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 mid- depths – 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 – Attached cameras (e.g., crittercam) Whale sharks forage at depths of 600 m

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