Since the previous update in June 2014, the project has largely completed analysing its first year of research results, with the second research season now underway. The main findings to date are available below.
This project was started in response to a lack of detailed information about the links between Eastern King Prawns (EKP) and estuarine habitat in NSW.
It has been known for some time that EKP spawn at sea, the larvae drifting south on the East Australian Current before moving into our coastal estuaries. The tiny prawns spend some months growing in the estuary, before heading out to sea and swimming back up north; where they continue their growth to full maturity and complete the breeding cycle.
However, until now there has been little detail about which parts of the estuary are more important to young EKP. Where do they live? What do they feed on? Are mangroves, seagrass, salt marsh or unvegetated habitats more important; or are they all just as critical? Are some river systems more important than others?
The project is a three year study at sites in the Hunter River, Lake Macquarie and the Clarence River. The Fisheries Research and Development Corporation is funding this exciting body of work, with the interest, involvement and support of the commercial EKP fishing industry.
In the lower Hunter alone;
As reported in the last project update, the final phase of rehabilitation of Hexham Swamp (a major rehabilitation program being undertaken in the Hunter River, our main study site) occurred in October 2013. Opening the final two floodgates restored full tidal connectivity to the swamp through Ironbark Creek for the first time in four decades.
Monitoring of the assemblage in the rehabilitated habitat included continuing seine surveys above and below the Ironbark Creek floodgates, and at adjacent reference sites. As reported in April, the lower Ironbark Creek sites, supported greater abundances of juvenile eastern king prawn relative to adjacent reference sites (on Ash Island). The latest monitoring period has seen the presence of eastern king prawns extend much further up the tributary, with strong recruitment in 2013/14. Similar trends were found for school prawn. This finding is significant because it provides the first clear demonstration of the impact of restoring connectivity with estuarine wetlands for commercial species of prawn in New South Wales.
Preliminary analysis of all Season 1 results has now been conducted (excluding data analysis from the Clarence River). These findings are summarised below, however the final outcomes may change as further results emerge throughout the course of the project.
Stable isotope analysis and nursery habitats: There was no significant difference in the isotope composition of school prawn and king prawn collected at the same habitat sites. Significant differences were detected between different nursery habitat sites, and isotopic composition was found to be highly correlated with salinity in the Hunter River. The isotopic composition of emigrating prawns was found to be independent of their size. There was no significant difference in the isotopic composition of emigrating prawns sampled on different nights, indicating that correlation between the distance of the supposed habitat of origin from the estuary mouth, and the night on which an emigrating prawn was sampled at the mouth of the estuary was unlikely.
Analysis of the sample data from Lake Macquarie (see Figure 1) indicates that prawns captured from different habitat sites were isotopically distinct and showed a remarkably low degree of within-site isotopic variation. Seagrass habitats on the eastern side of the lake were contributing proportionally more emigrating prawns than other areas in the lake. Eastern king prawns were almost completely absent from the south-western corner of the lake, where catches were dominated by greasyback prawns (Metapenaeus bennettae). There was reasonably strong positive correlation between the abundance of Eastern king prawns collected at a particular site, and the proportion of emigrating prawns assigned to that site. The seagrass habitats which were contributing more include primarily Zostera beds, but also with some potential contribution of Posidonia beds in the vicinity.
Figure 1: Proportional contribution of different habitat sites to emigrating Eastern king prawns in Lake Macquarie (shown as yellow circles of size proportional to the relative contribution) on the basis of stable isotope composition. Sites that were surveyed but did not contribute are shown as grey triangles.
In the Hunter (see Figure 2), on the basis of their isotopic composition, School prawns captured at the mouth of the river originated from along the monitored length of the estuary, almost as far as Raymond Terrace. There appeared to be more emigrating prawns originating from the south arm of the Hunter River and also the vast shallow sedimentary habitats of Fullerton Cove. A small proportion of emigrating school prawns appeared to originate from Ironbark Creek.
The majority of emigrating Eastern king prawns appeared to be originating from Fullerton Cove and also the vast mangrove lined habitats that dominate the lower reaches of the north arm of the river. Almost no emigrating king prawns originated from the extensive mangrove and saltmarsh habitats further up the north arm of the estuary. Not surprisingly, the highest contributing habitat sites all lay within the salinity range 25 – 33 ppt; a window that closely reflects the range through which eastern king prawns can best osmoregulate (i.e. effectively manage fluid concentrations within their tissues).
In conclusion, on the basis of Season 1 data the sites that contributed most to emigrating Eastern king prawns appear to represent a combination of vicinity to mouth (i.e. sites that could allow good supply of coastally spawned larvae), salinity, and mangrove/shallow soft-sediment habitats. Data analysis for the Clarence River is ongoing.
