September 2016

Key research findings

First years sampling has been completed with data from the period been downloaded and processed. Analysis of findings is well underway and the project’s progress to date against its objectives is summarised below.

1. Examine school prawn recruitment to different areas within the Camden Haven estuary, to determine if recruitment limitation in certain areas is likely

  • There is high variation in prawn abundance at different times and places.
  • The western part of Watson-Taylor Lake had the highest densities of prawns especially after significant rainfall

  • By March, most of the prawns had run to sea for spawning

2. Evaluate whether post-recruitment processes in Camden Haven estuary may be adversely affecting School Prawn growth and survival

  • There were many instances of low-dissolved oxygen water discharging into both Watson-Taylor Lake and Queens Lake, especially from John’s River which impacts on the eastern parts of Watson-Taylor Lake
  • The cause of these events is unclear, as they are not directly correlating with rainfall events
  • More acidic waters increase the mortality of prawns caused by aluminium, compared to when the water is less acidic

  • There may also be a range of impacts that affect prawns health without resulting in their immediate mortality

3. Synthesise research findings to provide recommendations to catchment, habitat and fishery managers regarding restoration of school prawn productivity

  • As the project is currently in the data collection phase, no new extension activities have occurred during this milestone period.

Detailed Progress report September 2016

The project is progressing well against the stated objectives, and is now just over the one year mark. This milestone period dealt primarily with routine sampling, data processing, mapping of findings and release of information.

Project progress against Project Objectives

1. Examine school prawn recruitment to different areas within the Camden Haven estuary, to determine if recruitment limitation in certain areas is likely

During this milestone period, routine sampling has been ongoing in both the Camden Haven estuary and Wallis Lake (reference estuary). To summarise, surveys are conducted every second month, during the daytime, using 70-100 m tows of a benthic sled towing a 26B-5C net. Every other month, local fishers tow the same research net (but not in a quantitative fashion) at a subset of sites to allow for a continual time series of length frequencies in the main regions of the lake. During each trip, 18 sites are surveyed for School Prawn abundance in Camden Haven, with four replicate tows performed at each site, and a 5th tow conducted where prawns are required to be placed in liquid N2 for later metabolomics analysis. An additional 4 reference sites are surveyed in Wallis Lake at the same level of replication, to yield abundance estimates in an adjacent system with which to compare recruitment levels in Camden Haven. Data from all field trips to date are reported here (November 2015, Fig. 1; January 2016, Fig. 2; March 2016, Fig. 3; May 2016, Fig. 4). Due to staffing issues the July 2016 field trip was rescheduled to August, and is currently being undertaken.

All samples to May 2016 have been sorted, and prawn density enumerated. Catch data has been standardised for gear efficiency and the area trawled (and thus represent absolute densities), and are presented as # 100 m-2. School Prawn density maps across the estuary are presented below (Figs. 1, 2, 3 and 4), and show a great deal of spatial and temporal variation in abundance. As highlighted in the previous milestone report, in November 2015 there were moderate-low densities of School Prawn across most of the estuary (Fig. 1), with negligible densities in Queens Lake and eastern sections of Watson-Taylor Lake. The extensive seagrass beds in Queens Lake did not support high densities of School Prawn as we had expected, and Heron’s Creek did not yield any prawns whatsoever. The western region of Watson-Taylor Lake supported the greatest densities of prawns. Following a large rainfall event in association with an east coast low in the first week of January 2016, the majority of the estuary had relatively low densities of School Prawn, with the exception of the western region of Watson-Taylor Lake which showed exceptional densities (Fig. 2). This is likely an aggregative effect following flushing of the main tributaries following rainfall, although moderate densities were still present in Camden Haven River (Fig. 2). In March 2016, overall densities were greatly reduced (Fig. 3), with the greatest densities still present in western Watson-Taylor Lake and John’s River, and small peaks in density in the seagrass beds of both eastern and western sections of Queens Lake. In May 2016, high-densities were only detected in the shallow unvegetated area to the west of the lake near its exit into Stingray Creek (May 2016). Maximum densities in March and May 2016 were about 10% of the maximum densities detected in January 2016, most likely as a result of prawns maturing and migrating from the estuary to ocean waters to spawn.

Figure 1. Heat map representing the distribution of School Prawn across the Camden Haven estuary during November 2015. In general, densities were relatively low across Queen’s Lake, the lower estuary and eastern Watson-Taylor Lake.

