INTERNATIONAL COUNCIL FOR THE ICES CM 2006/Q:16 EXPLORATION OF THE SEA Use of data storage tags to reveal aspects of fish behaviour important for fisheries management Development and Application of ‘RAFOS Fish Tags’ for Studying Fish Movement by Conrad W. Recksiek, Godi Fischer, H. Thomas Rossby, Steven X. Cadrin, and Prasan Kasturi Abstract The newly developed ‘RAFOS fish tag’ reverses the tracking process of conventional acoustic tags by receiving acoustic signals from moored sound sources, allowing triangulation of geographic position during deployment on fish. We report progress in developing this archival tag for geolocating juvenile and adult demersal shelf fishes. The tag and navigation system are similar in concept to those of isopycnal RAFOS floats, in which arrival times of low frequency tones broadcast from anchored sources are archived and later retrieved for retrospective positioning. The principal differences between the RAFOS fish tag and RAFOS floats is that the tag is small enough to be attached to or implanted in fish about 50 cm or larger, and the tags must be recovered from the tagged fish to download data. Prototype RAFOS fish tags are being deployed on adult yellowtail flounder, Limanda ferruginea, on Georges Bank to study movement in the vicinity of an offshore area that is closed to fishing. Deployment of sound sources will be on or along the edge of the continental shelf where detection ranges appear to be on the order of 100 to 120 km for sources generating a sound pressure of 180 dB re 1 μP. The size of the prototype is governed by dimensions of a cylindrical housing which functions as the hydrophone. Within this is a full-custom 0.5- μm feature size receiver chip, memory chip, timing crystal, two batteries, and pressure sensor (temperature sensor is on-chip). The receiver chip consumes 36 μW at 3 V with an expected data storage life of several months to two years. Keywords: acoustic positioning, acoustic tracking, archival tags, data storage tags, electronic tags, RAFOS fish tag, passive listening, retrospective positioning, yellowtail flounder Not to be cited without prior reference to the authors Conrad W. Recksiek: Fisheries, Animal, and Veterinary Science, Woodward Hall, University of Rhode Island, Kingston, RI 02881, USA [tel: +1 401-874-2334, fax: +401-874-6160, e-mail: [email protected]] Godi Fischer: Electrical and Computer Engineering, Kelly Hall, University of Rhode Island, University of Rhode Island, Kingston, RI 02881, USA [tel: +1 401-874-5879, fax: +1 401-782-6422, email: [email protected]] H. Thomas Rossby: Graduate School of Oceanography, University of Rhode Island, South Ferry Road, Narragansett RI 02882, USA [tel: +1 401-874-6521, fax: +1 401-874-6728, email: [email protected]] Steven X. Cadrin: NOAA/UMass CMER Director, School for Marine Science and Technology, 838 South Rodney French Boulevard, New Bedford, MA 02744-1221, USA [tel: +1 508-910-6358, fax: +1 508-910- 6396, email: [email protected]] Prasan Kasturi: Electrical and Computer Engineering, Kelly Hall, University of Rhode Island, University of Rhode Island, Kingston, RI 02881, USA [tel: +1 401-874-5861, fax: +1 401-782-6422, email: [email protected]. 0 Development and Application of ‘RAFOS Fish Tags’ for Studying Fish Movement Introduction We report progress in developing an archival tag capable of identifying sounds broadcast from distant, moored sources where the sounds’ arrival times are used for estimating a (tagged) fish’s position. The tag and sound source system we describe here is designed for geolocating juvenile and adult demersal shelf fishes; we hope to test the system on yellowtail flounder, Limanda ferruginea, of Georges Bank. The system concept could be applied to pelagic nekton offshore and into deep waters; and it could be used, within the physical constraints of the system, for retrospectively tracking most anything in the water column. General principles The technical approach builds on decades of research and development for tracking ocean currents (Rossby 2003) by means of subsurface drifters capable of broadcasting or receiving sound. Beginning in the 1950’s oceanographers used the deep sound or SOFAR (SOund Fixing And Ranging) channel to geolocate subsurface floats (Stommel 1955). Given the time of the float’s sound emission and the time of the emitted sound’s arrival, the float’s range may be estimated. A second receiver provides a fix. A ‘SOFAR float’ system is closely analogous to conventional systems for acoustically tracking aquatic animals, in which acoustic tags transmit to a receiving array (e.g., Urquhart and Smith 1992) or tracking vessel. In the early 1980s oceanographers reversed the tracking system concept by mooring the sound sources and letting the floats do the listening. This led to coining the word, RAFOS, SOFAR spelled backwards, to indicate the