This paper describes the design and deployment of a new type of underwater stereo camera capable of triggering when animals are present in the field of view. Unobtrusive evaluation of the camera view field for potential targets is achieved using far-red illumination invisible to most fishes. The triggered camera (TrigCam) system is designed to be low cost by incorporating off-the-shelf commercial camera and computer components. It also incorporates several novel software and hardware developments such as the Cannon Hackers Development Kit which provides a high degree of control over the cameras, and a Raspberry Pi computer-on-board module for low-power, cost-efficient computing. The innovative triggering algorithm for fine control of the size and intensity of targets necessary to trigger an image is described in detail. Stereo image analysis provides estimates of fish size, position, and orientation to provide quantitative data from images. Test field deployments demonstrate operational capacity and illustrate potential applications for sampling marine organisms through example descriptions of analytical methods, including the trigger process and stereo image analysis. The TrigCam is intended to be an open source project to encourage continued development within the marine research community, with design information available through internet posts.