Habitat Loss: Spatial Ecology

Many elasmobranchs use inshore nursery areas for the advantages of ample prey and protection from predators, and the availability of such habitats may be a limiting factor for some shark populations.  The shallow, mangrove-fringed lagoons and creeks of Bimini, Bahamas serve as lemon shark nurseries.  In Bimini’s North Sound nursery, the construction of a resort and marina has created a significant anthropogenic disturbance in the form of large-scale mangrove removal and dredge-and-fill activities.  The effects of this disturbance on the movements of nursery-bound juvenile lemon sharks are unknown. The objective of this study, therefore, was to compare movement data from before and after habitat loss to determine if there were changes in spatial patterns of nursery use in response to the disturbance.

1. Do juveniles continue to exhibit site fidelity to the degraded nursery?

2. Is there an increase in size and extent of home range to areas beyond the disturbed North Sound?

3. Has there been a shift away from extensive shoreline use following mangrove removal, dredging and filling in the North Sound? 

In Bimini, juvenile lemon sharks are obligate residents of specific natal nurseries. Tracking data show that despite large-scale habitat destruction, juveniles remain site-attached to the disturbed nursery and continue to use the degraded habitat in a manner similar to pre-disturbance use.   Despite suitable alternative habitats within the insular Bimini system, juveniles continue to remain within the disturbed natal nursery. As the only available nursery habitat on the western edge of the Great Bahama Bank, there is no viable alternative within a reasonable distance of Bimini. Therefore, juveniles are bound to the degraded North Sound and are particularly vulnerable to the anthropogenic disturbances within.  It is important to consider coastal zone management scenarios that protect these essential nursery habitats and weigh future development plans against species-specific and ecosystem-wide conservation and sustainability goals.

Between 2009 and 2011, a total of 28 juvenile lemon sharks ranging from Age-0 to Age-2 were surgically implanted with acoustic transmitters in two nurseries: the disturbed North Sound and the undisturbed South Bimini.  Data from a total of 17 juvenile lemon sharks ranging from Age-0 to Age-2 tracked between 2003 and 2005, prior to habitat loss in the North Sound, were used for comparisons. 

 

1. Do juveniles continue to exhibit site fidelity to the degraded nursery?

Yes.  Site fidelity was confirmed for each shark by comparing its observed tracking locations to 100 simulations of correlated random walks (see figure below).  Each shark’s movements were significantly different than randomly generated tracks.  In addition, there was no movement by any shark between the North Sound and South Bimini nurseries.

 

2. Is there an increase in size and extent of home range to areas beyond the disturbed North Sound?

No.  The size and extent of shark’s home ranges were not different after habitat loss.  Sharks from the North Sound do not appear to be emigrating from the degraded nursery. 

 

3. Has there been a shift away from extensive shoreline use following mangrove removal, dredging and filling in the North Sound?

No.  Sharks continued to maintain the same distances from shore, despite the deforestation and filling of wetlands in the North Sound. 

Principal Investigator:

Dr. Kristine Stump – University of Miami, US

Collaborators:

Jon Battïg, M.S. – University of Basel, Switzerland

Elena Salim Haubold, M.S. – Universidad de Granada, Spain

Prof. Samuel H. Gruber - University of Miami, US

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