Ecological effects of bottom trawling on the structural attributes of fish habitat in unconsolidated sediments along the central California outer continental shelf


Bottom trawling has been shown to affect the seafloor and associated biological communities around the world. Considerably less is known about the dynamics of im- pacts to structural attributes of fish habitat, particularly in unconsoli- dated sandy sediments of the con- tinental shelf. We collaborated with commercial fishermen to conduct experimental trawls, with the type of small-footrope trawl required for trawling on the continental shelf, along the 170-m isobath in an area off Morro Bay in central California. The bottom trawling intensity we applied was based on the historical range of fishing effort in the study area and included low-intensity and high-intensity treatments. A remote- ly operated vehicle was used to col- lect continuous video and still photo- graphs in trawledtrawled and in untrawled control plots, before trawling and at 2 weeks, 6 months, and 1 year af- ter trawling. Scour marks from the heavy doors of the trawl were ob- served in the seafloor and persisted for at least a year. Although data extracted from the collected imag- ery showed some smoothing of the seafloor in trawled plots, the mini- mal differences between trawled and control plots in microtopographic structure on the seafloor were sta- tistically significant only during one sampling period. Further, there were no significant differences between trawled and untrawled plots with respect to structure-forming inver- tebrates (e.g., sea whips) and mobile invertebrates (e.g., sea stars). The results of our study, part of ongoing efforts to understand and manage fishing impacts, indicate that bottom trawling with a small-footrope gear may have limited effects in some sand habitats.

Fishery Bulletin.