9 – Human Impact on Coral Reefs
87
the cities of Honolulu, Miami and Jakarta have all impacted very significantly on nearby coral reefs through
sewage discharge, increased sedimentation, industrial
pollution, overfishing, and so on. In comparison, the
land area adjoining the GBR has relatively few people,
and most of the reefs are tens of kilometres offshore. In
contrast, the reefs of Jamaica and elsewhere in the Caribbean are typically 100 m or less from the beach, and
millions of people live next to them. Arguably, reefs at
risk should be the highest priority for conservation and
management efforts.
Reefs closest to the mainland or fringing populated
high islands are more likely to be at risk than reefs on
outer continental shelves or unpopulated oceanic atolls. For example, on the GBR inshore reefs generally
have more human impacts than elsewhere (see Chapter 11). Nutrient and sediment runoff from farming activities on land have impacted many of the reefs closest
to the mainland. The chemical signals in annual growth
bands of century-old coral skeletons reveal a sharp increase in coastal runoff following the arrival of cattle
and sheep and large-scale land clearing in the 19th
century. Coastal development and recreational fishing
have also significantly impacted nearshore reefs. Historical photographs of mainland reefs show vibrant
stands of corals along the Queensland coast that are increasingly degraded today.
N TYPES OF HUMAN IMPACTS
Human impacts vary in scale (how big they are, and
whether they are acute or chronic), and the response
of reefs is correspondingly variable. Next is a brief
overview of some of the major categories of human
activities that affect reefs: overfishing, runoff from
land, climate change, coral harvesting, and recreational impacts.
Overfishing
Coral reefs support highly productive, diverse and economically important fisheries. However, human population growth and growing market demands for seafood
have depleted many targeted stocks. On the GBR, earlier export fisheries that flourished following European
colonisation (e.g. for sea cucumbers, pearl shell, Trochus
snails, and turtles) have collapsed or are no longer commercially viable. Harvested tropical megafauna (e.g.
whales, dugongs, crocodiles, turtles, and sharks) are
severely depleted worldwide. For example, the number
of dugongs on the GBR has declined by more than 90%
in the past 30 years. Larger species of carnivorous fishes
(e.g. groupers and snappers), especially those with vulnerable spawning aggregations, have also been heavily
overfished. Even on the GBR, where fishing pressure is
relatively modest compared to most coral reefs, the biomass of carnivorous fishes has been reduced by 4–5
fold on fished reefs compared to adjacent reefs that are
zoned as ‘No-Take Area’.
Top-down effects. In a classic example of ‘fishing
down the food chain’ many reef fisheries today rely
heavily on herbivores and planktivores. Australia is
unusual because it lacks a large human population reliant on subsistence fishing, and there is virtually no
recreational or commercial fishing of tropical reef herbivorous fishes. Over-harvesting of herbivorous fishes
and addition of nutrients both promote blooms of
fleshy algae that outcompete adult corals and impede
new recruitment. The depletion of herbivorous fishes
on Caribbean reefs has caused ecosystem collapse,
with unchecked algal blooms replacing corals. On
some reefs, overfishing has reduced levels of predation and competition from fishes, triggering unsustainably high populations of grazing sea urchins. This
top-down effect is unstable because of emergent diseases that cause mass mortalities of super-abundant
sea urchins, and because bioerosion of the substrate
by huge numbers of sea urchins can exceed the accretion rate of the reef (see Chapter 8).
