Diet and Feeding

Overview
Photo Credit: Matthew D Potenski
 
Sharks have a long-held reputation as opportunistic, asynchronous predators that feed on the most abundant prey. Dietary studies in sharks often compare prey composition with qualitative observations of prey communities; and these semi-quantitative reports of prey preference suggest sharks can be either opportunistic or selective feeders. Prey preference has been quantified in a range of marine animals including teleosts, mammals, birds, reptiles and invertebrates. However, few studies have quantified prey preference in elasmobranchs, and subsequently there is a paucity of data for an entire vertebrate class.
 
Prey species and size selection are typically a function of predator body size, and consequently ontogenetic diet shifts have significant effects on both predator and prey communities. Ontogenetic shifts in diet are almost universal in vertebrates and have been particularly well documented in teleost fish. Quantification of diet shifts and prey preference provide insights into niche utilisation as well as impacts of selective predation on ecosystem structure and functioning. Ontogenetic shifts in diet are almost ubiquitous in sharks, but despite recent attention, the mechanisms influencing variations in diet with shark size are often qualitatively inferred from dietary analysis.
  1. Do lemon sharks feed indiscriminately on available food, or exhibit a preference for certain prey species or size of prey.
  2. Do dietary composition, prey size and prey preference of nursery bound lemon sharks change with ontogeny?
Determining which prey is selected more often than others provides fundamental information on how predators meet their bioenergetic or ecological requirements for survival. Prey preference may play an important role in the growth and survival of a predator, and may in turn affect shark population fitness and adult recruitment. Subsequently, understanding the feeding behaviour of sharks is a necessary step in assessing their ecological requirements for habitat and prey species, ensuring their effective management and conservation.
  1. Stomach contents were quantitatively analysed from 396 lemon sharks captured in two populations, Bimini Bahamas.
  2. Yellowfin mojarra Gerres cinereus dominated the diet of juvenile lemon sharks (>50% by weight and percentage index of relative importance, %IRI), even when present in lower abundances in the environment.
  3. Juvenile lemon sharks do not feed indiscriminately, but exhibit prey preference and size selection.
  4. High overlap between shark diet and mangrove communities revealed the importance of mangroves to lemon sharks and their prey.
  5. Sharks therefore can be highly plastic foragers, capable of selective feeding, but will switch to more opportunistic foraging when environmental conditions deteriorate. For example in the less environmentally favourable North Sound nursery, where prey communities were less dense or diverse.
  6. Juvenile lemon sharks exhibited weak ontogenetic variation in dietary composition with high levels of dietary overlap. Variation in prey preference with ontogeny was complex, but revealed a continuous shift from predominantly opportunistic benthic foraging as neonates to more selective piscivory with increasing shark size while in the nursery.
  7. Lemon sharks demonstrated a discrete ontogenetic shift in the number of prey consumed and stomach content weight, as well as prey size. All sizes of sharks exhibited positive size selection of prey.
 
This study highlights the need to quantitatively compare shark diet with prey communities at different densities, rather than make statements on the prey preference (or lack of) exhibited by a species of shark based solely on the species diversity and composition of dietary data. 
Principal Investigator
Dr. Steven P. Newman  - Newcastle University, UK
Dr. Kristine Stump - University of Miami, US
 
Collaborators
Dr. Richard D. Handy - University of Plymouth, UK
Dr. Samuel H. Gruber - University of Miami, US
Mr. A. Reeve - University of Plymouth, UK
Ornella Weideli - University of Basel, Switzerland
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