THE USE OF FISHERY WASTE BY GULL POPULATIONS AROUND THE BRITISH ISLES
R.W. FURNESS, K. ENSOR & A.V. HUDSON
Applied Ornithology Unit, Department of Zoology, University of Glasgow, Glasgow G12 8QQ, United Kingdom.
INTRODUCTION
The enormous growth in numbers of Herring Gulls Larus argentatus in western Europe since 1900 has often been attributed to a large extent to extra food provided by man in the form of offal and discarded fish from fishing boats and edible waste at refuse dumps (Cramp et al. 1974). The first detailed study to test this idea (Spaans 1971) showed that although adult Herring Gulls in the northern Netherlands fed predominantly on intertidal invertebrates during the breeding season, chicks were fed mainly on fish. About one-fifth to one-third of the food of chicks was obtained as fishery discards, and Spaans argued that this was likely to be important in determining breeding success of the population.In Shetland, United Kingdom, most seabirds appear to feed their chicks by preference on sandeels Ammodyles marinus when these are available (Furness & Hislop 1981, Heubeck 1989). However, adult Gannets Sula bassana, Great Black-backed Gulls L. marinus and Great Skuas Stercorarius skua feed extensively on fish discards from trawlers and Fulmars Fulmarus glacialis take some discards and most of the offal discharged (Hudson & Furness 1989).
Quantitative studies of the exploitation of discards by seabirds at Shetland indicated that competition for discards is severe in summer and may determine scavenging seabird population sizes (Furness et al. 1988, Hudson & Furness 1988). Bergman (1982) also invoked changes in discarding by fisheries to explain the increase in numbers of Lesser Black-backed Gulls L. fuscus early in the twentieth century in Finland and argued that reductions in the fishery and the increase of Herring Gulls due to increased availability of garbage caused the subsequent decline in Lesser Black-backed Gull numbers. Changes in seabird community composition have also been attributed to increased fishery waste off southern Africa (Abrams 1985) and in Queensland, Australia (Blaber & Wassenberg 1989).
Many seabirds, especially gulls, associate with fishing boats all around the British Isles. In this paper we consider the quantities of waste made available to seabirds by fishing boats around the British Isles, seasonal patterns of exploitation of discards and offal by gulls, influences of interspecific competition, and evidence from pellets and regurgitates indicating the importance of discards for breeding gulls.
METHODS
Quantities of fish caught in defined sea areas around the British Isles are reported each year by the International Council for the Exploration of the Seas (ICES) in 'Bulletins Statistique des Peches Maritimes'. From these figures it is possible to derive estimates of quantities of offal and discards produced, from published data on discard rates and fish composition, and from observations of fishermen working at sea.Numerous trips on fishing boats in Shetland and in the Clyde, west Scotland, were made by AVH and KE during which they recorded the amounts of fish and shellfish caught, the amounts of offal discharged and whole fish discarded, and time spent by the crew in each fishing activity. Numbers of seabirds at the boat were counted and identified into plumage classes, at intervals of about 30 minutes, and samples of the fish being discarded were identified, measured and then experimentally discarded and observed.
Experimental discarding was done as the boat trawled for the next catch and while the fishermen were themselves sorting and discarding the previous haul. For each experimental discard fish we recorded among other things whether it sank, whether it was picked up by a bird, and if so, the species and age-class of bird, whether it was dropped or stolen, and in the last case, the species and age-class of the kleptoparasite.
To compliment the data on foraging at sea, food samples regurgitated by gull chicks and pellets regurgitated by adult gulls were collected at colonies to allow diet composition to be assessed. Identification and measurement of otoliths from pellets allowed us to identify the species and size classes of fish consumed. Fish livers and intestines (offal) cannot be detected from pellet analyses, but observations from boats showed that gulls rarely obtained offal because this was almost all taken by Fulmars.
RESULTS
Availability of fish wasteBoats catching herring Clupea harengus, mackerel Scomber scomhrus, sandeels, or other industrial fish (such as Norway pout Trisopterus esmarkii) normally provide little food for seabirds other than fish stomach contents and small numbers of small fish forced out of the net as it is lifted from the water. Factory ships processing pelagic fish may discharge offal and scraps that attract Fulmars, Kittiwakes and Larus gulls, but many of these retain such material for industrial use.
