Author: James Wong
Image: Jim ready to sample some fishes in Kingston Lagoon at Norfolk Island.
A quick disclaimer and a preface – I’m a fish scientist studying reef-associated fishes, so the focus of this post is on the role of subtropical reefs as it pertains to fishes. And if you’re wondering why I switch between ‘fish’ and ‘fishes’: ‘fish’ is the correct plural when referring to multiple individuals of a single species, while ‘fishes’ refers to multiple species.
**Fish** - 🐟
**Fish** - 🐟🐟🐟
**Fishes** - 🐟🐠🐡
Image: The Spot-fin Porcupinefish (Diodon hystrix) is a species of Tetraodontid porcupinefish that is found circumglobally including in temperate NSW. They can get up to 90cm - pretty huge!
Now, back to the program – subtropical reefs.
We often think of the ocean in a binary way – warm or cold. Reefs are typically viewed as tropical ecosystems bursting with life and diversity, much like a big, salty, underwater rainforest teeming with fishes. On the other hand, temperate ecosystems, such as those found along coastal southeastern Australia, are dominated by rocky structures and kelp forests. But in between these extremes lie subtropical reefs.
Subtropical reefs generally occur between **23°-35° latitude**, both north and south of the equator. They are characterized by lower annual mean temperatures and greater daily temperature fluctuations than tropical reefs. You can find them in places like Japan, Hong Kong, Florida, and right here in Australia, at Lord Howe and Norfolk Island—where I’m conducting PhD research on fishes.
These environmental differences make subtropical coral reefs unique. The temperature variability affects coral growth processes, influencing how they build their skeletons. This impacts not only how fast the reefs grow but also their structural complexity, including relief, dominant taxa, and overall biodiversity. But why does this matter? If a reef has a different shape or structure, what difference does it make?
Just as the diversity and ecological roles supported by an old-growth forest differ from those in a sparse shrubland, we see similar patterns in fish assemblages on reefs. The structure of a reef influences which fishes are present and how they interact with the ecosystem. Subtropical reefs may also respond differently to disturbances like coral bleaching, just as different forest types respond differently to wildfires.
The ecological niches supported by a reef determine which species can survive there. Most fishes start life as planktonic larvae, drifting over vast distances. If they arrive at a suitable habitat, they can settle, thrive, and establish populations. However, many larvae perish due to unfavorable conditions—such as water that's too cold to survive the winter, insufficient reef structure to avoid predators, or the absence of specific corals or algae they depend on for food.
Image: The Toadstool Grouper (Trachypoma macracanthus) is a species of Serranid grouper found in the South Pacific, including the Lord Howe Island group. They're also called strawberry rockcod because of their red colour and spots.
Subtropical reefs, positioned between tropical and temperate zones, provide critical habitats for ‘tropical vagrants’—fishes that stray into cooler waters. These reefs offer a place to settle and potentially establish breeding populations, enabling range expansions into more temperate climates. As oceans continue to warm, more species will be able to overwinter in higher latitudes. Subtropical reefs thus act as ecological 'stepping stones,' facilitating the gradual expansion of species further south. Over time, these reefs may become source populations, impacting temperate ecosystems and reshaping local species assemblages. No ecosystem exists in isolation—everything is interconnected, and, as they say, life finds a way.
In summary, subtropical reefs serve as important refuges for species undergoing range expansions in response to a warming ocean. This process, in turn, influences temperate ecosystems. Beyond their ecological importance, these reefs support local economies, deepen our understanding of reef function, and provide insights into coral biology at the edges of their natural ranges. However, my primary focus is on the fish side of things—what I get to study for my PhD. Specifically, I’m researching the fish assemblages at Norfolk Island, examining how these communities change over time and space, how coral-eating fishes responded to a recent bleaching event, and investigating some behavioral and trophic ecology dynamics—but those are stories for another day.
Thanks for reading along 😊
– Jim
Image: The Blackback Butterflyfish (Chaetodon melannotus) is a corallivorous fish and found in the Red Sea and Indo-Pacific. They're unusual amongst corallivores by being specialists on soft corals which can be quite toxic.
Comments