In March I was invited by Drs Ocativo Aburto and Andrew F Johnson to visit Cabo Pulmo, to use my hydroacoustic equipment and methods to examine the fish abundance and biomass inside and outside the marine park.
The study used non-destructive hydroacoustic methods to further examine the success (in terms of fish biomass) of the Cabo Pulmo National Park (CPNP). Although the benefits of hydroacoustics as a methodology, and marine reserves as a management tool are well acknowledged, there has been surprisingly little research relating the two inside Marine Protected Areas.
The CPNP was the ideal site to conduct such a comparison, due to the size of the reserve, the effective enforcement of its no-take status and due to the large increases in fish biomass since its inception as described by Aburto-Oropeza et al. (2011). My study ultimately aims to compare acoustic biomass estimates with those gained via diving (such as those noted in Aburto-Oropeza et al. (2011))and to compare values inside and outside the area afforded protection. An examination of habitat type (substrate) on the distribution of fish in the CPNP will also be undertaken.
To count how many fish were inside versus outside, and at different reefs of the park, an acoustic transducer was mounted to the side of the panga and parallel survey lines were driven east to west in the survey area. The acoustic equipment pings out sound, which travels through the water, bounces off the seabed and the echo is measured by the same equipment back at the surface. If the travelling sound meets a fish on the way, the echo “acknowledges” it counts it and records its size. With this information total estimates of abundance and biomass can be made. That is a lot of data for the whole of Cabo Pulmo!
The system I use allows fish to be seen on an echogram and this data can be exported so that numbers and biomass can be clearly plotted onto maps of the survey area.
The acoustic data collected will be analysed to provide numbers of individuals, biomass, average sizes and average lengths. It may also be possible to link this study to a concurrent fish tagging study (conducted by Timothy Rowell from GCMP) which may reveal important information on fish usage and residency times within the Cabo Pulmo park. We are hoping all of the results will be worked up and made available within the year so keep your eyes peeled for results from thus new fish census at Cabo Pulmo.
Working at Cabo Pulmo was an incredible experience working in Cabo Pulmo and during the long days bobbing around in the Panga I was entertained by breaching humpback whales, grey whales, dolphins, mobula rays and Brian Castro – the amusing and skilled skipper. I also got the chance to dive on one occasion and was lucky enough to witness the spectacular tornado of Bigeye jack (Caranx sexfasciatus) – those very same fish that made for the famous ‘David and Goliath’ taken by Octavio.