Comparing Autonomous Underwater Vehicle (AUV) and Vessel-based Tracking Performance for Locating Acoustically Tagged Fish

Autonomous underwater ve hicles (AUV’s) are increasingly used to collect physical, chemical, and biological information in the marine environment. Recent efforts include merging AUV technology with acoustic telemetry to provide information on the distribution and movements of marine fi sh. We compared surface vessel and AUV tracking capabilities under rigorous conditions in coastal waters near Juneau, Alaska. Tracking surveys were conducted with a REMUS 100 AUV equipped with an integrated acoustic receiver and hydrophone. The AUV was programmed to navigate along predetermined routes to detect both reference transmitters at 20–500 m depths and tagged fi sh and crabs in situ. Comparable boat surveys were also conducted. Transmitter depth had a major impact on tracking performance. The AUV was equally effective or better than the boat at detecting reference transmitters in shallow water, and signifi cantly better for transmitters at deeper depths. Similar results were observed for tagged animals. Red king crab, Paralithodes camtschaticus, at moderate depths were recorded by both tracking methods, while only the AUV detected Sablefi sh, Anoplopoma fi mbria, at depths exceeding 500 m. Strong currents and deep depths caused problems with AUV navigation, position estimation, and operational performance, but refl ect problems encountered by other AUV applications that will likely diminish with future advances, enhanced methods, and increased use. Introduction Detailed information on the movements and distribution of marine species is necessary for stock assessments and, when combined with habitat information, for the implementation of ecosystem-based management (Cadrin and Secor, 2009; Goethel et al., 2011). Establishing baseline data is also becoming increasingly important for assessing changes in population and community structure in relation to environmental shifts in ocean conditions (Perry et al., 2005; Rose, 2005; Doney et al., 2012; Aschan et al., 2013). Emerging technologies, including acoustic telemetry and remote monitoring with autonomous underwater vehicles (AUV’s), can enhance efforts to address research and management issues. Acoustic telemetry, defi ned here as the detection of fi sh and other aquatic macrofauna tagged with acoustic transmitters and located using submerged hydrophones (Winter, 1996; Arnold and Dewar, 2001), can provide detailed information on the distribution, movements, and habitat use of marine species. This approach has many advantages over conventional tagging (i.e., identifying individuals or groups of fi sh based on external marks or tags) and recovery methods, which provide information based solely on where the tagged individuals were recaptured and can be biased by disproportional recovery efforts, vulnerability to capture, and variable reporting rates (Seber, 1982; Buckland, 1980). Individuals tagged with acoustic transmitters can be repeatedly located without having to be captured. This factor can be a major advantage particularly when working in isolated areas or studying species that are sensitive to handling, diffi cult to catch, or associated with substantial bycatch. Despite these advantages, the utility of acoustic telemetry can be offset by the limited reception range (typically hundreds of meters) and the tradeoffs between transmitter size, transmitting power and operational life (Arnold and Dewar, 2001; Pincock and Johnston, 2012), and the vast distances, depths, limited access, and adverse conditions (e.g., rough seas, stratifi cation) often encountered in marine waters. To counter these limitations, a variety of methods have been used to actively locate acoustically tagged fi sh from vessels (Holland et al., 1985; Marsac and Cayre, 1998; Pepperell and Davis, 1999; Brill et al., 2002; Meyer and Holland, 2005; Ng et al., 2007; Taggart et al., 2008). However, high operational costs and limited vessel availability can signifi cantly reduce vessel-based efforts, especially when working in remote areas. Inclement weather can further impact the amount of time that tracking is feasible, particularly for smaller vessels, and substantially reduce acoustic signal reception (Goodman, 1990; Winter, 1996; Ross and Lueck, 2003). Tracking from larger vessels can be logistically challenging, electronically noisy, and often confl icts with other scheduled tasks or research activities. Alternatively, stationary receiver/ hydrophone arrays have been used to