Rockfish Adventures in 3-D

by Louise P. McGarry

Dr. Louise McGarry first arrived at FHL in 2003 as a student in the Marine Bioacoustics course and has returned biennially to participate as a TA and most recently as an instructor in Marine Bioacoustics. Her scientific specialty involves the use of acoustic methods to investigate animal behavioral ecology in the marine environment. Louise completed her Ph.D. at Cornell University and is a scientist-in-residence at FHL with the rockfish acoustic telemetry study. Her collaborators on this project are Dr. Charles Greene and Dr. Ian Brosnan. Chuck Greene is Director of Cornell’s Ocean Resources & Ecosystems Program, and is an FHL alumnus and Advancement Board member. Dr. Ian Brosnan earned his Ph.D. at Cornell University with advisor Dr. Greene, and is an alumnus of UW’s School of Marine Affairs. Currently, Ian is Assistant Division Chief of Earth Sciences at NASA Ames.

During early October 2014, an acoustic telemetry study designed to track the fine-scale, three-dimensional movements of copper rockfish (Figure 1) was installed off Cantilever Point at Friday Harbor Laboratories (FHL). This state-of-the-art system was developed collaboratively by Cornell University and Hydroacoustic Technology Incorporated (HTI, a private Seattle company), and will run for a year. Next summer, the ongoing acoustic telemetry study will be supplemented with a high-resolution, photomosaic mapping survey of the seafloor conducted with an autonomous underwater vehicle (AUV). The goal of this project is to evaluate the use of the acoustic telemetry system in investigations of the behavioral responses of copper rockfish to the fine-scale, three-dimensional features of their environment. This type of technology has tremendous potential for enhancing the study of fish ecology, including the design of marine protected areas.

The advantage of acoustic telemetry systems is their persistent monitoring of mobile marine organisms, allowing us to study behavior, movement, and resource selection. The organisms carry an acoustic tag that emits a sound pulse at pre-programmed intervals. The sound pulse travels through the water and is detected by hydrophones: underwater microphones installed on mounts resting on the seafloor. The position of the organism can be determined if the position of the hydrophones is precisely known and if the arrival time of the signal at each hydrophone is precisely determined. Existing marine telemetry systems are limited by positional errors on the scale of ten meters and the number of targets that can be simultaneously tracked is limited to twelve. This newly installed system is expected to reduce the positional error to less than a meter, increase the number of unique tags that can be simultaneously tracked to 500, and provide a detection range of up to 1 km. This marine system, built by HTI, builds on their experience of more than a decade developing acoustic telemetry systems for freshwater environments.

Copper rockfish were selected for the field trial of this new marine system because their high site-fidelity and small home ranges provide ample opportunity to track individual fish behavior, while testing our ability to characterize the movements of a species of interest to management authorities. In this initial phase of the study, acoustic tags were surgically implanted in ten fish. Individual fish were captured by research divers (Figure 2) and placed into cages resting on the seafloor, which were then brought to the surface slowly where surgery took place onboard a waiting boat (Figure 3). The fish were anesthetized and administered an analgesic for managing post-surgical discomfort. Each fish was placed in a sterile surgical field with a continuous flow of anesthetic seawater over its gills, and the acoustic tag was implanted in the body cavity. After surgery, initial recovery took place in a tank of seawater and then each fish was lowered to the seafloor in its protective cage for full recovery. The fish were then released back into their habitat by the divers. The four hydrophones used to detect the signals from the acoustic tags were deployed using small boats and the R/V Centennial (Figure 4).

These fish will be tracked until the energy reserves in the tag batteries are depleted in about a year. In order to relate the tracked, three-dimensional movements to ecologically meaningful habitat features, results from the telemetry study will be used to prioritize the fine-scale mapping to be conducted with the AUV. This portion of the project will involve collaboration among six partners: Cornell University, Friday Harbor Laboratories, Hydroacoustic Technology Incorporated, the Northwest Fisheries Science Center, the Tombolo Mapping Lab, and the Woods Hole Oceanographic Institution.