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You are here: Fisheries » About Fisheries » Publications and Resources » Fisheries Reports » Port Phillip Bay fish in our waters

Port Phillip Bay: fish in our waters

Featuring the King George whiting and snapper stories

Port Phillip Bay: fish in our waters

Published by the Department of Primary Industries Fisheries Research Branch, April 2010

ISBN 978-1-74264-105-8 (print)
ISBN 978-1-74264-106-5 (online)

Photography: DPI, Marc Ainsworth, Sean Blake, Guy Werner, Andy Longmore, David Ball, Paul Hamer, Silvana Acevedo, Greg Jenkins, Therese Stokie, Greg Parry, Simon Heislers, Marine Discovery Centre, Laura Hill, David Hobday, Julia Kent, Neil Hickman, Steve Colquitt and the Integration and Application Network.

Contents

Port Phillip Bay
Essential components

Fish in our waters
Producing fish
Sustaining production
Naturally dynamic

King George whiting
Snapper
Monitoring fish
Summary
Further reading
Acknowledgements

Port Phillip Bay

At the end of the last ice age, rising sea levels flooded the Yarra River, Werribee River, Little River and Kororoit Creek, and Port Phillip Bay (PPB) was formed.

Map courtesy of Melbourne Water and CSIRO

Description

Port Phillip Bay:

• is almost land locked
• has a coastline of 264 km
• covers an area of 1,950 km²
• is unusually shallow for its size with over half of PPB less than 8 m deep
• receives freshwater mainly from the Yarra River, Maribyrnong River and the Western Treatment Plant
• loses almost as much water from evaporation as it receives in freshwater
• is warmer than the open ocean in summer and cooler in winter.

PPB is a marine embayment. The waters of PPB are salty (similar to the open ocean) and well mixed. After prolonged drought, the waters of PPB can be slightly saltier than those of Bass Strait.

Winds and tides drive the movement of water in PPB, creating a clockwise movement of water (gyre) in the north and an anticlockwise gyre in the south.

Water that enters PPB stays for about six to twelve months.

Water exchange between PPB and Bass Strait is restricted by ‘The Great Sands’ and ‘The Rip’ - the narrow 3 km wide entrance between Point Lonsdale and Point Nepean. Tides affect circulation in the southern quarter of the bay but winds control circulation north of The Great Sands.

Only about 4% of the water in PPB is replaced with each tide because much of the water leaving PPB on the falling tide reenters PPB on the rising tide.

Value to Victorians

PPB is a major natural asset supporting sustainable multiuse relating to high value:

• Recreational activities PPB is one of Victoria's most popular recreational destinations. Every year millions of people enjoy beaches, coastal and marine parks and sanctuaries, boating, fishing and diving. PPB is the most important embayment in Victoria for recreational fishing, with 55% of fishers who hold a recreational fishing licence nominating the bay as their most frequent fishing destination
• Economic activities PPB provides the major shipping channel to the Port of Melbourne (Australia's busiest port) and the Port of Geelong. PPB also sustains the most diverse and most productive commercial bay and inlet commercial fishery in Victoria, along with commercial charter boats, diving and general tourism activities. PPB supports aquaculture (farming) industries growing mussels and abalone
• Domestic activities Over 3.2 million people live around its shores making PPB Australia's most densely populated catchment
• Industrial activities The cities of Geelong and Melbourne, located around PPB, have extensive secondary industries. The rural areas of the Werribee corridor and the Mornington and Bellarine Peninsulas support diverse primary industries, particularly for horticulture and wine production
• Biodiversity PPB provides habitat, including rocky reef, seagrass beds, sandy substrates and high quality seawater to over 1000 species of marine plants and animals, including over 300 species of fish. It is also home to important Marine National Parks and Marine Sanctuaries where all biodiversity is protected.

PPB is highly regarded by Victorians for these social, economic and environmental uses and values.

Essential components

Epiphytes growing on seagrass

Seagrass

Seaweeds

PPB has four essential components:

• physical and chemical environment
• producers
• consumers
• decomposers.

Essential components

The physical and chemical environment is:

• the water with dissolved minerals, gases, nutrients and suspended organic matter
• the seabed with its mineral and organic sediments and associated physical habitat such as rocky reef and variable depth (including sandbars, mudflats, deeper channels etc)
• gases in the air above (nitrogen, oxygen, carbon dioxide are the most important).

Maintaining a healthy seabed for purposes of cycling nutrients from organic matter is critical to maintaining water quality and the overall ecological health of PPB.

The producers are the plants:

• microscopic drifting plants (phytoplankton) consisting of mainly the simplest forms of algae
• large seaweeds (macroalgae) that attach to rocks and other hard substrates
• seagrass (a type of flowering plant) that lives in shallow marine waters
• epiphytes (plants growing on other plants).