Boats trawling for Norway lobster Nephrops norvegicus discard large quantities of fish, generally 5-25 cm in length, that are predominantly flatfish, whiting Merlangius merlangus, Norway pout, or poor cod Trisopterus minutus (Watson 1981). The discarding rate (mass of fish) was about twice the mass of Nephrops caught for boats in the Clyde and in the Hebrides (Furness et al. 1988).
Boats catching whitefish in Shetland (predominantly the gadoids haddock Melanogrammus aeglefinus and whiting) were estimated to discard 27% of the mass of whitefish caught in 151 catches in 1984 and 1985 (Hudson 1986). Jermyn & Robb (1981) estimated that about 20-33% of haddock and whiting catch and 1-10% of cod Gadus morhua catch was discarded by Scottish fishing boats in the North Sea in the late 1970s. Discards from whitefish boats tend to be much larger in size than those from nephrops trawlers. In Shetland, the median lengths of haddock and whiting discards (which represented 70-80% of all discards) were 27 to 29 cm in 1985 and 1987, with ranges from 15- 34cm, although few discards were less than 20 cm (Hudson 1986, RWF).
Offal mass represents about 10-15% of body mass of gadoid fish and 6-7% of flatfish mass (Boswall l960, Bailey & Hislop l978, Furness et al. 1988). The mass of offal that may be available to seabirds can be estimated by multiplying the data on catches in fisheries areas by the above figures for offal mass as a proportion of fish mass. However, a small part of the catch of demersal fish will be landed ungutted, and it is becoming increasingly common for offal to be retained for industrial use rather than discharged at sea. Small fishing boats without shelter decks tend to discharge a steady trickle of offal as fish are cleaned, but larger boats (which have come to form an increasing proportion of the fleet) tend to accumulate offal and hose large quantities overboard after fish processing. In this case some offal may sink before seabirds can reach it.
Assuming that most offal is discharged and based on fish catch data for 1982 (as a typical year) the amounts potentially available to seabirds over a year are large: 40,000 tonnes in the northern North Sea (ICES area 4a), 34,000 tonnes in central North Sea (ICES 4b), 10,000 tonnes in northwest Scotland (ICES 6a), 9700 tonnes in southern North Sea (ICES 4c), and rather smaller amounts around south and west England and Ireland (ICES 7a-k). Similar calculations to estimate the amounts of fish discarded on the basis of whitefish and Nephrops catch statistics and observed discard rates, indicate that whitefish discards may amount to 44,000 tonnes in the northern North Sea, 39,000 tonnes in central North Sea, 31,000 tonnes in northwest Scotland 21,000 tonnes in the Irish Sea and rather smaller amounts further south and west. In the Irish Sea, most discards come from Nephrops trawlers and so are of small fish. In north-west Scotland roughly similar amounts come from whitefish and Nephrops boats while in north, central and southern North Sea areas most discard mass derives from whitefish boats (Furness et al. 1988).
Given that fish offal has an energy value of about 7-13 kJ-g-l and discard gadoids a value of about 4-5 kJ-g-l (Harris & Hislop 1978, Hudson 1986) the energy value of offal and discards in all the sea areas around the British Isles could, in theory, sustain over 2 million seabirds (assuming the typical scavenging seabird weighs 1000 g and requires a food intake of three times basal metabolic rate throughout the year). There is relatively little seasonal variation in fish catch biomass, but it is evident that the amounts of offal and discards are highest in the northern North Sea, which is an area occupied by particularly high numbers of large seabirds (Tasker et al. 1987).
Seabirds at fishing boats
Around Shetland in summer, rather few seabirds attend trawlers while they are just towing the net, but numbers rapidly increase as the net is hauled up, and large flocks follow boats that are gutting and discarding (Hudson & Furness 1989: Table 1). Once the catch has been processed the numbers of birds following the boat fall dramatically. Some 3776 experimentally discarded fish were swallowed by seabirds and the proportions taken by each species indicated great differences in foraging success between species. It is clear that Gannets obtained far more of the discards than predicted on the basis of their relative numerical abundance, whereas Fulmars obtained very few (Table l).
Table 1. Foraging success index (experimentally discarded fish swallowed per bird of each species present, on average, over all fishing trips) for different seabirds feeding on fish discarded from trawling fishing boats in Shetland during summers of 1984-85.
