Why were 30 sharks hanging out together in a South African river?

For any security guard, keeping an eye out for interlopers is an expected part of the job. But sometimes, those interlopers come in an unusual guise. While patrolling an estate in St Francis Bay, a village in South Africa's Eastern Cape province, security guard Lawrence Everton recently discovered that the local stream had been infiltrated by visitors of the finned kind.

According to reports, around 30 sharks had swum into a creek that feeds the Krom River, a 109-kilometre waterway that meets the Indian Ocean through a system of estuaries. The impressive shark party sparked alarm among some local commenters online, but the presence of these predators in the river system is nothing out of the ordinary.

It's easy to jump to the conclusion that the sharks in the video are bull (or "Zambezi") sharks, which are known for their habit of swimming upstream to pup in shallow nursery areas. But these apex predators aren't the only shark species that can manage the transition from salt to "sweet" water.

The aggregation in the video is actually made up of sharptooth houndsharks (also known as "gully" or "spotted gully" sharks). The telltale clues are those broad, sickle-shaped fins (the scientific name for the species, megalopterus, roughly translates to "large wing").

These animals pose very little threat to humans: despite their menacing common name, sharptooth houndsharks eat small crustaceans, fish and cephalopods. A diet comprised of coastal species means the sharks spend most of their time in the shallows, but they're not known for approaching human bathers, let alone acting aggressively towards them.

Several hunches are floating around about why this group of sharks was spotted in the river.

Some commenters postulated that the sharks may have become stuck in the waterway after a large swell pushed them into the river, but this scenario is unlikely. Not only have local tides been relatively consistent, but according to Everton, the sharks moved off without any trouble later that day.

Other reports suggest that an upwelling event (which brings cold, nutrient-rich water up from the deep sea) resulted in a sudden temperature drop in St Francis Bay. This could have sent the sharks upstream in search of warmth, but South African shark scientist Dr Alison Kock notes that other factors could also have played a part.

"This [upwelling-related migration] has happened numerous times with a range of species," Kock said. She added, however, that other drivers, like food or reproduction, could not be ruled out.

Houndsharks elsewhere in South Africa are known to group together in sandy estuaries during the summer months, and scientists have observed large numbers of pregnant females during such aggregations. This shiver of sharks, therefore, may have been mating or pupping. (This would explain the splashing and grouping behaviour seen in Everton's clip: as we've discussed before, shark sex can get rough.)

Local resident Sulise Leja Human-Wells, who lives near the mouth of the bay, has seen such aggregations here before. "The sharks are there every year," she wrote on Facebook. "I hope no one tries to catch them."