Interactions Between Scavenging Seabirds and Commercial Fisheries
Around the British Isles
Robert W. Furness, Anne V. Hudson, and Kenneth Ensor
Department of Zoology University of Glasgow, Scotland
Human exploitation of living marine resources has provided an increasing
opportunity for some seabirds to take advantage of foods that would
otherwise be unavailable to them. Around the British Isles, for example,
adult demersal fish such as cod (Gadus morhua), haddock
(Melanogrammus aeglefinus), and whiting (Merlangius
merlangus) do not normally occur in the diet of the seabirds that are
capable of diving to the seabed in shallower areas of the sea.
Nevertheless, at some localities and at certain times of year, these fish
may represent the bulk of the diet, by mass or energy content, of certain
seabirds. These are species incapable of diving to the seabed, but able to
exploit the activities of commercial fishing boats; species such as great
black-backed gulls (Larus marinus), herring gulls (L.
argentatus), lesser black-backed gulls (L. fuscus) great skuas
(Catharacta skua), and northern gannets (Sula bassana).
Kittiwakes (Rissa tridactyla) may also make extensive use of this feeding
opportunity, and smaller numbers of several other species also join flocks
at fishing boats. The only source of demersal fish for these species must be
from man's activities, the fish being scavenged from behind commercial
fishing vessels or, in the case of some gulls, being stolen after the
catches have been landed. In addition, offal (fish livers and intestines)
from gutting operations carried out on board may be made available to
seabirds around vessels and is particularly sought after by northern fulmars
In many respects the interactions between seabirds foraging at fishing
vessels are similar to those between seabirds foraging in flocks over shoals
of small fish prey available at the surface. However, studying seabirds at
fishing boats has a number of advantages. It is possible to conduct
experiments where the availability of food is controlled, and so to examine
foraging behavior in relation to identified prey species, quantified prey
sizes, and controlled rates of presentation of food. This gives great scope
for the investigation of foraging efficiency and the effects of intra- and
interspecific competition. It also provides a useful model for the natural
relationships between foraging seabirds and fish schools at sea such as
discussed elsewhere in this volume.
There has been much dispute over the role of human exploitation of fish
and marine mammal populations in causing the observed population changes in
seabirds, James Fisher (1952, 1966) was convinced that food provided by man
during whaling and trawling operations caused the southward spread and
population increase of the northern fulmar in the northern Atlantic.
However, his view has been challenged. Wynne-Edwards (1962) suggested that
the spread was a natural result of genetic or behavioral evolution, and
nothing to do with man. Salomonsen (1965) suggested that it was due to a
gradual warming of the northeastem Atlantic which favored the large-billed
boreal population of the species. Brown (1970) also felt that oceanographic
factors were of prime importance since he found little association between
northern fulmar distribution in the northwest Atlantic and the distribution
of the fishing fleet. Dietary studies at Scottish colonies of northern
fulmars showed an enormous difference in food types taken by breeding birds
at a colony to the west of Scotland compared with food types taken by
breeding birds at a colony to the north of Scotland. Many of the food items
important in the diets at these locations were not obtained from fishing
boat activities (Furness and Todd 1984). Studies of the densities of
northern fulmars through the North Sea (Blake et al. 1984) provided
only weak evidence for an influence of fishing boat distribution on northern
fulmar distribution at sea. Rather little attention has been paid to the
ways in which seabirds exploit fishery waste, and competitive interactions
between different scavenging seabird species and age classes, the amounts of
offal and discarded whole fish made available to seabirds, or the proportion
of this that is exploited. Recent work in Queensland, Australia from prawn
trawlers in Moreton Bay indicates that scavenging seabirds (gulls, terns,
and cormorants by day and gulls and terns at night) obtain about 30-35
percent of discarded fish during the day and about 16 percent at night,
while dolphins account for about 40-50 percent of the discards, and the
remainder sink (T.J. Wassenberg, personal communication). Rees (1963)
observed scavenging behavior of seabirds in the Gulf of St. Lawrence and the
Strait of Belle Isle, and Rodriguez (1972) observed scavenging seabirds at
fishing boats off the southwestern coast of Africa. According to the
observations of Rodriguez (1972), some 125 freezer trawlers fished in
June-July 1967, predominantly for hake (Merluccius), in the
444-by-222 kilometer rectangle bounded by 23o-27oS, 13o-15oE, and discarded
a total of 576 metric tons of waste fish and offal daily. Much of this was
consumed by huge numbers of associated scavenging seabirds, of which an
estimated total of 46,000 albatross and large petrels predominated,
particularly black-browed albatross (Diomedea melanophris) and
white-chinned petrels (Procellaria aequinoctialis), which fed
almost exclusively on the waste. Rodriguez describes the clearly defined
differences in the positions behind trawlers taken by each species
apparently as a result of differences in competitive abilities and flight
characteristics. Abrams (1983) also concluded that the availability of
trawler waste had influenced the distributions of scavenging seabirds,
particularly black-browed albatross, in the Benguela Current off South
Africa and concluded that scavenging behind trawlers had been a factor
causing an increase in numbers of black-browed albatross as well.
