Plettenberg Bay seal reproduction rising
While visitors and residents have been complaining about rotting seal carcasses causing off-putting beach experiences in Plettenberg Bay over the summer, a recent report by a local marine biologist has shed some light on the matter.
The aim of marine biologist Frikkie van der Vyver’s report was to look at the impact on seal pup mortality by a low-flying helicopter on 30 November last year.
He found that while the helicopter flight may have caused a disturbance at the colony along the Robberg Peninsula leading to the drowning of several pups and subsequent stranding on Robberg Beach, this impact was minor compared to evidence of natural mortality most likely related to high easterly wind and swell.
“Future data analysis will investigate the effect of various environmental factors in order to test this hypothesis,” Van der Vyver stated in his report.
Increase in preferred prey species
He noted there was evidence of an increase in pup production at this breeding colony, most likely related to a local increase in the availability of the Cape fur seal’s preferred prey species – small pelagic fish such as anchovy, sardine and mackerel.
“Previous studies have shown how the quantity and quality of prey affect the body condition of marine top predators, including seals, which affects the timing of puberty, implantation of embryos, mortality of foetuses, and therefore reproductive rates of females. Based on this it is very possible that the increasing trend in Cape fur seal pups washed ashore on Robberg beach over the past three breeding seasons (2017-2020) may be related to an increase in the reproductive rates of females.” Van der Vyver added that the potential influx of adult females from other areas may also be a contributing factor. “Either way, it is suspected that any further increases in pup production will naturally be mirrored by increases in easterly storm-related pup mortality, and therefore carcasses washed ashore.”
‘Carcass increase could be expected’
He said that if the observed trends in seal pup production continue, one could expect further increases in the numbers of carcasses washing ashore on Plett beaches in the coming years.
The report, which has been submitted to the Bitou Municipality, suggests that major municipal beach cleanup efforts should be scheduled after periods of high easterly wind and swell had passed as these conditions appear to cast the highest number of carcasses ashore.
Van der Vyver said earlier that pups had an especially difficult time in Plett with the peninsula’s exposed easterly terrain.
Lack of shade at a breeding site
The scientist said pups are born with a black coat, not yet insulated for survival in cold ocean water. They spend the first three months of their lives on land while suckling milk from their mothers that take care of them between short foraging trips to sea.
Due to the lack of shade at the breeding site and being blocked off from cool southwesterly winds, pups often have to cool themselves down in rock pools close to the water’s edge.
On their way down many stumble over steep cliffs and end up in the ocean while others are knocked off the rocks by big easterly swells. Those that can’t make it back up drown from either exhaustion or hypothermia, many of whom wash up dead a few days or even weeks later on Robberg Beach.
“According to boat-based pup counts, at least a third of all pups born on the Robberg Peninsula last season died this way,” Van der Vyver said. The carcasses are sampled for research and then marked with spray paint. This is to indicate that they had been sampled and are ready for disposal.
Seals of Robberg in Plettenberg Bay. Photo: Brendon Morris
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Source: Knysna Plett Herald News