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 drift 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. There are also a number of other project partners who are providing additional support to the research program. These include Hunter Water, Newcastle Ports Corporation, Port of Newcastle Authority, Origin Energy, Hunter Local land Services, North Coast Local Land Services, Professional Fisherman's Association, University of Newcastle, Griffith University, Murdoch University and Oceanwatch.
The project is now just over two years since commencing and has completed its second research season. Analysis of those findings is well underway and preliminary results are summarised below.
The project is progressing well against the stated objectives, and is now just over the two year mark. This milestone period dealt primarily with Objectives 2, 3 and 4, however, work is progressing against all objectives simultaneously. The progress reported against Objective 1 has a strong focus on the Hunter River in this report, as this is the primary study system for the project, and has the most “mature” dataset. Work on the other estuaries is also progressing. Progress against each of these Objectives is summarised under the respective headings below.
The stable isotope design conducted in Season 1 was repeated in Season 2, and 608 samples were submitted for Stable Isotope analysis. These results were received back from the lab in late October, and will be analysed over the summer. In addition to the abundance data derived from quantitative sampling, reported in the previous milestone report, analysis of length and condition among putative nursery habitats is currently underway. In the Hunter River, the somatic condition of juvenile EKP sampled among the habitats in different months in the Hunter River differed significantly (ANCOVA Fmonth*habitat*length = 2.44, df 4, 3935, P=0.045). Overall, juveniles in March 2015 were in poorer somatic condition across all habitats (Fig. 1), while juveniles sampled from bays and marshes in November and January were in better somatic condition (Fig. 1).
In contrast, in Lake Macquarie the variation in somatic condition was largely due to month (ANCOVA Fmonth*length = 5.74, df 2, 3014, P=0.003), with juvenile prawns collected in October 2014 in better somatic condition than those collected in December 2014 and January 2015 (Fig. 2).
The previous milestone report included preliminary findings from a new sub-subcomponent of the broader project which evaluated the use of intertidal marsh and sub-tidal creeks by EKP in the Hunter River. The basic premise was that juvenile EKP directly move into intertidal marshes similarly to other penaeids in tropical habitats; however, this has not been confirmed by studies undertaken within temperate estuaries. The Hunter River contains significant intertidal marsh habitat intersected by numerous sub-tidal creeks, with the aim of this project component to evaluate whether prawns utilise these intertidal marshes with comparisons made to the sub-tidal creeks. The full dataset has now been compiled and analysis on both the abundance of EKP and community analysis on the nekton found within the marsh and creeks has been completed. Full analysis of the community data using PERMANOVA showed that the nekton community did not differ among the three spatially distinct wetlands sampled across the Hunter River.
Preliminary findings in the previous report indicated very few EKP utilised the intertidal marsh habitat, however significant numbers of other nekton were collected. Numbers of School prawn (Metapenaeus macleayi) within intertidal marshes were significantly greater than EKP, however even these numbers are still relatively low compared to penaeid samples collected in other habitats across Australia and the world.
Greater numbers of EKP were sampled using cast nets within sub-tidal creeks, with similar numbers collected at the fringing edges and the middle of creeks (Fig. 3). A similar pattern was found for school prawns. Large numbers of other, non-commercial crustaceans were also sampled, which largely consisted of Acetes sibogae australis. Although abundances of EKP were similar between creek habitats, they were found to differ among the three focal wetlands within the Hunter River. Carapace length of EKP ranged between 3.5 – 14.0 mm with a mean of 7.3 mm ± 0.4 S.E. No difference was found in the carapace lengths of EKP between the adjacent and middle habitats of creeks, however, carapace lengths of School prawns were significantly greater in middle as opposed to edge habitats. Length data collected from the intertidal marsh habitat with the fyke nets was not compared statistically with data from the creeks due to differences in gear; however these sizes were similar to both adjacent and middle habitats for EKP, but only similar to School prawn size within the adjacent habitat (Fig. 4). This may indicate a potential ontogenetic shift in the distribution of School prawns, with larger, older prawns moving away from the marsh onto the un-vegetated substrate of the creeks. Increased swimming ability of larger individuals is another potential explanation for these patterns, with the edges of the creeks and intertidal habitats providing refuge from tidal flow for smaller individuals compared to the middle of the channel. A PERMANOVA test indicated the nekton community within sub-tidal creeks as a whole was not found to differ between the two creek habitats, although there were some differences among creeks themselves, indicating there is smaller spatial variation within the Hunter River which may be due to variation in the tidal flushing rates among creeks.
As outlined in the project proposal, inter-annual variation in the catch of EKP could potentially be the result of recruitment limitation linked to significant rainfall events. EKP are relatively stenohaline, and are expected to have a negative response to rapid changes in salinity that occur in estuaries, and low salinity conditions could substantially alter the availability or quality of potential nursery habitats within estuarine systems. Anecdotal information from industry indicates that wetter years generally lead to poorer yields of EKP. Temperature and conductivity loggers were placed in estuaries to measure the rates-of-change, and the duration of low-salinity conditions experienced by free-ranging EKP. Prawns (CL 7.22 ± 1.24mm (S.D.)) were exposed to a broad range of rates-of-decline in salinity that were measured in the field, which ranged between <0.01% h-1 and 19% h-1 decline scenarios (Fig. 5). A total of 180 prawns were used, with 10 prawns randomly allocated to a tank for each rate of salinity decline. At the end of the dilution, each tank was maintained at its final salinity for an additional five days at 24°C; the mean summer water temperature of the Hunter River estuary. On completion of the fifth day, all surviving prawns were subject to measurements of their metabolic rate.
