Sodwana acoustic telemetry field trip

By Bruce Mann

From the 1-5 June 2020 the Linefish and Marine Protected Area (MPA) research team from the Oceanographic Research Institute (ORI) undertook a field trip to Sodwana Bay in the iSimangaliso Wetland Park. The team included Bruce Mann, Ryan Daly, Gareth Jordaan and Brent Chiazzari, accompanied by Thor Eriksen from the South African Institute for Aquatic Biodiversity (SAIAB). The purpose of the trip was to roll-over acoustic telemetry receivers that were attached to underwater moorings to “listen” for fish and sharks swimming past that had previously been tagged with acoustic transmitters. The trip coincided with the nation moving to Level 3 of the COVID-19 national lockdown, and a suitcase full of permits was required from a host of different organisations to enable the trip to take place.

Suitably equipped with our PPE and cognisant of social distancing at all times, we sallied forth with our trusty Ford Rangers (supplied by the Ford Wildlife Foundation) and our semi-rigid “ORVIII” research boat in tow and arrived at a deserted Sodwana Bay on Monday afternoon. Once clearing security at the gate and confirming that the boom gate would be opened to allow us to launch the following morning, we settled into the ORI cottage and got the telemetry equipment, boat and diving gear ready for the next day.

First light on Tuesday found us good to go, being the only vessel on the beach at Sodwana! After a quick risk assessment and an uneventful launch in a relatively calm sea, we headed for the Acoustic Tracking Array Platform (ATAP) line of three receivers off Jesser Point. The deepest of these receivers lies at a depth of about 64 m (too deep for diving retrieval), so a clever little device called an acoustic interrogator is required. With the boat positioned on the GPS mark, a transducer is lowered over the side and the receiver is dialled up using the interrogator. Once the signal is confirmed, the instruction to ‘release’ is sent and after a few minutes of tense expectancy, the device bobs up suspended under floatation buoys. Once the old receiver is secure on the boat, a new mooring and receiver is deployed on the same GPS mark so that “listening” continues uninterrupted. We quickly did the next ATAP acoustic release receiver located about 40 m down, and then Ryan and Thor retrieved the third one in 20 m water by diving.

After a welcome cup of hot coffee, the team was ready for the next challenge. We made our way up to Nine-mile Reef and rolled-over our two receivers, one in 30 m of water and the other at a depth of 18 m. That done we headed back to Two-mile Reef and did the same procedure for the next two receivers. Using the last two full diving cylinders, Ryan and I then undertook an observational drift dive along Two-mile Reef. There had been much speculation that after two months of no diving on Two-mile, the fish community would have changed. Surprisingly, we found the fish community very similar to that which we are accustomed to seeing on Two-mile and there was no visible evidence of increased numbers of predators such as sharks and potato bass.

Wednesday morning greeted us with a fresh north-easterly wind and a choppy sea. We made our way down to Diep-gat Canyon and did an acoustic release of a receiver that had been placed at a depth of about 70 m at the head of the canyon. That done, we diver-retrieved a second receiver at about 30 m depth further away from the canyon head. These receivers were part of the African Coelacanth Ecosystem Project (ACEP) Marine Canyons project, investigating the productivity associated with submarine canyons that occur on the shelf edge within the iSimangaliso MPA. That done, we headed down to the boundary of the sanctuary area off Red Sands and rolled-over our two receivers in about 30 and 20 m water by scuba diving. With the wind now strengthening, we called it a day and banged our way back to base at Sodwana.

Thursday saw the arrival of a light south-westerly and we made our way down to Leadsman Canyon to retrieve another one of the canyon receivers. Despite multiple attempts and double checking the marks and codes we had been given, there was no response from the receiver, so it had either disappeared or we had been given the wrong codes. Out on the shelf edge with the strong Agulhas Current flowing southwards and being met by a now cheeky south-westerly, the waves increased in size and were quite intimidating in our little boat! Nevertheless, we donned our oilskins and persevered southwards down to Leven Canyon where we successfully retrieved the last of the canyon acoustic-release receivers. Gratefully we then moved inshore out of the current, and diver-retrieved the receiver near the canyon head at a depth of 30 m. Sneaking in behind Leven Point we retrieved our shallow receiver in 16 m of water and replaced the mooring which had become heavily fouled and was unlikely to last another year. That done, we made our way to the sandy gap between Leadsman Shoal and Red Sands Reef to roll-over our deep and shallow receivers in this area.  With the job basically done, we made our way back to Sodwana on the backline, enjoying the magnificent coastal dunes and beaches without a solitary human in sight!

Back at camp we discovered that the reason for us not finding the Leadsman Canyon receiver was because we had been given the wrong codes. After a quick meeting with the team, we unanimously decided to head back to Leadsman early on Friday morning. As it turned out Friday was the best day of the week and we were greeted by a flat sea and a light, variable breeze. We made quick work of getting down to Leadsman Canyon and this time everything went like clockwork and within minutes we had the receiver safely in the boat. After a well-deserved cuppa we headed back to Sodwana, washed and packed our gear and hit the road for Durban. After a slow but uneventful trip we arrived safely back in Durban, tired but satisfied with our success.

At the end of the trip we had managed to successfully retrieve 18 receivers (12 by diving and 6 by acoustic release). Ryan did a quick download of the information stored on some of our receivers and it revealed that they were packed to the brim with detections from a host of species including potato bass, kaakap, Zambezi sharks, tiger sharks and many others.  Analysis of all these data is going to be a big job but it will hopefully reveal many of the hidden secrets about the movement patterns of some of our important fish, shark and ray species.

I would like to thank the team for their great camaraderie during the trip and for staying safe. Thanks also to Paul Cowley and Matt Parkinson (SAIAB/ATAP) for their ongoing support and for providing an incredible research platform. We are grateful to the KZN Department of Economic Development, Tourism and Environmental Affairs (EDTEA) for the provincial grant-in-aid which has made this research project possible.