aquatic food production systems

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  • Aquatic food production systems
    • factors affecting marine productivity
      • Light - needed for photosynthesis which provides energy to support the food web.
        • cant penetrate very deep so autotrophs anchored on the seabed are only found in shallow waters - these are the are algae or seaweeds such as kelp
      • Nutrients - marine autotrophs absorb nutrients directly from water.
        • nutrients in cells of planktonic organisms are carried to the seabed when they die
          • reduce nutrients in photic layer, reducing productivty
    • Fish population dynamics
      • fish populations are a renewable resource.
      • fish are evolved to have a natural breeding rate that exceeds a rate needed to sustain the population. but there is no long term population growth because of the carrying capacity
      • if too many adult fish caught, adult population decline will happen as not enough young are there to replace the adults
    • maximum sustainable yield
      • estimating is difficult because it requires difficult info to collect
        • current total biomass
        • annual biomass growth
        • breeding rate
        • survival rates of each age group
    • overfishing
      • fishing above the MSY
        • if a fish reaches a catch able size before a breeding age, them its possible to catch all the breeding fish adults and destroy the populatipn
        • strategies to reduce overfishing - catch quotas, minimum catchable size, fishing effort limits, no-take zones.
    • fishing techniques
      • pelagic fishing- pelagic species live at a mid-water level or near the surface. include many single species shoals such as tuna
        • methods - drift nets , ,purse seine, mid water pair trawl
      • demersal fishing - demersal species live on or just above the seabed, often in shoals of mixed species making it impossible to be selective with what you catch
        • methods - demersal trawl, bottom trawl, longline, lobster pots
    • environmental impacts of fishing
      • bycatch
        • immature fish too small to sell
        • shrimp trawl nets trap many species as the net is small
        • pelagic lon lines can catch albatrosses. reduced by fishing at night and weighting lines to stop birds
        • drift nets can kill whales, dolphons and turtles. reflective disks help the whales and dolphins
        • species with no commercial value
        • demersal long lines cab catch many other fish species such as sharks as well as turtles
        • catch quota already reached
      • seabed disturbance - disturb the structure, organisms on the sea bed and kill them
      • food web -  reducing the number of any organism will affect the food web. competitor species may become more common. food species more common whilst predators become rarer or vice versa
      • ghost fishing- lost equiptment can continue to catch organisms, they will become bait to attract more organisms. some countries use biodegradeable rope in lobster pots so if they get lost they fall apart. lost gear can also cause physical damage to coral reefs
      • oil spills
      • factory fishing - large vessels made to carry up to 5000 tons of frozen fish that travel for weeks on end, discharge effluent from processing the fish that can cause deoxygenation
  • Light - needed for photosynthesis which provides energy to support the food web.
    • cant penetrate very deep so autotrophs anchored on the seabed are only found in shallow waters - these are the are algae or seaweeds such as kelp
  • nutrients in cells of planktonic organisms are carried to the seabed when they die
    • reduce nutrients in photic layer, reducing productivty

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