The plants use sunlight, carbon dioxide, nutrients and water to build sugar, starch and cellulose (carbohydrates). Carbohydrates provide energy in the form of food for animals and plants.

Nitrogen levels in waters of PPB are naturally low and limit the growth of plants. The productivity (the rate of production of plant matter) of PPB is naturally low.

Plants produce a fixed amount of food that is shared by all the animals in PPB. The tiniest plants (the phytoplankton) make the greatest amount of food in PPB. Seagrass and seaweeds also contribute to the food supply and provide critical habitat and physical cover for many fish and invertebrate species.

The plants also produce oxygen, which dissolves in the water column. Oxygen is essential to all animals within PPB.

Scallops filter phytoplakton from the water to eat

Microscopic herbivores (zooplankton) are eaten by larger animals (carnivores)

The consumers are animals:

• herbivores (primary consumers) feeding on phytoplankton and other plant materials
• carnivores feeding on herbivores (secondary consumers) or on other carnivores (tertiary consumers).

The largest group of herbivores in PPB is the microscopic free-swimming animals (the zooplankton) which, along with filter feeders such as bivalve shellfish, eat phytoplankton. Zooplankton are eaten in turn by larger animals, including fish and crustaceans.

Phytoplankton, seagrass and seaweeds are also eaten directly by some fish, shellfish and crustaceans. Seagrass and seaweeds are more usually consumed after they have died and been broken down by the decomposers.

The decomposers

• are mostly bacteria and fungi and various invertebrates that feed on dead plants and animals and on animal wastes that settle to the seabed.

The inter-relationships of living things based on what they eat and how energy is transferred through PPB is called a food web. The food webs of PPB support healthy populations of fish, and are based on the producers, consumers and decomposers.

Fish food web in Port Phillip Bay

Fish in our waters

King George Whiting

Port Phillip Bay sustains the most diverse and most productive bay and inlet commercial fishery in Victoria and is the most important embayment in Victoria for recreational angling.

Commercial fishery

PPB provides 48% of the catch from Victoria's commercial bay and inlet fisheries.

In 2007/08 the commercial catch of fish from PPB was 658 tonnes valued at $3.6 million. Most is sold as fresh table fish to Victorian consumers in local markets. Some is sold as bait or aquaculture food.

The main target species are:

• King George whiting
• snapper
• calamari
• anchovy
• rock flathead
• garfish
• pilchards
• gummy shark.

Haul seines, mesh nets, long lines and purse seines are used to catch these fish.

The top five commercial species in 2007/08 Recreational fishery

Species	Catch (tonnes)	Value ($,000)
King George Whiting	104	1,477
snapper	94	668
calamari	30	335
anchovy	81	249
rock flathead	34	213

Recreational fishery

Sand flathead

PPB sustains 20–25% of all recreational fishing effort in Victorian waters.

Each year recreational anglers catch, on average, 2.5 million fish in PPB.

The main target species are:

• snapper
• King George whiting
• flathead (mostly sand flathead)
• calamari
• garfish
• Australian salmon
• gummy shark.
•  
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Anglers catch almost all of these fish from boats (95%) using lines and hooks. A small amount of fish is caught:

• by anglers fishing from jetties
• by divers spear-fishing
• by hand collection (e.g. scallops).

Victoria has the largest recreational snapper fishery in Australia, most of which is caught from PPB. Recreational catches of snapper, flathead and King George whiting from PPB are usually larger than the commercial catch of these species.

Black bream are also caught in the river estuaries within PPB.

Producing fish

PPB has many habitats - specific places or natural conditions - where animals occur. Not all habitats are of equal importance to fish. Indeed, many fish are reliant on habitats they never occupy. Seven broad habitats contribute directly or indirectly to the production of fish in PPB.

Installing loggers that continuously monitor the water quality in Port Phillip Bay

Water

Water is fundamental to all fish species in PPB.

The water, the pelagic habitat of PPB, supports fish production:

• directly by
  • providing a habitat for many fish at some or all stages of their life
• indirectly by
  • supporting the phytoplankton that fuel most of PPB’s animal communities
  • supplying the chemicals required for life (oxygen and nutrients)
  • maintaining the physical conditions required for life (temperature, pH and salinity)
  • sustaining all other habitats including seagrass, seaweed, sediment and reef habitats
  • providing mechanisms of dispersal through tides and currents for juvenile stages, such as eggs and larvae.

Key fishery species directly dependent on the pelagic habitat are:

• anchovy, which live in the water column and eat the plankton
• anchovy, snapper and flathead, which release eggs and sperm into the water column, where fertilisation and development takes place
• anchovy, snapper and flathead larvae, which live in the water column feeding and growing as part of the plankton.

Important fishery species indirectly reliant on the pelagic habitat include:

• King George whiting, rock flathead and calamari, which eat animals that live in bottom habitats but which feed in the water column
• snapper and flathead, which feed through food chains sustained by phytoplankton.

