title: Agent-Based Modelling of Short-Term Juvenile Bull Shark Movement in a Semi-Enclosed Gold Coast Estuary
Creators: name: Mortensen, Jonas Brandi
Publisher: name: Griffith University
Year: 2012
Subjects: subject: Juvenile bull shark
subject: Tallebudgera Creek
subject: Carcharhinus leucas
subject: Juvenile C. leucas
subject: Eulerian-Lagrangian agent-based modelling

description: This project investigated the value and future potential of a coupled Eulerian-Lagrangian agent-based modelling approach as an alternative method of investigating the movement and habitat use of juvenile bull shark Carcharhinus leucas in small peri-urban estuaries. Through the use of the MIKE21 modelling suite (DHI), a depth-averaged two-dimensional hydro-dynamic model was developed and implemented as a means to capture the spatio-temporal variation in hydrodynamics of the semi-enclosed Tallebudgera Creek estuary. This system provides a suite of habitats comprising artificial residential canals, polyhaline and brackish creek sections in a peri-urban setting. The hydrodynamic model served as the dynamic foundation of a spatially heterogeneous agent-based model (ABM) developed for juvenile C. leucas. The movement formulation of juvenile C. leucas was represented as a kinesis search for optimal conditions, while a random walk model served as a control. The hydrodynamic model performed satisfactorily in terms of capturing the variations of key physical conditions of Tallebudgera Creek. Modelled values of surface elevation and flow dynamics were in good agreement with measured data sets. Simulated mean levels of salinity and temperature were likewise in good agreement with measured means; however, model analysis revealed a high sensitivity to increased freshwater influxes, and a delay in model response time. Three neonate and juvenile individuals of C. leucas were captured and attached with acoustic tags for tracking of movement in Tallebudgera Creek. Short-term continuous tracks of a juvenile C. leucas were successfully collected as a means to relate observed movement to out-puts of the hydrodynamic model and measurements of water quality, while consecutive data-points of animal position served as validation data for the agent-based model. Analysis of C. leucas track data revealed a high site preference for the middle reach of the system over the course of the tracking campaign, even during periods when salinity levels were &lt; 1 PSU. However, an avoidance of high salinities &gt; 27 PSU was evident. Significant movement of the animal in a downriver direction only occurred after a period of increased flow velocities and turbidity, suggesting that these parameters may play an important role in directing shark movement in conjunction with salinity. The agent-based models in their current developmental stage performed unsatisfactorily in capturing observed movement, and their predictive ability was generally poor. The current ABM formulation of C. leucas movement is therefore deemed insufficient to capture the observed pattern of behaviour. However, unforseen technical difficulties originating from the narrow and shallow nature of the Tallebudgera Creek system prevented a full assessment of the ABM results. Despite current technical issues that were impracticable to be resolved under the available timeframe, this study represents a first attempt to construct and implement agent-based modelling to investigate bull shark movement and habitat use in a spatially and temporally dynamic hydrologic environment. It is predicted that once these technical difficulties are overcome, agent-based modelling as a research tool holds great promise for future investigation of the habitat ecology of C. leucas to benefit its conservation and management.