Evaluating seal–seabird interactions in southern Africa: a critical review

Abstract

Through predation and displacement, the Cape fur seal Arctocephalus pusillus pusillus poses a threat to several seabird species that breed in southern Africa. Measures such as the culling of 'problem' seals have been introduced to negate the effects on these species, but there have been calls for stronger measures to be taken against seals. In this review, past evidence for direct impacts of seals on seabird populations was scrutinised. It was found that frequently the interpretation of seabird numer ical trends, or of anecdotes on seal-seabird interactions, appear biased against seals. It is mooted that, as with seal-fishery interactions, the conspicuous nature of seals and some of their behaviour may have resulted in their effects on seabird colonies or populations being over-emphasised in the past. The poor conservation status of the impacted seabird species has been influenced by numerous factors, foremost of which are human interference and historical bad management practices. Culling or displacing of seals, at any scale, are unlikely to reverse trends in declining populations of seabirds, especially if conducted in isolation of other management actions, such as measures to enhance or expand the breeding habitat of seabirds. (90 ha) islands (see Figure 1) (Rand 1963a(Rand , 1963b. Whereas seal and seabird colonies co-existed on many of the islands three to four centuries ago, at the time when Europeans began to make their mark in the region (Shaughnessy 1984), the physical characteristics and faunal composition of many of the islands have altered radically since then. Between the 17th and the early 20th centuries, seals were hunted to extinction at many of the larger islands, including the aforementioned three and at least 20 other islands (Shaughnessy 1982). Many of these sites were subsequently manned on a year-round basis and managed for the production of seabird products such as guano and eggs, to the exclusion of seals. Hence, it became customary to refer to 'seal islands' and 'seabird islands' (or 'guano islands') exclusively. Seal islands included mainly small islands of 2 ha or less in area, which were inaccessible or

Spatio‐temporal shifts of the dynamic Cape fur seal population in southern Africa, based on aerial censuses (1972–2009)

“…As is the case with fur seal populations elsewhere in the world that have recovered from past over‐exploitation ( e.g., Boyd 1993, Arnould et al 2003, Hofmeyr et al 2006, Kirkwood et al 2009) there is interest with regard to the dynamics of the Cape fur seal population including trends, its relationship to the precommercial sealing population and to the carrying capacity of the environment ( e.g., Butterworth et al 1988, Kirkman 2009). This study used spatio‐temporal trends in pup counts as indicators of the changes in size and distribution of the Cape fur seal population, as has been done in several studies of seal populations elsewhere ( e.g., Hofmeyr et al 2005, Kirkwood et al 2005, Hofmeyr et al 2006).…”

“…Examples are the establishment of Atlas Bay and Wolf Bay colonies due to displacement from nearby Long Islands (Rand 1972). Other cases that have been cited include the establishment of a colony on Mercury Island and a large influx of seals to the colony at Cape Cross in the 1980s following large‐scale disturbance of the colonies at Atlas Bay and Wolf Bay (Crawford et al 1989, Wickens et al 1991), and the development of the colony at Dolphin Head on the Namibian mainland when seals were deliberately displaced from nearby Mercury Island in the interests of seabird conservation (Crawford et al 1994, Kirkman 2009). Except in the latter case, it was known that human disturbance at the purported source colonies was in the form of sealing during the breeding season ( i.e., November–December), the time of year when seal breeding colonies are most vulnerable to disturbance (Boren et al 2002).…”

Predator‐avoidance behaviour in a nocturnal petrel exposed to a novel predator

“…Among seabirds, reproductive behaviour is strongly influenced by predators (Nelson 1989, Mougeot & Bretagnolle 2000a, Votier et al 2004, Le Corré 2008, Matias et al 2009. The evolutionary origin of colonial nesting is unknown, but there is little doubt that this behaviour provides protection to many species of seabird via predator swamping (Harris 1974, Walker & Elliott 2005, Fauchald 2009, Kirkman 2009. Ground-nesting seabirds are at risk of nest predation by land mammals and therefore many species nest only in areas inaccessible to such predators, for example on cliffs and remote islands (Camphuysen & de Vreeze 2005, Barrett 2008, Jovani et al 2008.…”

Factors that influence Cape fur seal predation on Cape gannets at Lambert’s Bay, South Africa

“…Although only a small proportion of seal individuals actively prey on seabirds, the impact thereof can rapidly threaten a seabird population ( Swan et al., 2020 ) through desertion of breeding attempts ( Wolfaardt & Williams, 2006 ), decreased survival ( Horswill et al., 2016 ) and a decrease in recruitment ( Sherley et al., 2019 ). In southern Africa, predation by Cape fur seals on seabirds has been recorded around six islands, with an additional five locations in Namibia ( David et al., 2003 ; Kirkman, 2009 ; Makhado et al., 2009 ). In the Benguela ecosystem off the West Coast, Cape fur seals are known to prey on at least five seabird species ( David et al., 2003 ), four of which are endangered.…”