The humpback whale is one of the largest animals on earth. Humpbacks migrate through the Bass Strait in winter seeking warmer waters to breed. A species of baleen whale, humpbacks eat tonnes of zooplankton a day. Photograph by NOAA. CC BY 2.0.

​

The first layer of the web is microscopic algae and plants that drift in the sea’s sunlit surface. They harness the nutrients brought up from the deep waters and use them to convert sunlight into energy, a process called photosynthesis, which causes them to grow. When nutrient and sunlight conditions are just right, the algae grow rapidly and multiply in an algal bloom. Sometimes they grow so fast and become so dense that satellites can see their blooms as vibrant green patches on the sea. These algae and plants not only form the basis of the food web but also create key habitats.

The next layer in the food web is zooplankton. Zooplankton are small animals that drift through the water. Some are the size of a single cell, but some are much bigger and more complex. They come in an immense diversity of life stages, some are larvae that will grow into much bigger animals, while some are already fully grown. But all zooplankton rely on these drifting algae either directly, by eating the algae, or indirectly, by consuming other zooplankton that feed on the algae.

Zooplankton are themselves food to numerous groups of animals, from simple sea sponges, crustaceans, and fish to some of the biggest animals on earth, like baleen whales. All these animals are adapted to sifting and sieving zooplankton from the water. Baleen whales, for instance, have fine bristles for teeth which they use to filter these near microscopic zooplankton. These giant baleen whales are estimated to eat sixteen tonnes of tiny zooplankton a day! The animals that feed on zooplankton can also become food for predators like sharks, carnivorous fish, seals, dolphins and orcas.

Given the right sun and nutrient conditions algae can multiply rapidly on the ocean surface. These algal blooms become so large they can be seen from space. Here is an algal bloom in the Balkans in the Northern Hemisphere. Photograph by the European Space Agency. CC BY-SA 2.0.

Down on the seafloor of the strait, life also flourishes. It is the perfect habitat for benthic (seafloor dwelling) organisms such as crustaceans, molluscs, and even worms. Along shallower coasts, where these benthic habitats are within reach of sunlight, lush seagrass meadows take root and flower. A variety of fish come to these seagrass habitats to feed and hide from predators. These meadows are a nursery, essential to the early development of various species. In turn, a diversity of predators come to feed on the animals that live in these benthic habitats, including some of the largest seal colonies in Australia.

Despite the relatively shallow waters, several species of marine predators are found in the Bass Strait. These range from sharks such as mako and great whites, to seabirds like shearwaters and little penguins. The area also plays host to a wide variety of marine mammals, the most common of which are Australian fur seals. They are commonly found on offshore islands in the strait or traversing the waters to feed along the continental shelf, preying mainly on fish, octopus, squid, and crustaceans such as Southern rock lobsters.

Seagrass meadows, like the one pictured here off Flinders Island in the Bass Strait, become rich habitats for fish and other marine life. Photograph by IMAS. CC BY-NC-ND 4.0.

An Australian fur seal pops its head out of the water. Australian fur seals mostly breed on the islands and coasts of the Bass Strait and are a common sight throughout the region. Photograph by Ed Dunens. CC BY 2.0.

The two largest islands in Bass Strait are King Island on the north-west of Lutruwita/Tasmania, separating the Great Australian Bight and Bass Strait; and Flinders Island on the eastern Pacific side. These islands were once high peaks on the land bridge connecting Tasmania to the mainland. Their waters are now hubs for marine life and each benefit from the powerful upwelling of the Bass Cascade. Flinders Island has a warm eastern current flowing from the Australian mainland that supports lace corals and sponge gardens, providing habitat for an assortment of fish species. Orcas and sharks forage in the area, alongside Harrison’s dogfish – an endangered species of deep-sea shark which inhabits the steep continental slope and deep canyons of the sea off the east of the strait.

These deeper eastern waters are also part of the migration route for the humpback whale, which feeds in the Antarctic seas in summer and travels to southeastern Australia in winter to breed. Humpbacks are no longer listed as endangered, thanks to the ban on commercial whaling. But they are not the only whale found in these waters. Vulnerable and endangered whale species can be found here as well: southern right whales, Antarctic blue whales, and pygmy blue whales all migrate through the Bass Strait. Healthy populations of dolphins also thrive in the Strait including species like the common and the bottlenose dolphin.

Around King Island to the west, there is an even greater range of marine animals, including at least 408 species of coastal marine molluscs alone. Tall forests of bull kelp form a large part of the habitat in these waters. Bull kelp is particularly common around King Island where it is a key primary industry. Locals sustainably harvest the kelp when it washes ashore, and this is then processed to make commercial food additives and thickeners.

Underwater, the kelp forests are mostly inhabited by ground dwelling benthic organisms such as urchins, sea stars, and rock lobsters. Animals higher up on the food chain, such as seals and small whales, use the forests as a source of nutrition, feeding on the abundant crustaceans that live within the dense seaweed. Small predators also rely on the forests to hide from larger hunters, such as orcas. Pods of orca have been sighted in the Bass Strait as recently as 2022. However, the delicate balance in the food web of these kelp forests is falling apart due to global warming. Following warming waters, kelp-devouring sea urchins have spread down Australia’s east coast. Urchins have already destroyed many kelp forests in the Bass Strait. Because kelp forests support such a diversity of life, their absence has created a domino effect in the ecosystem, all brought about by a rise in temperatures from climate change.

The food web of the Bass Strait spans from tiny, microscopic algae to gigantic whales. It includes whole underwater worlds of kelp forests, seagrass meadows, sponge gardens, and a diversity of marine predators drawn to the flourishing life. All of which emerged in the brief geological blink of an eye since the land bridge was reclaimed by the sea.

Life flourishes in the shallow coastal area of Bass Strait off northeast Lutruwita/Tasmania. Photograph by Bush Renegade. CC BY-SA 2.0.

​A yellowstriped leatherjacket swims in the shallow Bass Strait reefs off southern Victoria. Photograph by Julian Finn. Courtesy of Museums Victoria. CC BY-NC 4.0.

A mako shark glides just beneath the surface. Photograph by Ed Dunens. CC BY 2.0.

This photo was taken in Alaska, but pods of orcas have been spotted in the strait. Photograph by David Ellifrit. Courtesy of NOAA, NMFS and AKFSC. CC BY 2.0.

The short-tailed shearwater, or muttonbird, is a major predator of fish in the strait. Each year shearwaters migrate in huge numbers to the Bass Strait to breed. Photograph by Ed Dunens. CC BY 2.0.

Common dolphins are also attracted to the abundant food sources of the strait. Photograph by Ed Dunens. CC BY 2.0.