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

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

In a recent study, the majority of haddock and whiting in the bycatch of the industrial

fisheries of Denmark and Norway were of age 3 or less (ICES, 2003c). The mortality

of haddock caught as bycatch by the industrial fisheries was small for age groups 0

and 1 (less than 1 percent by number and weight), while the mortality percentages

of older fish aged 2 and 3 were more varied. The percentages of whiting caught were

generally higher. However, the mortality due to industrial fishing was considered small

in comparison with the total estimated survivors for the year classes and considering

that the natural mortality of haddock and whiting is very high.

Seabirds

Bycatch mortality: The methods for catching fish species depend on the behaviour of

the fish. Many fish species shoal, and small-mesh trawls and gillnets are used to capture

the shoaling fish. Many of the feed-fish fisheries use trawls, and birds are less likely to

be caught by this type of gear (Tasker et al., 2000). A study in the Baltic Sea assessing

the bycatch of common guillemot (Uria alga) indicated that a small unquantified

degree of mortality could be attributed to trawls, but the researchers did not identify

the trawls as specifically targeting an industrial fish species (Österblom, Fransson and

Olsson, 2002). Bycatch of birds is potentially an issue in the purse-seining for anchovy,

but the level of interaction is little researched (Majluf et al. 2002), and there are only

anecdotal reports of bycatch (S. Austermühle, Mundo Azul, personal communication,

2003).

Availability as prey: Seabirds are long-lived, producing few fledglings that only

breed if they survive several years, and normally have various mechanisms to overcome

periods of low food supply. Specialist seabirds, such as small, surface-feeding species

with energetically expensive foraging methods are the most vulnerable to local

depletion and (natural) variability in prey availability. The relationship between the

reproductive success in black-legged kittiwakes on Shetland and sand-eel abundance

has been proposed as an indicator of local sand eel availability in the North Sea (ICES,

2003c). Potential conflicts between fisheries and seabirds are likely to arise only on a

local or regional scale (Tasker et al., 2000). Industrial fisheries can affect seabirds by

reducing prey stock biomass, leading to declining recruitment or alterations in the food

web structure. Although seabirds consume only an insignificant proportion of North

TABLE 19

Landings and bycatch from four Danish North Sea industrial fisheries, 1998–2001 (average) and 2002

Catch species

composition

Landings of four industrial feed fisheries (thousand tonnes)

Sand eel Sprat Norway pout Blue whiting

1998–2001 2002 1998–2001 2002 1998–2001 2002 1998–2001 2002

Sand eel 564.3 622.1 6.1 4.1 0.0 0.0 0.1 0.0

Sprat 6.6 1.0 152.8 140.6 0.2 0.0 0.0 0.0

Norway pout 1.6 0.0 0.4 0.2 53.8 43.2 3.5 3.7

Blue whiting 1.4 0.7 0.0 0.0 2.6 4.7 31.1 21.1

Herring 2.6 1.6 11.2 16.6 1.8 3.2 0.8 0.2

Cod 0.2 0.1 0.0 0.0 0.1 0.0 0.0 0.0

Haddock 0.7 1.2 0.1 0.0 0.9 1.5 0.2 0.1

Whiting 1.8 1.5 1.4 2.5 1.3 1.7 0.1 0.1

Mackerel 0.4 0.4 0.4 0.7 0.1 0.0 0.1 0.0

Saithe 0.0 0.0 0.0 0.0 0.0 0.2 0.0 0.1

Other species 2.2 1.4 1.8 2.7 0.9 0.4 3.3 1.6

Total 581.8 630.0 174 .2 167.4 61.5 54.9 39.2 26.9

% bycatch 3.0 1.0 12.3 16.1 12.5 21.5 20.9 22.1

Source: Adapted from Frid et al. (2003)