Consumption of discards by seabirds in the North Sea
Final report
EC DG XIV research contract 92/3505 Camphuysen, C.J., B. Calvo, J. Durinck, K. Ensor, A Follestad, R.W. Furness, S. Garthe, G. Leaper, H. Skov, M.L. Tasker & C.J.N. Winter. 1995.
Abstract
Seabird distribution was studied by means of strip- transect counts. The attraction of fishing vessels for seabirds was investigated by means of counts of birds asssociated with fishing research vessels and commercial fishing vessels. Trawler numvers were assessed by systematic counts using radar equipment. Discard experiments, in which the fate of measured discarded fish was studied were designed to assess consumption rates. Discarding practices were described from direect observations, interviews with fishermen and others, and litterature.
Eight species of seabird utilised fishery waste on a large scale, at least during part of the year. Consumption rates by seabirds, which were higher in winter than in summer, ranged from 95% for offal (entrails of gutted fish), to 80% for roundfish, 20% for flatfish and 6% for benthic invertebrates. All length classes of discards which occur normally in commercial fisheries can be consumed by seabirds. The median length of experimentally discarded roundfish consumed by seabirds ranged from 15cm in a small species such as the Kittiwake to 25cm in Gannets and Great Skuas. Fulmars and Kittiwakes were specialised feeders on offal. Discards size selection by different species of seabirds overlapped, leading to severe inter-specific competition. Many discards were stolen from smaller birds by larger species, with the result that success rates of the smaller birds were usually lower. Around 1500 active commercial fishing vessels were estimated to occur in the North Sea. The distribution of different types of fishing vessels are described. For none of the scavenging seabirds could the spatial distribution be fully explained by the presence or absence of commercial fishing vessels.
The data obtained can be used to provide detailed advice when the potential effects of measures to reduce fishing effort in general or discards production in particular are evaluated. The effects of different measures can be predicted, using the information on feeding success rates, vulnerability to robbery and prey selection. Anticipated scientific papers resulting from these studies include the distribution of seabirds in the North Sea, the distribution of fisheries in the North Sea, inter- and intra-specific relationships of scavenging seabirds, the analysis of inter-observer variance, and quantified information on discards utilisation by scavenging seabirds.
Key words: Marine birds, census, diets, discards, ecological balance , feeding behaviour, feeding experiments, foos prferences, interspecific relationships, nature conservation, marine fisheries, fishing effort, fishing gear, fishery management, ANE, North Sea, Skagerrak, Kattegat
Summary for non-specialists
Ornithological observations were performed onboard fisheries research vessels engaged in the International Bottom Trawl Survey in February 1993, May, August and November 1994. The observations on board were designed to assess seasonal patterns in the spatial distribution of scavenging seabirds (birds which utilise fishery waste) in the North Sea and to study the attraction of fishing vessels for these birds. The selection and consumption of discards by seabirds was quantified during sessions of experimental discarding. Dominance hierarchies were studied to obtain information on the vulnerability to robbery for each species. The spatial distribution of fishing vessels in the North Sea and discard practices were investigated and the results used to analyse the attractiveness of different fisheries for seabirds, and to calculate the total number of active boats in different regions in the North Sea.Seasonal fluctuations in spatial distribution patterns and the attraction to fishing vessels for eight common species of seabirds (Fulmar, Gannet, Great Skua, and five species of gulls) are described. Total numbers of these seabirds in the North Sea were estimated to vary between 2 and 6.5 million individuals.
Total numbers of fishing vessels in the North Sea were estimated to vary between 800 and 1800 boats, although sampling was not very good in the coastal zone. Beamtrawlers and shrimpers were the most numerous type, followed by stern and otter trawlers, set net fishing vessels (or long liners), purse seiners, pair trawlers and anchor seiners. Up to several thousands of seabirds were observed in association with commercial fishing vessels. The highest numbers were seen associated with otter trawlers, relatively small numbers of seabirds were associated with purse seiners, beamtrawlers and anchor seiners.
