A bioenergetics-based index of habitat suitability: Spatial dynamics of foraging constraints and food limitation for a rare estuarine fish

Abstract

Objectives: Habitat suitability models can guide restoration efforts in freshwater and estuarine ecosystems by synthesizing important habitat attributes in a single index. Several important elements of the foraging arena of Delta Smelt Hypomesus transpacificus, a small, zooplanktivorous fish, were quantified using a model that combined bioenergetics with physical habitat attributes. We applied the model to evaluate spatiotemporal variation in the suitability of Delta Smelt foraging habitat as indicated by growth potential. Methods: Bioenergetics-based estimates of daily potential growth, given constraints on daily foraging due to temperature, turbidity, day length, and prey availability, were compared to reference growth, estimated separately from length-at-age data. We defined suitable foraging arenas for Delta Smelt as times and places when potential growth equaled or exceeded reference growth. Result: Among six contiguous regions of the upper San Francisco Estuary, temperature was consistently lowest and turbidity highest in the most seaward locations. As a result, predicted growth rates were greater there compared to landward, upstream regions that were warmer and less turbid. Conclusion: Simulated habitat augmentation of prey and turbidity revealed that the effects of potential restoration actions could vary regionally as the factors limiting Delta Smelt foraging varied along longitudinal and seasonal gradients. Models of the spatiotemporal variation in Delta Smelt habitat may be substantially improved by directed laboratory studies; however, this research provided a data-driven example of how dynamic estuarine habitat attributes interact to influence the suitability of foraging arenas experienced by a pelagic fish.