Commensal Cape fur seals in Cape Town docks
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Cape fur seals regularly frequent Alfred Basin, an inner section of the docks of Table Bay Harbour, Cape Town, where fish packed loose in ice are discharged from trawlers. An average of 10,2 seals (with standard deviation of the mean 0,48 and range of 030) was counted on 122 occasions between April 1974 and April 1979. Numbers were generally greater in winter (12,50,71) than summer (7,80,48). Many of the seals were resting at the surface. Almost half of them occurred singly; group size averaged 1,7 with a maximum of 14. The seals consumed fish that fell into the water during discharging operations. Water samples from both the surface and near the bottom were collected during 1981 from two stations in Alfred Basin and two in adjacent areas closer to the open sea. Temperature and salinity measurements showed the existence of a wedge of colder, denser water at the bottom of the docks. Nutrient levels and levels of oxygen absorbed were indicative of a polluted area. Oil levels varied widely but were three or four orders of magnitude higher than in the open sea. The regular presence of seals in Alfred Basin suggests that the appeal to them of a consistent food supply outweighs any disadvantage that might stem from a polluted habitat. However, it is not possible to determine from this study whether human activity in Alfred Basin will have any long-term beneficial or detrimental effects on the seals there. 1984 Taylor & Francis Group, LLC.