In this paper we will consider two topics. First, how much food is made
available to scavenging seabirds by the fishing industry around the British
Isles and to what extent is this utilized by seabirds? Second, how do the
scavenging seabirds interact when feeding at boats and do the interactions
suggest that the population dynamics of scavenging seabirds may be affected
by competition for food at fishing boats?
Study Area and Fish Catch Data
The study area is defined by the division of the North Atlantic and North
Sea as indicated in figure 7.1. Based on the Food and Agriculture
Organization (United Nations) map, and used to delineate the Intemational
Council for the Exploration of the Seas (ICES) fishing areas. Quantities of
each species of fish and shellfish harvested in each of these areas are
listed in the statistical bulletins of the ICES, which are published
annually. The most recent bulletin, published in 1985 and dealing with fish
catches in 1982, has been used as the primary source of information from
which we have derived estimates of the amount of food (discards and offal)
made available to seabirds in each of the areas in figure 7.1. Comparison
with volumes for the years 1977 to 1981 shows that quantities have been
fairly consistent between years so that the 1982 data are taken to be
indicative of the general pattern for the years immediately before, and
probably after, 1982.
Figure 7.1. ICES fishing areas around the British Isles: 4a-northern North
Sea; 4b-central North Sea, 4c-southern North Sea; 6a-northwest Scotland;
7a-lrish Sea; 7bc-western Ireland; 7de-English Channel; 7f-Bristol Channel;
Discard and Offal Quantities
In order to estimate consumption of fishery waste by seabirds we need to
know how we can use data on fish landings in order to assess the quantities
of offal and discards (fish of no commercial value plus fish of commercial
value that are of a size not worth landing or below the legal size limit)
that are thrown back over-board by fishermen and so made available to
scavenging seabirds. Quantities of offal and discards produced depend
firstly on the fishing method employed and the ways in which the fish are
processed, so that observations were made on a number of different types of
fishing vessels. Clearly it is not possible to make observations, as ideally
required, from all types of fishing vessels of each size and of each
nationality in all areas around the British Isles in each session of the
year over several years. We have been limited to sampling from British
vessels, principally of short trip duration (generally one-day fishing trips
from Shetland, the Clyde Sea area, and northwest Scotland), and we have
assumed that discarding practices are comparable among vessels of other
countries and in other areas around Britain and Ireland. This assumption may
not be correct, and so our estimates of quantities of offal and discards may
need to be revised if foreign vessels treat their catches appreciably
differently from the way that they are processed by British boats.
We also need to determine how much of the available offal and discards is
consumed by seabirds. These questions were addressed by making observations
from trawlers around the Shetland Islands and in the Clyde Sea area and
around the Small Isles Rhum, Eigg, Muck, and Canna) in northwest Scotland
(figure 7.1) during their normal fishing activities. We gutted a number of
fish of various species in order to estimate the weight of offal as a
proportion of fish weight, allowing the total quantity of offal to be
calculated from total landing figures. The sizes of catches made at each
haul, and quantities of fish discarded, were estimated in terms of boxes of
fish, each boxful weighing on average 45 kilograms (Hudson 1986). The
quantity landed at market was recorded and this, once corrected to allow for
the weight of offal removed, provided an independent check of our estimate
of the number of boxes of marketable fish, since this gave the quantity of
fish discarded at sea by subtraction from the estimated total catch.