Seabirds consumed 95% of the offal (intestines of gutted marketable fish), 80% of roundfish, 20% of flatfish and 6% of benthic invertebrates. The competition for fishery waste increased between summer and winter and from south to north. Larger scavenging seabirds tended to consume larger discards than those selected by smaller species and many discards were stolen from smaller birds by larger species, resulting in the success rates of the smaller birds tending to be lower than those of larger species. All size classes in the discards fraction of commercial fisheries in the North Sea, except some of the larger flatfish, can be utilised by seabirds. The total amount of discards and offal in the North Sea has been estimated as 70,000 t of offal, 120,000 t of roundfish, 200,000 t of flatfish and 180,000 t of benthic invertebrates. Between 2.5 and 3.5 million seabirds are potentially supported by fishery waste in the North sea.
A general reduction in fishing effort (and therefore of catch and discards) will probably lead to more intense competition and thus would therefore lead to a decline in foraging success of species of with the highest vulnerability to robbery. The effects of a reduction in discards and offal discharges would be greater under adverse conditions (prolonged storms, winter) than in summer. The development of commercial fisheries this century has provided an important extra source of food, as a result of which several species have apparently increased in numbers and spread over larger areas. If circumstances arise when discards and offal were no longer produced or discharged, and if these birds could not turn to alternative foods, these populations would be negatively affected. The results of this study may be used to help predict the possible effects of measures to reduce the amounts of discards directly or to reduce fishing effort for seabirds on a regional scale and on species level.
English Summary
Fisheries research vessels engaged in the International Bottom Trawl Survey in February 1993, May, August and November 1994, were joined by ornithologists from six institutions based around the North Sea. On-board observations were designed to assess seasonal patterns in the spatial distribution of scavenging seabirds in the North Sea and to study the attraction of fishing vessels for these birds. The selection and consumption of discards by seabirds was quantified during sessions of experimental discarding. Dominance hierarchies were studied to obtain information on the vulnerability to robbery for each species. The spatial distribution of fishing vessels in the North Sea and discard practices were investigated and the results were used to analyse the attractiveness of different fisheries for seabirds, and to calculate the total number of active boats in different regions in the North Sea.Seasonal fluctuations in spatial distribution patterns and the attraction to fishing vessels for eight common species of seabirds, including Fulmar Fulmarus glacialis, Gannet Morus bassanus, Great Skua Catharacta skua ,Common Gull Larus canus, Lesser Black-backed Gull Larus fuscus, Herring Gull Larus argentatus, Great Black-backed Gull Larus marinus , and kittwake Rissa tridactyla are described. Total numbers of these seabirds in the North Sea were estimated to range from 1.5-4.0 million individuals in February to 3.0-6.5 million individuals in November. Most abundant were Fulmars and Kittiwakes. Fulmars, Gannets and Kittiwakes were numerous all year round. Common Gulls, Herring Gulls and Great Black-backed Gulls were comparatively scarce in summer, whereas Great Skuas and Lesser Black-backed Gulls were scarce in winter. Generally the seasonal and spatial variations in relative abundance and species composition of seabirds at sea were reflected in the numbers and species composition of scavenging seabirds attracted to the fishing research vessels. A notable exception was the Kittiwake which appeared to be more readily attracted by fishing vessels in winter than in summer whereas numbers at sea peaked in summer.
Total numbers of (active, not on transit) fishing vessels in the North Sea were estimated to vary between 800 and 1800 boats. However, these numbers do not adequately assess fishing vessels working close to the coast due to poor sampling in this zone. Beamtrawlers and shrimpers were the most numerous type (28%), followed by stern or otter trawlers (27%), set net fishing vessels (or long liners; 11%), purse seiners (7%), pair trawlers (7%) and anchor seiners (2%). Beamtrawlers, shrimpers, set net fishing vessels and anchor seiners predominated in the southeastern half of the North Sea and these vessels were rare in the northwest. Purse seiners, pair trawlers, stern trawlers and otter trawlers were the most common types in the central, northern and northwestern North Sea.
