Tidal impact on breeding African Black Oystercatchers on Robben Island, Western Cape, South Africa

Abstract

African Black Oystercatchers, Haematopus moquini, breed along the coast and on the offshore islands from southern Namibia around the Western Cape to southern Kwa-Zulu Natal (Martin 1997). They breed mainly during the austral summer, from October to March, with breeding sometimes extending into autumn, as late as May. The Western Cape has a Mediterranean climate, so that they breed during the dry season when weather conditions are normally mild and dry, but occasional cold fronts bring wind, cold temperatures, rain, and stormy sea conditions. A potential result of this generally mild weather is that the oystercatcher breeding season is long and not particularly synchronous, and breeding attempts can be found relatively late in the breeding season (Summers and Cooper 1977, Hockey 1983). The tidal range in South Africa is relatively small; the mean predicted tidal range is 1.59m at spring tides, and 0.56m at neap tides (Anon 2003, 2004). Meteorological conditions, in particular wind and atmospheric pressure, can generate substantial differences between the astronomically-predicted tide heights and the observed sea levels. Onshore winds along the South African coast can increase sea level above predictions by 0.3m. The tidal predictions are based on mean atmospheric pressure; each millibar of pressure departure from the mean causes a sea level change of c. 1cm. Maximum air pressure anomalies of +23 to –27 millibars occur along the South African coast; therefore sea levels can be up to 0.27m above that predicted. The effects of wind and pressure are additive; low pressures and onshore winds frequently occur together so that tidal anomalies as large as 0.5m can potentially occur. Thus, significant wave height in excess of 2m can occur on the coast of the Western Cape when there are no spring tides. Extreme tide events have occurred on the southern coast of the Western Cape in the past, e.g. the ‘storm of the century’ in May 1984 (Searson and Brundit 1995). Most of these, however, have been recorded in winter, outside the oystercatcher breeding season. Robben Island (33°49’S 18°22’E) lies in Table Bay, 11km from the port of Cape Town, and 7km from the nearest mainland at Bloubergstrand (Barnes 1998, Crawford and Dyer 2000). On Robben Island, African Black Oystercatchers nest on the barren rocky shore, just above the high tide line, and feed in the adjacent inter-tidal zone. Their nests vary from an indentation on a sandy or shelly beach to a scrape carefully lined with small pieces of shell in amongst the rocks, and their speckled eggs are well camouflaged (Hockey 1982, 1983). Their choice of nest sites may be influenced by predation risk, vegetation type and the structure of the shore, competition for breeding and feeding territories, and closeness to feeding grounds. The interior of Robben Island is well vegetated (from directly above the spring high-tide level) with grassland and alien tree species (Crawford and Dyer 2000). As a result, oystercatcher breeding territories are restricted to a narrow strip of 5–20m in width along the shore. During a three-year study of breeding oystercatchers on Robben Island, two tidal events were observed to impact upon the nesting success of breeding birds: one during the 2002–2003 breeding season and another during the 2003–2004 breeding season. These events will be described further.