Species Mean number Total of Success
at boat fish swallowed index
_____________________________________________________________________________________
Gannet 9 452 50
Great Skua 12 347 29
Great Black-backed Gull 234 2753 12
Lesser Black-backed Gull 6 32 5.3
Herring Gull 30 107 3.6
Fulmar 485 85 0.2
Kittiwake 3 0 0.0
_____________________________________________________________________________________
Herring Gulls had a slightly but significantly (x2 = 4df = 1, P < 0.05) lower success index than Lesser Black-backed Gulls and both did less well than Great Black-backed Gulls or Great Skuas. In part, this was due to Herring Gulls and Lesser Black-backed Gulls dropping 35% of fish they handled, whereas Great Black-backed Gulls dropped only 5 % . However, Great Skuas dropped 16% of the discards they obtained. Their higher success index was largely due to their aggressiveness and consequent ability to displace all birds except Fulmars from the best positions behind boats. Herring Gulls and Lesser Black-backed Gulls tended to remain on the periphery of flocks at trawlers, apparently unable to compete with Great Black-backed Gulls, Gannets or Great Skuas, which took central positions close to where discards were dropped. Kittiwakes rarely attempted to join seabird flocks at fishing boats in Shetland, and obtained no experimental discards.
In most cases Fulmars obtained almost all offal discharged, being so numerous and aggresslve that other seabirds avoided them. On rare occasions when Fulmar numbers were small, Herring Gulls were the most successful of the other seabirds in taking offal (Hudson & Furness 1989).
Statutory returns to the Department of Agriculture and Fisheries for Scotland (DAFS) give information on landings of demersal fish, shellfish, hours of fishing and number of boats landing for each of the ports in the Clyde Sea area each month. By assuming discard mass to be twice the landed mass of shellfish and 25% of whitefish catch (but 15% in March and April when the catch is predominantly of cod) the discard mass each month can be estimated (Fig. 1).
Fishermen were discarding waste fish for 49% of the time (276 hours of observations during 41 fishing trips) between lifting of the first haul of the day and the end of the day's fishing. This allows the total number of birds of each species attending boats in the Clyde to be estimated by multiplying the mean number counted during discarding at boats each month by the mean number of boats fishing that would be discarding fish each day. The resulting estimated total numbers of seabirds (which, except in the case of Herring Gulls, are predominantly of birds in adult plumage) following boats can be related to local breeding numbers within the relatively self-contained Clyde Sea (Table 2).
Fig. 1. Estimated masses of fish (tonnes) discard each month from fishing boats in the Clyde Sea area over the years 1983-85 (source: DAFS Annual Reports, data converted using discard ratios discussed in the text), and total numbers of seabirds estimated to be associated with fishing boats in the Clyde Sea area each month over 1985-87 (see Table 2).
Clearly discards are important for Gannet when they come back to their breeding colony in the Clyde in February and March, but cease to be so in May-June, when Gannets take shoaling fish (Tasker et al. 1987). Numbers of Kittiwakes at boats in the Clyde are larger than found at boats in Shetland, but represent a small part of the local population in most months. Lesser Black-backed Gull numbers at boats in summer seem to be high in relation to local breeding numbers and suggest that discards are a more important food source for this species than for Herring Gulls.
Table 2. Numbers of boats trawling and discarding each day for each month (from DAFS data for years 1983-85), mean numbers of birds at each boat during discarding of fish (data from 300 counts made during 38 fishing trips between 25 September 1985 and 29 April 1987), estimated total numbers of seabirds at these boats in the Clyde Sea area each month in relation to the sizes of the known local breeding populations (breeding pairs) in the early 1980s (numbers in parentheses, sources: Monaghan & Zonfrillo 1986, B. Zonfrillo). The numbers of Herring Gulls (all ages) at 12 main refuse tips in and around Glasgow each month (Shedden 1983) are shown for comparison (GBG = Great Black-backed Gull, LBG = Lesser Black-backed Gull, Kit = Kittiwake, Ful = Fulmar, HG ad = Herring Gull adults, HG imm = Herring Gull immatures).