Seabirds Using Fishery Waste
During each fishing trip we counted numbers of seabirds associating with
the vessel and behind other vessels; we also observed boats from the
mainland and, in the vicinity of Shetland, from a light aircraft. Although
results differ between boats of different sizes and fishing methods, between
areas and at different times of year, constraints on fieldwork meant that we
had to concentrate effort at sea largely on small whitefish trawlers around
Shetland in summer and on Norway lobster (Nephrops norvegicus) boats in the
Clyde Sea area and around the Small Isles of northwest Scotland throughout
the year (figure 7.2). Other studies have dealt with other types of boat and
other areas (Irish Sea: Hillis 1971, 1973 and Watson 1981; North Sea: Anstey
1984 and Blake et al. 1984; northwest Scotland: Boswall 1960, 1977
and Lockley and Marchant 1951; southwest England: Lake 1984).
Figure 7.2. Gulls dropping onto discarded fish behind a trawler in the Clyde
In order to convert quantities of discards and offal available to
seabirds into a measure of the numbers of birds that may be sustained by
scavenging at boats we measured the energy value of offal and discards by
bomb calorimetry and estimated the energy requirements of seabirds from
bioenergetics equations following the results presented by Ellis (1984).
Feeding Behavior and Interactions
The ability of different age classes and species of seabirds to exploit
offal and discards was assessed by experimental discarding performed by
ourselves while fishermen were engaged in normal fishing activities (i.e.,
sorting, gutting, and/or discarding, and/or trawling for the next catch).
Individual fish that were about to be discarded by the fishermen were taken,
identified as to species, measured, and then discarded. Thus our
observations of seabirds taking measured fish were made while birds were
also feeding on fish being discarded in the normal way by fishermen.
Attempts by seabirds to obtain each individual fish were filmed using a JVC
KY19OOEL10 video camera with a synchronized sound track and a JVC CR4400E
portable recorder. Subsequently, when the methodology had become routine,
the video equipment was replaced by a portable tape recorder and stopwatch
system which allowed data to be transcribed much more quickly with little
loss of accuracy or information. The following data were recorded: fish
species; fish length (mm); species and age (where this could be determined
from plumage) of the seabird attempting to take the fish; the outcome of the
attempt (fish missed, swallowed, dropped, or stolen); the time taken between
fish being grabbed by the bird and either swallowed, dropped, or stolen;
whether the bird swallowed the fish on the water or in the air; if birds
fought for the fish; the species and age of kleptoparasite; and whether the
fish was eventually swallowed or sank. Because some fish were tackled by a
series of birds the sequence could occasionally be complex and could only be
recorded by video film or by describing events and recorded stopwatch
timings into a tape recorder.
Swennen and Duiven (1977) found that the maximum body width and depth (or
cross-sectional area) of fish was the key measurement used by alcids in
choosing optimal prey sizes. This may be a more important feature of a fish
than its length for prey selection by scavenging seabirds as well, but it is
difficult to measure maximum fish depth or breadth as accurately or as
easily as one can measure fish length, and both depth and breadth measures
tend to be influenced by the handling of the fish during capture, since
these measurements are more sensitive to effects of pressure change and
compaction which result, for example, in the stomachs of many of the fish
being forced out through the mouth or the swim bladder ruptured. Also,
relationships between fish length and weight or otolith length have been
published for many marine fish, but few exist allowing fish breadth or depth
to be estimated from otolith dimensions. In this paper we have considered
only fish length, as measured, since within a given fish species, fish
breadth or depth is closely related with the length of the fish. Behavioral
records were analyzed by use of the SPSS and SPSS-X packages on the Glasgow
University ICL 2988 computer. Where statistical tests have been performed,
significant means p < 0.05.
Fish Discards and Offal Made Available
Observations from fishing boats catching sand eels (Ammodytes
marinus) or other fish for industrial purposes (reduction to oil and
fish meal) showed that these fisheries provide very little food for
seabirds. Gulls do attend sand eel boats and pick fish from the net when it
is hauled, but the total quantity of food obtained is very small by
comparison with amounts available from whitefish boats Similarly, boats
fishing for herring (Clupea harengus) or mackerel (Scomber
scombrus) do not normally provide seabirds with feeding opportunities,
although on occasion quotas can result in discarding of these species.
