Social mapping of Australian bays and conservation of Fish Aggregating Bryozoans

Guest Blog By Dr. Adrian Flynn(a) and Dr. Travis Dutka(b)

(a) Marine Ecologist and Director at Fathom Pacific

(b) Senior Lecturer at La Trobe University Department of Ecology, Environment, and Evolution.

The waters of Western Port in southeastern Australia are a recreational fishing haven and hidden beneath its turbid waters, a unique fragile seafloor community has been newly described.  Here, bryozoans, skeleton-forming filter-feeding organisms also known as ‘lace corals’, form expansive areas of reef that support a high diversity and abundance of macroinvertebrates important to snapper and other prized recreational fish species.

Figure 1: Underwater imagery of the bryozoan reefs revealed remarkable biogenic reef structures with abundant invertebrate life surrounded by large areas of bare sediment.

The area, known to recreational fishers as ‘The Corals’, was known to consist of coral-like growths that come up on anchors or fishing lines.  Thanks to dedicated research effort, we now know that the bryozoan colonies form large “biogenic” (i.e., a substance produced by living organisms) reefs in linear and patch formations.  Bryozoan reefs of this size and vertical relief have not been described from anywhere else in the region

Figure 2: Living in the ‘goldilocks’ zone of low light and lower current than surrounding areas, the Western Port bryozoan reefs are a unique biotope. Now mapped by high resolution multibeam echosounder, a ‘baseline’ is established which can be analysed into the future and compared against crowd-sourced data

But the colony structures that make these reefs so attractive to fish (i.e., Fish Aggregating Bryozoans or FABs) are also slow growing and brittle.  Like sugar glass decorations on a cake, the net-like calcareous skeleton crumbles at the lightest touch.  Bottom trawls from commercial fishing operations have been reported for deep-water bryozoan ‘thickets’ in New Zealand that have not been able to recover after decades of protection due to physical substrate alteration.  In the neighbouring Port Phillip Bay, historic oyster dredging drove biogenic shellfish reefs to extinction and millions of dollars are now being spent to build new shellfish reefs.  In Western Port, we have the opportunity to protect what is still present – a unique biogenic reef that supports recreational fish stocks, including spawning migratory species, and a wide range of other valuable ecosystem services.

While there is no longer any commercial bottom trawling in Western Port threatening these FABs, anchoring activity by recreational fishers is identified as a potential threat, along with other stressors such as sedimentation.  At the height of snapper season, recreational fishers target The Corals location, using echosounders and plotters that can easily identify FAB structure. It is not unusual to see 50-100 boats anchored over the site.  Fishing practices in Western Port normally involve fishing over a tidal change and moving to multiple locations in a fishing session.  Scaled up to a fishing season and hundreds of anchoring events multiplied by hundreds of anglers, physical damage from anchors and chains is a concern. Furthermore, growth in recreational fishing activities continues and is encouraged by government for its benefits to health and wellbeing.

A collaborative research team has studied the extent and distribution of the FABs, the invertebrates associated with the reefs (fish food!), the fish populations associated with the reefs and the growth characteristics of colonies.  Reports can be found here – https://fathompacific.com/project-spotlight/western-port-bryozoan-reef-project/

These studies have been completed to investigate the ecological values of the reef systems and characterise the vulnerabilities and threatening process that could impact this spectacular ecosystem.  The studies have also informed management and monitoring options.  Through this research project, the team also identified (social or crowd-sourced) data collection opportunities by the recreational fishing community, education about safe anchoring practices and demonstration of station-keeping trolling motor technologies (e.g., “anchor” features; Figure 3).

Figure 3. New generation trolling motors (Lowrance Ghost pictured here) have embedded GPS and “anchor” features that keep a boat in place without physically anchoring.

FAB monitoring

Recognising that acoustic monitoring will be part of the solution, due in part to the turbid nature of these waters, scientists used multibeam echosounding to map the extent of the bryozoan reefs and provide a basis from which to embark on citizen science monitoring.  It was recognised that recreational fishers are major stakeholders in this environment and that crowd-sourcing acoustic data files from recreational fish finders could help solve the dilemma of continually funding expensive multibeam surveys.

A proof-of-concept study was completed which used a Simrad® NSS Evo3 sounder (Figure 4) and Lowrance® TotalScan transducer mounted on a scientific research vessel and the Victorian Fisheries Authority’s compliance monitoring vessel.  Dedicated and opportunistic surveys were completed with the fish finder set to record data during other operations, mimicking the activities of recreational fishers travelling to the area, finding a fishing spot and moving throughout the day.

Figure 4. An off-the-shelf Simrad NSS Evo 3 chartplotter/depth finder system was used to passively record sonar over bryozoan reefs which could be read an analysed by BioBase to create bottom hardness maps

Using the BioBase Automated Mapping system and the seafloor hardness algorithm, and comparing those results onto the multibeam record, we were able to demonstrate that the fish finders accurately mapped the bryozoan reefs.  The highly differentiated acoustic returns from the bryozoan reef skeleton compared to the bare soft sediments and algae and seagrass beds in the general area create an easily detected ‘acoustic signature’(Figure 5).

Figure 5. Echosounder tracks over the bryozoan reef areas were processed using the BioBase Automated Mapping system to detect seabed hardness characteristics. These data were analysed against known reef location from multibeam echosounding.

The result is a large pool of echosounder raw data, distributed randomly throughout the bryozoan reef area.  By processing these data through the BioBase system, and spatially selecting data that overlay pre-established monitoring grids of known (baseline) bryozoan reef habitat, we have developed a method of monitoring the seabed acoustic properties to detect changes that are indicative of reef coverage and density reduction (see full report here).

Angler participation in community mapping

Anglers and boaters around the globe can upload Simrad and other compatible brand Lowrance sonar logs to create custom maps and contribute to the C-MAP Genesis Social Map. Fortuitously, these maps not only provide anglers with custom fishing maps, but also provide important habitat data to researchers in line with Navico’s sustainability program (Figure 6). Navico is the maker of the brands Lowrance, Simrad, C-MAP (and BioBase and Genesis which fall under the C-MAP brand)

Figure 6. Community mapping by anglers and boaters can help bryozoan research and conservation, while also providing up to date fishing maps of The Corals.

Looking Ahead

Ensuring the long-term sustainability of The Corals/FABs will depend on enduring partnerships between citizens, governing authorities, industry, researchers, and non-profits. More tangibly, Fathom Pacific and La Trobe University will be working together with OzFish and Navico to develop the next phase of this research. Stay tuned!

Author: biobasemaps

BioBase is a cloud platform for the automated mapping of aquatic habitats (lakes, rivers, ponds, coasts). Standard algorithms process sonar datafiles (EcoSound) and high resolution satellite imagery (EcoSat). Depth and vegetation maps and data reports are rapidly created and stored in a private cloud account for analysis, and sharing. This blog highlights a range of internal and external research, frequently asked questions, feature descriptions and highlights, tips and tricks, and photo galleries.

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