Month Boats Birds Numbers of birds at all boats Herring
at Gulls
boats at tips
Gannet GBG LBG Kit Ful HG ad HG imm
(21000) (360) (3600) (2000) (600) (7200)
Jan 23 345 90 650 90 70 0 4500 2450 8700
Feb 22 331 2950 155 140 75 0 2550 1400 11000
Mar 25 330 4800 30 30 80 40 2200 1000 11500
Apr 19 377 2600 40 140 400 30 1400 1050 5300
May 16 188 1250 0 900 20 45 530 85 3300
Jun 18 175 530 25 1150 65 0 1100 180 4800
Jul 18 259 85 70 1200 0 45 2350 750 4200
Aug 23 157 460 15 900 5 50 820 1180 5300
Sep 23 284 1000 110 180 165 95 1850 2900 8100
Oct 27 339 200 205 145 850 0 2200 5450 7600
Nov 35 282 0 520 350 160 0 4150 4900 7300
Dec 28 231 5 530 160 225 5 2700 2900 8300
Shedden (1983) counted numbers of Herring Gulls at 12 major refuse tips in and close to Glasgow, a study area adjacent to the Clyde Sea area (Table 2). Although numbers of Lesser Black-backed Gulls at boats in May and June exceeded numbers of adult Herring Gulls, Shedden (1983) found that Herring Gulls outnumbered Lesser Black-backed Gulls by more than two to one at refuse tips, supporting the idea that Lesser Black-backed Gulls have a more marine feeding habit than Herring Gulls.
Foraging interactions between species
Seasonal changes in numbers of Herring Gulls in the Clyde differ in detail from those recorded by Shedden (1983) on tips. Herring Gull numbers appear to be depressed in the Clyde in those months during which Gannets attend fishing boats in largest numbers (Fig. 2), and during the six months of smallest Gannet numbers the ratio of Herring Gull numbers on the Clyde to numbers on tips was higher than in the six months of largest Gannet numbers (Mann-Whitney U66 =2, P < 0.01). Herring Gulls appear to move away from the Clyde Sea in the face of increased competition with Gannets. The foraging success index of adult Herring Gulls is more than halved by the presence of large numbers of Gannets at boats (Fig. 3). Similarly, the success index of immature Herring Gulls is much less in the presence of large numbers of Gannets, and consistently less than that of adults (Fig. 3). Lesser Black-backed Gulls achieved a success index greater than that of adult Herring Gulls during all six summer months. By contrast, Kittiwakes had a very low success index, which was usually below that of any of the Larus gulls.
Fig. 2. Total numbers of Herring Gulls (all ages) estimated to be feeding at fishing boats in the Clyde Sea area each month (closed circles: more than 500 Gannets present at boats, open circles: less than 500 Gannets present at boats) plotted against total numbers counted at 12 refuse tips in and around Glasgow for the same calendar month (from Shedden l983). The line is a linear regression of numbers at boats (NB) on numbers at tips (NT) for all 12 months (NB = 0.465 NT + 911, r = 0.46, P < 0.05).
Fig.3. Mean feeding success index (percentage of experimentally discarded fish below 30 cm in length that were swallowed by Herring Gulls of each age category divided by the percentage of all birds at the boat that were Herring Gulls of the same age category) each month for adult (closed circles) and immature (open circles) Herring Gulls in relation to the mean number of Gannets at each boat in the Clyde Sea area during that month.
Table 3. Contents of pellets (expressed as % by numbers) regurgitated by gulls at breeding colonies in summer, with each pellet assigned to the prey type of which it was predominantly or entirely composed (HG = Herring Gull, LBG = Lesser Black-backed Gull, GBG = Great Black- backed Gull).
Prey item Rhum 1985 Clyde Forth Shetland
1983 1979-81 1983-85
HG LBG GBG HG HG HG
Whitefish discards 14 42 66 18 52 6
Nephrops norvegicus 3 9 25 0 18 0
Intertidal invertebrates 80 47 5 77 9 91
Shoaling fish 0 0 0 0 1 0
Terrestrial invertebrates 0 0 0 2 7 0
Refuse 1 2 0 3 12 1
Rat 1 0 3 0 0 0
Bird 0 0 2 0 0 1
Number of pellets 268 45 61 93 488 10182
Pellets and regurgitates
Pellets regurgitated by adult Herring Gulls at colonies in Shetland in summers 1983-85 consisted almost entirely of intertidal invertebrates (Table 3). However, as previously found by Spaans (1971) Herring Gull chicks were fed extensively on discarded whitefish. Of 211 food samples regurgitated by Herring Gull chicks in Shetland in 1983-85, 48% consisted of whitefish, 21% of sandeels, 11% of terrestrial invertebrates, 11% of food remains from refuse tips, 4% of intertidal invertebrates, 2% of bird meat and 2% of other items. There was also evidence of a seasonal change in diet, with a higher proportion of regurgitates comprising sandeels before 25 June than after and with the predominance of whitefish greater in July (Hudson 1986). Great Black-backed Gulls nesting in Shetland feed on a wide variety of items, including whitefish discards, intertidal invertebrates, rabbits, seabirds, eggs and sandeels. Pellets of whitefish discards represented 31% of a sample of 640 from Noss in 985 (Hudson 1986). Non-breeding Great Black-backed Gulls loafing on headlands on Foula took some sandeels but fed primarily on discards and have fed almost exclusively on discards since 1985 when sandeel availability declined (Furness in Heubeck 1989).