Scavenging seabirds obtain most fishery waste from whitefish and Norway
lobster boats. All areas around the British Isles except for the Irish Sea
are dominated by catches of industrial species, or of herring or mackerel,
so that total fish catch data provide little indication of the availability
of fishery waste to seabirds. Observations from whitefish and Norway lobster
boats indicate that the amounts of fish discarded vary greatly between
catches, between boats, between areas, and seasonally. Our data and data of
the Department of Agriculture and Fisheries for Scotland (DAFS) suggest that
variation between catches is almost as great as variation between areas and
seasons (table 7.1), so we have used the same factors to estimate discard
quantities from landings data for each fishery region. We have assumed that,
for whitefish boats, the discard mass averages 20 percent of the mass of
landed haddock and whiting, 10 percent of the mass of landed hake
(Merluccius merluccius), ling (Molva molva), pollack
(Pollachius pollachius), torsk (Brosme brosme), monkfish
(Lophius piscatorius), gumards (Triglidae), and 5 percent of the
mass of landed cod and saithe (P. virens). Norway lobster boats,
on average, discard about twice as much fish as the quantity of Nephrops
caught (table 7.1). These values may be exceeded in certain years when good
cohorts recruit into the fishery (for example, DAFS data indicate that as
much as 42 and 48 percent of haddock weight caught by seine net vessels in
1975 and 1980 was discarded because of the strong 1974 and 1979 cohorts).
Thus, in some years the total amount of discards may be elevated by such
occurrences. However, good years for one fish species may not be good for
others, so that total discard amounts will fluctuate much less than the
discard amounts of particular species.
Table 7.1. Quantities (weights) of fish discarded from fishing boats.
catches Average Rate of
Fishing Method Locality Sampled Discarding Fish Source
Nephrops trawl Clyde 18 1.8 x mass of Nephrops This study
catch (range 0.5 x
to 3.0 x)
Hebrides 9 2.2 x mass of Nephrops This study
catch (range 1.0 x
to 3.0 x)
Seine net North Sea > 50 33% of haddock caught Jermyn and
25% of whiting caught Robb (1981)
10% of cod caught
Motor trawl North Sea > 20 20% of haddock caught
20% of whiting caught
1% of cod caught
Light trawl Shetland 151 27% of whitefish Hudson
caught (range 1% to (1986)
85% of catch)
NOTE: Discard rates are given either as mass of fish as a multiple of the
measured quantity of Nephrops landed by Nephrops trawlers or as a percentage
of the total mass of whitefish or whitefish species caught by whitefish
boats. Sample size is the number of separate catches that were examined.
By applying these values to ICES landings data for 1982 we can obtain a
crude estimate of the annual total mass of discards in each fishery area
(table 7.2). From analyses of the mass of offal in relation to fish mass
(table 7.3) we can convert the same fish landings data to quantities of
offal discharged, on the assumption that all of these species are gutted at
sea and that all offal is discharged (table 7.2). In practice, some fish
are landed whole and a few boats may retain offal, but these practices
probably have little effect on the overall calculations.
In the Irish Sea, Norway lobster boats are the main source of discard
fish, while total discard quantities are much the highest in the north and
central North Sea. Offal mass is also much greater in these last two regions
than elsewhere (table 7.2).
Norway lobster trawlers use a much smaller mesh net than whitefish
trawlers and as a result the fish discarded from Norway lobster trawlers
tend to be smaller than those discarded from whitefish trawlers. Around
Shetland, most discards were of haddock and whiting with median lengths of
28 and 29 centimeters respectively (Hudson 1986). Discards from Norway
lobster boats in the Clyde and around the Inner Hebrides tended to be
predominantly 10-20-centimeter flatfish, 10-25-centimeter whiting, Norway
pout (Trisopterus esmarkii) or haddock (Ensor and Furness,
unpublished data). In the Irish Sea most discards are whiting, dab
(Limanda limanda), poor cod (Trisopterus minutus), or
Norway pout, and are generally 5-24 centimeters in length (Watson 1981).
Table 7.2. Quantities of offal and fish discards from whitefish and Norway
lobster boats around the British Isles.