Maximum numbers of individuals seabird species in association with commercial fishing vessels ranged from just over 20 Great Skuas to 3500 Fulmars. The highest total numbers of seabirds associated with commercial fishing vessels were found near otter trawlers and stern trawlers, relatively small numbers of seabirds were associated with purse seiners, beamtrawlers and anchor seiners. Great Black-backed Gulls, Herring Gulls and Lesser Black-backed Gulls (in summer) were the only species of which the overall distribution in the North Sea was clearly positively influenced by the presence of fishing vessels. The spatial and temporal variations in the overall distribution of Fulmar, Gannet, Great Skua and Kittiwake could not be explained by differences in fishing vessel abundance.
Generally, consumption rates of offal and discards were higher in February and November than in May and August. Overall consumption rates were estimated at: 95% for offal, 80% for roundfish, 20% for flatfish and 6% for benthic invertebrates. The competition for fishery waste increased between summer and winter and from south to north. In comparison to proportions of each species present, Fulmars and Kittiwakes obtained more offal than expected. For experimentally discarded gadids, feeding success of Fulmars and Kittiwakes was comparatively low and Gannets had very high success rates. Lesser Black-backed Gulls and Herring Gulls were the most important consumers of gadids in the southeastern North Sea. Of experimentally discarded clupeids, Fulmars obtained considerably less than their numerical abundance might predict. Gannets obtained significantly more clupeids than gulls in most seasons and subregions, but particularly in the northern and western parts of the North Sea. Kittiwakes, Lesser Black-backed Gulls and Herring Gulls were the most important consumers of clupeids in the southeastern North Sea.
Larger scavenging seabirds tended to consume larger discards than smaller seabirds. The median length of roundfish ranged from 14cm in Common Gulls to 25cm in Gannets and Great Skuas. The median width of flatfish consumed by seabirds ranged from 3.5cm in Fulmars and Kittiwakes to 6.5cm in Gannets and Great Black-backed Gulls. Many discards were stolen from smaller birds by larger species, resulting in the success rates of the former tending to be lower than those of the latter. Gannet, Great Black-backed Gull and Great Skua were consistently the species highest in rank in the dominance hierarchy, whereas Fulmar and Kittiwakes were most vulnerable to robbery. Small species such as Kittiwakes and Common Gulls usually avoided fights for discards. Great Black-backed Gulls and Great Skuas obtained most discards through Kleptoparasitism.
The most prominent types of fishing vessels in the North Sea are beamtrawlers, bottom trawlers, pelagic or midwater trawlers, pair trawlers, purse seiners, anchor seiners, Scottish seiners, long liners and small boats using gill nets or tangle nets. Demersal fisheries for gadids by trawl and seine, pelagic fisheries for clupeids and mackerel, and beamtrawl fisheries produce large amounts of discards and offal.
The total amount of discards in the North Sea has been estimated as 120,000 t of roundfish, 200,000 t flatfish and 180,000 t benthic invertebrates. Beamtrawl fisheries produce comparatively large amounts of discards of which a relatively small proportion (roundfish, smaller flatfish, offal) can be utilised by seabirds. Demersal and pelagic fisheries for roundfish produce comparatively small amounts of discards which can all be utilised by scavenging seabirds. The annual amount of offal produced in the North Sea fisheries (11% of the mass of roundfish and 6.5% of the mass of flatfish) was estimated at nearly 70,000 tonnes. Between 2.5 and 3.5 million seabirds are potentially supported by fishery waste in the North Sea.
A general reduction in fishing effort (and therefore of catch and discards) will probably lead to more intense competition and thus would therefore lead to a decline in foraging success of species of with the highest vulnerability to robbery. The effects of a reduction in discards and offal discharges would be greater under adverse conditions (prolonged storms, winter) than in summer. The development of commercial fisheries this century has provided an important extra source of food, as a result of which several species have apparently increased in numbers and spread over larger areas. If circumstances arise when discards and offal were no longer produced or discharged, and if these birds could not turn to alternative foods, these populations would be negatively affected. The results of this study may be used to help predict the possible effects of measures to reduce the amounts of discards directly or to reduce fishing effort for seabirds on a regional scale and on species level.
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