Dietary analysis of Cape fur seals through fecal analysis

By virtue of their position at the apex of the food chain, marine top predators integrate processes occurring at lower trophic levels and hence are potentially good indicators of ecosystem changes. In the light of ecosystem changes, related to anthropogenic effects (e.g. over-fishing, pollution), global warming or other possible causes, marine top predators such as seals are increasingly being used to monitor marine ecosystems, making use of measures such as reproductive success, foraging behaviour and diet composition. In the context of the Benguela Ecosystem, which stretches between the Eastern Cape, South Africa and Angola, monitoring the diet of the Cape fur seal was been put forward as a priority for a proposed monitoring programme in the region based on top predators.

In recent years, the sampling and analysis of fecal products (scats) has been the preferred technique for investigating seal diet. The technique is cheap, is not destructive or intrusive, and large sample sizes can be obtained relatively easily from seal colonies. The technique is based on the premise that certain undigested prey remains recovered from scats can be used to estimate the consumption of the different prey groups ingested. While the technique is subject to numerous biases, it can provide important information on spatial and temporal trends in the relative consumption of the main prey species. However, in the case of the Cape fur seal, any diet monitoring study would need to be designed with adequate replication to account for the high levels of spatial and temporal variability which occur throughout the seal’s range.