Passive acoustics as a tool to quantify/characterize vessel activity at fish spawning aggregation sites

Abstract

The vulnerability of fish spawning aggregations (FSA) to fishing pressure has led to seasonal fishing bans for certain aggregating reef-fish species and seasonal or permanent closures of known FSA sites throughout the Caribbean. While discerning the degree of compliance with fisheries regulations is essential in evaluating their success, this information can be scarce when opportunistic surveillance from enforcement vessels is minimal or absent. We used passive acoustic recorders to determine the temporal patterns of vessel activity at three red hind, Epinephelus guttatus, spawning aggregation sites off western Puerto Rico with varying levels of protection (Buoy 4: no regulation, Abrir la Sierra (ALS): seasonal closure, and Mona: permanent closure) during the closed (January and February) and open (March) periods for the capture of red hind. We proposed that dynamic vessel sounds (e.g., gear shifting, sudden changes in speed) at known FSA sites and during aggregating periods could be indicative of fishing activity. A band-limited energy detector was used to detect vessel sounds, with detections grouped into discreate vessel events and classified as having only constant vessel sounds or containing dynamic vessel sounds. While most of the vessel events consisted of constant vessel sounds at all sites, events with dynamic vessel sounds were greatest at Buoy 4, followed by ALS and only one detected at Mona. Average events with dynamic vessel sounds were greater during the closed than the open period at Buoy 4, while the opposite was found at ALS, and red hind calling activity was significantly correlated with vessel detections during 3 years at Buoy 4. Results suggest fishers may have been targeting red hind at Buoy 4 despite red hind's seasonal closure, and the site-specific regulations at ALS may have served as a deterrent and divert fishing effort towards other areas. Acoustic records can reveal high-resolution temporal patterns of vessel activity and of events with dynamic vessel sounds, which could be used to infer about fishing activities and complement other surveys to provide estimates of compliance.