Figure 2. Heat map representing the distribution of School Prawn across the Camden Haven estuary during January 2016. Similarly to November, the distribution was non-uniform, but overall densities were higher which is to be expected given January falls du

Figure 3. Heat map representing the distribution of School Prawn across the Camden Haven estuary during March 2016. Densities were lower than in January, but this sampling point included the greatest densities yet detected in John’s River. Note that colou

Figure 4. Heat map representing the distribution of School Prawn across the Camden Haven estuary during May 2016. This sampling event saw the lowest densities yet detected. The majority of School Prawn present were concentrated in Queens Lake. Note that c

2. Evaluate whether post-recruitment processes in Camden Haven estuary may be adversely affecting School Prawn growth and survival

As introduced in the previous milestone report, water quality monitoring conducted under the project includes several components. Dissolved oxygen (DO) and temperature loggers are deployed at the mouths of Heron’s Creek, at the mouth of John’s River, and two are deployed at eastern and western sections of the delta at the mouth of the Camden Haven River. These loggers continuously monitor the DO levels in the water flowing out of these major tributaries to the system (and two of these logger stations have recently had conductivity loggers added to them). The second component of the monitoring program involves profiling the water quality at each of the sampling stations in the Camden Haven estuary. This occurs during normal sampling trips, and also following major freshwater flow events (where possible). Finally, water samples are also collected at these stations and analysed for aluminium.

The full time-series of dissolved oxygen logger data is presented in Figs. 5, 6, 7 and 8, and show multiple instances of low dissolved oxygen water flowing into both Watson-Taylor and Queens Lake. This occurs most frequently in John’s River. Observed low-DO events generally have a duration of 2-4 days (Figs. 5, and 6), however there was an exceptional event which occurred in late December 2015 that lasted for 2-3 weeks in John’s River and Heron’s Creek. The cause of these events are currently unclear and do not appear to be correlated with rainfall, but the duration of the flow of low-DO water from John’s River in particular would likely have an effect on entire eastern section of Watson-Taylor Lake, which was evident in some of our DO monitoring across this region (this was presented in the previous milestone report).

Figure 5. Dissolved oxygen logger data from John’s River, a tributary to Watson-Taylor Lake in the Camden Haven estuary (6 month time-series from project commencement shown). Several low-DO events are obvious, the most severe being the persistent flow of

Figure 6. Dissolved oxygen logger data from Heron’s Creek, a tributary to Queens Lake in the Camden Haven estuary (6 month time-series from project commencement shown). Several low-DO events are obvious, the most severe being the persistent flow of anoxic

Figure 7. Dissolved oxygen logger data from the eastern mouth of the Camden Haven River, the main tributary to Watson-Taylor Lake in the Camden Haven estuary (6 month time-series from project commencement shown). Dissolved oxygen generally remained above

Figure 8. Dissolved oxygen logger data from the western mouth of the Camden Haven River, the main tributary to Watson-Taylor Lake in the Camden Haven estuary (6 month time-series from project commencement shown). Dissolved oxygen generally remained above

The sampling over the summer/autumn of 2016 tracked almost an entire recruitment season for School Prawn. While temporal changes in abundance follow the general paradigm of recruitment to the nursery, juvenile growth, then emigration to the sea, the variation in spatial distribution across the estuary is likely reflecting changes in water quality observed across the system during the study period, particularly dissolved oxygen. For example, the periodic flow of low-DO water from John’s River likely affects the low abundance estimates observed in the eastern region of Watson-Taylor Lake. The absence of prawns in this area is surprising, because it represents warm, shallow sedimentary habitats, with a good biofilm coverage on the sediments. Despite this, the highest densities of prawns detected in March 2016 (which were substantially lower than November 2015 and January 2016), were captured in John’s River on 21-22 March 2016, nested neatly between two short low-DO events. The freshwater flow in early January clearly aggregated prawns that were present in high densities in the Camden Haven River in western Watson-Taylor Lake. These results reveal that there is some flux in both the recruitment and distribution of School Prawn across the lake during the summer/autumn period.

As outlined in the project application, a key area of work under Objective 2 was to assess the lethal and sub-lethal effects of Aluminium and pH on School Prawn. Work on this objective has now been completed, with over 90 trials run at different pH levels, and the results are presented in Fig. 9 (with the exception of 24 trials which are currently awaiting data analysis). The data is highly variable, but several patterns are apparent. Firstly, at lower pH levels aluminium appears to be lethal at lower concentrations than at higher pH levels. It should be noted however, that field sampling has not yet detected extremely low pH levels. Secondly, it appears that an LC-50 of approximately 80 mg L-1 total aluminium is likely at pH 8, but this will modelled once data collection is complete. Th results also indicate that exposure to aluminium levels detected in the field (Fig. 10, note that these are dissolved aluminium levels, not total) can cause mortality in School Prawn at both high and low pH (Fig. 9). Over the next two weeks, histological preparation and examination will be conducted on School Prawn exposed during the course of these experiments, to establish the sub-lethal effects of aluminium on the gills and hepatopancreas of prawns. This data will be reported at the next milestone.

Figure 9. Compilation of results from LC-50 trials at two pH levels (pH 5 and pH 8), shoeing the effect of the concentration of total aluminium on survival of juvenile School Prawn.

Figure 10. Dissolved aluminium concentrations across 18 sites sampled in Camden Haven estuary during November 2015 (1511), January (1601) and May (1605) 2016. The horizontal red line indicates the hypothetical level at which deleterious effects on aquatic

3. Synthesis of research findings to provide recommendations to catchment, habitat and fishery managers regarding restoration of school prawn productivity

As the project is currently in the data collection phase, no new extension activities have occurred during this milestone period.