Migration, Habitat Use, and Predator-Prey Dynamics of Coastal Sharks in the Northeast Gulf of Mexico

Drivers of animal movement, including abiotic factors such as environmental conditions or climate and biotic factors such as species interactions and reproduction, are classic topics in ecology and relevant to both basic and applied scientific questions. Understanding phenomena such as seasonal migrations are important in fisheries management, and describing habitat use and ecological interactions is a primary goal in fisheries ecology as the field moves toward holistic, ecosystem-based approaches. Here I present the results of research investigating the drivers of movement in coastal sharks, from scales of seasonal migrations and temporal patterns of abundance to fine-scale movement of a single species as it relates to the distribution and density of its prey and predators. Iused a 10 year data set of long-term fishery-independent gillnet and longline surveys to explore the effects of abiotic variables on temporal patterns of community structure of fishes and correlates of migration at two seagrass shoals off the FSU Coastal and Marine Laboratory. I examined community structure using non-metric multidimensional scaling (NMDS) with environmental fitting, compared temporal community structure using permutational multivariate analysis of variance (PERMANOVA) of monthly catch rates in both gear types, and tested for cyclicity in community structure. I also used generalized additive models (GAMs) to explore the effects of environmental variables (e.g. temperature and photoperiod) on immigration and emigration phases of dominant taxa. I found strong seasonality and cyclicity in assemblages captured by both gear types, with depauperate winter communities and diverse assemblages in warmer month - especially late summer and fall. My results suggest temperature may determine the timing of immigration and duration of the residency period of dominant taxa, including the juvenile life stages of some coastal sharks, but photoperiod may cue immigration and emigration in adults when the purpose of those migrations includes predicable reproductive functions (e.g. parturition). I found evidence of partial migration in juvenile life stages of some coastal sharks, and hypothesize that rising temperatures due to climate change may have variable effects on residency patterns over ontogeny. I conducted similar fishery-independent sampling in Apalachicola Bay from 2018 to 2020 during the summer to study habitat overlap in multiple life stages of a group of sympatric coastal sharks. I also used data shared with me by the Florida Fish and Wildlife Conservation Commission to explore the relationship of habitat use in coastal sharks to prey distribution and density. My analyses suggested this relationship may vary among species and life stage, as predation risk or environmental tolerances may drive habitat use in early life stages of some species. Prey distribution may be more important for dietary specialists. Interestingly, habitat use of species life stages with increasing reported dietary overlap appeared to converge. Using acoustic telemetry, I described movement and habitat use of bull sharks (Carcharhinus leucas), bonnetheads (Sphyrna tiburo), and gafftopsail catfish (Bagre marinus) in Apalachicola Bay and St. George Sound. I used linear mixed models (LMM) to integrate the tracking information with blue crab (Calinectes sapidus) density data from the same fishery-independent surveys conducted by FWC to explore the predator-prey dynamics of blue crabs, bonnetheads, and bull sharks. All three fishes were resident throughout the summer and were philopatric. Bull sharks used more of the habitat than bonnetheads or gafftopsail catfish. Bonnetheads mainly used seagrass habitats, while gafftopsail catfish used muddy habitats around freshwater inputs. There were relationships of size and the proportion of the acoustic array each species used, with a positive correlation of size and space used in both species of sharks and a negative correlation in gafftopsail catfish, which may be driven my differences between sexes. Bonnetheads were not detected in habitats where blue crabs were most dense, but the probability of bull shark presence was highest. Results of the LMM analyses suggested a significant negative relationship of bonnethead habitat use and bull shark presence probability, as well as a significant negative relationship of the interaction of bull shark presence and salinity (bull shark presence probability was highest at sites with the lowest average salinities). These results suggest habitat use in bonnetheads may follow predictions of optimal foraging theory and the ideal free distribution under the constraint of predation risk, which may be informative in understanding the roles of predation and prey availability in the habitat use of marine mesopredators.