Acoustic Survey of an Estuarine Marine Protected Area and of its Close Vicinity: Analysis and Monitoring Prospective

Abstract

Facing the loss of fish biodiversity and the decrease of fish stocks since several decades, a proposed solution is the multiplication of Marine Protected Areas (MPA). In Senegal, one MPA has been implemented in 2003 in one of the channels composing the Sine Saloum estuary. A monitoring program was carried out based on hydro-acoustic survey and the two main metrics extracted from hydro-acoustic data: Fish biomass proxy and fish size distribution proxy. The surveys were performed using vertical beaming at different seasons from 2003 to 2010. In the MPA, a light increase of the fish biomass proxy and an increase of the fish size proxy during the dry season were observed. The biomass proxy recorded in a close but not protected tributary were higher than in the MPA, probably due to a strong connection with open marine water, aggregative behavior of preys in the MPA due to higher predation, and a stronger avoidance phenomenon in the MPA. The high variability of the MPA environment submitted to both estuarine and marine influences and the size of the MPA may explain the lack of clear results. However, the learned lessons enable us to make recommendations to monitor fish population in MPAs using hydroacoustic methods. Evaluation procedures using these methods are proposed highlighting the importance of a rigorous survey protocol to insure the exploitation of the entire acquired time series.