The winner of the long format essay competition is of course a Queenslander, Dave Roberts. The main picture shows Dave in his natural habitat in more primitive times using the standard dip net in the upper Brisbane River catchment. Dr Roberts is understandably ecstatic with the win.
How Tapioca pearls helped develop a non-destructive demersal egg sampling method.
By Dr David Roberts
The problem: How to quantitatively sample the large demersal eggs of a nationally listed threatened species, the Australian Lungfish without harming the eggs or the habitat it uses for spawning.
Working with the Australian Lungfish has presented researchers with many challenges to overcome. One that recently presented itself to me was how to quantitatively describe the progress of a spawning event that occurred in a large impoundment environment. It was important to be quantitative as well as being non-destructive, as one of the key objectives of the study was to track the viability and developmental stages of the eggs over the very long development period that occurs in lungfish from the spawned egg to the free swimming larvae, being upwards of 50 days. We wanted to return the undamaged eggs to the spawning substrate they were collected from and also maintain the fragile spawning substrate (macrophytes and flooded grasses) they were using during the study period.
After a few days scratching around the Web for similar problems and equipment to do the job, it became apparent that this was just another one of the challenges that lungfish throw up. As far as I could determine, a method for sampling quantitatively and non-destructively eggs, larvae and habitat hadn’t been used in this type of application before. So how did we approach the problem?
The quantitative side of the problem was dealt with by deciding on a 1 m2 quadrat enclosure (Fig. 1). 1 m2 was chosen as lungfish egg densities are not overly high and this was a compromise between size and manageability. The standard lungfish egg survey methods developed by Peter Kind and Steve Brooks employs a semi-quantitative push net method using a 1 m2 frame with fine mesh stretched over it and trawling this through dense macrophyte habitat to collect eggs. While this method can be standardised in terms of frame size and length of area trawled for eggs, this method is relative and can be subject to operator variations in deployment method and effectiveness of trawling the substrate for eggs. Using a fixed area of 1 m2 overcomes this problem and allows for true replication of the sampling unit. There were also two other design features incorporated into the quadrate frame including one wall being made of a 1 mm mesh cod end that served to collect the eggs and larvae washed from the sample area. The opposite wall also had a small window of 1 mm mesh in it below the water line that served to improve the laminar flow of water out of the quadrat into the cod end when using a jet of water as described next.
Despite having the quadrat, there is still the need to collect the relatively large eggs of the lungfish (~6–8 mm Ø, see Fig. 2) that are also negatively buoyant and sometimes loosely attached to the spawning substrate. This was achieved by using gentle but constant water pressure from a water jet delivered by a small 1-hp fire-fighting water pump. This pump delivered a relatively constant rate of water flow that can be used to gently but thoroughly agitate the substrate to suspend the eggs and allow collection both from the water column and in the cod end. Collection from the water column was achieved by sweeping a 1mm mesh dip net through the water column while maintaining constant agitation from the pump. Secondly a laminar flow was created into the code end by directing the water jet toward the cod end while agitating the substrate. This pumping and sweeping action was continued until three separate sweeps of the dip net yielded no more lungfish eggs (Fig. 3). This whole process would take 3–5 minutes per quadrat, so is relatively rapid. All material collected in the dip net and cod end are then sorted in trays for eggs and larvae and the eggs are returned to the quadrat they were sampled from.
Fig.1: Diagram of sampler on dry land showing cod end net and small mesh opening to allow laminar flow through quadrat.
Fig. 2: A typical lungfish egg showing dark embryo and clear outer egg capsule. The scale of this egg is shown as the 1mm grey line in the bottom left of the picture. Source of photo is Kemp 1994.
Fig. 3: Sampler in operation showing dip net used to sweep for suspended eggs and hose pipe from water pump used to lift the eggs from the substrate.
Validation of the Method
We wanted to validate and establish the efficiency of the methods. We did not wish to use real lungfish eggs for fear of damaging them from double handling and transferring from substrate to quadrat and back to substrate. Using real lungfish eggs would also reducing our capacity to replicate any validation trials due to their relative scarcity. So we looked far and wide for a surrogate for lungfish eggs. We thought about green peas, plastic beads, chick peas, but nothing was quite the right size or had the same buoyancy and fragility of the lungfish egg. But an epiphany happened one day while I was sitting at the food court of a shopping centre and seeing people walking away from the local juice bar with drinks full of small 8 mm clear rubbery balls. That’s, it tapioca pearls (Fig 4.). They were a similar size, and were quite gelatinous and fragile, but most importantly had a very similar negative buoyancy. Using 50 tapioca pearls randomly added to replicate quadrats and allowed to settle for 5 minutes, allowed us to determine a recovery rate of over 95%.
Fig. 4: Inspirational tapioca pearls in their natural habitat.
We did it! We now had a non-destructive, quantitative method to survey lungfish eggs that allows us to repeatedly survey spawning habitats without damaging either the eggs or the substrate and allowing eggs to be returned unharmed. This method will be the subject of a manuscript in the near future once I finish this bubble tea. Yum.