis, the southward migration of sardinella during austral summer is interpreted as an adaptive response to the seasonal deterioration of the feeding and spawning conditions in its primary habitat, located north of 13 S. Because the interannual MHWs observed in the Angolan waters- the Benguela Niños, also occur during austral summers. they amplifies the usual habitat pressures associated to the seasonal arrival of warm water, implying a stronger response in fish distribution during Benguela Niño years. Furthermore, the sardinella stock comprises two species differing in plasticity to salinity, the stenohaline S. aurita and euryhaline S. maderensis. Awo et al. (2022), using a NEMO model, showed that seasonal warming and low salinity intrusions occur synchronously. This synchrony suggests stronger migratory responses to MHW by S. aurita compared S. maderensis . Task 2: Overall, 93 MHW events were identified in the Angolan coastal ocean between 1994 and 2015. The gravest MHWs matched the timing of the Benguela Niños. Different metrics may be applied to categorize these events. The maximum intensity occurred during the 1995 Benguela Niño. The maximum cumulative intensity, in contrast, characterized the 2010/11 event. The MHW of lesser durations (5-15 days) were observed across the entire time series. Still, these tended to occur during austral summers, rather than winters, and showed a higher frequency of this occurrence during the warm pentads of 1995-1999 and 2006-2010. Task 3: The peaks in the Angolan sardinella population size were observed during the same warm period periods. In the case of the 2006-2010 warming, the Angolan peak matched a comparable drop in the biomass estimated for Gabon and Congo, thus suggesting the ocean-warming-induced distributional shift of sardinella towards the relatively colder coastal waters of Angola. Within the Angolan waters, the most extreme changes in the sardinella range (southward and offshore shifts) were observed during the gravest MHW events of the 1995 and 2010/11 Benguela Niños. These abnormal distributional shifts were transient, with the sardinella stock returning to the normal distribution within its primary habitat during the following winter. One exception from this was a massive sardinella recruitment event that took place in the wake of 2010/11 Benguela Niño, owing to a favourable post-extreme event climatic scenario in 2011/12. A publication (Vaz Velho et al., under preparation) provides the full results and analyses of these observations. Task 4: In March 2022, a visit from IMR to the Angolan partner took place aimed at building capacity for local scientists to monitor MHWs. On a meeting at the Regional Fisheries Authority of the Namibe Province these scientists presented information on MHW impacts to local fishing community members. Summary: Although the coastal ocean off Angola was warming at a range of 0.25-0.3 degrees per decade during the investigation period, no decadal-scale shift in the sardinella stock range towards the southern latitudes was observed. Notwithstanding, large fluctuations in fish range, causing variability in the availability of sardinella to coastal fishermen, were commonly observed in connection with occurrences of extreme MHWs events.