opposite direction of acoustic signaling (Rossby et al. 1986). The RAFOS tacking system has been applied extensively in oceanographic studies, e.g., Bower et al. 2002. The RAFOS system was designed for studying ocean currents by retrospective analysis of a listening/recording float’s archived sound source arrival time data. We have simply applied the same idea to studying fish movements, using ‘RAFOS fish tag’ to describe the system. The RAFOS sound source (Rossby 2003, Rossby et al. 1986) amounts to an aluminium, open, 35-cm diameter, 2-m pipe with a transducer inside and a power supply attached. For open ocean work the sources are typically deployed near where sound speed will be at a minimum, the SOFAR channel; in shallow waters at mid-depth. There is usually no surface marker; the float and mooring may be retrieved with an acoustic release or may simply be left where it is (because the cost of recovery exceeds the value of the source). Typically three sources are deployed. RAFOS sound sources, insonifying the deep ocean, may be detected at distances of one to two thousand kilometers, depending upon the sound channel characteristics. Where the thermocline is absent, such as in boreal oceans, minimum sound speed is at the surface. The RAFOS source emits a low frequency tone centered at 260 Hz, having a duration of 80 sec, on a predetermined schedule. For tracking RAFOS floats in the Gulf Stream, for 1 instance, tones would be broadcast twice daily. The tone, known to the community of oceanographers studying currents this way, is referred to as a pong. The tone changes frequency steadily, i.e., it sweeps, over its broadcast interval, viz., increasing from 259.375 to 260.898 Hz over 80 sec. The strength of the signal at 1 m is on the order of 180 dB re 1 μP. The motivation for ramping the frequency, or sweeping the frequency, is that such sound is unique in the ocean; the chance of confusing that pattern, i.e., a ramped, low-frequency sound, with ships and natural sounds, physical and biological, is minimal. The RAFOS float package (or hull) (Rossby 2003, Rossby et al. 1986) is a 15-cm diameter, 2-m PYREX glass pipe. It is ballasted to float vertically and is engineered to enable it to drift along isopycnal surfaces thereby tracking fluid motion (advection and diffusion) – hence the appellation isopycnal. By means of a hydrophone on the bottom end, the float listens and when the unique, ramped low-frequency signal is detected the arrival time is recorded. Knowing when the RAFOS source emits its tone, together with sound arrival time, yields a time difference between float and source; a range may be obtained given an appropriate speed of sound. With sound arrival times from a second source, a fix may be obtained (Figure 1). Note that if three or more signals are available, hyperbolic navigation may be employed to obtain a fix, since the locus of equal sound arrival time difference is a hyperbola (as with LORAN). Besides sound source arrival times, the float records ambient pressure and temperature. At the end of its mission, a ballast weight falls away, the float reaches the surface, and, through an antenna inside the glass pipe, its data are uploaded to an ARGOS satellite. The float track and temperature/pressure records are retrospectively derived from the data set recovered from ARGOS (see http://www.po.gso.uri.edu/rafos/ for additional details on RAFOS technology). The RAFOS fish tag system works the same way. The sound source side of the system is identical. The principal difference is that the tag is small enough to be attached to or implanted in fish about 50 cm or larger, and the tags must be recovered from the tagged fish to download data. The capability of breaking free from the fish at mission’s end is of course feasible but an antenna and RF system would make the device more costly and larger. The size of such a device, i.e., one capable of broadcasting radio frequencies at the surface, would be a function of the power needed to communicate with satellites. RAFOS fish tag system and operational environment A study of fish movements using RAFOS would be, on the ‘tagging side,’ approached and conducted like an archival tag study, e.g., Cadrin et al. 2005, Cadrin and Westwood 2004, NMFS 2005. The logistics amount to tagging and releasing animals and waiting for the fishery to return tags. Clearly, the closest operational environment to a RAFOS-based study of ocean currents would be a fish movement study on pelagic species of the deep sea. Indeed RAFOS technology may ultimately prove most fruitful there because physical oceanic gradients used to geolocate conventional archival tags can be very slight. Consequently we have 2 chosen to implement the prototype system on the continental shelf, on a flatfish species in particular – in our case Georges Bank yellowtail flounder (Figure 2).