Pumping coral-spawn slicks for large scale reef rehabilitation

Abstract

Coral reefs are degrading across the entire Great Barrier Reef. Rehabilitation of the Great Barrier Reef is crucial for Australia, both socially and economically because it provides $6 billion in revenue and 63,000 jobs. The Queensland state government issued a challenge within the "Small Business Innovation Research" program to quickly restore ecological functions provided by the reef. Van Oord and CSIRO participated in this challenge by testing a concept designed to scale-up rehabilitation efforts. This concept involves rehabilitation of the reef by increasing the number of larvae deployed onto the reef. To achieve this, coral-spawn slicks should be collected from areas with healthy coral populations that show a redundancy of embryos. During transport, these slicks should then be stimulated to grow into settlement competent larvae. Subsequently, they can be deployed at a degraded coral reef. The collection phase should be executed with pumps to reach considerable volumes of coral embryos for significant ecological scale. Unfortunately, the mortality rate of coral embryos due to pumping is so far unknown, yet crucial to determine. Therefore, the objective of this research is to develop a method for designing a pumping system that can pump coral embryos and larvae into a vessel-based container or hopper with maximal survival rates.
To achieve this objective, the strength of coral embryos and larvae as well as the stressors in a pumping system are investigated. In order to design and optimise the pumping process for survival, it is useful to have a framework that estimates the balance between strength of coral embryos and larvae and stressors. In literature, comparable strength and stress balances are known in, for example, thermal stress onto corals. The cause of failure (or mortality) is a combination between the strength, the exposed stress (or load) for a certain period of time and the capability to regain strength. Failure happens when the strength cannot withstand the stress (or load). These failure effects are also described through threshold limit values (TLV) which indicate a limit that the stresses may not exceed. Examples of such limits are time weighted average, short-term exposure limit and ceiling limit. Within the design of a pumping system, these values are expected to be acute (e.g. in the pump) and chronic stress (e.g. in the pipeline). In order to distinguish between pumping systems, the stressors in the system can be calculated by using tools such as a mathematical model.

The coral life cycle commences with gametes that contain eggs which are fertilised in the slick at the ocean surface, before developing into embryos. Because the buoyancy of coral embryos is positive until 36h following spawning, dispersion is limited. Hence, they are easiest collected within this period of time. The pumping related strength of coral embryos and larvae is therefore investigated in this research. The assumption is made that the strength of the embryos and larvae differ in the first period following spawning. In order to gain insight in the strength limits of coral embryos and larvae within this period, strength tests are executed. During the mass-spawning event in November 2018, fertilised eggs were collected and used in a Couette-rheometer test at the Heron Island Research Station. These tests consist of a cylinder rotating at different speeds in a container filled with water and embryos. Due to the rotation of the cylinder, the coral embryos and larvae experience stress. The living coral embryos are counted before and after each experiment by use of a microscope experiment in order to define the survival rate. After 5-7 hours the embryos have developed a considerable strength (larger than can be applied by the Couette-Rheometer). Therefore, it is recommended to start the collection process after 5-7 hours following spawning. Nevertheless, the tests do not define the exact strength of the embryos. Hence, it is advisable to investigate the exact strength in further research.

To design a pumping system that can pump coral embryos and larvae with low mortality, the stressors in the system should be minimal. The main technical criteria of the pumping system include low shear stresses, low-pressure fluctuations and low flow accelerations. Practical criteria such as availability, scalability and handling should also be considered in the design. A mathematical model was designed to calculate the previously described factors by calculating the magnitude of the stressors in a pumping system. Based on the model, it can be concluded that the pipeline configuration of a coral slick collection system should contain minimum pipe length, maximum diameter, minimal surface to encounter (e.g. bends and connections), low rpm and flow velocities and submerged in- and outflows openings.

Laboratory tests in the Netherlands and field tests in Australia have been executed in order to validate the relation between the strength of coral embryos and larvae and stressors in a pumping system. During laboratory tests, pumping system aspects that contribute most to damage have been investigated, and potential practical issues have been identified. Damage rates were estimated for different pumps and pipeline configurations using different proxies such as, hydrogell balls, peas, berries and fish eggs. From these tests, the Hidrostal and Diaphragm pump resulted in low damage rates and were selected to be applied during the field tests. Additionally, a skimmer (intake) to collect the floating particles was designed and tested, both in the laboratory as well as the field.

The field test has been executed in the Southern Great Barrier Reef around the Heron and Wistari reef. Currently, this part of the reef boasts the highest level of coral cover throughout the entire reef, and is almost at its historical maximum. Therefore, it was chosen as research location because of its expected large supply of coral spawn. The main goal of the field study was to investigate the possibility to pump coral embryos following the mass spawning in November 2018. The experimental setup consisted of a tug vessel with two pumping systems, each with a different pump (i.e. the Hidrostal and Diaphragm pump) and the same configuration. A total of total twelve tanks was used for cultivation of which six plastic and six steel. The total number of living coral embryos that have been pumped was approximately 29 million from which 19% developed into competent larvae within five days. The competent larvae were pumped through the same system to investigate their survival rate, which was around 88%. This indicates that deployment of larvae onto degraded reefs should be possible by pumping without much loss and that coral larvae are less fragile compared to coral embryos.

The objective of this research was to develop a method for designing a pumping system that can pump coral eggs, embryos or larvae, from the sea surface onto a vessel housing an aquaculture facility. By using the previously described criteria, it is concluded that coral embryos and larvae are able to survive pumping related stressors. Further research should aim to find the exact strength of coral embryos after which a more precise model can be created to estimate mortality. Furthermore, limitations for pressure differences should be determined. For now, the assumption is made that for designing a pumping system, atmospheric pressure should be the minimum local pressure which is conservative. Additionally, the controlled deployment of the competent larvae onto degraded reefs should be further investigated because this project was not focused on that phase. To conclude, the project needs up-scaling towards hopper sizes, with a broader range of coral species and various weather conditions.

Given the previous described challenge, this research proofs that it is possible to pump coral embryos, after which they can grow into competent larvae and be deployed onto degraded reefs. Significant quantities of new recruits can be collected at healthy reefs and by using hopper size vessels, the collection of coral embryos from slicks become promising.