Abstract
Under the impact of natural and anthropogenic climate variability, upwelling systems are known to change their properties leading to associated regime shifts in marine ecosystems. These often impact commercial fisheries and societies dependent on them. In a region where in situ hydrographic and biological marine data are scarce, this study uses a combination of remote sensing and ocean modelling to show how a stable seasonal upwelling off the Kenyan coast shifted into the territorial waters of neighboring Tanzania under the influence of the unique 1997/98 El Niño and positive Indian Ocean Dipole event. The formation of an anticyclonic gyre adjacent to the Kenyan/Tanzanian coast led to a reorganization of the surface currents and caused the southward migration of the Somali–Zanzibar confluence zone and is attributed to anomalous wind stress curl over the central Indian Ocean. This caused the lowest observed chlorophyll-a over the North Kenya banks (Kenya), while it reached its historical maximum off Dar Es Salaam (Tanzanian waters). We demonstrate that this situation is specific to the 1997/98 El Niño when compared with other the super El-Niño events of 1972,73, 1982–83 and 2015–16. Despite the lack of available fishery data in the region, the local ecosystem changes that the shift of this upwelling may have caused are discussed based on the literature. The likely negative impacts on local fish stocks in Kenya, affecting fishers’ livelihoods and food security, and the temporary increase in pelagic fishery species’ productivity in Tanzania are highlighted. Finally, we discuss how satellite observations may assist fisheries management bodies to anticipate low productivity periods, and mitigate their potentially negative economic impacts.
Highlights
Living marine resources of the Western Indian Ocean (WIO) are under an ever-increasing threat from the impacts of climate change and increasing coastal populations [1]
The effects of the super El Niño and strong +Indian Ocean Dipole (IOD) in 1997/98 on the regional biological productivity along the East African coasts are unraveled from historical remote sensing observations and modelling outputs
We show that unique anomalies of wind stress curl over the central Indian Ocean have led to a rearrangement of the surface currents in the Western Indian Ocean
Summary
Living marine resources of the Western Indian Ocean (WIO) are under an ever-increasing threat from the impacts of climate change and increasing coastal populations [1]. Ecosystem changes and declining fish populations are important drivers responsible for fisheries conflicts in East Africa [8,10] These have respectively caused 25% and 55% of the disputes between fishers and governments in Tanzania between 1990 and 2017 [10], and tense competition between a variety of users in Kenya [11]. A similar situation was recorded in eastern Tasmania, where oceanic warming decreased phytoplankton blooms by up to 50%, causing a change in regional fish composition [16] Another example is that seasonal changes in the Canary Current upwelling system are known to lead to the latitudinal migration of Remote Sens. 2020, 12, 3127 small pelagics, spanning the borders of three countries in the eastern North Atlantic [17] Such changes can lead to significant socio-economic conflicts and even “fish wars” between nations over rich fishing grounds [18]. Climate change impacts are likely to make similar scenarios more commonplace in all regions of the world ocean, and the East African waters are not an exception
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