Nekton

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Nekton
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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

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Planet Earth Deep Sea
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What groups are included among nekton?

• Fish
• Large invertebrates (e.g., squid)
• Reptiles
• Mammals
• Birds

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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

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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
  mid-depths
• Insufficient ambient light for orientation in
  aphotic zone

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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

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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

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How do we know about patterns of mobility?

• Older technology tag recapture
• Newer technology
• Time-depth recorders (TDRs)
• Acoustical tags
• Satellite tags GPS
• Attached cameras (e.g., crittercam)

Whale sharks forage at depths of 600 m
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Example of satellite tag on a marlin
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Vertical foraging movements of a marlin
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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.
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Tiger Shark track 50 days
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Tracks of Atlantic tuna (l) Ridley turtle (r)
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Shifts in tuna location with life history
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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?