Spatial and temporal variation in the occurrence of bottlenose dolphins in the Chesapeake Bay, USA, using citizen science sighting data

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Lauren Rodriguez, Amber D. Fandel, Benjamin R. Colbert, Jamie Testa and Helen Bailey, Chesapeake Biological Laboratory, University of Maryland Center for Environmental Science, Solomons, MD

Background/Question/Methods
Bottlenose dolphins (Tursiops truncatus) are migratory marine mammals that live in both open-ocean and coastal habitats. Although widely studied, little is known about their occurrence patterns in the highly urbanized estuary of the Chesapeake Bay, USA. The goal of this study was to establish a community network of observers to determine the spatial and temporal distribution of bottlenose dolphins throughout this large estuarine system and use statistical modeling techniques to determine how their distribution relates to environmental factors. Three years (April-October of 2017-2019) of dolphin sighting reports from a citizen-science database, Chesapeake DolphinWatch, were analyzed using Generalized Additive Models (GAMs). Passive acoustic monitoring of dolphin echolocation clicks within the middle of the Chesapeake Bay was used to identify whether there were any biases in the temporal pattern of reported sightings from the citizen scientists.
Results/Conclusions
The dolphins had a distinct temporal pattern, most commonly sighted during summer months, peaking in July. This pattern of observed occurrence was confirmed by the systematic, passive acoustic detections of dolphin clicks. Using GAMs for each of three regions in the Chesapeake Bay (lower, middle and upper), dolphin presence was found to be significantly correlated to spring tidal phase, warm water temperatures (24-30 °C), and salinities ranging from 6 – 22 PPT. Predictions from these GAMs were statistically correlated to the actual number of dolphin sightings reported to Chesapeake DolphinWatch in a separate testing data set. These models for dolphin presence could be implemented as a predictive tool for species occurrence and inform management of this protected species within the Chesapeake Bay.