Offal Discarded Whitefish Boats Nephrops Boats
ICES Fishing Ground (metric tons) (metric tons) (metric tons)
Northern North Sea 40,000 41,000 3,000
Central North Sea 34,000 30,000 8,800
Southern North Sea 9,700 5,300 10
Northwest Scotland 10,000 13,000 18,000
Irish Sea 3,700 1,400 20,000
Western Ireland 1,600 1,800 2,500
Eastern English Channel 1,200 1,000 0
Western English Channel 1,900 900 20
Bristol Channel 1,000 1,100 10
Southwest Approaches 4,300 4,500 7,300
NOTES Data for 1982; for methods of calculation and assumptions made see text.
Calculations of the Numbers of Seabirds Supported by Fishery Waste
The best measurements of the daily energy expenditure of seabirds are
recent studies using isotopically labeled water. Ellis (1984) reviewed seven
studies of seabirds where such methods indicated daily expenditures of 2.6,
3.0, 3.1, 3.3, 3.4, 4.8, and 5.2 times Basal Metabolic Rate (BMR).
Subsequent studies (Gaston 1985; Roby and Ricklefs in press; Birt et
al. unpublished data) also indicate that the daily energy expenditure
of seabirds is generally between 3 and 4 times BMR. Since most of these
studies are of breeding seabirds, and nonbreeders have a lower energy
requirement (estimated to be 20 to 30 percent lower in thick-billed murres
(Uria lomvia) and black guillemots (Cepphus grylle);
Gaston 1985) we have chosen 3 BMR as a suitable estimate of the daily energy
requirement of seabirds over the whole year. According to Ellis (1984)
seabirds from high latitudes have a greater BMR than tropical seabirds.
Herring gulls and great skuas have a measured BMR 1.25 and 1.27 times that
predicted by an allometric equation based on data for all seabirds. We
assume a BMR in line with these deviations from a common regression, which
gives a value of 478 kilojoules per day for a l,000-gram seabird.
Table 7.3. Offal mass as a proportion of total fish mass.
Source of Data Relationship Given
This study Offal = 11.0% of mass of gadoid fish caught
This study Offal = 6.5% of mass of flatfish caught
DAFS* (A. S. Jermyn, personal Offal = 11.1% of mass of fish caught
communication) Offal = 12.5% of mass of gutted fish
Boswall (1960) Offal = 12.5% of mass of fish caught
Offal = 14.3% of mass of gutted fish
Bailey and Hislop (1978) Offal = 10-15% of gadoid body mass
*Department for Agriculture and Fisheries (Scotland).
From measurements of the calorific value of fish offal and tissues (table
7.4), we have calculated on the basis of offal containing 11 kilojoules g-1
and discards 5 kj g-1. Assuming a food utilization efficiency of 75 percent
(derived from Kendeigh, Dolnik, and Gavrilov 1977) we can estimate that a
l,000-gram seabird could survive on 64 kilograms of offal per year or on 120
kilograms of discards per year. Most scavenging seabirds around the British
Isles are northern fulmars or herring gulls which weigh close to 1,000
grams, while great skuas and great black-backed gulls weigh from 1,200 to
1,800 grams. Kittiwakes at 360 grams and northern gannets at 3,000 grams are
rather far from a l,000-gram standard, but represent relatively smaller
numbers of the birds obtaining food at fishing boats.
By combining these data with the quantities of offal and discards
available (table 7.2), we can estimate the maximum number of l,000-gram
seabirds that could be sustained by offal and discards in each area (table
7.5). In theory, if all of the offal and discards were taken, then some 2.8
million l,000-gram seabirds might be supported by this food supply. However,
this total exceeds the number of seabirds that could actually be supported.
In practice, most offal is taken, although on the occasions when large
quantities of offal are washed out of the scuppers at once, much tends to
sink before seabirds can reach it. Some discards are too large to be handled
or are of species that are not easily swallowed (see below), so that the
available offal and discards could only support somewhat smaller numbers
than these calculations suggest.
Table 7.4. Caloric value of fish offal and discards.
