Sometimes we all need a sea change, though few take this as literally as the critters riding ocean currents from one reef to the next in search of a new home. The Illawarra’s kelp reefs have received some unexpected visitors over the past few years, and they may be overstaying their welcome.
The East Australian Current – affectionately known as “the EAC dude”, thanks to Crush the turtle from Finding Nemo – carries warm water from northern Australia onto the east coast’s temperate reefs via a series of interlinked and dynamic circular currents called “eddies”. The EAC is an important oceanographic system that regulates water temperatures and redistributes nutrients, with profound effects on the distribution of marine plants and animals.
Ocean warming has caused the EAC to intensify, meaning warmer water is more frequently being transported further south for longer incursions. Consistently warmer water in temperate ecosystems has changed the composition of biological communities by facilitating colonisation by tropical and sub-tropical species that could not have survived previously.
From the plankton to the fishes, community transitions of this kind can be swift and dramatic! Climate-related “range expansions” can threaten existing species by introducing new predators or increasing competition for food and resources. For example, the explosion of long-spined sea urchins (Centrostephanus rodgersii) has depleted kelp beds across the Great Southern Reef. However, some of the recent changes in Illawarra’s temperate reef systems have been much more subtle.
Illawarra collectors, spear fishers, snorkellers and divers can now encounter military turban snails, imperial shrimps, and a variety of tropical fishes at their favourite local spots. These new kids on the block are past the southern point of their expected ranges and have not yet been documented on the South Coast.
Although tropical fish have visited the Illawarra before, they could not tolerate winter temperatures and dropped off at the end of Autumn. Now, it appears they are here to stay.
Spearfisher's surprise find
Stanwell Park diver Duncan Leadbitter – The Illawarra Flame's Hello Fish columnist – has noticed some unusual shells.
“I was out spearfishing around Bellambi Point late last year and found some shells I had not seen before despite many years of collecting and spearing around Wollongong,” Duncan said.
After consulting with a mollusc expert from the Australian Museum, he confirmed that the shells were from a military turban (Turbo militaris) snail, which had only been documented north of Sydney! This made Duncan’s next discovery even more surprising.
“I subsequently went out snorkelling off Currarong on the north side of Jervis Bay and can confirm militaris down there as well. I did not do a search specifically for them but found several easily, so I guess that there are more.”
Duncan said the invasive snail is rather tasty, despite the tedious preparations! Keep an eye out for this spiky mollusc next time you hit the water.
New sight for experienced diver
On the more ornamental side is the stunning imperial shrimp (Zenopontonia rex). Native to the Indo-Pacific region, this little guy has several colour forms and is tiny at less than 3cm long.
Local diver Carly Jacobsen did well to spot the critter on a winter dive at Bass Point. Rarely at a loss when it came to identifying local sea creatures, she had no idea what to call this one!
Although the imperial shrimp has been documented as far south as Sydney, they are most often found on coral reefs. Imperial shrimps are frequently observed hitching a ride on nudibranchs, sea stars and sea cucumbers.
Tropical fish in the Illawarra
Last but not least, we are starting to accumulate a wealth of colourful tropical fish, from yellow tangs to butterflyfishes! Local diver Max Dall’Asta couldn’t believe it when he spotted at least three different species sheltering behind the same rock in The Gutter.
What have we got to lose?
Although it is exciting to discover new characters in our underwater neighbourhood, the presence of these species indicates change is in the water. If tropical species are moving south, then it follows that some cold-water species may also be moving into new, restricted ranges.
Duncan Leadbitter can’t remember the last time he saw a bastard trumpeter or a southern coral fish. Species that are dependent on cooler water, such as blacklip abalone, may also be suffering and their recovery from past overfishing put at risk by warmer water.
There is a critical research gap in the Illawarra. We need to understand how climate change will affect our temperate reef ecosystems. This will be important for protecting vulnerable species and habitats as the waters continue to warm into the future.
Sources: Seashells of NSW, Journal of Experimental Marine Biology and Ecology, World Register of Marine Species
