Janet C. Coetzee

Influences of the abundance and distribution of prey on African penguins Spheniscus demersus off western South Africa

Off South Africa, anchovy Engraulis encrasicolus and sardine Sardinops sagax are the main prey of African penguins Spheniscus demersus. The combined spawner biomass of these fish increased from less than one million t in 1996 to more than nine million t in 2001 and then decreased to four million t in 2005. The combined biomass of young-of-the-year of these species increased from 0.2 million t in 1996 to 3.2 million t in 2001 before falling to 0.4 million t in 2005. There was a large eastward shift in the distribution of sardine between 1999 and 2005. The number of African penguins breeding in the Western Cape Province increased from 18 000 pairs in 1996 to more than 30 000 pairs from 2001 to 2005 before falling to 21 000 pairs in 2006, as the availability of fish decreased near breeding localities. Numbers of penguins breeding and numbers of birds in adult plumage moulting were significantly correlated with the young-of-the-year biomass of anchovy and sardine and with the available biomass of spawning sardine. The increase in the number of penguins breeding was mainly attributable to a greater proportion of birds breeding and improved breeding success. The decrease probably resulted from high mortality. Delayed first breeding and abstinence from breeding during periods of food shortage may both increase survivorship when food is scarce and enable seabirds rapidly to take advantage of improved feeding conditions. Although long-lived seabirds are buffered against short-term variability in food supplies, environmental change that influences the abundance and availability of prey can have severe consequences for central-place foragers, such as penguins, if there is long-term displacement of prey to regions where no suitable breeding localities occur.

Are Cape gannets dependent upon fishery waste? A multi‐scale analysis using seabird GPS‐tracking, hydro‐acoustic surveys of pelagic fish and vessel monitoring systems

Summary 1. Climate change and fishing impact marine ecosystems, potentially modifying the availability of small pelagic fish to marine top predators. Some seabirds that primarily rely upon these resources have switched to feeding on fishery waste. It has therefore been argued that seabirds might become dependent upon this artificial resource. 2. To test this hypothesis, we studied the foraging behaviour of Cape gannets Morus capensis breeding off the coast of South Africa using high-resolution Global Positioning Systemtracking in relation to the availability of pelagic fish assessed by acoustic at-sea surveys, and fishing effort by the two main south African fisheries (purse seiners that compete with seabirds for pelagic fish, and demersal trawlers that process fish at sea and discharge fish waste) tracked with vessel monitoring systems. Conjoint seabird, fish and fisheries information were analysed at mesoscale (c. 100 km) and sub-mesoscale (c. 10 km) in years of high (2002), medium (2009) and low (2005) pelagic fish biomass within gannets’ foraging range. 3. We found substantial inter-annual variability in spatial use by breeding gannets, which was driven primarily by pelagic fish availability. At the mesoscale, birds and purse seiners exploited similar marine areas, but no fine-scale dependence of birds on purse seiners was detected. Crucially, fine-scale dependence of gannets upon trawlers producing fishery waste was only detected in 2005, when pelagic fish biomass was lowest, indicating a direct effect of trawlers on gannet foraging behaviour in the absence of natural prey. 4. Further overlap analyses of gannet and trawler foraging areas during 2002–2010 confirmed that breeding birds only seek trawlers when pelagic fish availability is low, strongly suggesting reversible seabird dependency upon fishery waste. 5. Synthesis and applications. Our study demonstrates that seabirds such as Cape gannets depend on fishery waste when their natural prey is scarce, but revert to feeding on natural resources whenever available, showing highly flexible foraging behaviour. These results have important implications in the context of the anticipated legislation banning at-sea disposal of fishery waste in different regions, including European seas, highlighting the necessity to concomitantly promote sustainable fishing allowing the restoration of pelagic fish stocks.