Herring, Lesser Black-backed and Great Black-backed Gulls at Rhum, west Scotland, show differences in diet, as indicated by pellets, with Great Black-backed Gulls taking most discard whitefish and Herring Gulls least (Table 3). Herring Gulls at colonies in the Clyde also appear, on the basis of pellets, to feed largely on intertidal invertebrates in summer (Table 3).
DISCUSSION
Discards and offal provide a large quantity of food for seabirds. At present, Fulmars, Gannets and Great Black-backed Gulls exploit this food source particularly successfully. Discard rates in the period l900-70 are not known, but rates recorded in the 1970s (Jermyn & Robb 1981) appear very similar to those in the 1980s (Hudson 1986, RWF). Total catch of demersal fish in the North Sea remained around 0.5 million tonnes from 1910 to 1960 and then more than doubled during the 1960s and 1970s. The increase was largely attributed to reduced predation on whitefish larvae by depleted stocks of herring and mackerel (Hempel 1978). Demersal fish landings from the North Sea have declined somewhat in the 1980s following a decline in stocks of most whitefish from the 1970s peak. Thus the amounts of offal and discards in the North Sea may have peaked in the 1970s but were lower in the 1980s. However, numbers of Fulmars, Gannets and Great Skuas in the northern North Sea have increased greatly over this century, with by far the largest absolute increases between 1970 and the present (Tasker et al. 1987). This would imply that the species that now have difficulty in competing for offal and discards in that area (Herring Gull, Lesser Black-backed Gull and Kittiwake) may have found it much easier to obtain this food before the 1980s. Future changes are likely to accentuate his trend. Not only are numbers of Fulmars, Gannets and Great Skuas at Scottish breeding colonies continuing to increase, but reductions in discharging of offal and discards are likely to occur as more use is made of these for industrial purposes and net mesh design and size is altered to conserve whitefish stocks. Such changes may be expected to lead to Herring Gulls, Lesser Black-backed Gulls and Kittiwakes having to rely less on fishing boats and if alternative foods are not available then the numbers of these species may decline. Given the extensive use of discards by Herring Gulls to feed chicks, reductions in breeding success may also be expected.Further south, the seabird species composition changes. There are fewer Fulmars, Great Skuas and Gannets in the southern North Sea (Tasker et al. 1987) and so the Lesser Black-backed Gull, Herring Gull and Kittiwake can probably make more use of fish discards there. Kittiwakes seem to be particularly numerous at fishing boats in the south central North Sea in winter (M. Leopold, M.L. Tasker). It would be useful to make counts of scavenging birds at fishing boats in these areas to allow any future changes due to changing fishing practices to be recorded.
Several studies have compared the feeding success of different gull sexes, age-classes or species in flocks at refuse tips (Monaghan 1980, Burger 1981, Greig et al. 1983, 1985). Flock feeding by gulls has also received considerable attention at sea (Porter & Sealy 1981, 1982, Braune & Gask 1982a, 1982b, Götmark et al. 1986, Hudson 1986 Hudson & Furness 1988,1989). Apart from a clear and usually pronounced, tendency for adults to have a higher feeding success than immatures, the studies at tips and at sea show some interesting contrasts. Herring Gulls dominate Lesser Black-backed Gulls and Great Skuas on land, but are less successful than both these species in competing for discards at fishing boats (see also Noordhuis & Spaans 1992). The success of Herring Gulls seems to arise when they can form dense flocks of their own species; other gulls and skuas appear to prefer to feed in less dense groupings.
