Pemba Island Bruv Surveys
KENNEDY OSUKA & PETER MUSEMBI
Researchers, Fisheries
Baited remote underwater video systems (BRUVs) are becoming extensively adopted as a non-extractive technique for sampling fish diversity, relative abundance and the size structure of fish assemblages. The technique offers the advantages of eliminating the need for divers, reducing the risk of incorrect fish identifications, reducing observer and inter-observer inconsistency. BRUVs can also be deployed at greater depths that usually are inaccessible by SCUBA surveys. Nevertheless, the use of bait is considered to increase the relative abundance and diversity of fish observed, particularly predatory species that are often targeted in many fisheries.
The journey through Pemba Island was itself interesting and breathtaking living to its name as the “green Island”. Upon landing at Pemba Airport I passed through meandering tarmac roads fringed by trees bearing mangos, coconuts, bananas, and cloves of which Pemba is well-known for. The tarmac road ended up near Ngezi Forest Reserve, which had dense indigenous trees forming a remarkable arch over a murram road and leading all the way to Manta Resort, which was located in the far northeastern of Pemba. This would be our research base for the BRUV surveys from 18-24th November 2019.
The survey contributed to the SOLSTICE-WIO project, which is being coordinated by the National Oceanography Centre (NOC). The activity was conducted in collaboration with CORDIO East Africa, University of Dar es Salaam – Institute of Marine Sciences (IMS), Ministry of Agriculture, Natural Resources, Livestock, and Fisheries Pemba, and University of York. Kennedy Osuka, a Ph.D. student at the Department of Environment and Geography University of York led the surveys. He together with Peter Musembi represented CORDIO while Dr. Saleh Yahya and Mr. Shariff Mohamed Faki represented IMS and the Department of Fisheries Pemba respectively. The aim of the survey was to sample reefs off west of Pemba Island in order to measure the fish population including the vulnerable species such as sharks and rays. Short training on the assemblage of BRUVs units and sampling methodology preceded these surveys.
The activities of the first day involved paying a courtesy visit to Pemba Fisheries Office in Wete, purchasing of bait, the assemblage of BRUV units, and discussion on site selection. Sites were chosen to include reefs within Pemba Community Conservation Area and to be representative of different reef habitats. Each sampling was intended to deploy a series of 5 BRUV drops at an interval of ~500 m. Exact stations would be selected using real-time depth echo sounder on the boat to ensure that deployment occurred at desired depths and habitats.
On the second day, under the guidance of our skipper Rob Kuenstle and his crew and hoping for the best weather and sea conditions, we set out to start our sampling. At the end of the day, a total of 19 BRUVS had been deployed on Scorpion Secret, Paradise and Njao reefs. Currents picked up during the day making sighting of buoys attached to BRUVs, impossible. Deployment seemed slower since participants were new to BRUV methodology. At the end of each day, video footage were downloaded off memory cards onto raw and back-up hard drives.
On the third day, the team deployed 9 BRUVs at the southernmost sites in Mkoani and Misali reefs. Moderate rains made camera operations difficult and coupled with strong currents and winds that seemed to pick up made the team to stop deployments after just nine drops. Moreover, the presence of steep slopes and drop-offs was a challenge in getting the required depths.
On the fourth day, the team deployed 20 BRUVs in a south-north direction starting from Vikunguni, Uvinje and Fundo Outer reefs. The sea conditions were good with good visibility and low to medium wind speeds. However, the sudden changes in depths and seafloor geomorphology seemed to be our biggest challenge leading to the toppling of some BRUV units.
The team resampled Fundo Outer reefs at an interval of 250 m on the fifth day and later moved to Mandela and Njao reefs giving a total of 18 BRUVs. One BRUV unit was found dragged 1.5 km south of where it was deployed due to variable currents in Fundo Outer.
It rained heavily on the morning of the sixth day making camera operations difficult at northernmost reef sites: Swiss, Shimba, Manta, and Scorpion Secret. Currents also picked up making sighting of our first drop difficult. Despite this, the team spirit was high leading to the deployment of 15 BRUVS. We could not believe the last day had finally reached and it was time to disassemble our BRUV units, clean and pack the gear and ensure all valuable data were compiled and saved in raw and back-up hard drives.
Despite the unpredictable terrain of Pemba Island’s seafloor and weather, the team deployed 81 BRUV drops over 5 days covering depths from 5.8 m to 47 m. This was a great achievement thanks to the tremendous zeal and team spirit. As we begin analysis of the videos we are as curious as you are what was captured and what lies in those depths, walls, and drop-offs.