Maritime traffic trends around the southern tip of Africa – Did marine noise pollution contribute to the local penguins' collapse?

Abstract

The rapid increase in seaborn trade since the 1990s has resulted in an increase in vessel-derived noise pollution, yet there is little evidence linking these activities to a decline in many marine taxa, such as seabirds. Algoa Bay, South Africa, is a marine biodiversity hotspot, providing habitats for the largest populations of endangered African Penguins (Spheniscus demersus), as well as other endangered seabirds, cetaceans and seals. The bay is situated on a major shipping route and since 2016 has hosted the first offshore ship-to-ship (STS) bunkering operations in the country, i.e. the supplying of fuel from one ship to another outside of harbours. Using Automatic Identification System (AIS) data, we estimated noise emissions from vessels as a proxy for underwater ambient noise levels within the core penguin utilisation area. Frequency of vessels using the bay doubled during our study, with numbers of bulk carriers increasing ten-fold. Ambient underwater noise levels were generally high in the bay (ca 140 dB re 1 μPa since 2015) but significantly increased by 2 dB SPL after the initiation of STS bunkering in 2016, corresponding to double the underwater noise intensity. This increase coincided with a significant and dramatic decline by 85% in penguin numbers from St Croix Island since 2016. Algoa Bay is now one of the noisiest bays in the world. This is the first study to assess the potential impact of vessel-derived underwater noise levels on a seabird population. Penguins, like marine mammal species, are known to be sensitive to marine noise pollution and urgent management interventions are required to mitigate this recent disturbance, to preserve the remaining stronghold of the African penguin and the marine mammals' populations sharing the penguins' habitat.