Studies of Herring Gull activity budgets at refuse tips, and seasonal changes in weight (Coulson et al. 1983) suggest that Herring Gulls have little difficulty in meeting their energy demands in winter, but may encounter a critical period of food shortage in the late chick-rearing period. The counts of Herring Gull numbers at fishing boats in the Clyde show a peak in July after rather small numbers using discards as food in May and June (Table 2). This might suggest that Herring Gulls are able to find other foods in the early summer but have an increased need for discards to rear chicks successfully and to survive through the later part of the summer. Watson ( 1981 ) counted largest numbers of Herring Gulls at fishing boats (in the Irish Sea) in July, and Tasker et al. (1987) recorded that Fulmars and Herring Gulls in the North Sea showed a closer correlation with fishing boat distribution in July than in earlier months. Although fishery waste is clearly a large food supply for scavenging seabirds, geographical variations in its use by gulls are not well known and deserve further study in these times of continued pressure for better conservation of fish stocks through measures that would reduce the need for wasteful discarding of fish.
ACKNOWLEDGEMENTS
This study was supported by grants from the Natural Environment Research Council and the Nature Conservancy Council. We are most grateful to the skippers and crews of the many fishing boats that allowed fieldwork to be carried out during their normal working trips. Their hospitality and tolerance were quite outstanding.REFERENCES
Abrams, R.W. 1985. Pelagic seabird community structure in the Southern Benguela region: changes in response to man's activities? Biol. Conserv. 32:33-49. Bailey, R.S. & J.R.G. Hislop 1978. The effects of fisheries on seabirds in the northeast Atlantic. Ibis 120: 104-105. Bergman, G. 1982. Population dynamics, colony formation and competition in Larus argentatus, fuscus and marinus in the archipelago of Finland. Ann. zool. fennici 19:143-164. Blaber, S .J .M. & T.J. Wassenberg 1989. Feeding ecology of the piscivorous birds Phalacrocorax varius, P. melanoleucos and Sterna bergii in Moreton Bay, Australia: diets and dependence on trawler discards. Marine Biol. 101:1-10. Boswall, J. 1960. Observations on the use by sea-birds of human fishing activities. Br. Birds 53:212-215. Braune, B.M. & D.E. Gaskin 1982a. Feeding methods and diving rates of migrating larids off Deer Island, New Brunswick. Can. J. Zool. 60:2190-2197. Braune, B.M. & D.E. Gaskin 1982b. Feeding ecology of nonbreeding populations of larids off Deer Island, New Brunswick. Auk 99:67-76. Burger, J. 1 98 1. Feeding competition between Laughing Gulls and Herring Gulls at a sanitary landfill. Condor 83:328-335. Coulson, J.C., P. Monaghan, J. Butterfield, N. Duncan, C. Thomas & C. Shedden 1983. Seasonal changes in the Herring Gull in Britain: weight, moult and mortality. Ardea 71:235-244. Cramp, S., W.R.P. Boume & D. Saunders 1974. The sea-birds of Britain and Ireland. Collins, London. Furness, R.W. & J.R.G. Hislop 1981. Diets and feeding ecology of Great Skuas Catharacta skua during the breeding season in Shetland. J. Zool., Lond. 195:1-23. Furness, R.W., A.V. Hudson & K. Ensor 1988. Interactions between scavenging seabirds and commercial fisheries around the British Isles. In: J. Burger (ed.), Seabirds and other marine vertebrates - competition, predation and other interactions: 240-268. Columbia University Press, New York. Götmark, F., D.W. Winkler & M. Andersson 1986. Flock-feeding on fish schools increases individual success in gulls. Nature 319:589-591. Greig, S.A., J.C. Coulson & P. Monaghan 1983. Age-related differences in foraging success in the Herring Gull (Larus argentatus). Anim. Behav. 31: 1237-1243. Greig, S.A., J.C. Coulson & P. Monaghan 1985. Feeding strategies of male and female adult Herring Gulls (Larus argentatus). Behaviour 94:41-59. Harris, M.P. & J.R.G. Hislop 1978. The food of young Puffins Fratercula arctica. J.Zool.,Lond. 185:213236. Hempel, G. 1978. North Sea fisheries and fish stocks a review of recent changes. Rapp. P.