Why there are fewer fish at Ricketts Point in winter

The density of fish at marine parks like Ricketts Point, in northern Port Phillip Bay, peaks in the late summer and declines in winter. The contrast is stark. You can swim in July, August or September and see almost nothing. Compare this to January onwards when there are large schools of Zebra Fish, Snapper and a host of other species. The reason why there are fewer fish at Ricketts Point in winter can be explained. It doesn’t necessarily explain their almost complete absence.

Fish don’t move as a result of colder water. The fact that the water is colder in winter is mere coincidence. The real reason for fish absence in the northern bay in winter is a fundamental shift in the oceanographyThe study of the physical, chemical and biological properties of the ocean. More of the bay.

A change in gradients, not temperature

In the mid 2000s, I was part of a team of scientists and we discovered an oceanographic change between summer and winter. I’m not sure this is a particularly widely known phenomenon. But it’s really important. Knowing this alters the significance we place on our marine parks.

Northerly winds in winter set up a counter-current in the middle of the bay. This happens from about June to August (see Figure 1). Overall, nutrientA substance that contains the raw materials for life. At a chemical level, these are contained inside compounds that are absorbed into the body and essential energy-containing molecules are extracted, so that energy can be transformed into other chemical processes that use the energy for living. More production is reduced, but what little nutrient there is, is squeezed in between two rotating gyres on a line roughly between the Bellarine Peninsula and Beaumaris. This feature is so significant, that almost all of the adult Little Penguins from Phillip Island, feed here in winter.

The intensive seabird activity along this line – which includes many of the seabirds you see roosting at Beaumaris on high tide in winter – recharges the food chainA single thread in a food web illustrating the chain of animals that eat each other. At the base of the food chain are small high-energy (fast metabolism) animals and at the other end large low metabolism animals. An example would be whales eating krill that eat plankton that eat algae. Or lions that eat gazelles that eat grass. More over winter. What little algae can grow in the cooler winter conditions, fuels zooplankton growth, which feeds filter-feeding pilchards and anchovies. Snapper and other animals move in to feed on them. My best guess is that the Banjo Sharks, Morwongs … everything … move into deeper water to forage on the sandy bottom and deeper reef flats just offshore.

As a consequence, there are few fish left at Ricketts Point.

Figure 1: Winter (left / top) and summer (right / bottom) oceanography in Port Phillip Bay

Loading up in winter and early spring

After September, when predominant winds shift to the south, the counter-current disappears. It’s replaced by a single rotation clockwise around the bay (Figure 1). At the same time, spring rainfall from the Yarra River and nutrient run-off from the Western Treatment Plant at Werribee combines with sunlight, to create algal blooms. This new system floods the outer margins of the bay with nutrientsEnergy and nutrients are the same thing. Plants capture energy from the Sun and store it in chemicals, via the process of photosynthesis. The excess greenery and waste that plants create, contain chemicals that animals can eat, in order to build their own bodies and reproduce. When a chemical is used this way, we call it a nutrient. As we More.

You can see this happening. Particularly last year (2021) which was a La Niña year. The extra freshwater input caused huge blooms of jellyfish and algae. This is something we might also expect to happen this coming season. Another La Niña is likely to be on its way.

You might think more nutrient in summer means more plentiful feeding but no. All of these resources are also less concentrated. It all comes back to gradients again. In actual fact it’s harder to find food in summer, because the nutrients are plentiful everywhere but too dilute.

In one of many ecological paradoxes, it’s easier to find food when there is less nutrient, as all the animals are forced to head to the areas of highest gradient. This is what happens in winter. There is more abundance concentrated in small areas (along the line between the two rotating water masses in Figure 1).

The boom and bust of life at Ricketts Point

We’ve learn that in summer, nutrients are plentiful, but food is scarce. Last year’s snapper are now maturing and all the animals that were feeding in the middle of the bay are starting to get hungry.

