Spatial Ecology

Shark nursery areas have been well studied but information on long-term, fine-scale movements within nurseries and between nursery areas is lacking.  We tracked 47 juvenile lemon sharks at Bimini, Bahamas over three years (2003-2005) within two nursery areas.  Using both manual and automated telemetry, we examined the movement patterns and habitat selection of juvenile sharks in the waters surrounding Bimini.  Sharks consistently showed a high degree of site fidelity to their natal nursery areas and there was no detectable immigration or emigration between nurseries.  Juvenile lemon sharks had small home ranges and there was a positive relationship between home range size and distance from shore with body size.  Resource selection functions calculated during both the wet and dry seasons from 2003 to 2005 showed that shark locations were most correlated with increased prey biomass and decreased distance to the shoreline.

Examining both movement patterns and habitat selection from two distinct nurseries over a long time period demonstrated that juvenile lemon sharks in Bimini utilized the habitat in their natal nursery seemingly to minimize predation risks while maximizing prey availability.  Integrated approaches using long-term data to understand how juvenile sharks utilize nursery areas can significantly improve how essential fish habitat is defined and thereby strengthen conservation and management plans for these species.  

  1. How is the nursery habitat in Bimini utilized?  Delineation of nursery habitat in Bimini to determine if individual sharks remain in their natal nursery area or if they utilize all potential nursery habitat around Bimini.
  2. How do movement patterns change as sharks age and grow?  Examination of movement parameters for individual sharks as they are becoming larger.
  3. Do shark movement patterns vary spatially when comparing one nursery to another?  Do patterns vary temporally (i.e. year to year, season to season)?  Analysis of movement patterns on a spatio-temporal scale.
  4. What factors in the environment are most correlated with shark movements within their nursery areas?  Construct resource selection models using abiotic and biotic factors to elucidate important factors for juvenile lemon sharks.

The Magnuson-Stevens Fishery Conservation and Management Act (1996) requires the collection of data related to essential fish habitat (EFH).  It defines EFH as “waters and substrate necessary to fish for spawning, breeding, feeding, or growth to maturity.”  Due to an unreasonable amount of habitats being defined as EFH, Musick (1999) recommended defining EFH based on the specific utilization by the species, availability of the habitat, and vulnerability to exploitation.  Because most shark nurseries are relatively small areas located in coastal habitats, they become habitats of particular concern (Grubbs, 2001).  Amendment 1 to the Highly Migratory Species Fisheries Management Plan stresses research needs with respect to EFH of sharks and specifically calls for research on shark nursery areas.  Due to the importance of nursery areas to the life history of sharks and the fact that many nurseries are in areas under threat of anthropogenic impacts, there is an urgent need for research to define EFH for juvenile sharks.  A clearer understanding of the habitats that serve as nursery areas for sharks will aid in the conservation and management of sharks (Heupel et al., 2007).

There was no immigration or emigration between the North Sound and the South Bimini nursery.  Although we tracked individual sharks traveling up to 38 km within their home range in a 24 hour period, sharks from the NS never ventured to SB.  Thus the two lemon shark nurseries studied at Bimini must be considered entirely separate and distinct nursery grounds rather than a single continuous unit.   This finding must be taken into account when designing and implementing conservation strategies.  The decision by a full-term shark of exactly where parturition occurs may have important implications for the long term survival of cohorts.  Even small-scale destruction/degradation of nursery habitat may have far-reaching implications to populations if nurseries are geographically separated within a small area.  In conclusion, large areas that support juvenile sharks may well consist of many small geographically and ecologically distinct primary nurseries.  These results demonstrate the importance of fine-scale studies of shark nurseries to accurately delineate space use by juvenile sharks and how such results may affect conservation strategies.

Comprehensive tracking of juvenile lemon sharks in two primary nursery areas provided a detailed description of their home range and movement patterns as sharks age and grow.  There was a significant positive relationship between home range size (using two home range estimators) and age/size in almost all instances demonstrating that sharks do increase space use as body size increases.  This may be due to a combination of factors including an increased energy requirement causing an increase in range to find adequate food and/or a decreased amount of predation pressure thereby opening previously restricted areas due to elevated predation risk.

There was significant spatial and temporal variation in movement patterns among the juvenile sharks in the two nursery areas studied.  Sharks from the North Sound nursery area had larger home ranges and ventured further from shore compared to sharks in the South Bimini nursery area.  This may reflect differences in biotic and abiotic factors among the two nursery areas.  Sharks in the North Sound nursery may have to increase space use because of less stability in terms of productivity and environmental factors such as temperature and salinity.  Conversely, sharks in the South Bimini nursery may be able to minimize their space use because of the stability in the environment thereby conserving energy.  These differences may be reflected in the variation in growth rates seen in the two nurseries (DiBattista, 2007; pers. obs.), where sharks in the South Bimini nursery area grow approximately 2-5 cm per year faster than similar aged sharks in the North Sound nursery area.

There were some clear patterns in the resource selection models  with most sharks during most seasons in all years having movement patterns that were correlated with areas of high prey biomass and areas close to shore.  The North Sound nursery area was the safest area in terms of predator density compared to all other locations around Bimini.  Therefore, if predation by larger conspecifics accounts for a substantial portion of mortality rates in juvenile sharks in Bimini, then a pregnant adult female can increase the survival rates of her offspring by choosing to give birth in this nursery area.  The fact that juvenile lemon sharks disproportionately utilized areas close to shore demonstrates the importance of mangroves to the early life stages of lemon sharks.  The protection offered by the structure of the mangroves and the abundant prey found in the mangroves maximizes the fitness of young lemon sharks in their primary nursery areas.

Principle Investigator

Dr. Bryan Franks - Florida Southern College, US

Collabrators

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

Dr. Steven P. Newman  - Newcastle University, UK

Dr. Steven Kessel - Windsor University, US

Sam Collin - Cardiff Universtiy, UK

Kelsie Jackson - Murdoch University, Australia

Dr. Alastair Harry - James Cook University, Australia

Dr. Kristine Stump - Unversity of Miami, US

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