Use of manned submersible and autonomous stereo-camera array to assess forage fish and associated subtidal habitat

Abstract

Forage fish and fish associated with particular benthic habitats (e.g., rockfishes, sand eels, sand lances) may be particularly difficult to assess through standard survey methodologies. Stereo-cameras, video, and automated visual data may serve as useful complementary tools to provide insight into the dynamics of these species. Visual methods may be used not only to estimate abundance and distribution, but also to inform important biological metrics and life history attributes. We explored the application of these methods to assess Pacific sand lance (Ammodytes personatus), a forage fish associated with benthic sediments, using a combination of directed observations from a manned submersible and quantitative analysis of fixed image footage obtained with a stereo-camera. This research provides a better understanding of how in situ observations and automated image analysis might complement other methods to estimate fish abundance, distribution, habitat, and behavior. Visual data were compared to data collected via directed sampling using physical extraction methods at the same site in the same year. Submersible observations provided new insights on the physical conditions and habitat. Visual observations confirmed wavefield morphologies previously identified through multibeam acoustic imagery and measured attributes relevant to the physical oceanography of the water column above this benthic habitat feature. Visual observations also informed understanding of light penetration, relevant to diurnal cues for seasonal progression and diel vertical migration and foraging. Submersible observations provided insights into abundance, schooling dynamics, and behavioral attributes, including avoidance in response to physical disturbance and aggregation in presence of artificial light. Quantitative analysis of stereo-camera data in center-edge and north-south transects determined that fish abundance and length distribution was relatively uniform throughout this particular benthic habitat. Estimates of measurement error associated with stereo-cameras were calculated and correction factors identified. Mean lengths estimated in visual data and in physical specimens were closely matched, though variance in visual data measurements was far greater. This error was reduced when filtering data on the basis of orthogonal position or incidence angle relative to the camera. Our research provides important insights to the presence, distribution, abundance, and movement of Pacific sand lance within benthic sand wavefield habitats. Our research also provides insight to the applications, opportunities, and constraints to observation-based sampling methods, including the use of manned submersibles and automated stereo-cameras.