Fish production in PPB is, in the end, dependent on the quality of its water habitat.

Seagrass

Seagrasses are flowering plants that root in the sediments of PPB. Seagrasses have stems and leaves, inconspicuous flowers, form fruits and set seeds. Below the sediment, rhizomes (tuberous root-like structures) connect the different bundles of stems.

Seagrass requires good quality clear water, with access to nutrients, to grow. Seagrass is mostly restricted to shallow waters (<3 m) where light penetrates. Seagrass form meadows in both intertidal and subtidal areas of PPB.

Seagrass is found mainly in western and southern parts of PPB (Swan Bay, around the Geelong Arm, off Werribee). Areas of seagrass are also found on the eastern side of PPB.

Seagrass monitoring

Seagrass meadows

There are three main species of seagrass in PPB:

• Heterozostera nigricaulis (dominant species)
• Zostera muelleri (confined to intertidal areas)
• Halophila australis (found in deeper water than Heterozostera).

The seagrass habitat in PPB supports fish production:

• directly by
  • providing a habitat for some fish at some or all stagesof their life
  • constituting the major food item of some fish
• indirectly by
  • underpinning detritus food chains which sustain animal communities in unvegetated sediments
  • supporting a rich diversity of invertebrates and epiphytes (‘fouling’ plants and animals) providing food for some fish
  • stabilising and trapping sediments
  • maintaining water quality.

Important fishery species directly dependent on seagrass habitat in PPB include:

• calamari, which attach eggs to seagrass leaves
• King George whiting, which live in and among seagrass as newly settled fish.

Important fishery species indirectly reliant on the seagrass habitat in PPB include:

• King George whiting juveniles and young adults, which feed through food chains sustained by seagrasses.

Not all seagrass meadows have equal importance to fish. Importance depends on:

• seagrass species
• location
• depth
• physical exposure (waves, currents etc).

Sediments

Benthic chambers measure the nutrient recycling processes that occur in the sediment

Sediment

Sediment habitats occupy most of the floor of PPB and therefore directly or indirectly, support most of the fish communities in PPB.

In PPB sediment habitats can be broadly divided into three types:

• sand
• clay
• silt.

Each type supports distinct animal groups.

Important fishery species directly dependent on sediment habitat in PPB include:

• sand flathead, which live on the sediment surface
• King George whiting, sand flathead and snapper (adults and juveniles), which feed on animals that live on and in the sediments.

Sediment habitat indirectly supports all fish production in PPB. The organisms (invertebrates, algae and bacteria) living in and on sediments are important in recycling nutrients.

These recycling processes involve animals and bacteria that convert the nitrogen compounds, that can act as fertilisers, to nitrogen gas which is then lost to the atmosphere. The loss of this ‘fertiliser’ reduces the likelihood of algal blooms and maintains water quality in PPB.

Reefs

Reefs are areas of rocky substrate. Reefs provide habitat for mussels, sponges, oysters, other invertebrates, and a variety of plants, mainly seaweeds.

There are four broad types of sub-tidal reefs in PPB:

• basalt reefs, found mainly between Williamstown and Avalon
• sandstone reefs, distributed between Port Melbourne and Rickett’s Point
• granodiorite reef, located between Balcombe Bay and Safety Beach
• limestone reefs, found south of Indented Head and Point King.

Sub-tidal reefs cover a small area of PPB but support the production of key fisheries species:

• directly by providing shelter
• indirectly by providing habitat for food species.

Important fishery species directly dependent on reef habitat in PPB include:

• abalone and rock lobster, which live on reefs
• juvenile snapper, which shelter and feed on animals that live on reefs.

Fish associated with reefs may also be reliant on other habitats adjacent to the reef.

Seaweeds

Ecklonian kelp - a type of seaweed

Typical saltmarsh habitat

Seaweeds are large, multi-celled algae. There are three major groups of seaweed: the red, the green and the brown.

Many species of seaweed are found in PPB. Some anchor into the sediment or attach to shell fragments or rocks. Others, the drift algae, lie on the bottom and are moved by the water currents.

Seaweeds support the production of key fisheries species:

• directly by providing food for some fish such as luderick, mullet, wrasse
• indirectly by
  • providing habitat for a large range of animals that are eaten by fish
  • providing surfaces for epiphytes that are also important sources of food for some fish
  • contributing to the primary production of PPB
  • providing shelter (the 3-D structure of the algae) for fish

Salt marsh

Salt marsh is an area occupied by salt tolerant grasses, reeds, sedges and small shrubs. Salt marsh occupies flat regions between the extreme high water mark and the land.

Salt marsh is usually water-logged and on high tides, covered by salt water.

Salt marsh does not underpin the food webs driving fish production in PPB.

