Acoustic seabed mapping in central Chile : evaluation of echo sounder and description of kelp bed under different management and environmental regimes

Abstract

The importance of kelp ecosystems is widely known both in economic and ecologic terms. Therefore, the identification and description of kelp forests is an essential step towards the understanding and future management of the species and the coastal ecosystem that they inhabit. Particularly, in Chile it has been a major source of income with a major focus on both the North of the country. Here, four sites of the Central coast of the country have been sampled to assess the distribution and structure of Lessonia trabeculata under different levels of human pressure and wave exposure. Even though kelp is not currently harvested in any of these areas, the description of its characteristics in spatial terms can be useful for potential use by fishers of the region. Acoustic imaging by means of a single-beam echo sounder was used to identify the algae patches. Assessing the reliability of the acoustic data was the first step in the process, several spots were sampled with the echo sounder, video recordings and direct observations to see how similar the information provided by these three sources was, depth and height values proved to be more accurately measured than coverage (r2 0.75 and 0.65 vs. 0.48). Several transects were carried out in each of the study areas and then interpolated to produce a map of the whole site enabling the description of the sites and comparisons between them. L. trabeculata showed denser and taller structures in areas protected from wave action regardless of the management regime. The findings offer new insights from a broader perspective than traditional methods and its applications can be beneficial for scientists, managers and decision makers as well as general public.