Wild fish and other aquatic organisms as feed in aquaculture in Europe 241
necessarily a concern to managers of all stocks. ICES (2003c) 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 the
region of 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 and 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 removed
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 teleost feedfish, is, therefore
considered to be higher than industrial fisheries removals, and this is especially true in
the sand-eel fisheries.
The ICES stomach sampling project in 1981 showed that sand eel, Norway pout
and sprat provided more than 50 percent of the food of saithe and whiting and
between 1 and 30 percent of the food of cod, mackerel and haddock (Gislason, 1994).
Greenstreet (1996) investigated the diet composition of the main predators in the
North Sea; Table 20, which gives the consumption of industrial species, shows that
industrial or feed-fish species are a valuable food resource for predatory fish.
However, while bioenergetic estimates of sand-eel consumption in the North Sea
show that fish are important predators, predation on sand eels is declining (Furness,
2002), as stocks of large gadoid predators are weak and their spawning stock biomass
is declining (Sparholt, Larsen and Nielsen, 2002). Sparholt, Larsen and Nielsen (2002)
tested the hypothesis that a reduction in consumption of industrial fish by gadoids
such as cod, whiting and saithe should lead to a measurable reduction in the predation
mortality of their prey (Norway pout) and found the total mortality of Norway pout
for ages 1 and 2 had declined between the 1980s and 2000.
If small pelagic species have become more dominant in marine systems, resulting
from a decline in demersal fish predators due to fishing, then there is an argument for
management to allow larger harvests of industrial species due to the reduced natural
predation pressure on these stocks. However, Naylor et al. (2000) argued that in the
North Sea, exploitation of sand eel and Norway pout is implicated in the decline of
cod. It has been suggested that a reduction in fishing effort on industrial fish stocks
will benefit higher trophic predators (including gadoids) (Dunn, 1998; Cury et al.,
2000; Furness, 2002). The more recent assessments of the Norway pout stocks in ICES
TABLE 20
Diet composition (%) of the main predators in the North Sea
Prey Predator
Cod Haddock Whiting Saithe Mackerel Atlantic horse mackerel
Norway pout 7.7 6.3 8.9 32.2 7.3 0.0
Herring 4.1 0.1 6.6 0.6 3.7 8.8
Sprat 2.1 0.3 9.4 0.4 3.2 0.4
Sand eel 7.3 7.2 27.3 9.7 16.6 0.0
Source:^ Recalculated from Greenstreet (1996)