Figure 2: Proportional contribution of different habitat sites to emigrating Eastern king prawns in the Hunter River (shown as green circles of size proportional to the relative contribution). An interpolated salinity surface and major aquatic habitat types are also shown.
Salinity impacts: The effects of low salinity levels on prawns (principally arising from freshwater inundation of estuaries) are relatively unknown. Professional fishers have observed and reported decreases in EKP catch rates, and also slower growth, in years of high rainfall. EKP have been proposed as being unable to tolerate wide variations in salinity when compared to other commercial prawn species in NSW (i.e. greasyback and school prawn). Juvenile eastern king prawns can osmoregulate down to about 10 ppt; however as they grow to adults they lose this ability and are poor osmoregulators across the spectrum of salinities.
Osmoregulation is an energetically expensive process, and this may eventually result in decreased growth rates, thus providing some explanation of the observations from professional fishers. Consequently, freshwater inflow to estuaries may have a direct impact on metabolism, appetite, available prey, and habitat availability; and consequently the growth and survival within those habitats. As salinity in the estuary decreases, several things may occur. If the rate of decrease in salinity is far more rapid than the rate at which the prawns can effectively osmoregulate, they may either die or emigrate from the estuary early. If the rate is moderate, they may simply endure the lower salinities through osmoregulation, over short or long periods. These factors may have effects on recruitment to the fishery and associated yields of EKP. An experiment has been designed (and has commenced over summer 2014/15) to test the response of EKP to varying salinity levels and temperatures.
Historical and recent spatial imagery have now been analysed from the lower Hunter River in the context of the data and relationships derived from the first years sampling. This GIS analysis has derived that ≈21 hectares of channel habitats have been lost from the lower Hunter River between the 1950s and 1990s. Using densities of Eastern king prawn captured in Season 1 sampling (and making many assumptions), this loss of habitat could have contributed to the loss of between 100,000 and 500,000 recruits to the emigrating stock on an annual basis.
About 1426 ha of saltmarsh has been lost from this area of the estuary. Whilst it is difficult to convert this into a loss of recruits (as saltmarsh is only periodically inundated), it is likely that the loss has affected overall primary productivity in the estuary. This has flow on impacts on the productivity of other areas of the estuary such as downstream tidal creeks and channel/embayments where it seems the prawns are concentrated. Our stable isotope analysis in Season 2 and Season 3 will help to further reveal and quantify this impact.
The overall loss of waterway area in tidal creeks that drain saltmarsh is around 12 ha, which could contribute to the loss of up to 500,000 recruits from the fishery annually. Happily, connectivity between 825 ha of saltmarsh in the Hexham swamp and the estuary has recently been restored through the opening of the floodgates on Ironbark Creek in 2013, and the creeks which drain this saltmarsh are now recolonizing with both Eastern king prawn and School prawn.
Interestingly, the lower Hunter River gained 400 ha of mangrove habitat over the same period, which will likely have positive impacts on prawns by the provision of nutrients to support lower trophic levels through microbial breakdown of organic material. Spatial analyses of habitats will be ongoing in the Hunter River and other study estuaries over the next 12 months.
On the 30th July 2014, a project partner update meeting was hosted by NSW DPI staff at the Newcastle Fishermen's Coop. Invitees included Hunter Water, North Coast Local Land Services, Hunter Local Land Services, Griffith University, Newcastle Ports Corporation, Port of Newcastle Authority, Origin Energy, Professional Fishermen's Association, Newcastle Fishermen's Co-op, and OceanWatch Australia. This meeting provided an opportunity for project staff to discuss the project's progress, answer any queries and explore ways to more closely engage the partners in the project. Dr Craig Boys (DPI) discussed preliminary findings from recent Hexham biological sampling.
The following presentations have been conducted:
1. Taylor, MD (2014) The impact of habitat loss and rehabilitation on recruitment to the NSW eastern king prawn fishery. Project Inception Meeting, Newcastle Co-op, 30th July 2014;
2. Russell, K. (2014) Why understanding Eastern King Prawn habitat is important. Project update meeting, Newcastle Co-op, 30th July 2014;
Links to the project were published in the following sources:
Association (PFA) magazine
500 hard copies distributed
plus website availability
The results from the survey of EKP fishers (presented in the June 2014 Progress report) were used to inform the development of our Communication Plan.
Different methods have been identified to get the key messages to each of those groups. Timeframes and responsibilities for specific actions have also been delegated within the Communications Plan.