there are many cases of ∼10–40% residuals of the once unfished or lightly fished
stocks of large pelagic predators in the sea. It is expected from lake and terrestrial
ecology that these reductions must have had and continue to have, trophic cascade
effects. When hunters kill nearly all the coyotes, wolves, and carnivorous bears in a
temperate ecosystem, populations of large herbivores such as deer expand, overgraze
the vegetation, starve, and eventually die back. This has been well and repeatedly
observed. Stocking of large piscivorous fish in a lake without them can sharply reduce
populations of planktivorous fish. Zooplankton then increase and graze down the
phytoplankton, “cleaning” the water to improve the view (and sometimes the air
quality) for nearby luxury homes. Presumably, starvation then moves up the food
web, but eventually a new equilibrium becomes established. Such an induced cascade
is an anti-alga strategy that sometimes works.
(^) Exactly what effects the dramatic reductions of apex predators are having on
oceanic ecosystems is not particularly clear. The dramatic population shift of krill
predators in the Southern Ocean from over-exploited rorquals to seals and penguins,
mentioned above, is a top-down effect. Tuna fisheries, to continue that example,
mostly take larger, older age classes that eat a preponderance of smaller tuna-like
fishes (Scombridae) and squid. Those in turn eat small fish that primarily eat
zooplankton. This oceanic food web has at least one additional step in these levels
compared to adding or removing fish predators in a lake. In areas with lots of
nutrients the zooplankton reduction might increase populations of large phytoplankton
and lead to more rapid exhaustion of nutrients by them. By and large, however,
oceanographers did not gather detailed data about ocean biota during the draw-downs
of large predatory fish that would allow us to see the resulting cascades at lower
levels. There must have been and continue to be such effects.
(^) A widely cited (e.g. Baum & Worm 2009; Perry et al. 2010) case study by Shiomoto
et al. (1997) is a possible interaction of pink salmon (Oncorhynchus gorbuscha) with
zooplankton in the subarctic Pacific and Bering Sea. Pink salmon have a two-year life
cycle, and all life stages eat zooplankton. Stocks of pinks spawning in coastal rivers of
northeastern Asia alternate year-on-year between strong and weak year classes, and
the data seem to show that zooplankton from 1989 to 1994 (three cycles) were low
when pink salmon catches (CPUE) were high. Unfortunately, the salmon data came
from the central Bering Sea, while the zooplankton estimates were from south of the
Aleutians in the Alaska stream. Shiomoto et al. show data (also from the Alaska
stream) that perhaps show the cascade extended to chlorophyll concentration. At least
the oceanic migration patterns of pink stocks from Kamchatka to Anadyr Bay include
both areas.
(^) Strong evidence for top-down trophic effects comes from two studies by Frank et al.
(2005, 2006) of fisheries on eastern Canadian shelves. These ecosystems are not
strictly pelagic, but involve the interaction of demersal (near bottom, partly bottom-