Nekton
What do we need to know?
• What are the characteristics of pelagic
ecosystems used by nekton?
• How are nekton adapted to diverse
challenges?
• What influences nekton abundance?
– Bottom-up influences
– Top-down influences
What groups are included among “nekton”?
• Fish
• Large invertebrates
(e.g., squid)
• Reptiles
• Mammals
• Birds
Mobility allows nekton to use more
ocean volume & diverse habitats
• Light declines with depth
• Pressure increases with depth
• Lack of shelter, structure
• Patchy distribution of resources (horizontal)
• Resource density declines with depth
Vertical variation in pelagic habitats
• Depths to 6000 m for abyssal plains
– 10,000 m for deep oceanic trenches
• Mid-oceanic rises (sea mounts, volcanic
islands)
• Gradient for reduction in light
– Low plankton below compensation point
– Just enough light for visual orientation at middepths
– Insufficient ambient light for orientation in
aphotic zone
Advantages to size and mobility
• Access to higher volume for foraging
• Access to specialized habitats and niche
separation
– E.g, pilot whales, beaked whales, dolphins
• Ability to utilize patchy prey
• Better ability to escape predators
• Ability to select different habitats for different
stages of life history
A major life history consideration
• Large animals come from single-cell eggs
• Small stages require different resources than
adults; often are planktonic
• Small stages are exposed to more predators
and different kinds of predators than adults
How do we know about patterns of mobility?
• Older technology: tag & recapture
• Newer technology:
– Time-depth recorders (TDR’s)
– Acoustical tags
– Satellite tags & GPS
Whale sharks forage at depths of 600 m
– Attached cameras (e.g., crittercam)
Example of satellite tag on a marlin
Vertical foraging movements of a marlin
Great White Shark movements
While they sometimes dove as far as 2,040 feet (680 meters) below sea level, the
animals seemed to prefer swimming at two discrete depths -- one within 15 feet (5
meters) of the surface, the other 900 to 1,500 feet (300 to 500 meters) down.
Tiger Shark track – 50 days
Tracks of Atlantic tuna (l) & Ridley turtle (r)
Shifts in tuna location with life history
What is a “population” of tuna?
• How do we define “population”?
– spatial component
– reproductive component
– in fisheries, “stock” is a synonym
• Why do we want to know the size &
distribution of populations?
• What criteria can be applied to delimit
populations of oceanic nekton?
Planet Earth – Deep Sea