Lure-assisted visual census: a new method for quantifying fish abundance, behaviour, and predation risk in shallow coastal habitats

Abstract

Understanding the causes of variation in faunal abundance and diversity across shallow coastal habitats is a fundamental goal of marine ecology. Field methods for inferring a habitat effect on population density and predation risk are informative only if method biases are equal across habitats and species. We hypothesised that observation of fixed lures has a species by bias interaction if sampled species have different modes of predation, and that these biases are overcome by use of moving lures. We tested this hypothesis by observation of fixed and moving lures within seagrass and bare sediment in the Novigrad Sea, Croatian Adriatic. Both methods showed that ambush predators peaked in seagrass, wait–chasers peaked over bare sediment, and move–chasers were abundant in both. Stationary lures underestimated wait–chase and wait–ambush predators relative to moving lures, whereas moving lures did not underestimate the density of predators. These results indicate that stationary lures can underestimate both fish abundance and predation risk in the presence of waiting predators, and that if waiting predators are more abundant in structured habitat, then stationary lures will underestimate the predation risk within such habitats. Use of moving lures may be preferable for comparing habitats differing in structural complexity and frequency of predation modes.