Out of the 18 tanks in the first three runs of experiments, the surviving prawns from 17 were subject to the respiration experiments as one tank suffered 100% mortality. A fitted GAM model of metabolic rate against the rate-of-decline and condition accounted for 25.6% of the observed deviance in the data. The rate-of-decline had a significant non-linear effect on the metabolic rate. The metabolic rate had a negative relationship with condition, with the animals in the poorest condition having the highest metabolic rates. The GAM found that rate-of-decline in salinity had a significant non-linear effect on the water concentration of the prawns that had survived the full experimental duration and explained 34.8% of the observed deviance in the data. The water content was higher in animals that had experienced high rates of decline in salinity, with an intercept of 80.3% water coinciding with animals that had experienced salinity decline of 2.6% h-1 and salinity around 20. Varying salinity declines also influenced mortality rates of prawns, with Chi-square tests showing that increasing the rate-of-decline significantly increased the mortality of prawns during the 24 h transition (Fig. 6).
A second experiment conducted during July 2015 evaluated the effects of temperature on the respiration of EKP and determined Q10 values. Five temperature treatments (10°C, 15°C, 20°C, 25°C and 30°C) were included with six replicate prawns exposed to each treatment. Over 13 h, the temperature was increased from ~14.2°C to 18.2 ± 0.37°C. This temperature was maintained for 48 h to allow prawns to acclimate to the same temperature before starting the experiment. Experimental alteration in the temperature of each treatment group were altered by a maximum of 3°C over 24 h, until the target temperature was reached. Once reached, the temperature was maintained for 48 h prior to assessment of metabolic rate.
Survival was 100% over the course of this experiment. Temperature significantly affected the rate of oxygen consumption of prawns, with rates of oxygen consumption increasing with water temperature. Animals held at 10°C and 15°C had metabolic rates 52.0% and 61.4% slower than those held at 20°C, 69.9% and 75.5% slower than animals at 25°C and 72.4% and 77.6% slower than animals at 30°C respectively (Fig. 7). The differences in metabolic rates had corresponding Q10 values of 0.666 between 10°C and 15°C, 6.69 from 15°C to 20°C, 2.5 from 20°C to 25°C and 1.189 from 25°C to 30°C. The Q10 value for the full temperature range (10°C to 30°C) was 1.908.
In summary, these results provide evidence that rapid declines in salinity and temperature associated with flood events are likely to compromise the survival of juvenile EKP, with the potential for the loss of entire cohorts of juveniles which may cause a subsequent reduction in recruitment to the fishery. These periods of high freshwater inundation ultimately reduce the availability of nursery habitats within estuarine systems, and may also have non-lethal effects (e.g. reduced growth, at certain salinities). These findings are currently being prepared for submission to the scientific literature, and will be presented to stakeholders at the next project update.
Mapping for the Hunter and Clarence River is now complete. This has involved digitising and rectifying old aerial photo imagery, and analysing the imagery collected at different times in ArcGIS (Fig. 8). Mapping the Clarence River has occurred to the upper tidal limit (40 km upstream of Grafton). The changes in the habitat of the Hunter River have been reported previously.
There have been substantial changes in the abundance of macrophyte habitat in the Clarence River between 1942 and 2009. Overall, the area of mangrove habitat has slightly increased by 6% to 765 ha. The area of saltmarsh and seagrass in the estuary has decreased substantially, with a loss of 64% of all saltmarsh habitat to 290 ha in 2009, and a loss of 79% of all seagrass habitat to 82 ha in 2009.
Over the coming summer, mapping of Lake Macquarie will be finalised. In addition, if time permits, we hope to conduct similar mapping of Wallis Lake and Tuggerah Lake as well.
Unfortunately, due to circumstances beyond our control, the PhD student engaged to address this objective through Murdoch University has resigned from their candidature. Despite this, progress has continued on Objective 4, and we have elected to focus on both EKP and School Prawn (in that order of priority). Three modelling approaches are being undertaken:
A simple cohort model which will allow the translation of the recruitment subsidy provided from rehabilitation in a particular estuary to be translated into potential harvest and economic benefit. This model develops the approach employed in Blandon and zu Ermgassen (2014), and is currently being written in Matlab (almost completed). Unlike Blandon and zu Ermgassen (2014), the simulation structure will follow a standard Monte-Carlo Analysis of Uncertainty (MCAoU), which will allow us to incorporate parameter uncertainty in our estimates;
Dr. Matt Ives (Oxford University) is being engaged to run the models he developed for both EKP and School Prawn in NSW. These models will incorporate field data derived under Objective 1, and be used to evaluate rehabilitation scenarios at the estuary level, and we aim to develop a decision analysis to establish the minimum “area of rehabilitation” to have a noticeable effect on the fishery. Most parameters are now in place for these models, and preliminary simulations should be completed by March/April 2016.
As proposed in the last milestone report, we intended to use the Enhancefish (http://fisheriessolutions.org/projects/enhancefish/) to model the impact derived from the recruitment subsidy provided from rehabilitation in a particular estuary on potential harvest and economic benefit. While this was to be undertaken by the discontinued PhD student, we are now negotiating with the author of the program (Prof. Kai Lorenzen) regarding his potential involvement in the application of this tool.