Salt marsh provides habitat for a range of crabs, snails and other animals. Fish also enter salt marsh on the high tide to feed and young fish may use salt marsh for shelter.

Salt marsh also maintains water quality in PPB by:

• stabilising and protecting the shoreline from erosion
• capturing sediment
• removing nutrients present in run-off.

Estuaries

Black bream

Estuaries are transition zones between fresh and salt water and between the land and the sea.

Estuaries are subject naturally to continual change from:

• sedimentation and erosion
• tidal cycles
• flood events.

Estuary habitat in PPB occurs at the mouths of rivers and creeks. Within each estuary many habitats may occur including:

• pelagic habitat
• sediments
• macroalgae
• salt marshes.

The Yarra River estuary is the most important estuary within PPB. Other smaller estuaries are associated with Werribee River, Little River and Patterson River.

The estuaries of PPB are characterised by relatively low productivity.

Estuaries within PPB attract adult black bream and mulloway, which are important recreational species and provide important spawning areas for black bream.

Exotic species

Exotic species are outsiders - species that enter new ecosystems beyond its historical or natural range. They come in all shapes and sizes, from tiny algae and larvae that can only be seen under a microscope, to fish, seastars, molluscs and aquatic plants.

All water bodies in the world, including PPB, contain exotic species. Exotic species have the potential to affect habitats, food chains, the ecosystem and our enjoyment of the marine environment.

Management actions in Australia are focussing on stopping the arrival of new exotic species. "Prevention rather than cure" is far more effective than trying to deal with them after they are established.

Sustaining production

The PPB ecosystem is healthier and cleaner than comparable bays near other large cities. Maintaining the key environmental processes is the cornerstone for sustainability.

A typical pristine catchment area

Recreational craft in Port Phillip Bay

Ecosystems

An ecosystem is the dynamic complex of plant, animal and micro-organism communities and their non-living environment interacting as a functional unit.

When these relationships are in balance the ecosystem is said to be ‘healthy’.

Ecosystems change naturally over time.

Ecosystems can also change in response to human activities such as development. This impact can break down the underpinning relationships and affect the balance within ecosystems, rendering them dysfunctional. Changes can then occur to the diversity, composition and abundance of species within an ecosystem, thereby causing further impacts.

Human activities, nutrients and PPB

The catchment and shores of PPB have been substantially altered since the mid-1800s, because of European settlement and the clearing of land for agriculture, cities and industry.

Such development has substantially increased the amount of nitrogen entering the system.

In PPB nitrogen is naturally scarce. This scarcity limits the growth of plants and structures the animal and plant communities present in PPB.

Increasing the amount of nutrient in PPB increases plant production. Any increase in the amount of nitrogen has the potential to increase the growth of some species at the expense of others.

While the nutrient load in PPB has increased substantially since European settlement, the Port Phillip Bay Environmental Study (1992-1996) (PPB ES) concluded that overall PPB was ‘relatively healthy'. The PPB ES recommended the nutrient load entering PPB be capped and management actions to address this are well underway. More recently (2008-09), Baywide programs designed to monitor the health of PPB have confirmed the overall good health of the bay.

Self sustaining

PPB water quality is healthy in large part because of its ability to remove excess nitrogen from the system.

Nitrogen does not build up in PPB. Of the 6,000 to 8,000 tonnes (t) of nitrogen entering PPB each year, about:

• 650 t is incorporated into PPB's plants
• 700 t is exported to Bass Strait
• 1200 t is buried in the sediments.

The rest is lost from PPB.

The key to PPB's good health lies in its diversity of plant and animal life and in their interactions.

The process causing the loss of nitrogen takes place in the sediments. It arises from the coupling of two microbial processes, called nitrification and denitrification. Because of these processes, nitrogen is converted from forms readily available for plant growth, to a form that is lost to the atmosphere.

PPB is unusually efficient at denitrification. Maintaining the high efficiency of these processes is essential to its continuing health.

One way to maintain high efficiency is to restrict the discharge of nutrients. Improvements to wastewater treatment and the prolonged drought within the catchment have reduced nutrient inputs to PPB by about a third over the past decade.

Current Health Status

Water Quality monitoring is conducted monthly across PPB by Victoria's Environment Protection Authority.

As a whole, water quality throughout PPB remains high and has not deteriorated in the past 20 years.

Drought has impacted water quality in PPB by reducing inputs of fresh water and river-borne nutrients, chemical contaminants and sediments.

Drought has also increased the residence times of water in PPB, changed circulation patterns and increased salinity.

As anticipated, there were changes in turbidity in PPB as a result of the 2008-09 channel dredging. All changes were temporary and consistent with predicted effects.

Naturally dynamic

PPB fisheries are characterised by large fluctuations in fish abundance that are driven by natural changes in environmental conditions.

Fluctuating fish abundance

The abundance of any species of fish is known to vary over time and space within the marine environment.