So they spread out to the bay’s margins. But where specifically do they go? They now concentrate around physical structures. The reefs at Ricketts Point are alive with a diversity of seaweeds and rocky reefs where nutrient-laden currents pool, dropping resources onto the seabed, and fuelling more localised food chain processes. The consequence is a massive increase in the biomassThe weight of living organisms. Biomass can be measured in relation to the amount of carbon, the dry weight (with all moisture removed) or living weight. In general it can be used to describe the volume of energy that is contained inside systems, as the size of animals relates to their metabolism and therefore, how much energy they contain and More and diversity of fish by summer.

This is why these marine parks are critically important!

Is the complete absence of fish in winter natural?

What we don’t know is whether the total absence of winter fish is natural. I doubt it.

Many of our marine parks are fished right up to the margins and resident reef fish are mostly absent throughout the year. These are the species most at risk from localised fishing and spearfishing. Because most of these have been eliminated, how will we know?

Historical evidence is lacking, as John Neville has pointed out. His anecdotal accounts are important to how we manage our impacts on the bay’s ecosystemsHow ecosystems function An ecosystem is a community of lifeforms that interact in such an optimal way that how ecosystems function best, is when all components (including humans and other animals) can persist and live alongside each other for the longest time possible. Ecosystems are fuelled by the energy created by plants (primary producers) that convert the Sun's heat energy More going forward.

“In those days, snorkelling beside the Hampton breakwater, I could count on catching enough fish in 20 minutes to feed five people – generally 3 to 5 fish between 0.35 and 1.0 kg in weight. Leather jackets and luderick were abundant, as were several other species of reef dwelling fish. Large flathead and flounder could be easily caught on the sand near the reefs. Like most others spearing fish, we simply assumed that the fish we took would be replaced by fish moving in from deeper reefs. We were wrong. In Port Phillip Bay, I found that several species appeared to have been entirely eliminated from accessible shallow reefs. Even tiny juveniles had disappeared.”

John Neville, 2006

But so too is an understanding of bay ecosystems and in particular, how wildlife diversity underpins everything.

There are some serious questions that need to be asked. Not least being:

• Are we over-fishing resident reef fish?
• How are we going to reintroduce and replace lost species to restore balance?

Where are the Blue-throated Wrasse?

Described as one of the most abundant fish on southern Australian coastal reefs, blue-throated wrasse seem uncommon at Ricketts Point. One of the places they seem most frequently observed is around the pylons that mark the southern boundary in front of teahouse. But that is also where vessels and kayaks congregate to fish. Removal of the largest breeding animals will quickly deplete a population.

“Wrasses are important carnivores in coastal reef ecosystems, and selective fishing on them may have serious effects on the structure and function(Of an ecosystem). A subset of ecosystem processes and structures, where the ecosystem does something that provides an ecosystem service of value to people. More of such ecosystems”.

Diet, feeding behaviour, activity and predation of the temperate blue-throated wrasse, Notolabrus tetricus Shepherd et al. (2001)

It is likely that this species has been almost eliminated by fishing around Ricketts Point. And there is very little consideration given by fisheries, about the importance of resident fish like this. In the 2001 paper cited above, the authors add ‘In South Australia, where about a third of a million people engage in recreational fishing, the effect of recreational fishing on fish communities ranges from light to severe, according to accessibility and distance from population centres’.

The Victorian Fisheries Authority has a plan to increase the number of fishers to a million but no plan to restore ecosystems by increasing fish abundance. This is the biggest threat to the future of our bay.

I’ll finish on this important quote from Clover’s latest book Rewilding the Sea.

‘To rewild the sea is to bring back lost and depleted species to our oceans and restore ecosystems that have been harmed by human activities – simply by stepping back and letting nature repair the damage, or by reintroducing species or restoring habitatsWhat is habitat for animals and people? Habitat, hence the word "habitable" describes the natural surroundings in which any animal (or human) lives, that houses basic needs, such as food and shelter. Vegetation, for example, is habitat for animals. On its own, habitat is not necessarily stable or sustainable, which is why it differs from an ecosystem. Habitat in disrepair More.’

Charles Clover in Rewilding the Sea 2022.

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