282 CHAPTER 11 The Ocean and Fisheries
Hugh Rose/Danita Delimontall floaters or swimmers. The upper level of the pelagic
environment is the euphotic zone, which extends from
the surface to a maximum depth of 150 m (490 ft) in
the clearest open ocean water (see Figure 11.4). Suf-
ficient light penetrates the euphotic zone to support
photosynthesis.
Large numbers of phytoplankton (microscopic
algae) produce food by photosynthesis and are the base
of food webs. Zooplankton, including tiny crustaceans,
jellyfish, comb jellies, and the larvae of barnacles, sea ur-
chins, worms, and crabs, feed on phytoplankton in the
euphotic zone. Zooplankton are consumed by plankton-
eating nekton (any marine organism that swims freely),
such as herring, sardines, baleen whales, manta rays, and
squid (Figure 11.9). These in turn become prey for
carnivorous nekton such as sharks, tuna, porpoises, and
toothed whales. Nekton are mostly confined to the shal-
lower neritic waters (less than 60 m, or 195 ft, deep), near
their food.
EnviroDiscovery
Otters in Trouble
Sea otters play an important role in their environment. They
feed on sea urchins, thereby preventing the urchins from
eating kelp, which allows kelp forests to thrive. Now scientists
have uncovered an alarming decline in sea otter populations
in western Alaska’s Aleutian Islands—a stunning 90 percent
crash since 1990—that in turn poses wide-ranging threats to
the coastal ecosystem there. The population of sea urchins in
these areas is exploding, and kelp forests are being devastated.
Strong evidence identifies killer whales, or orcas, as the culprits.
Orcas generally feed on sea lions, seals, and fishes of all sizes.
Sea otters, the smallest marine mammal species, are more
like a snack to the orca than a desirable meal. So why are the
orcas now choosing sea otters? Biologists suggest that it is
because seal and sea lion populations have collapsed across
the north Pacific.
In a scenario that is partly documented and partly
speculative, the starting point of this disastrous chain of events
is a drop in fish stocks, possibly caused by overfishing or
climate change. With their food fish in decline, seal and sea lion
populations have suffered, and orcas have looked elsewhere
for food. Even the terrestrial food chain has been affected, as
bald eagles shift away from fish and baby otter prey where
otters are scarce. The change in the orcas’ feeding behavior has
transformed the food chain of kelp forests, with orcas disrupting
the otters’ role as predators. In 2009, the U.S. Fish and Wildlife
Service designated more than 15,000 km^2 (nearly 6000 mi^2 ) of
the Aleutian Islands as critical habitat for the threatened sea
otter. The areas are near shore and in kelp beds, where they
might offer protection from predators.iÀÌVÊ«ÀÛViÊUÊ}ÕÀiÊ££°Ê
The opalescent squid—here photographed at night—is abundant
in the eastern Pacific, particularly off the coast of California, where
individuals gather by the thousands to breed. These animals are
active predators of planktonic crustaceans and small fish.Otters in Alaskan waters© Travis VanDenBerg/Alamy