In a comparatively shallow bay separated from the ocean and with relatively limited freshwater inflows, such as PPB, fish stocks are expected to vary continually as the conditions change.

Environmental conditions vary on many time frames:

• daily
• seasonal
• annual
• multi-year
• decadal
• in response to long-term climate change or climate shifts.

Drivers of fish abundance

In many cases, it is difficult to pinpoint why fish abundance varies. This is because the environmental influence that triggered the change rarely affects the fish directly. Environmental influences usually effect changes indirectly via a component of the ecosystem.

For example, daily tides structure the use of intertidal habitats such as seagrass and salt marsh. Tides do not directly exert an impact on the fish. Rather the tides control access. When high tides flood these habitats, fish move in to feed or to seek refuge from larger predators. As tides ebb fish retreat to deeper water.

Environmental variations can change the abundance of fish by influencing:

• dispersal
• migration
• growth
• survival
• the distribution and extent of important habitats that support fish.

Short-term and long-term drivers of change

Sensitive early-life stages

The early life stages of many fish are thought to be most sensitive to environmental change. Environmental factors can potentially influence survival of early life-stages by:

• dispersal to unfavourable environments
• effects on food supply
• effects on feeding ability and growth
• the abundance, distributions and susceptibility to predators
• rendering nursery habitat unavailable.

Natural mortality of the young life-stages of fish is incredibly high. Changes in the relative mortality of these life-stages from year to year drive future fishery strength.

This is seen yearly when stronger or weaker age classes - the number of fish spawned in the same year - enter a fishery.

Fisheries Productivity

Fisheries productivity varies at multi-year to multi-decadal scales. Often the environmental influences on fisheries at the multi-decadal scale, in response to climate shifts, have far greater impact on fisheries production and associated economies than variation on the inter-annual scale.

Fluctuations in PPB fisheries

Commercial and recreational catches of many species provide information on fish abundance.

Catch information from the commercial fishery in PPB for many species has been recorded since the early 1900s. Commercial catches of snapper and King George whiting from PPB have cycled over long periods.

The catch of King George whiting declined from the early1900s to the mid-1950s, followed by an overall rise with record catches during the 2000s.

Snapper catches were highest during the 1920s and 1970s at about 230 t. This is well above the long-term average of about 130 t (calculated for the period 1914 to 2008). Catches were lowest at about 65 t (half the long-term average) during the 1940s, 1960s and 1990s. Catch has increased during the 2000s.

Due to the time scales involved, the influences of climatic regime shifts on marine ecosystems and fisheries are generally poorly understood.

Example of the natural variability in a fish population

King George whiting

King George Whiting

Development of King George Whiting

Catches from PPB's King George whiting fisheries fluctuate in response to inter-annual and longer-term variations in winds and currents in the coastal waters of Bass Strait and to environmental factors which determine the extent of seagrass habitat in PPB.

Supply

In some years, fishers in PPB feast on bountiful numbers of King George whiting, whereas in other years this popular table-fish is scarce, and fishers experience lean times.

While PPB’s recreational and commercial King George whiting fisheries are based on two to three year old fish (i.e. above the legal minimum size limit), the number of fish in PPB (and therefore the amount available to be caught) is related to the number of larvae that entered PPB 2 to 3 years earlier from offshore spawning grounds.

Development

King George whiting spend the first three to five months of their lives as larvae in the water column. The larvae drift from the spawning grounds in South Australia into Victoria’s large central-coast bays and inlets including PPB, when they are approximately 20 mm in length.

Once in PPB these larvae settle onto seagrass, which provides food and shelter. King George whiting leave PPB when they are three to five years old and move to open coastal waters where they spawn.

Larval drift of King George Whiting.

Supply of larvae

Newly settled King George whiting

Link between westerly winds and King George whiting catch.

King George whiting settlement in PPB since 1996.

King George whiting larvae drift at the mercy of winds and currents.

The numbers of larvae entering and settling (taking up residence as post-larval stage) in PPB is determined by yearly and long-term climatic changes that occur in the open coastal waters of southern Australia.

Westerly winds

Historical records indicate that commercial catches of King George whiting appear to rise and fall on an 8–12 year cycle. These catch variations match cycles in the strength of westerly winds over Victoria, except that there is a lag of several years between the wind cycle and the catch trends (see graph below).

Currents and temperatures

The settlement of King George whiting in PPB is also strongly related to the sea-surface water temperatures west of Cape Otway. "Good" whiting settlement years in PPB correspond with warmer than average temperatures west of Cape Otway, which in turn are related to the eastward extension of warm waters from the Leeuwin current.

The Leeuwin current is a warm ocean-current that flows strongly southwards along the Western Australian coast, before turning eastwards at Cape Leeuwin and dissipating in the Southern Ocean, where westerly winds move warm surface water into the Great Australian Bight. Its influence sometimes extends as far as Tasmania.

