Fish as feed inputs for aquaculture: practices, sustainability and implications

242 Fish as feed inputs for aquaculture – Practices, sustainability and implications

Sub-area IV and Division IIIa (ICES, 2003d) 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 scale of the stock. It further noted that locally concentrated

harvesting may cause local and temporary depletions of predators and, therefore,

harvesting should be spread widely across large geographical areas.

The ICES Multispecies Forecast Programme (MSFOR) (reported in Gislason and

Kirkegaard, 1998) predicted that if there was a 40 percent reduction in the industrial

fishing effort in the North Sea, the harvested yield of sand eel would decrease by

19 percent (compared with the prevailing situation), while the spawning stock biomass

would increase by more than 50 percent (Figure 8). The model predicted that reducing

the fishing mortality of industrial species, and hence increasing the sand-eel stock,

would only have a small effect on predatory species. Such modelling must always

be interpreted with caution, as models can only make predictions based on the data

available. For example, the overfishing of predatory fish may have perturbed the

marine system to such an extent that the recovery of these stocks is unlikely even if

there is a reduction of the fishing effort on sand eels (Beddington, 1984). The lack of

appropriate modelling frameworks for establishing the ecosystem effects of fisheries is

well recognized (Robinson and Frid, 2003). However, it appears that fishing mortality

due to the sand-eel and Norway pout feed fisheries is sufficiently low to ensure that

prey items are available to predatory fish.

Teleost 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. Sand eel, Norway pout and capelin consume fish eggs

and larvae (www.fishbase.org), and sprat and herring prey on cod eggs (Stokes, 1992;

Total

Sole

Plaice

Sand eel

N. pout

Sprat

H erring

Haddock

Mackerel

Saithe

Whiting

Cod

-2 0^0 20 40 60

Yield^

SSB

Source: Gislason and Kirkegaard (1998)

FIGURE 8

MSFOR predictions of the percentage change in yield and spawning stock biomass

(SSB) in the North Sea for 11 North Sea species upon a 40 percent reduction in

sand-eel fishing