The Web of Life (22-3-21) Sardine Tongue Trap

 Sardine Tongue Trap

In the Indian Ocean off South Africa, the southern winter brings a spectacle of consumption. Billions of silvery sardines (aka pilchard) follow cold water that is rich in the phytoplankton that feed them. In doing so, the sardines become fodder themselves.

The Benguela Current is a counter-clockwise oceanic gyre in the Atlantic Ocean; a carrier of cool water. The fast Agulhas Current is an Indian Ocean equivalent, carrying warm water clockwise.

When the 2 currents meet at the southern tip of Africa at the beginning of the southern winter, the Agulhas Current has weakened somewhat. The Benguela gyre pushes a tongue of cold water from the Southern Ocean off Antarctica up along the coast of southwest South Africa.

An upwelling brings nutrients from the depth to the euphotic zone: the layer of water with enough sunlight for photosynthesis. Phytoplankton proliferate there.

In contrast, there is little mixing of surface and deeper waters in tropical oceans. Phytoplankton are relatively scarce in the tropics.

Massive schools of sardines, slaves to cool current because of its food supply, follow the tongue. The tongue will eventually flag into a return gyre of warmer water.

The pilchard run on the tongue is a trap. Other animals know this. Hordes of copper sharks follow in the wake of sardine shoals.

Bryde’s whale is a smallish whale with notably small flippers for its size. They typically feed on small fish and squid at depth but are opportunists that take advantage of herding done by another species.

Cape gannets, a large seabird, tirelessly patrol the skies at the right time of year, waiting for the shimmering slick of surface oil that the sardine shoals produce on the run. On their own, the gannets cannot harvest the sardines, nor can the sharks. So they wait.

The pilchard maestro of menace is the common dolphin, which normally feeds on a staple diet of squid in the depths. For the sardine run they come up for a feast.

A dolphin scout spots indications that the pilchard run is on – the signal may be the sardines, or groups of gannets or sharks. Great dolphin pods congregate, fanning out in a line. Once set, dolphin groups break off in a coordinated attack, breaking off sardine schools and driving them to the surface.

The sardines naturally form a tight ball as a defensive measure, concentrating in the classic stratagem of “safety in numbers confuses an enemy,” which backfires horribly in this instance. Individual dolphins take turns swooping in while others continue to herd the school ball.

The sharks too take bites from the surface balls, as do Bryde’s whales. As schools come to the surface, the gannets dive bomb for dinner. Other species, such as seals, join in.

Predators ignore each other, all focused on filching the pilchards. Dolphins host a dinner party that lasts for weeks.