When the sea surface temperatures in the Portland area are warmer than average, settlement of King George whiting in PPB is strong. When sea surface temperatures in the Portland area cool, fewer King George whiting settle in PPB.

The abundance of King George whiting settling in PPB fluctuates from year to year depending on the strength of the Leeuwin current and currents driven by westerly winds.

Importance of Seagrass

The availability of nursery habitats also influences the number of whiting larvae that settle, survive and grow large enough to subsequently enter (and 'recruit' to) the PPB fishery.

Seagrass is very important in providing physical shelter, protection from predators and food (the small crustaceans living in the seagrass) for juvenile King George whiting.

Seagrass habitat is known to fluctuate naturally - increasing and decreasing over multi-decadal timeframes.

In Western Port, the same 8-12 year cyclic fluctuations in commercial whiting catches have been observed, but the long-term upward trend in catches since the 1940s stopped in the mid 1970s and has since been declining. A major loss of seagrass occurred in Western Port in the mid to late 1970s.

In recent years, there is evidence of the return of seagrass in Western Port, which may partly explain the recent reports of improved whiting fishing in this area.

Researchers are interested to see if any changes to seagrass extent in PPB in the future will have any impact on the whiting catch.

Predicting future numbers

By measuring larval settlement, Fisheries Victoria can now confidently predict the population size of King George whiting in PPB 3 to 4 years in advance.

Being able to forecast strong or weak year classes entering the fisheries is vital to maintaining a sustainable fishery through adaptive management arrangements. This information also allows fishers to understand the natural variability associated with whiting abundance in PPB.

Snapper - Internal drivers

 

Hot spots for snapper larvae in PPB

Eggs, 1 mm 1 day

Hatched larvae, 3 mm 5 days

Developed larvae, 5 mm 12 days

Juvenile snapper 8 – 10 cm three months

Snapper catches fluctuate in response to environmental factors within PPB that influence spawning success and the numbers of larvae that survive.

Importance of PPB

Snapper spawned in PPB are the mainstay of the Victoria's western snapper fishery. This fishery extends along the coast from Wilson's Promontory to the South Australian border and includes Western Port and PPB.

Spawning

Snapper spawn in the late spring / early summer.

Since 2004 Fisheries Victoria has monitored the numbers of snapper larvae in northern PPB, the Geelong Arm, Port Phillip Heads and coastal waters in Bass Strait.

Snapper larvae are common during December/January in PPB. Very few larvae have been collected from Port Phillip Heads and coastal areas around the entrance to PPB, suggesting that in most years snapper spawning is confined to PPB.

The number of snapper larvae in PPB varies considerably from year to year. The processes that determine the fluctuations in the production and survival of snapper larvae are still poorly understood.

Newly settled snapper

Newly-settled snapper larvae live on sediments and feed on microscopic bottom dwelling crustaceans.

The successful spawning and survival of young snapper varies greatly from year to year. Ten to 20 fold variations in settlement success are characteristic of snapper.

What influences survival of larvae and newly settled snapper is still unknown. Researchers believe it could be related to fluctuations in environmental factors such as water temperature and nutrient supply, which in turn influences the food supply, growth and survival of larval and small juvenile snapper from November to February.

These processes ultimately determine the strength of the snapper fisheries in PPB.

Year class strength

Fisheries Victoria scientists survey PPB each March after the snapper spawning season to determine numbers of newly settled juvenile snapper less than 10 cm in length, as an indicator or measure of 'recruits' to the fishery in future years.

The strong recruitment of newly settled snapper to PPB in summer 2001 was responsible for the large numbers of snapper that first entered the fishery in 2006.

There was also another good spawning season in 2004 and provided the spring migrations of adult snapper into PPB from coastal waters remains consistent, the prospects of PPB's snapper fishery remain bright for the next year. Consistent with the natural dynamics of the fishery, after 2010 the fishery is expected to decline for a period, as settlement in 2006 was relatively weak.

Weak year classes are the norm, not the exception, for snapper in PPB. Since 1993, only six spawnings have resulted in strong to moderate year classes.

Fishing controls allow commercial and recreational fishers to take advantage of times when spawning has been successful and

Numbers of larval and newly settled snapper ('recruits') in PPB

snapper are abundant (such as now). Regulations may need to be adjusted in future to sustain adult stocks, taking account of fish survey results and changing patterns in commercial and recreational fishing effort.

Numbers of newly settled snapper in PPB

Monitoring fish

Annual trawling of PPB benthic communities

Measuring anchovies from PPB

Monitoring fish living in seagrass beds

Monitoring fish eggs and larvae

Examining anchovy ear bones to determine age rings

Examining anchovy ear bones to determine age rings

Recreational fishing for flathead

Fish caught as part of the seagrass monitoring

Banjo shark cruising benthic sediments looking for crustaceans

Surveying recreational catches is one of the many ways of monitoring fish in PPB

The fisheries resources of PPB are managed for long-term sustainability, to ensure fish now and into the future. To achieve such an outcome, policy and management decisions made about PPB fish stocks are underpinned by scientifically-based research, monitoring and assessment.

