26 Fish as feed inputs for aquaculture – Practices, sustainability and implications
food webs, and changes in the abundance and size composition of populations are
likely to lead to changes in the quantity and type of prey consumed (Frid et al., 1999).
However, these changes may not be predicted by simplistic models of predator-prey
interactions, as models do not account for prey switching, ontogenetic shifts in diet,
cannibalism or the diversity of species in marine ecosystems (Jennings and Kaiser, 1998;
Jennings, Kaiser and Reynolds, 2001).
Ecological dependence takes account of the ecological linkages in the marine systems.
However, assessing ecological dependence is problematic, as evidence for the effects of
strong ecological interactions on some stocks should not be taken as evidence that the
effects are necessarily a concern to managers of all stocks. ICES (2003b) suggested
that the current approaches for assessing ecological dependence could not be widely
applied and that fundamental research is needed to develop an appropriate method for
assessing and ranking the strength of ecological dependence of species.Commercial species as predators of feed-fish species
Feedfish tend to feed at or near the bottom of the food chain, so fisheries interactions
with the marine food web are more likely to affect their predators. Gislason (1994)
reported that the sand-eel and Norway pout fisheries of the North Sea took in about
20 percent of the annual production of these fish species. The consumption of sand
eels in the North Sea by fish that are targeted for human consumption, seabirds and
“other species” (including some fish species and marine mammals) has been estimated
as 1.9, 0.2 and 0.3 million tonnes, respectively (ICES, 1997). Bax (1991) reviewed the
fish biomass flow to fish, fisheries and marine mammals using a variety of data sets in
the Benguela system, on Georges Bank, in Balsfjorden, the East Bering Sea, the North
Sea and the Barents Sea and calculated that consumption of fish by predatory
fish was 5–56 tonnes/km^2 compared with fisheries (of all types), which caught
1.4–6.1 tonnes/km^2 ; marine mammals, which consumed 0–5.4 tonnes/km^2 and seabirds,
which consumed 0–2 tonnes/km^2. Fish predation on feedfish is, therefore, considered
to be higher than industrial fisheries’ removals, and this is especially true for the
sand-eel fisheries.
If small pelagic industrial feed-fish species have become more dominant in marine
systems as a result of a decline in demersal fish predators (commercial species) due
to fishing, then there is an argument for management to allow larger harvests of
industrial feed-fish species due to the reduced natural predation pressure on these
stocks. However, Naylor et al. (2000) argued that in the North Sea, exploitation of the
industrial species such as sand eel and Norway pout is implicated in the decline of the
higher trophic predator cod. It has been suggested that a reduction in fishing effort on
industrial feed-fish stocks will benefit higher trophic predators (including gadoids )
(Dunn, 1998; Cury et al., 2000; Furness, 2002). ICES assessments of the Norway pout
stocks in ICES Sub-area IV and Division IIIa indicate that fishing mortality is lower
than natural mortality, and multispecies analyses have indicated that when F (fishing
mortality) is below M (natural mortality), the fisheries are not causing problems
for their predators on the population size of the stock. It further noted that locally
concentrated harvesting may cause local and temporary depletions of predators and,
therefore, harvesting should spread widely across large geographical areas.Feedfish as predators of commercial species
The survival of the early planktonic phases of the fish life cycle is essential for stock
recruitment (Blaxter, 1974; Chambers and Trippel, 1997; Horwood, Cushing and
Wyatt, 2000). Even small variations in the mortality rate between egg fertilization and
recruitment can have a profound effect on the subsequent adult abundance (Jennings,
Kaiser and Reynolds, 2001). Many industrial fish species prey on the eggs and larvae
of commercial fish. In the North Sea in Europe, sand eel, Norway pout and capelin