-v. Reun. Cons. int. Explor. Mer 173:145-167. Heubeck, M. 1989 (ed.). Seabirds and sandeels. Proc. Seminar Lerwick, Shetland, 15-16 October 1988. Hudson, A.V. 1986. The biology of seabirds utilising fishery waste in Shetland. Ph.D. thesis University of Glasgow, Glasgow. Hudson, A.V. & R.W. Furness 1988. Utilization of discarded fish by scavenging seabirds behind whitefish trawlers in Shetland. J. Zool., Lond. 215:151-166. Hudson, A.V. & R.W. Furness 1989. The behaviour of seabirds foraging at fishing boats around Shetland. Ibis 131:225-237. Jermyn, A.S. & A.P.Robb l981. Review of the cod, haddock and whiting discarded in the North Sea by Scottish fishing vessels for the period 1975-1980. Mimeogr. report International Council for Exploration of the Seas, Copenhagen. Monaghan, P. 1980. Dominance and dispersal between feeding sites in the Herring Gull (Larus argentatus). Anim. Behav. 28:521-527. Monaghan, P. & B. Zonfrillo 1986. Population dynamics of seabirds in the Firth of Clyde. Proc. R. Soc. Edin. 90B:363-375. Noordhuis R. & A.L. Spaans 1992. Interspecific competition for food between Herring Gulls Larus argentatus and Lesser Black-backed Gulls L. fuscus in the Dutch Wadden Sea area. Ardea 80:115-132. Porter, J.M. & S.G. Sealy 1981. Dynamics of seabird multispecies feeding flocks: chronology of flocking in Barkley Sound, British Columbia, in 1979. Colonial Waterbirds 4:104-113. Porter, J.M. & S.G. Sealy 1982. Dynamics of seabird multispecies feeding flocks: age-related feeding behaviour. Behaviour 81:91-109. Shedden, C.B. 1983. Feeding and roosting behaviour of gulls: implications for water storage. Ph.D. thesis University of Glasgow, Glasgow. Spaans, A.L. 1971. On the feeding ecology of the Herring Gull Larus argentatus Pont. in the northem part of The Netherlands. Ardea 59:73-188. Tasker, M.L.. A. Webb, A.J. Hall, M.W. Pienkowski, & D.R. Langslow 1987. Seabirds in the North Sea. Nature Conservancy Council. Peterborough. Watson, P.S. 1981. Seabird observations from commercial trawlers in the Irish Sea. Br. Birds 74:82-90.
SAMENVATTlNG
Op de Noordzee worden achter vissersboten vaak grote aantallen zeevogels gezien die azen op overboord geworpen vissen (puf) en visafval. Jaarlijks wordt in de Noordzee en het zeegebied ten westen van Groot-Brittannie bijna 95,000 ton visafval en 135,000 ton vis overboord gezet. Theoretisch kunnen hiervan 2 miljoen zeevogels leven.Er bestaat tussen de verschillende soorten zeevogels een grote mate van concurrentie om deze voedselbron. Rond Schotland voert de Noordse Stommvogel Fulmarus glacialis als het om visafval gaat, de lijst aan. Bij puf zijn Jan van Gent Sula bassana en in iets mindere mate Grote Jager Stercorarius skua het beste af, Noordse Stormvogel en Drieteenmeeuw Rissa tridactyla het slechtst; Grote Mantelmeeuw Larus marinus Kleine Mantelmeeuw L.fuscus en Zilvermeeuw L. argentatus nemen een tussenpositie in. Voor de Jan van Gent is puf als voedselbron vooral in het vroege voorjaar belangrijk. Parallel aan de toename van de Jan van Gent achter de vissersboten in die periode neemt het aantal Zilvermeeuwen af en daalt het foerageersucces van deze soort met meer dan een factor twee.
Jan van Genten, Noordse Stommvogels en Grote Jagers zijn in absolute zin de laatste twee decennia sterk in aantal toegenomen. In de jaren tachtig is daarentegen de hoeveelheid puf en visafval afgenomen. Het ziet ernaar uit dat beide tendenzen zich in de naaste toekomst zullen voortzetten. Op grond daarvan wordt verondersteld dat Kleine Mantelmeeuw, Zilvemmeeuw en Drieteenmeeuw steeds minder zullen kunnen profiteren van deze antropogene voedselbron. Verwacht wordt dat deze soorten daardoor in aantal zullen kunnen afnemen en dat bij de Zilvemmeeuw, waarvan de jongen sterk afhankelijk zijn van puf, het broedsucces geringer zal worden.
Check out my Home Page
-