Overfishing has removed fish from marine ecosystems at astounding rates.
According to Richardson, this has made it possible for jellyfish to take
their place. This is because small fish appear to keep jellyfish in
check by predation (on jellyfish when they are very small) and
competition (when feeding). So, once we remove fish, jellyfish can
proliferate. Eutrophication is another human-caused change in the ocean
that has likely contributed to jellyfish explosions. Eutrophication is
an increase of nitrogen and phosphorus in the ocean, largely caused by
fertilizer and waste run-off. This leads to algae blooms, which lower
oxygen in the marine ecosystem, creating so-called 'dead zones', which
have been increasing dramatically around the world. According to
Richardson, these low-oxygen waters give jellyfish the advantage. 'Fish
avoid low-oxygen water but jellyfish, having lower oxygen demands, not
only survive but can thrive in these conditions as there is less
predation and competition from fish.'
1. Which **TWO** of the following are possible causes of an increase in jellyfish numbers?
请选择 2 个答案
A. A shortage of small fish in the oceans.
B. The dumping of chemicals into the oceans.
C. A decline in biodiversity in the oceans.
D. More competition among other fish in the oceans.