Lecture 14- Harvesting of the sea Flashcards
How much fish do fisheries catch per year?
- 90 million tons a year
- continuous shift to new species and new area to maintain catch levels
What is the increase in catch driven by?
-industrialization of fisheries and increased population
Are all oceans now fished?
- yes, few unutilized resources remain
- about 75% important commercial species fully or over exploited
How much do Australian fisheries catch?
- approx. 200 species caught for food
- almost half of Australia’s 70 principle fish species are fully fished or overfished
- low production by global standards due to low productivity of coastal areas
- major Victorian fisheries are fully fished (rock lobster, giant crab, abalone)
What does fully fished and overfished mean?
- fully- if remain at that level it is just maintaining
- overfished= decline in population, do not have time to replenish
PIC1What is trawling?
- type of fishing
- can be pelagic(floating) or demersal (touching the bottom)
What is the by-catch with trawling?
- high by-catch of small invertebrates and large invertebrates (turtles)
- prawn trawling catches 2% of the worlds total fisheries catch but is responsible for 33% of the world’s total by catch
What is a turtle exclusion device?
- in trawling so turtles do not get stuck in there
- works
- TEDs
- bars, the turtle bumps into it and it flips her out
What is the problem with trawling when demersal?
- habitat destruction as they use rollers or chains
- area of seafloor modified by trawling is greater that the global terrestrial area of deforestation
- before trawling= complex benthic communities, after=simple communities
What nets are used for fishing?
- gill nets (pelagic and demersal)
- seine nets (the one that close like a circleú
What is the problem with nets fishing?
- non-selective, leads to high by-catch of non-target species
- ghost-fishing problem
What is ghost fishing?
- when lose a net or trap and it still continues to catch fish and other animals
- the animals caught then act as bait and more are attracted and caught
What is line fishing?
- demersal and pelagic long-lines, trolling
- has thousands of hooks on a line
- by-catch of birds, sharks, turtles
What is fishing with traps?
- cages
- selective fishing technique
- limited by catch
- ghost fishing problem
What are the less used types of fishing?
- hand collection, wading, free diving, SCUBA
- eg. shellfish, crustaceous, sea cucumbers
- cyanide fishing-potential for local over-exploitation
- dynamite fishing=potential for habitat destruction when combined with cyanide and dynamite
What is the concept of fishing down marine food webs?
- first proposed by fisheries scientist Daniel Pauly in 1998
- mean trophic level of the species caught globally from 1950 to 1994 declined 3.4 to 2.6
- reflects a gradual transition in landings from long-lived, high trophic level, piscivirous bottom fish toward short-lived, planktivorous pelagic fish
- fishing down is a process occurring in 3 phases
What is Phase 1: Pristine of fishing down?
- the state of the ocean ecosystems before fishing made strong impacts
- for most place in the ocean, the baseline must be inferred from archeological data and historical accounts
- biomass of large fish was 10 to 100 times greater than present
- sea floor dominated by large filter feeders which reduce the phytoplankton and suspended particles
How much reduction in fish has occurred?
- 89% reduction of the pristine abundance of prized predatory fishes
- shark reductions by 40-99% in most ecosystems
- 91% decline in global oyster populations
What is phase 2:Exploited of the fishing down?
- characterised by declines in 1) the biomass of large predator fish 2) the diversity,size and trophic level of captured fish and 3) the benthos
- trawlers destroy the habitat built over many years on the seafloor by the filter and detritus feeders
- as these structures and animals that were filtering the phytoplankton and consuming the detritus disappear, they are replaced with the polyp stages of jellyfish
- jellyfish and other opportunistic feeders become abundant (the rise of the jellyfish)
What is the jellyfication of the oceans?
-when sea floor trawled, clear, the jellyfish can breed more
What is phase 3:Fully degraded of fishing down?
- the dead zone is the biological endpoint of a fully degraded marine ecosystem
- excessive nutrients in the water column, resulting in the depletion of oxygen and the elimination of multicellular organisms
- the abundant detritus and marine snow is processed by bacteria rather than by the benthic animals (the rise of microbes!)
- happened in Bohai Sea in China, northern Gulf of Mexico and northern Adriatic Sea
PIC2What was the example of trophic cascades and phase shift where the predators were removed?
- sea urchins not much effect on kelp as they are eaten by Cod, Sea mink, Sheep head,
- due to presence of predators there is more kelp as fewer urchins
after fishing-massive increase in sea urchins= eat all the kelp
-due to loss of the predators, the mink hunted out, Cod, fished out, …
What is the example of a trophic cascade and phase shift with corals?
- had lot of herbivorous fish, those people ate and fine but then a storm and the coral gone
- the balance not maintained anymore
- herbivorous fish are really important for maintaining the ecosystem
What happened to Cod in Canada?
cod= caught for 1000s of year, (vikings too)
- catch in 1000s of tonnes, population decline due to overfishing, complete collapse, now no fishing in the Canadian fishery
- top predator, things that it would eat= now increase= crabs and shrimp
- system dominated by crabs and shrimp instead to Cod (difficult for cod recovered, as they get eaten by teh crabs and shrimp)
