Shedding light on day-night habitat use of oyster reefs by fish

Oyster reefs provide key ecosystem functions, including the provision of habitat for fish. While their importance in supporting fish assemblages is widely accepted, an assessment of intertidal oyster reefs at night remains a knowledge gap. Documenting habitat use at night could reveal new species reliant on these systems and help uncover diel fish movements among structured and unstructured habitats. We hypothesise that habitat use will interact with time and habitat, whereby differences in fish assemblages between oyster reef and unstructured habitats would be greater during the day. In this study, we used infrared-capable remote underwater video stations (RUVS) to sample paired intertidal oyster reefs and unstructured habitat during day and night. Assemblages using these habitats during the day do not match those at night, consistent with diel shifts observed in similar habitats. Consistent with our hypothesis, during the day oyster reefs had higher diversity and richness than unstructured habitat, whereas both habitats had low diversity and richness at night. The high relative abundance of piscivores in oyster reefs during the day suggests reefs create food webs that extend beyond benthic matter and sustain higher trophic levels. Nighttime abundances and diversity in oyster reefs dropped to levels similar to those of unstructured habitats during the day and night, likely a product of temporal niche partitioning and dynamic predation risks. Nighttime studies were important in documenting a wider suite of species and can thus provide a more complete understanding of fish-habitat interactions. Furthermore, this study used novel methods that enabled depth and size estimation from a single camera unit. While still in its infancy, this can serve as a force multiplier in monitoring intertidal habitats and lead towards a cost-effective method for standardising day/night abundance measurements and obtaining fish size estimates.