Factors influencing density and distribution of African Black Oystercatchers in the Garden Route.

While reviewing August data of the Strandloper Project shoreline surveys and incidental sightings, one of the GIS maps that stood out was that of the distribution and density of African Black Oystercatchers observed between Wilderness East and Oysterbeds in the Goukamma Nature Reserve.

Distribution of African Black Oystercatchers relative to public beach access points between Wilderness and Platbank.

A total of 50 sightings were made and 138 birds counted. 24 pairs were recorded and 5 groups with between 6 and 13 birds in the group were observed.

When mapped out it is apparent that there are two high density areas, namely for 4.5km west of Gericke’s Point and between Platbank and Oysterbeds in Goukamma Nature Reserve.

Two notable factors influence the distribution and density of oystercatchers along the shoreline, namely food availability and human disturbance.

Along sandy beaches the dominant diet of oystercatchers is the White sand mussel (Donax serra). Territories along sandy beaches extend approximately 2km and are actively defended, even out of breeding season.
By contrast, along rocky sections of coastline, territories are approximately 200m per pair as there is a wider diversity and density of prey species available. Of interest is that this territorial distance has reduced from 800m/pair in 2000 as a result of the invasion of the exotic Mediterranean mussel which increased the calories per foraging effort due to it’s large size than the local Brown Mussel (Perna perna).

When the distribution of the oystercatchers are mapped out relative to beach public beach access points, it suggests that the zones of regular human activity also deters oystercatchers from those sections of shoreline, though the sandy nature of the popular shoreline is also a combining factor in the sparsity of oystercatchers in these regions.

In the map the larger shapes are a 1000m buffer from the popular public beach access points. The light green is a 500m buffer around the location of each oystercatcher sighting while the smaller vivid green is a 200m buffer around each oystercatcher sighting. The latter is to demonstrate the territorial range of pairs on the rocky shoreline.

I also noted that two pairs of oystercatchers on a sandy beach were disturbed by fishermen who were collecting sand mussels from where they had been trying to feed.

Oystercatchers can only feed at certain times of the tide, on the out going and incoming tides when their prey is exposed and in the case of mussels, when they are open. If you see an oystercatcher feeding please try not disturb it.

To find out more about oystercatcher and other surveys conducted by Strandloper Project Click Here

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