Northern Prairie Wildlife Research Center
D. Weihs and D. Levin, Department of Aerospace Engineering, Technion, Haifa 32000 Israel
The U.S. National Marine Fisheries Service, with CSIRO of Australia, are running a long-term project to monitor swordfish migratory motions in the open ocean, at the University of Hawaii. As part of this project, it is planned to tag several of these fish with archival tags that will record physical-oceanographic data of the environments these fish traverse. As the swordfish does not surface on a regular basis, techniques used for up linking the data collected to satellites, as developed for marine mammals and reptiles, are not applicable. The present paper describes the design of a new type of tag that will enable retrieval of the data. The approach chosen is of designing a tag that electronically processes and records the desired data on pressure, water temperature, light levels, and speed over a period of a year or more. This tag has to be external to measure the required variables, and the solution suggested is to include the tag in a streamline shaped body with stabilizing fins, which is attached to the fish body by means of a cable connected to an anchor embedded in the shoulder area. Swordfish travel through most of the world's oceans at depths of up to 1000 m and at speeds of up to 20 m/sec, so the tag needs to withstand hydrostatic pressures up to 100 atm, with additional dynamic pressures due to the swimming speed. The streamlining is required to reduce drag so that the anchor will not be pulled out by the hydrodynamic forces and the stabilizing fins cause the body to move in a straight line, even though the attachment point to the body oscillates as the fish swims. The tag collects and stores the data during the months it is attached to the fish, until a predetermined time, when an explosive device (squib) that is placed halfway along the cable is ignited by an electronic signal from the microprocessor. This squib activates a knife-edge that cuts through the wire connecting the tag body to the fish, releasing it. The body is designed such that its average density is slightly less than seawater and so it will float to the surface over a period of several minutes to several hours, depending on the depth at which it is released. The distribution of weights within the body is such that it will float head upwards, so that when it reaches the surface the cable remnant sticks out of the water and serves as an antenna. The fins now serve as stabilizers keeping the body vertical even in a wavy environment, and a light sensor, activated when the squib was ignited, starts broadcast transmission of the data collected. Transmission then continues until the batteries run out. We present the design and results of tests in a wind tunnel and towing tank of this tag body which is about 20 cm long and of about 100 cc volume with about 35 g payload (including antenna).