Rockfish sounds and their potential use for population monitoring in the Southern California Bight

Non-lethal methods are being developed to assess and monitor the depleted rockfish stocks off southern California. For example, data from multifrequency echosounders and underwater cameras have been combined to map the dispersions and estimate the abundances of rockfish at the historical fishing sites within this region. From August to October 2007, this ship-based technique was augmented with two passive-acoustic moored recorders. One collected data at the 43 Fathom Bank for 46 days, while the other was serially deployed at 13 locations for shorter periods (1-8 d). Passive-acoustic data were analysed for the presence of rockfish sounds. Potential sources of five pulsing sounds were identified from the optically estimated species compositions at each location, as well as from known rockfish recordings collected in aquaria. All sounds had a low frequency (<900 Hz). Some were short, individual pulses (0.1 s), others were repetitive. A repetitive pulsing from bocaccio (Sebastes paucispinis) was the most commonly recorded sound and it occurred mainly at night. The daily calling rates at each site were quantitatively compared with the rockfish abundance estimates obtained from the active-acoustic survey, and they were positively correlated. The feasibility of using passive-acoustic tools to monitor changes in rockfish populations efficiently is discussed.