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)
How can we stop the degradation of the oceans?
- ensure fisheries are sustainable (something we can do forever)
- minimize coastal pollution and eutrophication
- create a global network of Marine Protected Areas
How is tuna fished?
fishers would look for dolphins to catch tuna as they eat the same things
- tuna attracted to floating logs
- the logs drift into productive areas (usually)
- meeting point hypothesis= where the tuna meet so they are bigger school
- from boats they send people to see if tuna is there
- response to catching too many dolphins, means many other things attracted to it, dolphins not caught but now lot of by catch of pelagic sharks etc.
