Remote video methods for studying juvenile fish populations in challenging environments

Abstract

Understanding the dynamics of juvenile fish populations is a key component to understanding variation in adult populations, however, juvenile fish are frequently found in a variety of structurally complex habitats which are often not easily sampled. The use of baited (BRUV) and unbaited (RUV) remote underwater stereo-video to study juvenile fish populations is uncommon and thus far, no study has directly compared the efficacy of these two methods solely for juvenile fish assemblages.The macro-tidal Kimberley region in the north-west of Australia is challenging to sample using diver-based methods due to the strong currents and prevalence of saltwater crocodiles (Crocodylus porosus). We sampled 60 Stereo-RUVs and 60 Stereo-BRUVs across four shallow-water (1-6 m) benthic habitats: coral, mangrove, macroalgae, and seagrass, to contrast the effect of the presence or absence of bait, deployment period, in-water visibility and tidally driven water speed on estimates of relative total abundance, species richness, and composition of the juvenile fish assemblage.No difference was detected in the ability of stereo-BRUV or stereo-RUV to quantify the relative total abundance, species richness, or assemblage composition of juvenile fish. We also found little effect of in-water visibility or tidally driven water speed, likely as a result of stratifying our sampling to slack water periods.We found that a deployment period of 10 min for Stereo-BRUVs and 15 min for Stereo-RUVs was optimum for sampling the juvenile fish assemblage across all four contrasting habitats. Since no statistical significance was observed between 10 and 15 min, we recommend that Stereo-RUVs deployed for 15 min during tidal slack water conditions are an optimum way to provide consistent results for comparisons of juvenile fish assemblage metrics across the benthic habitats studied within this region.The controlled video methodologies described here provided an opportunity to access un-sampled juvenile fish habitats and assemblages where traditional diver-based methodologies are impossible. Applying these methods broadly can help to improve our understanding of the status of juvenile fish and infer the reproductive success and viability of their populations across a range of habitat and environments.