2. Target Species / Stock Description
5.3 Environmental Influences
5.3.1
Approximately 75 % of the catchment area of the PHE has been cleared of natural vegetation for agricultural purposes. Due to the natural low-nutrient levels of the catchment soil, fertilisers have been used to improve agriculture in the area throughout the past century. A large portion of these nutrients have been flushed into the estuary, causing extensive macrophyte growth and toxic algal blooms in the 1960 – 1970s (Fretzer 2011).
Due eutrophication, the condition of the estuary became a public concern and an extensive study of the estuary was initiated in the late-1970s. This study investigated various aspects of the ecosystem, including nutrient inputs, features of the catchment (e.g. physical features, rainfall and runoff), physical and biological characteristics and hydrodynamics of the estuary and production and abundance of Cladophora (Hodgkin et al. 1981).
The Dawesville Channel 5.3.1.1
To increase flushing and reduce nutrient levels in the PHE, an artificial entrance channel (the Dawesville Channel) was opened in 1994. The channel was expected to increase salinity in the estuary, making conditions unsuitable for the toxic phytoplankton species Nodularia. The channel delivered the predicted increase in water quality, with a reduction in primary production. The channel also benefited seagrass within the estuary due to the less variable salinity regime, maintenance of bottom marine salinities for extended periods and improved water clarity. Macroalgae biomass in the Peel Inlet was significantly lowered, with a change in growth and distribution with peak biomass currently occurring in spring (as opposed to autumn).
The Dawesville Channel had a significant impact on the fish and invertebrate communities within the estuary. The impact of the channel on western king prawns, blue swimmer crabs and other commercially-fished species was investigated by Potter et al. (1998). Relevant biological data were collected for these species in 1995 – 1998 and were compared with historical data collected for the same sampling sites in 1979 – 1988 (i.e. prior to the Dawesville Channel). Results indicated that blue swimmer crabs and western king prawns were more abundant and present for longer periods in the Harvey Estuary than prior to the channel. This increase was attributed to:
1. A direct connection between the sea and the Harvey Estuary, which is a shorter distance to travel from the ocean into the Harvey Estuary;
2. A greater tidal water flow into the Harvey Estuary providing a more effective means of transportation into this part of the system; and
3. Salinities in the Harvey Estuary remaining higher for longer periods, providing an environment condusive to the retention of blue swimmer crabs and western king prawns for protracted periods.
The increased tidal movement through the PHE also accounted for the following:
• Small juvenile blue swimmer crabs recruiting from the ocean into the PHE over a longer period of time;
• Female blue swimmer crabs emigrating from the estuary earlier once ovigerous; and
• Prawns often emigrating from the estuary at a smaller size.
Furthermore, the growth of blue swimmer crabs in the estuary became more rapid since the opening of the channel, which has resulted in an earlier attainment of sexual maturity (Potter et al. 1998).
Prior to the construction of the Dawesville Channel, the composition of the fish communities in the different basin regions of the PHE differed and did not change markedly throughout the year. Since the opening of the channel, the composition of the fish communities in the different regions have become more similar. They also undergo pronounced seasonal cyclical changes, presumably related to the increased strength of environmental cues that are provided by the exchange in water during each tidal cycle. The number and overall abundance of fish
species in the Harvey Estuary, particularly the southern region, were found to be greater than prior to the opening of the channel. However, there was evidence that the levels of recruitment of the juveniles of each of the main commercial fish species (i.e. sea mullet, yelloweye mullet, cobbler, King George whiting and yellowfin whiting) into the PHE was lower than prior to the construction of the Dawesville Channel. This decline may be due to the reduction in the volume of macroalgae in the estuary resulting in reduced food and areas of protection from predation (Potter et al. 1998).
A quantitative model using Ecopath with Ecosim and Ecospace has been applied to the PHE to identify the ecosystem impacts of the artificial entrance channel (Fretzer 2011). Two Ecopath models were developed for PHE consisting of 30 living functional groups, comprised of dolphins, sharks, waterbirds, teleost fish, invertebrates and primary producers, and describing the ecosystem before and after the opening of the Dawesville Channel. The ecosystem of the PHE was found to have declined drastically in total biomass since the opening of the channel, as well as declined in biomass at each trophic level and in the size of flows between the functional groups. Changes in flows and transfer efficiencies illustrate a change in the functioning of the ecosystem since the opening of the Dawesville Channel (Fretzer 2011).
The results of the Ecopath models indicate that the Dawesville Channel has markedly impacted species composition and dominance in floral and faunal communities. Estuarine fish species have decreased, and marine species have become more dominant in the estuary (Fretzer 2011). Ecosim was applied to the model to identify the impact of primary producers on functional groups of the estuary and the impacts of fishing on target and non-target species. Results indicate that primary producers, such as seagrass, have an influence on blue swimmer crab biomass (Figure 5.1). Furthermore, with high nutrient concentrations still present in the PHE from continued agricultural and urban runoff, phytoplankton blooms may potentially reduce the biomass of some fish species, such as sea mullet and yelloweye mullet (Figure 5.2), whereas others, such as Australian herring, may increase in biomass (Fretzer 2011).
Figure 5.1. Effect of 50 % increase and decrease of seagrass biomass on the relative biomass of blue swimmer crabs in the PHE (Source: Fretzer 2011).
Figure 5.2. Effect of phytoplankton blooms, simulating an increase in biomass of microscopic algae by factor 10, on the target fish species yelloweye mullet (Aldrichetta forsteri) and sea mullet (Mugil cephalus) in the PHE (Source: Fretzer 2011).
