Abstract

Phocoena sinus (vaquita) is a small marine harbor porpoise endemic to the Gulf of California that is believed to be the most endangered cetacean in the world as reported by the International Union for Conservation of Nature (IUCN) red list (see Rojas-Bracho et al. 2008). Simultaneous use of the same habitat by fishermen and Phocoena sinus has led to the precipitous decline of the porpoise. Phocoena sinus is easily entangled in fishing nets, resulting in drowning.
Our challenge was twofold: first, to understand the pathway by which Phocoena sinus receives sound and second, whether Phocoena sinus should be able to detect nets using their echolocation system. Recently, research cruises have focused attention on recording sounds present in the Phocoena sinus habitat. We used numerical analysis to understand how sound propagates through the anatomy of Phocoena sinus. Finite element modeling (FEM) offers the capacity to simulate what happens when anatomic geometry interacts with sound pressure waves. An example of a similar application of FEM can be found in Cranford et al. (2008) where they constructed a numerical simulation of the acoustic pathways in the head of a Cuvier’s beaked whale. This kind of simulation is valuable because marine mammal hearing is not fully understood and contradic- tory suggestions have been put forth to explain it. It is clear that toothed whales have a well- developed sense of hearing, probably as a result of selective pressures that compensate for the diminishing penetration of light in water. It is not yet clear how toothed whales detect, receive, filter, or amplify sounds.