L11 Pelagic oceans Flashcards
What is the pelagic and other terminology
pelagic = open sea neritic = zone above the shelf Oceanic = zone beyond shelf break
describe the highly heterogeneous environment of the pelagic
vertical and horizontal heterogeneity in light,
temperature, oxygen, wind-driven mixing, mixing of water masses.
(fronts = meeting of water masses
differing in physical properties).
In frontal regions, the sharp environmental
gradients can be over very narrow spatial scales, just a few metres
- Leads to differences in the spatial distributions of biota, from phytoplankton to large marine predators.
Where in the pelagic is there sometimes upwelling?
In the equatorial pelagic, as tradewinds cause Ekman transport, promoting the upwelling of cold, old and nutrient rich
water, in turn leading to high primary productivity and important fisheries
What happens in el nino years?
Usually, tradewinds drive surface water westerly across Pacific, due to high pressure in East (peru) and low pressure in west (Indonesia).
In el nino years, reversed.The weakening of the
tradewinds leads to intense heat in western pacific, leading to forest fires and warming seas (including
coral bleaching). There is also a weakening of the upwelling off Peru, leading to a warming of the water
and a change in the fish assemblage (and their fisheries).
who realised that fishery assemblage was linked to climate?
One of the first studies to identify a link between the prevailing climate and fisheries was by Chavez
et al. 2003, who found that the prevailing fish species in the Eastern pacific were linked to air
temperatures and other climate variables in the region.
Give an example of ontogenetic variation in diet
(juveniles feeding on different resources to adults).
Atlantic herring - feeds on larger planktonic prey as it reaches larger body sizes.
To fully understand what drives interannual variation in recruitment of fished stocks, we need to have
quantitative data on the availability of key food for larval fish.
What makes up the macrofauna of the pelagic?
dominated by large zooplankton: drifters, jellyfishes and oceanic nekton: active swimmers, large cephalopods, bony fishes, elasmobranchs, reptiles and
birds.
Mammals: Cetaceans, pinnipeds (seals and sea lions).
Nektonic reptiles: turtles and sea snakes.
Most birds simply visit to feed, but there are the flightless penguins.
What are some adaptations of pelagic orgs?
Buoyancy - Swim bladdersin slow teleosts, lpst in fast moving teleosts as they prevent rapid depth change.
Marine mammals may have accessory air sacs, and have air trapped in dense wool undercoats.
Fish can be rich in lipids to increase buoyancy.
Locomotion - adaptations either create propulsive force, or reduce resistance.
Defence - shoaling, camouflage, sensory systems.
Reproduction - spawning migrations
Why do some species migrate?
locating food, mates and/or to release young in the vicinity of suitable nursery grounds.
Difficult to locate mates and nursery grounds without visual landmarks.
Describe one circumglobally distributed pelagic species and why tracking it is significant. 2 studies.
Leatherback turtle, Dermochelys coriacea, is largest of marine turtles and feeds on zooplankton. Reach maturity at 8-15 years, and females mate every 2-3 years but can breed annually.
Use of satellite tags to gain understanding of their movements. Tracked 8 females in Carribean after egg laying, for 6-12 months. Showed little consistency in movements, and 50% of time spent below 10m.
Significance as shows measures need to be taken to prevent turtle by catch from longlines across the Atlantic basin.
Contrastingly, recent tracking of 46 leatherbacks showed consistency as they showed directional mvt away from breeding beaches in CRica, past galapagos and into S pacific gyre.
Shows clear migration corridor and shows habitat use is not random in this population. Presents areas for marine protected zones.
How do turtles navigate?
research on Green turtles suggests they navigate using geomagnetic cues. 20 turtles were tagged with satellite tags, displaced to the open sea 100-120km from the start position and tracked. 19 returned. Those with magnets on their heads had longer homing paths than those without.
Describe a large pelagic marine predator
Atlantic bluefin tuna Thunnus thynnus.
30 year lifespan, 300kg, 4m length. Matures at 4- 8 years.
Basis of v profitable fishery using longlines and purse seins.
what evidence is there that mid atlantic tuna are from both populations from the med and the gulf of mexico?
satellite tagging found 2 populations were present.
western population, breeds in gulf of mexico, and Eastern pop, breeds in the mediterranean.
Geochemical information has provided additional information for differences as the young show different C and O stable isotope ratios in otoliths. Could find out natal origin of adult tuna.
The stocks need to be managed as separate stocks but the overlap in foraging ground causes difficulties in fishery management.
What is a levy flight?
An optimal search strategy when searching complex prey landscapes.
Occasional long distance movements, and many local movements.
Levy like movements are present in sharks, teleosts, sea turtles and penguins.
Juvenile basking shark failed to show this - suggests learnt behaviour.
What has satellite tracking of megafauna shown, regarding macroecological patterns of space use?
- Many species are taking latitudinal movements, related to temperature and chlorophyll abundance.
- suggests they are
undergoing seasonal feeding migrations. - also showed patterns of niche partitioning related to
environmental variables - possible this pattern of space use has evolved
to reduce interspecific competition for food resources