Climate Change 5.3.2
Climate change has the potential to influence different aspects of the biology of species such as sea mullet and blue swimmer crabs. Increased water temperatures as well as changes to
seasonal rainfall patterns and the strength of oceanic currents could all potentially affect migration patterns, spawning success and recruitment of these species in south-western WA.
Changes to the strength of the Leeuwin Current that flows southwards and eastwards along the south-west corner of WA could influence the northward spawning migrations of sea mullet during the spawning season. Increased water temperatures may also lead to shifts in the distribution of sea mullet along the coast, which has been observed for several other finfish species in waters off south-western Australia (Smith et al. 2014).
The effects of climate change on blue swimmer crabs are likely to vary between fisheries in WA, based on the large latitudinal and longitudinal range of this species, and depending on the particular ecosystem the crabs inhabit. Long-term climate change predictions for the west coast of WA indicate that rainfall will decrease over time, potentially increasing hypersaline areas in coastal waters and shallow estuaries. Such a rise in hypersalinity may lead to increased mortality of juveniles and adults as blue swimmer crabs do not tolerate high levels of salinity. Declining rainfall could also negatively influence the movement of crabs out of the PHE to spawn, which normally occurs at the onset of winter rains flushing the crabs out of the estuary (Section 2.2.3.1).
The waters of the lower west coast of WA are at the southern extreme of the temperature tolerance of blue swimmer crabs and thus they are highly susceptible to fluctuations in temperature. Johnston et al. (2011a) reported cooler than average water temperature in August and September for four consecutive years in Cockburn Sound, which lead to poor spawning success in the subsequent spawning seasons. This was suggested as a major contributing factor in the decline of this fishery and thus needs to be evaluated in future management of blue swimmer crab fisheries in the south-west of WA.
Introduced Pests 5.3.3
The introduction and spread of marine pests in WA waters poses a serious threat to native biodiversity and can have widespread effects on both our economy and health. For example, the Asian paddle crab (Charybdis japonica) has the potential to outcompete native species such as the blue swimmer crab if it becomes established in Australia. The Department’s Marine Biosecurity Research and Monitoring group continue to implement a series of biosecurity-related projects to ensure early detection of the presence of introduced marine pests in the WCB (Fletcher & Santoro 2014).
Early detection of introduced marine pests is vital if any attempt at eradication or other management strategies is to be successful. When an Asian paddle crab was recently handed in to the Department’s Mandurah office without details of where it was captured, 100 traps were deployed in the PHE for several days but no further paddle crabs were found.
5.4 Urban and Other Developments
Significant growth is projected for the Perth and Peel regions over the coming decades.
Recent projections have estimated that by 2026 the State’s population will grow to
~ 3 million, with the Perth and Peel region projected to be ~ 2.3 million. In light of these
projections, in 2011 the WA Ministers for Planning and Environment and the Commonwealth Minister for the Environment agreed to undertake a Strategic Assessment of the Perth and Peel regions of WA. The Strategic Assessment3 is being led by the Department of the Premier and Cabinet, in partnership with the Commonwealth Department of the Environment (DotE). At a State level, the Department of the Premier and Cabinet is working on the Strategic Assessment with DPaW, the Department of Planning and the Office of the Environmental Protection Authority. The purpose of the Strategic Assessment is to:
• Reduce the need for project-by-project assessment under the EPBC Act in the Perth and Peel region;
• Deliver an effective long-term and strategic response to key environmental issues in the Perth and Peel region, e.g. water quality in the PHE;
• Provide greater certainty to industry on areas that be developed and associated mitigation, including environmental offsets; and
• Provide greater certainty in terms of long-term land supply to meet the needs of a city of 3.5 million people.
Point Grey Development 5.4.1
Recently, the development of Point Grey, a peninsula which separates the Peel Inlet from the Harvey Estuary (Figure 5.3), has been approved by the Commonwealth DotE. The Point Grey Development will include an urban zone for residential purposes, as well as a regional-level marina and associated facilities. The proposed marina is located on the western edge of the Point Grey peninsula, an area historically used for grazing. The proposal also includes the construction of a 2.5 km navigation channel across the Harvey Estuary from the marina to the Dawesville Channel, effectively linking the marina to the Indian Ocean (EPA 2011).
The marina waterbody will occupy 9.8 ha and will be excavated to a maximum depth of 3 m.
Excavation of the marina is expected to result in the generation of approximately 660 000 m3 of spoil, which will be used as fill within the Point Grey development and in the construction of two protective groynes adjacent to the entrance channel. The marina will accommodate up to approximately 300 boat pens and will be designed to accommodate boats of maximum length 15 m and draft 1.5 m. Access to the marina water body from the Harvey Estuary will be via a 100 m long and 120 m wide entry channel through the foreshore. Approximately 5.1 ha of foreshore will accommodate car parking requirements for 200 cars and four boat ramps. This area will include a portion of landscaped foreshore between the car park and the shoreline which will contain paths and public toilets, allowing the public to access and use the beach to the west of the marina (EPA 2011).
The potential impacts on the environment are ongoing operational impacts to estuarine water quality and sedimentation of the navigation channel. In addition, the proposal is considered to have localised and temporary direct impacts on estuarine fauna. Based on these issues, a number of recommendations and conditions have been imposed by the EPA (EPA 2011).
3 More information on the Strategic Assessment is available at:
http://www.dpc.wa.gov.au/Consultation/StrategicAssessment/Pages/Default.aspx.
Figure 5.3. Location of the proposed Point Grey Development and Marina (Source: EPA 2011).
MSC Principle 1
MSC Principle 1 (P1) focuses on maintaining, indefinitely, fishing activity at a level that is sustainable for the targeted populations (MSC 2013).