236 Fish as feed inputs for aquaculture – Practices, sustainability and implications
species are particularly vulnerable to this type of overfishing, as they are short-lived
(Lluch-Belda et al., 1989; Santos, Borges and Groom, 2001).
Beverton (1990) reviewed the collapse of stocks of small, short-lived pelagics by
examining the effect of fishing and natural extrinsic drivers. In four of the stocks
studied (Icelandic spring-spawning herring, Georges Bank herring, California sardine
and Pacific mackerel), the evidence indicated that the reproductive capability had fallen,
probably due to environmental conditions, but suggested that fishing accelerated the
collapse. Beverton (1990) concluded that although the likelihood of harvesting small
pelagic species to extinction was remote, a major population collapse may result
in subtle changes to the ecosystem that may change the biological structure of the
community.
Other researchers also consider that harvesting an entire industrial fish species to
extinction seems unlikely (Hutchings, 2000; Sadovy, 2001), but the treatment of stocks
as single, panmictic populations means that if there are relatively local and sedentary
stocks, overall catches could conceal community extirpation. This has implications for
instance, for the management of localized substocks such as the North Sea sand eel.Habitats
The pelagic gear and purse seines used to target many industrial fish species – such as
sprats, blue whiting and Peruvian anchovy – are deployed in the water column and
have minimal contact with the sea floor. Demersal otter trawls are used to catch some
species, such as sand eel and Norway pout, and these may have more of an impact on
the sea bed and benthos. The degree of impact depends on the targeted species and the
location, as specific gears will be used to target specific species, and the impact on the
sea floor will depend on both the substrate type and the physiology of the animals that
live there.
Typically in the sand-eel fishery, the trawl is kept close to the sea bed, which is
usually sandy (Wright, Jensen and Tuck, 2000), but actual contact is kept to a minimum.
The gear is also lighter than the other demersal trawls. The effect of this disturbance
on the more dynamic sand habitats is less significant than disturbance in areas of lower
energy such as muddy substrates and in deep water, as the level of natural disturbance
in the more dynamic areas is likely to be greater than that caused by fishing (Kaiser et
al., 1998).
Although the impact to the sea bed and benthos by each individual tow may be
less than with comparable demersal otter trawling operations, as the gears are lighter,
the way the fishery operates suggests that local impact on the sea bed and invertebrate
communities may be quite intense. This is because the same trawl path tends to be
fished repeatedly over a period of several days by several boats operating in any
particular region (Frid et al., 2003). Mitigating against this, however, is the fact that
these fisheries are seasonal. The local impact may be intense, but it is followed by long
periods of recovery. The fishery for Norway pout occurs primarily through the winter
months, with little fishing during the summer, which allows six to eight months of the
year for the benthos to recover. The sand-eel fishery is constrained by the hibernation
of the species in winter.4.1.2 Indirect effects of fishing
There are a number of indirect effects of fishing feed-fish stocks, largely due to their
foraging low in the food chain and, therefore, being preyed upon by fish, marine
mammals and seabirds of higher trophic levels. Changes to specific predator-prey
relationships may impact the whole food chain and lead to changes in the composition
of biological communities (Greenstreet and Hall, 1996; Rijnsdorp et al., 1996; Bianchi
et al., 2000; Hill et al., 2001). Removal of a species’ biomass reduces the buffering
capacity of the stock and makes the population more vulnerable to poor prey