Baywide monitoring

The health of the fish stocks in PPB is measured year round by fisheries scientists from the Department of Primary Industries (DPI).

In 2008 the monitoring undertaken by DPI was augmented by the Channel Deepening Project Baywide Fish Stock and Recruitment Program. The aim of this Program is to:

Detect changes in the distribution, abundance and types of fish in the Bay, their population structures and larval distribution and abundance, outside of expected variability.

This Program collects reliable scientific information about:

the benthic fish living on or near the sediments of PPB

• to identify long-term trends in fish abundance and biomass in the Bay. This information is based on annual (March) trawl survey results.
• to quantitatively assess abundance of four common and widespread bottom dwelling 'indicator' species used to monitor the health of the benthic fish community

the anchovy population and other key species which live in the water column of PPB

• to identify anchovy distribution and population structure (e.g. fish size and age) in the Bay. This information is based on annual (June) trawl survey results

the fish living in the seagrasses beds of PPB

• to provide information on the structure (species richness and abundance) of fish communities in both shallow and deep seagrass beds in the Bay from annual spring and autumn surveys

the eggs and larvae of fish that spawn in PPB

• to identify trends in fish egg and larval distribution and abundance, and potential future recruitment to fish stocks in the Bay, from annual (summer) surveys

the fish that recreational anglers catch

to provide information on the status of key recreational fish stocks in the Bay

to monitor and assess catch rates, growth and recruitment of snapper, King George whiting andflathead throughout the year to compare with historical catch and growth rates.

Where possible, all information collected is assessed against indicators of how stocks vary naturally. These indicators are derived from available historical data, and flag if fish stocks in the Bay are doing anything unusual or unexpected. If changes outside of natural variation are detected, management measures will be undertaken where appropriate.

Health of PPB fish stocks

The results of this monitoring for the period 2008-2009 indicated the majority of fish stocks and populations in PPB were within natural and historical ranges.

Scientific evidence

The scientific evidence gathered during this period showed:

• most fish stocks in PPB were relatively stable
  • changes in the total biomass of benthic fish in the Bay from trawl survey results were comparable with other year-to-year changes experienced in the past 20 years
  • the number (abundance) and weight (biomass) of three common and widespread benthic fish species (eastern shovelnose stingaree, sparsely spotted stingaree and spiny gurnard) did not differ significantly from historical levels and were within the range of natural variability expected for these species in the Bay
  • mean recreational catch rates for snapper and King George whiting indicated that these stocks were within historical levels for the Bay
  • the population structure of both snapper and KingGeorge whiting based on recreational catch were stable compared with historical levels in the Bay
• there was evidence of recent spawning of fish, including key species, in PPB
  • fish eggs and larvae from annual surveys were found in large numbers in the eastern area of the Bay
  • the numbers of fish eggs (including anchovy eggs) and fish larvae (including anchovy and snapper larvae) from these surveys varied each year within historical ranges for the Bay
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  •  
  • recreational angler catches indicated the presence of snapper and King George whiting just below the legal minimum length, which are expected to recruit to the fishery over the next few years
• changes in sand flathead abundance and biomass were outside of natural variability for PPB
  • the decline in sand flathead numbers in the Bay appears part of an on-going trend that can be traced to the mid-1960s
  • weak recruitment appears to be driving the recent decline in sand flathead abundance in the Bay
  • the reasons for this weak recruitment and the ecological significance of declining sand flathead abundance in the Bay are not known.

New insights

The scientific information collected as part of the research, monitoring and assessment of PPB fish stocks also fills in knowledge gaps of fish population dynamics and the importance of habitats to the health of fish stocks.

Anchovy

PPB is home to large numbers of anchovy.

Anchovy are found mainly from the centre and east of the Bay. The oldest anchovy found in PPB was 3 years old while the youngest was less than 12 months old. Most anchovies in PPB are less than 2 years old. Older anchovies migrate out of PPB.

Fish in Seagrass

Results to date suggest that shallow and deep seagrass beds in PPB have distinct fish assemblages. The fish assemblages present in seagrass beds change seasonally.

Shallow seagrass beds tend to be dominated by small schooling species that do not occur in deeper seagrass. The majority of species using seagrass as a nursery, such as King George whiting, Australian herring, and Eastern Australian salmon, have been found almost entirely in shallow seagrass beds.

Management action underpinned by scientific information

DPI actively considers all the scientific evidence and, when required, adjusts management settings to ensure fish stocks are not overexploited.

