Terrestrial systems

16.4.1 Terrestrial systems

ing into watershed systems, the importance of the At first glance, the ecosystem consequences of role of salmon carcasses has prompted manage- fishing in the marine environment might seem to ment action to ensure their continued presence. have little relevance to ecosystem processes in ter- For example, timber debris on which salmon car- restrial ecosystems. However, there is a substan- casses become snagged is introduced into river tial literature that has examined the importance of systems to avoid loss of the carcasses through the contribution of nutrients and energy to aquatic wash-out, and salmon carcasses from fish farms and terrestrial ecosystems by salmon carcasses are used to supplement nutrient inputs in some (See also Persson, Chapter 15, Volume 1, for a dis- systems (Cederholm et al. 1999). cussion of the role of fishes in nutrient cycling within freshwater systems.). Pacific salmon (On- corhynchus spp.) migrate to sea where they grow

16.4.2 Energy subsidies from

to maturity before migrating back to their native rivers to spawn and in many cases die (Cederholm

discarding practices

et al. 1999). Thus Pacific salmon are important While some fishing techniques are highly specific, vectors of marine-derived nutrients that are such as pelagic fisheries and single hook and moved upstream. Energy subsidies, such as those line fisheries, others yield bycatches or incidental provided by salmon carcasses, are only likely to catches of species that are not utilized by the have a significant ecological contribution if they fishers. These bycatches are usually discarded and occur frequently enough and in large concentra- are composed of animals that are dead, dying or in- tions (Polis and Strong 1996). Mass spawning ag- jured (Kaiser and Spencer 1995). Bycatches may be

Chapter 16

discarded for legal reasons, in the case of commer- consumed by sea birds each year and this partly ex- cially valuable species that are undersized. Alter- plains the increases in the populations and distrib- natively, bycatches are discarded because there is ution of some species (Furness 1996; Camphuysen insufficient money to be gained by sorting or land- and Garthe 2000). Sea birds are very choosy, prefer- ing them. There are even instances when fishers ring to eat offal that gives the greatest energetic will discard commercial species that are greater reward in preference to flatfishes or invertebrates than the minimum landing size so as to enable that yield lower energy returns or that are more them to fill their quota with a larger average size of difficult to swallow. Relatively high consumption fish. This phenomenon is known as high-grading rates for these less-preferred discard types occurs (Pascoe 2000). Global energy subsidies in the form in areas of the sea where the resource needs of the of bycatch material dumped back into the ocean sea birds at sea exceeds the amount of material dis- totals approximately 27 million tonnes per annum carded (Table 16.2). This suggests that competition which is almost equivalent to a quarter of the for fisheries discards is intense in some areas of the global annual landings of fish (Alverson et al. sea, but not in others. 1994). However, this figure does not account for

The northern fulmar (Fulmarus glacialis) is one those animals that are never landed on board ves- of the most common scavenging sea birds that sels but remain dead on the seabed. Thus, fishing are observed following fishing vessels in the North activities redirect energy in marine ecosystems Sea, and has shown the most spectacular increase by the production of carrion that then becomes in numbers (Fig. 16.3) and breeding range of any sea available to scavengers on the seabed and those bird found in the North Atlantic (Lloyd et al. 1991). that feed at the surface of the sea.

The expansion of the fulmar population has been attributed mainly to its success as an offal scav-

16.4.3 Changes in sea

enger, firstly associated with whalers in northern polar regions, and later in association with com-

bird populations

mercial fishing fleets. Field studies have ranked Anyone who has witnessed a fishing boat return- fulmars at the apex of the interspecific dominance ing to port will have observed the scrum of sea hierarchy, hence they are able to obtain the best birds that contest for the offal and discarded fish as quality food: offal or fish livers (Hudson and they are thrown overboard. In the North Sea alone, Furness 1988). While Tasker et al. (1987) conclud- approximately 235 000 tonnes of discarded fish are

ed that fishing activities were an important deter-

Table 16.2 The energetic requirements of sea birds at sea and the energetic content of available discards. (Source: from Camphuysen and Garthe 2000.)

Subregion of the Energetic

Potential shortage North Sea

Energetic equivalents

Index of

requirements of

required/available or surplus if scavengers

of discards and offal

discards were only ( ¥10 kJ)

6 ¥10 6 kJ)

discards

prey Northwestern

1.9 shortage Northeastern

0.7 surplus Central-western

1.4 shortage Central

0.5 surplus Central-eastern

0.2 surplus Southern

0.2 surplus Total

0.6 surplus

Ecosystem Effects of Fishing

All savenging sea birds

Fig. 16.3 Increases in the num-

bers of breeding pairs of a selection Kittiwake of scavenging sea birds breeding on

Gannet North Sea coasts. (Source: adapted from Camphuysen and Garthe

minant of fulmar distribution at sea, more recent Further investigations indicated that fulmars had studies, conducted at different scales, were less

a northerly, offshore distribution, over waters with conclusive and suggested that, although fulmars

a rather strong Atlantic influence and high con- were evidently attracted by fishing vessels, the centrations of zooplankton. This observation sug- spatial distribution of the main fisheries and gested that the distribution of zooplankton, their fulmars did not coincide as well as was expected natural prey, was a much more important parame- (Camphuysen et al. 1995; Stone et al. 1995). Also, ter that influences their distribution at sea. recent investigations have shown that the position

While offal and discards would appear to pre- in the dominance hierarchy at fishing vessels was sent sea birds with high-energy food that is easily not so high in other parts of the North Sea, as com- obtained, this does not always equate to higher re- pared with studies that had concentrated on the productive output as might be expected. Hamer et Shetland Islands (Camphuysen et al. 1993).

al. (1991) demonstrated that the chick growth In light of these findings, Camphuysen and index of great skuas on Foula (Shetland Islands) de- Garthe (1997) re-examined the distribution and clined considerably when more than half of the scavenging habits of northern fulmars in the prey delivered by the parents was comprised of North Sea using a large set of recently gathered discards and offal. When smaller proportions data. They calculated the availability of discards (20–30%) of offal and discards were included in

and offal per km 2 for each subregion in the North chick diets, growth rates were relatively high. Sea (Table 16.2). They found negative correla-

The insights into the dominance hierarchies tions for northern fulmar densities at sea versus that exist between different scavenging sea birds the availability of offal and all types of fisheries have important implications for fisheries man- wastes. There was no correlation between num- agers as they suggest that measures to reduce the bers of fishing vessels at sea and densities of amount of discards would affect the lower ranked northern fulmars at this scale, but the numbers sea bird species first. Increases in trawl-net mesh of fulmars observed at the stern of fishing vessels size would increase the mean length of discarded were highly correlated with the overall densities of fish and this in turn would lower the handling northern fulmars at sea. These results suggested efficiency of smaller scavenging sea birds that are strongly that commercial fisheries were not the less well adapted to consuming large fish (Furness, prime determinant of fulmar distribution at sea. Ensor and Hudson 1992). A total ban of discards

Chapter 16

and offal would lead to major changes in current sea bird diets. An overnight introduction of such a

(a)

ban would certainly increase the predation risk for smaller birds from larger predators such as great skuas (Camphuysen and Garthe 2000).