Applying Individual-Based Modeling Techniques To Address Potential Impacts of Essential Habitat Loss In A Lemon Shark Nursery.

Abstract:

Individual-Based Modeling (IBM) is a powerful heuristic tool used to understand complex ecological processes. “Agents” representing individuals are assigned rules that dictate their behavior and influence interactions with other agents and the environment. After multiple iterations of the model, population- and ecosystem-level patterns may emerge from interactions of independently-acting agents. The goal is to develop a model such that emergent patterns reflect patterns of interest observed in the field. In the present study at Bimini, Bahamas, the mangrove-fringed lagoon comprises several important nurseries for lemon sharks (Negaprion brevirostris). The study site is a critical nursery, as it affords ample prey and protection from larger predators. A significant volume of research exists concerning the life history, physiology, diet, bioenergetics, growth and behavioral ecology of lemon sharks in Bimini. This study will utilize a twenty-year database combined with ongoing field observations to create an IBM that combines the behavior and bioenergetics of juvenile lemon sharks with that of their prey and predators as they interact within the nursery ecosystem. The field-validated model will be used to describe the functioning of the nursery ecosystem and address actual and potential ecological impacts of planned coastal development in the area, serving as a tool to assess management alternatives. The model will help elucidate the role of top predators in a mangrove-fringed lagoon ecosystem, as well as the potential consequences of a decline or loss of such predators.

Effects of Nursery Habitat Loss on Juvenile Lemon Sharks.

Introduction

Mangroves provide critical habitat for many species, often during juvenile life stages (Nagelkerken et al. 2000, Laegdsgaard and Johnson 2001, Nagelkerken et al. 2002, Dorenbosch et al. 2004, Mumby et al. 2004,). In Bimini, Bahamas, 85km east of Miami, FL, the mangrove-fringed lagoon between two main islands comprises several important parturition sites and nursery areas for lemon sharks (Negaprion brevirostris), a large coastal species managed by the National Marine Fisheries Service Highly Migratory Species (HMS) Division. Within the North Sound nursery, juvenile lemon sharks are afforded ample prey, as well as protection from larger predators. Although this area had previously been proposed by the Bahamian Government as a high priority site for a Marine Protected Area (Sobel and Dahlgren 2004), it has already been impacted and now faces the threat of extensive habitat loss due to a large-scale coastal development project.

A significant volume of research exists concerning life history, physiology, feeding and diet, bioenergetics, growth and behavioral ecology of lemon sharks in Bimini. In addition, there have been numerous studies focusing on the ecosystems of Bimini itself. Due to the wealth of existing baseline knowledge about this system in an undisturbed state, the effects of a new, large-scale anthropogenic impact can be quantified. Over the next three years, this study will utilize over twenty years of data to 1) examine effects of nursery habitat loss on juvenile lemon shark (a) abundance, (b) first-year mortality, (c) multi-year survivorship, (d) growth rate and (e) habitat use and home range; 2) quantify ecological differences between areas adjacent to unmarred habitat and damaged habitat in terms of (a) species richness and (b) substrate composition; 3) compare differences between baseline measurements collected prior to and at different stages of habitat degradation; and 4) create an agent-based model that combines the behavior and bioenergetics of juvenile lemon sharks with that of the sharks’ prey and predators as they interact within the North Sound nursery ecosystem in Bimini, Bahamas. The field-validated model will ultimately be used to address actual and potential impacts of mangrove removal in the nursery area on the local juvenile lemon shark population. In addition, the ecosystem-based model will help elucidate the roles of this high-level predator in a mangrove-fringed lagoon ecosystem, as well as potential ecological consequences of a decline or loss of this predator.

Methods and Materials

In order to examine the effects of nursery habitat loss on juvenile lemon shark abundance, first-year mortality, multi-year survivorship and growth rate, data collection will follow a protocol, established in 1995 by the Bimini Biological Field Station (BBFS) for its annual tag-recapture census of juvenile lemon sharks within two Bimini nurseries. Following parturition by gravid females each spring, nursery areas are sampled with gillnets for six nights, until catch is zero or near zero on the final night (Gruber et al. 2001, Feldheim et al. 2002, Barker et al. 2005, DiBattista et al. 2007). This method has allowed for annual nearly absolute population estimates, with a high recapture percentage of the same individuals in subsequent years. In turn, this allows for determinations of growth rate and a host of additional parameters such as mortality/survival, as well as quantitative genetics of fitness determinations. In addition, due to the early life history characteristics of N. brevirostris, individuals in Year 1 or Year 2 not recaptured in the nursery the following year can be assumed to have died rather than have emigrated (Gruber et al. 2001, DiBattista et al. 2007). Therefore, sampling methods allow for direct estimation of first-year mortality and multi-year survivorship. Continuation of this sampling protocol over the next three years will provide updated annual population estimates, growth rates, first-year mortality estimates and multi-year survivorship figures, as well as other life history parameters that will be statistically compared against past values (prior to habitat degradation) to quantify impacts.

Juvenile lemon sharks rely on mangrove-associated prey species which will also likely be affected by the loss of mangrove shoreline. To assess changes in these prey assemblages, species diversity, abundance and biomass will be assessed in affected areas following methods employed in previous studies (Franks 2007, Jennings et al. 2007, Kessel et al. 2007, Newman et al. 2007, Jennings et al. 2008). New data collected after the onset of mangrove loss will be compared to data collected prior to habitat destruction in order to quantify impacts. An undisturbed nursery area in South Bimini (Jennings et al. 2008) will be sampled concurrently as a control site to determine if changes in the affected area are greater than natural variation observed in the control site (Underwood 1992).

Habitat use and home range have previously been studied using both passive and active telemetry tracking techniques, and various life-history parameters have been determined from tracking data (Morrissey and Gruber 1993a, 1993b, Franks 2007, Guttridge et al. submitted). For the past three years, BBFS has deployed an array of Sonotronics Submersible Underwater Receivers (SURs) at multiple sites around Bimini. Coupled with the usage of surgically-implanted Coded High Powered Transmitters, BBFS has gained valuable insights into the behavior and movements of neonate, juvenile, subadult and adult lemon sharks. To help determine impacts of habitat loss on movements, home ranges and habitat usage of lemon sharks in the North Sound, an array of SURs will be placed at overlapping range intervals for complete coverage of the 3km2 North Sound to capture large-scale movement within the nursery. SURs will also be placed just beyond the boundaries of the nursery area to determine emigration of individual juveniles as well as immigration and exposure to their main predator (i.e. subadult lemon sharks). A proportion of juvenile lemon sharks in the nursery (approximately 30 to 50 individuals) will receive transmitters to enable both passive and active tracking of individual movements at both large and fine scales. Movement data from the telemetry array will be used to parameterize the concurrently-developed agent-based model (Grimm and Railsback 2005), which will be applied toward elucidating the factors most critical to determining the sharks’ spatial patterns (and therefore survival) in a mangrove-fringed nursery habitat.

For more information, please contact Kristine Stump at kstump@rsmas.miami.edu.

< Back to Research List