Shining a light on the composition and distribution patterns of mesophotic and subphotic fish communities in Hawai‘i

As agencies shift from single-species management to ecosystem-based fisheries
management, ecosystem models are gaining interest for understanding species dynamics in relation
to oceanographic and ecological processes and human marine uses. However, information on
community structure or distribution of many species that occupy deep (>30 m) waters is largely
unavailable. We amassed a total of 24 686 fish observations of 523 species/taxa for the 30−410 m
depth areas surrounding the main Hawaiian Islands (MHI). We also obtained estimates of geomorphological
variables, including substrate type, slope, rugosity, and ridge-like features. Using these
2 data sources, we (1) identified distinct fish communities along the 30−410 m depth gradient, and
(2) generated relative biomass maps for fish functional groups. We showed that the mesophotic
zone ranges between 30 and 129 m, with a fish faunal break at 60 m. Beyond this zone, 4 subphotic
zones were identified: upper rariphotic (130−169 m), mid-rariphotic (170−239 m), lower rariphotic
(240−319 m), and upper bathyal (320−410 m). We assigned fish species to functional groups partly
based on identified depth ranges and fitted general additive models (GAMs) integrating geomorphological
covariates to the functional group relative biomass estimates to determine the environmental
variables that best predict the probability of encounter and relative biomass of each fish
functional group. Finally, GAM predictions were employed to map functional group relative biomass
distributions. These distribution maps showed a high relative biomass of many groups in the
center of the MHI chain. This study contributes to a better understanding of fish community structure
around the MHI and will inform ecosystem model parameterization.