Fish size spectra from imaging sonar reveal variation in habitat use across nearshore coastal ecosystems

Abstract

Structured coastal habitats provide foraging opportunities and refuge from predation for fish species of varying size and function. However, comprehensive assessments of fish communities among ecosystems are challenging because of biases arising across traditional sampling methods that target subsets of the community in different habitats (e.g. traps, seines, trawls, or visual census). Here, we used dual-frequency identification sonar (DIDSON) to examine shallow, nearshore habitat use by fishes in multiple structured habitats (seagrass, coral reefs, oyster reefs, rocky reefs, mangroves, woody debris, and docks) relative to soft-sediment habitat across 4 study locations ranging from tropical to temperate: Bocas del Toro, Panama, and Florida, Maryland, and California, USA. We then examined the distribution of individual fish sizes using size-spectra analysis. For temperate docks (Maryland) and eelgrass beds (California), size-spectra slopes were less steep than for soft-sediment habitats, indicating that larger fish associated with these structured habitats. No differences in slopes were identified for (sub)tropical Florida or Panama, although spectra intercepts from docks were higher in each location relative to soft sediment, denoting higher total abundance. Our results suggest geographically stratified habitat use with a tendency towards comparatively greater importance of structure in determining fish size distribution at higher latitudes, and greater importance of structured habitat in governing total abundance at lower latitudes. This study also demonstrates the potential of imaging sonar as a new tool for revealing variation in fish communities among habitats at local to continental scales.