DPI recently revised sand flathead recreational fishing regulations in light of the scientific evidence and:

• increased the size limit for all flathead (except dusky flathead) from 25 cm to 27 cm total length
• reduced the bag limit from 30 to 20 fish.

These changes became effective in March 2009.

DPI will continue to scrutinise scientific evidence and act appropriately to ensure fish now and for the future.

Summary

PPB is highly regarded by Victorians for the social, economic and environmental uses and values.

PPB is underpinned by phytoplankton, which provides most of the food for PPB's animal communities. Phytoplankton growth is naturally limited by the low levels of nutrients (nitrogen).

The key to PPB's good health lies in its diversity of plant and animal life and in their interactions.

As a whole, water quality throughout PPB remains high and has not deteriorated in the past 20 years. PPB has been affected by the prolonged drought.

PPB supports diverse commercial and recreational fisheries. The abundance of the fish in PPB varies in response to natural changes in aquatic habitat and environmental conditions.

Environmental fluctuations occur on short time scales and long-term time scales. These fluctuations drive fisheries production.

Fluctuations in catch of King George whiting are related to the strength of westerly winds and water temperatures of western Victoria. These changes influence the number of larvae that are delivered to PPB. The extent of seagrass habitat is also thought to determine the number of whiting that can live in PPB.

In contrast, environment conditions within PPB determine the numbers of snapper that live in PPB as larvae and juveniles (pinkies).

By monitoring the numbers of young King George whiting and snapper in PPB, the strength of the fishery in the future can be predicted. The outlook for King George whiting and snapper fisheries in PPB is for stable to declining catches over the next few years.

Scientific monitoring of PPB fish stocks underpins fisheries policy and management decisions to ensure fish now and for the future.

Further reading

Fisheries Information

Recreational Fishing Guide
The Victorian Recreational Fishing Guide is a handy resource that outlines all your rights and responsibilities to help you fish sustainably.

Fish-e-Fax
Subscribe on-line to DPI’s fortnightly two-page electronic newsletter for recreational fishers.

Fisheries Information Notes
From time to time, information about fisheries management, fish species and research projects are summarised into handy downloadable Fisheries Information Notes.

Get Hooked it’s Fun to Fish
The National Junior Fishing Codes Education Kit - Get Hooked it’s Fun to Fish is a fun, in-class program developed specifically for primary school age children.

Commercial Fish Production Information Bulletin
This annual report describing commercial fish production for the 2007/08 season is produced using data compiled from catch and effort records supplied by Victorian commercial fishery access licence holders. Commercial Fish Production Bulletin

Web sites:

Fisheries Victoria
www.new.dpi.vic.gov.au/fisheries

Marine Discovery Centre
www.dpi.vic.gov.au/mdc

Office of the Environmental Monitor
www.oem.vic.gov.au

Port of Melbourne Corporation
www.portofmelbourne.com
  www.channelproject.com

Environment Protection Authority
www.epa.vic.gov.au

Department of Sustainability and Environment
www.dse.vic.gov.au

Acknowledgements

The information that underpins this report arises from the research and monitoring activities commissioned by:

• Department of Primary Industries - Fisheries Victoria (DPI FV)
• Department of Sustainability and Environment (DSE)
• Environment Protection Authority (EPA)
• Port of Melbourne Corporation (PoMC)

Specifically information was gained from:

• Fisheries Victoria's PPB R&D Portfolio
  This diverse portfolio of monitoring and research provides recreational and commercial catch data, fisheries data (size and age) and information about the linkages between fish and habitat and the ecological processes that drive fish production in PPB.
  These research and monitoring activities have beenconducted since 1990.
• EPA's Water Quality Monitoring Program
  This program collects, analyses and synthesises information about the quality of the water in PPB monthly.
  This monitoring program has been conducted since1984.This program was augmented by the Baywide Monitoring Program in 2008.
• DSE's Nutrient Monitoring Program
  This program was set-up to inform the Environmental Management Plan for PPB in 2000.
  This program collects information on water quality continuously and on the nutrient cycling twice a year.
  This program was augmented by the Baywide Monitoring Program in 2008.
• PoMC's Channel Deepening Baywide Monitoring Program
  A suite of Baywide Monitoring Programs provides information on the status of key species, habitats and ecological processes in PPB.
  The overarching objectives of the Baywide Monitoring Programs are to assess the status of PPB and detect changes outside of expected variability.
  Expected variability includes both natural fluctuations and anticipated CDP-related changes as predicted in the risk assessment.
  Most of the Baywide Monitoring Programs build on existing government monitoring programs.
  • Seagrass
  • Water Quality
  • Nutrient Cycling (denitrification)
  • Contaminants in Fish
  • Algal Blooms
  • Little Penguins
  • Fish Stock & Recruitment
  • Ramsar Wetlands
  • Plume Intensity & Extent.