Human Problems (from a shark’s viewpoint) 191
When all fishes are surveyed for extinction risk, it becomes obvious that
freshwater fishes have suffered more extinctions and population depletions
than marine fishes. Human impacts, especially pollution and habitat de-
struction, have been more serious in freshwater because we use rivers and
lakes as waste disposal grounds, withdraw water for consumption and ir-
rigation, erect dams, dredge channels, and fish heavily. Not surprisingly,
fresh- and brackish-water elasmobranchs as a group are disproportionately
affected by human activities, putting many on endangered species lists. Of
the 47 elasmobranch species that depend on freshwater or estuary habi-
tats, 18 (38%) have been assigned to high-risk critically endangered, en-
dangered, or vulnerable categories by the IUCN.
Will sharks be affected by global warming?
Global warming is just one aspect of global climate change. Climate
change could influence many of the environmental factors that determine
where sharks live. These factors include temperature extremes, oxygen
availability, floods, droughts, major storms, and habitat loss. Some of these
may have direct effects on sharks; others may affect the distribution and
abundance of their prey, which will in turn impact sharks.
Global warming is likely to lead to alterations in wind direction and
strength, which will, in turn, alter ocean currents. Ocean currents deter-
mine the timing and locales of spawning of ocean fishes, the dispersal of
their larvae, and the distribution of adults. Altered currents could affect the
distribution and production of open-ocean species that make up 70% of
the world’s fisheries, including the prey of open-ocean sharks. In the North
Atlantic, where bottom temperatures increased only 1°C between 1977
and 2001, 24 of 36 fish species moved northward or deeper toward cooler
waters, indicating that shifts in species distributions have already occurred.
Because sharks follow their prey, distributional shifts in prey species could
affect shark distributions.
Increased temperatures are also a threat because fishes often live close
to the maximum temperatures they can tolerate, and because oxygen con-
centrations in water are reduced at higher temperatures at the same time
that metabolic rates increase. Coldwater sharks may be squeezed towards
higher latitudes to avoid warming ocean temperatures. Also, many pollut-
ants are more toxic at higher temperatures. Corals are also very sensitive
to slight increases in temperature, as was seen in 1998 when an increase of
only 1°C in average temperatures killed 50% to 100% of the corals in some
areas of the tropical Pacific. Many coral reef fishes are directly and indi-
rectly dependent on live corals, and these fishes declined in many regions.
Again, food availability for reef sharks could be an issue.