Seasonal and Annual Variation in the Extent of Suitable Habitats for Forage Fishes in Chesapeake Bay, 2000-2016
Virginia Institute of Marine Science
The sustained production of sufficient forage is critical to advancing ecosystem-based management in Chesapeake Bay. Yet factors that affect local abundances and habitat conditions necessary to support forage production remain largely unexplored. Here, we quantified suitable habitat in the Chesapeake Bay region for four key forage fishes: bay anchovy Anchoa mitchilli, juvenile spot Leiostomus xanthurus, juvenile weakfish Cynoscion regalis, and juvenile spotted hake Urophycis regia. We coupled information from 17 years of monthly fisheries surveys with hindcasts from a numerical model of dissolved oxygen (DO) conditions and a 3-D hydrodynamic model of the Bay that provided estimates of habitat conditions across 18 covariates of salinity, temperature, DO, depth, and current speed for the period 2000 to 2016. Sediment composition and distance to shore metrics were also considered. The hindcast covariates were subsampled at the times and locations of the fisheries surveys to provide dynamic habitat metrics that are not generally observed at the time of fish sampling (e.g., current velocity, salinity stratification). Hindcast covariates were also used to describe habitat conditions in areas of Chesapeake Bay that are not sampled routinely by fisheries-independent surveys such as the Potomac River and Mobjack Bay. Boosted regression trees were used to identify influential habitat covariates for each species, and these influential covariates were then used to construct habitat suitability models. Habitat suitability indices, which ranged between 0 (poor habitat) and 1 (superior habitat), were assigned to each location in the 3-D model grid for each season in 2000-2016. Based on the estimated habitat suitability index and using a GIS approach, we quantified suitable habitat (defined as habitats with a habitat suitability index > 0.5) throughout the Chesapeake Bay and its tidal tributaries. Furthermore, we validated the modeling approach using out-of-sample observations from Mobjack Bay in 2010-2012.
Suitable seasonal habitat extents for forage species exhibited strong seasonal and annual signals reflecting temporal heterogeneity in habitat conditions in Chesapeake Bay. Current speed, water depth, and either temperature or dissolved oxygen were identified as important covariates for the four forage species we examined, and distance to shore was important for three of the four species; thus, suitable habitat conditions resulted from a complex interplay between water quality and the physical properties of the habitat. In our study, two species exhibited a relationship between relative abundance and extent of suitable habitats – juvenile spot in summer and bay anchovy in winter; as such, estimates of the minimum habitat area required to produce a desired abundance (or biomass) of forage fish can be used to establish quantitative habitat targets or spatial thresholds that may serve as spatial reference points for management. In an ecosystem-based approach, important habitats may be targeted for protection (e.g., by limiting fishing activities that may incidentally capture or injure forage fishes) or restoration (e.g., by improving water quality conditions), thereby ensuring production of sufficient forage for predators. In addition, the consequences of aquatic habitat alterations, whether due to climate change or physical disturbances can be investigated using projections of environmental conditions and habitat suitability in the region, though these projections will introduce additional uncertainty.
Fabrizio, M. C., Tuckey, T. D., Bever, A. J., & MacWilliams, M. L. (2020) Seasonal and Annual Variation in the Extent of Suitable Habitats for Forage Fishes in Chesapeake Bay, 2000-2016. Virginia Institute of Marine Science, William & Mary. https://doi.org/10.25773/dyjy-mm73