Main human uses of ocean areas and resources, impacts, and multiple scales of governance - Ocean Sustainability in the 21st Century - page 38

2.3.1 Fishing
The recent rapid increase in fishing effort (Watson et al ., 2013 ) has been accompanied by
increasing use of technologically advanced fishing vessels, including bottom trawlers. The
combined effects of the volume of fishing and the fishing gear applied have resulted in
a number of environmental impacts including (i) overfishing of fish stocks (Pauly et al .,
2002 ) ; (ii) destruction of fish habitats (Sainsbury et al ., 1993 ) ; (iii) the fishing down of mar-
ine food webs (Pauly et al ., 1998 ) ; (iv) ecological disruption; and (v) by-catch problems
(Alverson et al ., 1994 ) .
Overfishing may cause ecological disruption and impact the health of the oceans be-
cause, when commercially valuable species are overexploited, there are negative ecosystem
effects through impacts on non-targeted species. For instance, when top predators such as
large shark species are fished out they trigger trophic effects in the shark food chain, which
in turn leads to increasing numbers of species, such as rays, that are prey for large sharks.
The result of this is declining stocks of smaller fish and shellfish that are eaten by these spe-
cies, which then impacts the survival of marine mammals (e.g. Hansen, 1997 ) and the breed-
ing success of seabirds (e.g., Anker-Nilssen et al ., 1997 ) . It is suggested that fishing may
even eliminate trophic groups or keystone species thereby altering the overall community
structure of an ecosystem (Botsford et al ., 1997 ). Fishing usually results in unintentional
killing of untargeted marine life, which can have significant effects on marine ecosystems,
e.g. impacts on fish community structure by altering predator-prey relationships (e.g. Mehl,
1991 ) .