Shark Attach

Abstract

Smooth dogfish are a near-threatened species of shark that primarily reside in coastal waters off of Central America and the Northeastern United States. As a result of dogfish being a bycaught species, having a late sexual maturity, and having small litter sizes, dogfish populations are now in danger. The species’ distinction as “near-threatened,” along with the fact that little is known about their fine-scale behavior, has encouraged scientists at the Woods Hole Oceanographic Institution (WHOI) to study dogfish in their natural habitat. To assist in this study, we were asked to design a biologging tag that can securely attach while being minimally-invasive on dogfish. To guide our design process and conceptualization of potential solutions, we developed a set of specifications that any solution must meet. This list includes securing the sensing electronics to dogfish while minimizing invasiveness, which is measured using metrics such as tail beat frequency and qualitative analysis of behavior by the researchers at WHOI. We further specified that the solution must be sufficiently durable to stay on the animal for 72 hours under typical environmental conditions and activity levels, characterized by depth and movement of the animals. The tag must also detach autonomously and float to the ocean surface for retrieval. Finally, the device must not contribute to ocean pollution and have an affordable cost. Through research, rapid prototyping, and physical testing, we developed a method to attach the necessary biologging electronics to dogfish. The final solution features a flexible harness around the circumference of the body with hydrodynamically efficient packages housing the electronics on either side of the body in line with the first dorsal fin. The harness closes around the shark by threading a strip of Nichrome wire through a clasp. The wire will break and release the tag when a strong electrical current is triggered at a time programmed by the researchers. We used a number of analytical and experimental approaches to verify our solution. Early in the design process, we used computational fluid dynamics (CFD) to study the hydrodynamic qualities of a virtual model dogfish. The package designs were hydrodynamically improved using CFD and the final package design adds roughly 5-7% drag to the baseline shark. A physical model of the dogfish was created and taken to the Michigan Hydrodynamics Laboratory to verify our analysis procedure and test a prototype tag in their tow tank. Through tow tank testing we verified the results from the CFD while also observing qualitative information about the attachment method. Finally, we mailed a prototype tag to WHOI for testing on live dogfish that they have on campus. Testing on the live dogfish highlighted potential areas for improvement in the design and confirmed that the attachment method was effective, but too invasive for long-term deployment. The current final prototype is not ready for deployment on dogfish in the wild. However, our design and analysis procedures have been verified and can be used in future work related to tagging small marine animals.