The relative importance of herbivory and nutrient
supply (‘bottom-up versus top-down control’) in regulating the biomass and composition of seaweed is an ongoing, sometimes contentious debate. Algal biomass is
often highest where herbivores are naturally scarce, for
example, on intertidal reef flats, in turbulent shallow water, or within the defended territories of pomacentrid
damselfish. Reef substrates that are protected experimentally from grazing rapidly become colonised by macroalgae, which in turn inhibit recruitment and growth of
corals. Previous experimental approaches to explore herbivore-algae-coral interactions have used a variety of
87
the cities of Honolulu, Miami and Jakarta have all impacted very significantly on nearby coral reefs through
sewage discharge, increased sedimentation, industrial
pollution, overfishing, and so on. In comparison, the
land area adjoining the GBR has relatively few people,
and most of the reefs are tens of kilometres offshore. In
contrast, the reefs of Jamaica and elsewhere in the Caribbean are typically 100 m or less from the beach, and
millions of people live next to them. Arguably, reefs at
risk should be the highest priority for conservation and
management efforts.
Reefs closest to the mainland or fringing populated
high islands are more likely to be at risk than reefs on
outer continental shelves or unpopulated oceanic atolls. For example, on the GBR inshore reefs generally
have more human impacts than elsewhere (see Chapter 11). Nutrient and sediment runoff from farming activities on land have impacted many of the reefs closest
to the mainland. The chemical signals in annual growth
bands of century-old coral skeletons reveal a sharp increase in coastal runoff following the arrival of cattle
and sheep and large-scale land clearing in the 19th
century. Coastal development and recreational fishing
have also significantly impacted nearshore reefs. Historical photographs of mainland reefs show vibrant
stands of corals along the Queensland coast that are increasingly degraded today.
N TYPES OF HUMAN IMPACTS
Human impacts vary in scale (how big they are, and
whether they are acute or chronic), and the response
of reefs is correspondingly variable. Next is a brief
overview of some of the major categories of human
activities that affect reefs: overfishing, runoff from
land, climate change, coral harvesting, and recreational impacts.
Overfishing
Coral reefs support highly productive, diverse and economically important fisheries. However, human population growth and growing market demands for seafood
have depleted many targeted stocks. On the GBR, earlier export fisheries that flourished following European
colonisation (e.g. for sea cucumbers, pearl shell, Trochus
snails, and turtles) have collapsed or are no longer commercially viable. Harvested tropical megafauna (e.g.
whales, dugongs, crocodiles, turtles, and sharks) are
severely depleted worldwide. For example, the number
of dugongs on the GBR has declined by more than 90%
in the past 30 years. Larger species of carnivorous fishes
(e.g. groupers and snappers), especially those with vulnerable spawning aggregations, have also been heavily
overfished. Even on the GBR, where fishing pressure is
relatively modest compared to most coral reefs, the biomass of carnivorous fishes has been reduced by 4–5
fold on fished reefs compared to adjacent reefs that are
zoned as ‘No-Take Area’.
Top-down effects. In a classic example of ‘fishing
down the food chain’ many reef fisheries today rely
heavily on herbivores and planktivores. Australia is
unusual because it lacks a large human population reliant on subsistence fishing, and there is virtually no
recreational or commercial fishing of tropical reef herbivorous fishes. Over-harvesting of herbivorous fishes
and addition of nutrients both promote blooms of
fleshy algae that outcompete adult corals and impede
new recruitment. The depletion of herbivorous fishes
on Caribbean reefs has caused ecosystem collapse,
with unchecked algal blooms replacing corals. On
some reefs, overfishing has reduced levels of predation and competition from fishes, triggering unsustainably high populations of grazing sea urchins. This
top-down effect is unstable because of emergent diseases that cause mass mortalities of super-abundant
sea urchins, and because bioerosion of the substrate
by huge numbers of sea urchins can exceed the accretion rate of the reef (see Chapter 8).
The relative importance of herbivory and nutrient
supply (‘bottom-up versus top-down control’) in regulating the biomass and composition of seaweed is an ongoing, sometimes contentious debate. Algal biomass is
often highest where herbivores are naturally scarce, for
example, on intertidal reef flats, in turbulent shallow water, or within the defended territories of pomacentrid
damselfish. Reef substrates that are protected experimentally from grazing rapidly become colonised by macroalgae, which in turn inhibit recruitment and growth of
corals. Previous experimental approaches to explore herbivore-algae-coral interactions have used a variety of