Biology Reference
In-Depth Information
21.4 Overfishing
Drastic changes in consumer abundance due to the overharvesting of commercially
and recreationally important fish, mammal, and invertebrate species have occurred
in many marine ecosystems throughout history (Jackson et al.
2001
). A recent study
shows approximately 90% of predatory fish stocks have been removed by fishing
fleets worldwide (Myers and Worm
2003
) with cascading effects down to lower
trophic levels (Hughes
1994
; Jackson et al.
2001
; Mumby et al.
2006
). Here, we
focus on the role overfishing plays in the occurrence of macroalgal blooms in
marine coastal waters. A classification of the different type of seaweed grazers
and an overview of algal-herbivore interactions can be found in detail in Chap.
8
by Iken.
Most cases where overfishing has directly impacted macroalgal growth occur in
coral reef and kelp ecosystems. This is due primarily to the number of relatively
larger and ecologically important consumers that have been removed from these
ecosystems compared to others (Jackson et al.
2001
). Overharvesting of herbivo-
rous fish, marine mammals, and invertebrates that directly graze on macroalgae
have led to increased occurrence of macroalgal blooms in coral reefs (Hughes
1994
;
Jackson et al.
2001
; Mumby et al.
2006
). The opposite trend has occurred in kelp
forests, where the removal of large consumers has often led to increases in inverte-
brate grazer abundance which then reduce macroalgal cover in these systems
(Estes and Duggins
1995
; Jackson et al.
2001
).
It has been argued that shifts in benthic and pelagic communities due to
overfishing long predated coastal eutrophication problems in marine systems
(Jackson et al.
2001
; Lotze and Milewski
2004
; Heck and Valentine
2007
).
A review paper by Heck and Valentine (
2007
) stressed the importance of
overfishing in regulating ecosystem structure by indirectly leading to altered food
webs. They support that many of the negative effects on coastal ecosystems
attributed to coastal eutrophication may actually be confounded by indirect effects
of historical changes in consumer abundances. They state that most manipulative
experiments that test the importance of nutrients and consumers on benthic
ecosystems today are doing so in ecosystems that have already been long devoid
of apex predators and many other consumers lower in the food web. Another study
(Eriksson et al.
2009
) showed that declines in predatory fish were promoting
blooms of macroalgae in the Baltic Sea through cascading effects down the food
web. Through surveys they showed that macroalgal cover was highest when
abundances of large piscivorous fish were low, which led to higher abundances of
smaller fish that feed on invertebrate grazers. Further manipulative experiments
supported the notion that these predators when abundant exerted a strong top-down
forcing on the food web (Eriksson et al.
2009
).
