Life in the mosaic: Predicting changes in estuarine nursery production for juvenile fishes in response to sea-level rise with a landscape-based habitat production model

Background/Question/Methods

Identification of critical habitat in estuarine fish nursery areas is an important conservation and management objective, yet response to changes in critical habitat is both equally important and harder to predict.  Habitat can be viewed as a mosaic of both temporally variable environmental features and spatially variable structural features that combine to define habitat.  Effective models of population responses to habitat change should account for individual response to change, as well as the full suite of temporal and spatial habitat variability.  We extended a spatially-explicit individual-based model of small-scale movement to describe habitat choices of an index juvenile fish in an estuarine nursery system.  The objective was to compare responses in movement and production across a spectrum of spatial and temporal heterogeneity as a precursor for predictions of population-level responses to habitat change.  The model was validated based on an examination of movement, distribution, and net production of immigrating juvenile spot, Leiostomus xanthurus in the Pascagoula river estuary, MS, USA.    The validated model was used to explore responses of juvenile fishes to predicted changes in coastal marsh habitat resulting from a predicted 0.24 m sea level rise (SLR) between 2007 and 2100 in this northern Gulf of Mexico estuary.  

Results/Conclusions

Movement of juvenile fishes was influenced by both spatial and temporal patterns in habitat quality, and it was a balanced mix of both that resulted in a match to observed distributions.  Fishes that perceive habitat on a scale smaller than the scale of spatial heterogeneity may respond to temporal change as a movement cue allowing for deterministic distributions at larger scales despite perceptual limitations.  Model results suggest a hierarchical approach is best for describing responses to habitat changes.  Sea level rise is predicted to result in increased marsh fragmentation, edge density, and a shift from high to low density marsh composition.  Simulations exposing cohorts of juvenile fish to SLR conditions at multiple time points out to a 93-year time horizon predicted that movement rates are the primary driver of population production responses.  Gross production was minimally affected by predicted habitat change even at maximum SLR used.  Instead, movement rates were reduced as edge habitat increased resulting in lower predicted mortality and higher predicted growth.  Habitat changes associated with SLR alters distributional patterns, which can reduce growth, but the net effect of these changes on population production is highly dependent on behavioral responses of fish to change at the landscape level.