Date

Spring 2025

Document Type

Master's Thesis (Open Access)

Degree Name

Master of Science (M.S.)

Department

Moss Landing Marine Laboratories

Abstract

Management of marine ecosystems requires accurate, quantitative, and spatially explicit information on species distributions at scales that are relevant to the management process (J. Ward et al. 1999). This study investigates fish assemblages at depths between 30 and 40 meters, which are relatively understudied compared to shallower ( < 30 m) waters, surveyed extensively via SCUBA, and deeper (>40 m) waters, which are surveyed using submersibles. This research addresses ecological and management-based questions about seasonal and depth-related shifts in groundfish community structure, the role of oceanographic variables in predicting these shifts, and microhabitat selectivity as predictors of community assemblage. To evaluate species abundance, biomass, diversity, and microhabitat associations associations on a mesophotic reef in the Carmel submarine canyon, diver-held stereo video was used to collect species information and quantify habitat characteristics. Oceanographic variables were recorded via a SeapHOx sensor throughout the study period. My analyses revealed seasonal variation in fish communities, with the highest densities occurring in summer, fall, and winter, largely due to small schooling species such as halfbanded and squarespot rockfish (Sebastes semicinctus and S. hopkinsi). Biomass peaked in fall and winter, driven by larger species like blue rockfish (Sebastes mystinus) and lingcod (Ophiodon elongatus). While species diversity showed no seasonal or depth-related differences, species richness was lowest at the shallower transect, likely due to the dominance of a single species, blue rockfish. Fish densities were higher in conditions of elevated temperatures, low pH and low upwelling, which are typical of late summer through winter. Habitat selectivity was evaluated for species driving differences in community structure, with most species preferring hard or mixed hard substrate, medium to high relief, and medium to high rugosity. Species were more selective when looking at the individual transects rather than the site as a whole, indicating that habitat selectivity is highly dependent on habitat availability and suggests that factors that inform selectivity could shift with available habitat. This study provides insights into the seasonal dynamics and habitat preferences of a nearshore mesophotic reef community, contributing valuable information to support management decisions for marine resources.

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