Fish

There is a high diversity of fish in the bays and they can be categorised into a range of habitats.

Reef and seagrass habitats in temperate marine waters are dominated by three particularly diverse families:

• the wrasses
• leatherjackets, and
• morwong.

Fishes characteristic of soft-bottom habitats include:

•     flounders
•     flatheads
•     gurnards
•     rays, and
•     skates.

Coastal open water fish include several larger species such as:

•     Australian salmon
•     barracouta
•     short-fin pike, and
•     silver trevally.

Smaller, schooling open water species include several families including:

•     the sardines and sprats
•     anchovies
•     scads and jack mackerel, and
•     hardyheads and silversides.
  
   

Three key species of fish can tell us a lot about the state of the bays are:

• King George whiting
• snapper
• sand flathead

These species indicate the health of fish in the bays more generally, as well as other processes (for example, seagrass health, snapper as an important predator etc.).

The focus is on juvenile fishes of the three indicator species because this information can help predict the size and sustainability of the fish population in the future.

The health and numbers of juvenile fishes of each species is linked to changes in environmental condition, habitat and adult breeding.

King George whiting have shown a long-term increase in numbers in Port Phillip Bay since 1978-79.

King George whiting spawn in ocean waters during the late autumn and early winter. During spring, the small larvae (post-larvae) drift for up to 100 days before they enter Port Phillip Bay (and other bays and estuaries throughout Victoria). At this time, the small larvae are about 20-30 mm length and settle into the bays’ fringing seagrass beds - their primary nursery habitat and refuge from predators.

The newly settled fish take about two and a half years to grow to a size where they can be legally caught, and by about four years of age they leave the bay to complete their adult life in coastal waters, never to return.

Due to its critical importance as a juvenile nursery habitat for King George whiting, reductions in the area and condition of seagrass beds in Port Phillip Bay is an ongoing threat for fish stocks and ecosystem health.

Threats to the fishes’ seagrass habitats are discussed in the Seagrass topic.

We don’t know enough about the spawning sources, movement or migration patterns of the King George whiting so it’s hard to predict how warming sea surface temperatures will affect them.

Climate change can have a positive or negative effect on King George whiting:

• If rising SST promote larval growth and therefore survival, then climate change may have a positive effect.
• However, if rising SST leads to less favourable currents then climate change may have a negative effect.

Snapper spawn in Port Phillip Bay over the late spring and throughout summer. Current surveys focus on snapper juveniles less than five months of age, and less than 15 cm total length.

The survey is designed to measure the relative abundance of juveniles from the previous spring and summer spawning seasons, providing an annual understanding of the health of the fish stocks.

The data is used by Fisheries Victoria to predict future fisheries production, as well as in studies of relationships between the environment, the climate and birth rate.

The survival of snapper from juvenile to adulthood varies. Research is ongoing to understand the relative contributions of the pressures of fishing, nutrient inputs, water quality, food availability (particularly in the larval and juvenile stages), and habitat availability in shaping patterns of survival and birth rates in young snapper.

Sand flathead stocks in Port Phillip Bay declined by 89% between 2000 and 2010 but had recovered to 30% of 1990s levels, and 50% of 1980s levels, by 2012-13.

Similar to snapper, sand flathead spawn in Port Phillip Bay over late spring and summer. Sand flathead are considered to have a largely resident, isolated stock within Port Phillip Bay – with limited exchange with the coastal population. Therefore the monitoring indicates replenishment of the bay's population only.

What we know

Sand flathead birth rate in Port Phillip Bay has been well below its long-term average for all but two years since 2001.

Much of the yearly variation is likely related to environmental influences on survival of the early life stages (i.e. river flow, nutrient supply, plankton food chain dynamics), but the long-term trend is thought to be related to prolonged drought impacts on the bays.

What we need to know

Interestingly, sand flathead in Port Phillip Bay grow more slowly and reach a smaller maximum size than sand flathead collected from Bass Strait and south-east Tasmania. The reasons for this are unclear, but may reflect differences in the activity on the sea floor that the fish live in or genetic changes to the Port Phillip Bay sand flathead population.

Anchovy and pilchards are important prey items for little penguins. The decline in the pilchard population in the mid to late 1990s has likely increased the ecological importance of anchovy in Port Phillip Bay.

Port Phillip Bay and Western Port are important spawning areas for anchovy. Adult anchovy migrate into the bays between October and December to spawn and return to Bass Strait between February and May. Juvenile anchovy remain within the bays for up to two years after hatching, before migrating offshore to join the adult population.

Juvenile pilchards also use Port Phillip Bay as a nursery, entering the bay as mostly juveniles in late spring/early summer, but do not attain sexual maturity within the bay. Unlike anchovy, pilchards spawn primarily in offshore coastal shelf waters and rarely in Port Phillip Bay.

It is suggested that the spatial segregation of spawning areas for anchovy and pilchards in South Australian coastal waters may have evolved as a result of competition avoidance between these two species.

The development of indicators for open sea fish status in both Port Phillip Bay and Western Port based on anchovy or pilchard birth rate/growth is required.

Despite their proximity to Victoria’s largest cities, Melbourne and Geelong, Port Phillip Bay and Western Port are quite healthy overall.

To measure the environmental health of the bays, the Commissioner for Environmental Sustainability worked with marine scientists who identified 36 indicators across the six key topics you see on this website.

The indicators were chosen because they best ‘indicate’ whether that key topic is healthy.

Here’s how the fish measured up: