Estimating the abundance and size structure of an estuarine population of the sparid Pagrus auratus, using a towed camera during nocturnal periods of inactivity, and comparisons with conventional sampling techniques

Abstract

An estimate of the absolute abundance of a population of the sparid Pagrus auratus (‘snapper’) was obtained by towing a dropped underwater video (DUV) camera close to the sea bed during darkness when this species is inactive (asleep), counting the number of fish present and estimating their sizes. This method was used in an estuary and compared with active (beam trawl, Danish seine, SCUBA visual counts) and passive (multi-panel gillnets, pots, Sabiki jigs, baited underwater video) fish sampling methods. The DUV detected a wide snapper size range (2–70 cm), which from around 5 cm and up appear to represent absolute abundance estimates. For snapper 5 cm and below, density estimates were less than those obtained by fine-mesh beam trawl, which sampled snapper down to 17 mm fork length (recent settlers from the plankton). Total DUV estimates of 0+ snapper abundance (3–9 cm) within this estuarine system were 64,000 ± 20,000 (standard error), compared to 105,000 ± 17,000 by beam trawl. For snapper 10 cm and larger (1+ and older), the DUV population estimate for the harbour was 166,000 ± 28,000. These larger snapper were not vulnerable to the beam trawl, while Danish seine sampling returned estimates of 20,000–80,000 (5000–20,000) fish, depending on the swept area assumptions used. None of the passive methods were able to provide relative or absolute estimates of the overall snapper population size, and each returned strongly skewed snapper length frequencies. The DUV was also able to collect detailed imagery of seafloor habitat features, at the spatial scales of both sampling transects, and individual sleeping fish. This combination of new technologies and specific fish behaviour (sleeping inert on the seafloor) offers a significant new sampling tool for assessing absolute fish abundance and associated population size structures. There is now a strong and ongoing focus by the marine science and management community, on seafloor habitat mapping, and associated Geographic Information System (GIS) resource inventorying. The addition of night deployed DUV sampling into such field deployments offers significant additional fish-habitat information gains to be made within these frameworks, at little additional cost.