Fish Length Sample Value
Sample Analyzed (cm) Size (kJ/g) Source
Offal (saithe) 28-29 3 7.3 this study
Offal (whiting) 18-28 6 12.6 this study
Saithe (whole) 28-30 2 5.2 this study
Whiting (whole) 18-28 5 5.8 this study
Saithe (whole) 3-4 8 5.1 Harris and Hislop (1978)
Whiting (whole) 4-6 3 4.1 Harris and Hislop (1978)
Saithe (gutted) 29 1 4.9 this study
Whiting (gutted) 18-28 5 4.4 this study
Cod (fillet) - 3 3.2 Paul and Southgate (l975)
Haddock (fillet) - - 3.1 Paul and Southgate (1975)
Plaice (whole) - 8 3.8 Paul and Southgate (1975)
Seabirds Using Waste in Different Areas
The numbers of each species of seabird in flocks foraging at fishing boats
vary considerably between areas and to some extent between seasons (table
Foraging Efficiency of Different Age Classes
In the Clyde Sea area juvenile herring gulls missed (i.e., failed to get
hold of) 4.4 percent of the discards they attempted to pick up.
Significantly fewer were missed by older birds: one-year-old birds missed
0.8 percent while two-year-olds and older birds missed only 0.4 percent (n=
1,365 fish, x22 = 26.7, p < 0.005). In addition, juvenile herring gulls
were more likely than adults to drop the fish they did pick up (9.7 percent
dropped by first-year birds versus 6.2 percent by adults, x22 = 3.97, p <
Table 7.5. Numbers of 1,000-gram seabirds that
could be supported by offal and discards around
the British Isles.
Potentially Supported By
Fishing Ground Offal Discards
Northern North Sea 620,000 310,000
Central North Sea 530,000 277,000
Southern North Sea 150,000 38,000
Northwest Scotland 172,000 220,000
Irish Sea 58,000 153,000
Western Ireland 25,000 31,000
Eastern English Channel 19,000 7,000
Western English Channel 30,000 6,000
Bristol Channel 15,000 8,000
Southwest Approaches 67,000 84,000
All areas 1,686,000 1,134,000
Table 7.6. Numbers of seabirds associated with fishing boats in different areas.
Irish Sea Clyde Sea Shetland Summer Eastern Northwest
_________________ ___________ ______________________ Scotland Scotland
Summer Winter Winter Inshorer Offshore Winter Summer
Seabird (18) (32) (149) (72) (40) ( ) (1)
Northern fulmar 23 3 0 485 721 1 155
(0-100) (0-30) (0-3) (10-2,500) (0-6,000) (0-4)
Northern gannet 11 6 6 9 3 15 5
(0-50) (0-50) (0-167) (0-200) (0-100) (0-51)
Herring gull 213 251 241 30 0 87 15
(0-800) (10-900) (0-1,340) (0-400) (0-380)
Lesser black-backed gull 2 0 3 6 0 2 1
(0-10) (0-1) (0-88) (0-20) (0-28)
Great black-backed gull 5 16 6 234 19 2 90
(0-30) (0-60) (0-80) (10-1,000) (0-300) (0-24)
Kittiwake 75 78 24 3 0 0 35
(0-200) (2-400) (0-285) (0-50)
Great skua 0 0 0 12 1 0 0
(0-6) (0-6) (0-1) (0-50) (0-20)
SOURCES: Irsh Sea: Watson 1981; Clyde Sea and Shetland: this study; eastern Scotland: Anstey 1984;
northwestern Scotland: Boswall 1960.
NOTES: Summer = April-July, winter = October-February; numbers of counts are given in parentheses below
the location heading; minimum and maximum counts are given in parentheses below means except where all
counts were zeros.
Adult herring gulls in the Clyde obtained more fish per bird than did
immature herring gulls (table 7.7). Adult herring gulls were also more
selective in the sizes of fish they took. While mean fish lengths were
almost the same (first year birds, 24.6 cm; two- to four-year-olds, 24.5 cm;
adults, 24.9 cm), the variance of fish length was greater for first-year
birds (13.76) than for two-to four-year-old birds (11.70) or adults 18.46).
Adults were less likely than juveniles or immature birds to take
particularly small or particularly large fish = 1.626, p < 0.001, F448,404
= 1.383, p < 0.05).
Around Shetland in summer the largest numbers of discard fish were taken
by great black-backed gulls, many of which were immature birds. For both
adults and immature birds the handing time of fish (time taken from grabbing
the fish to swallowing it) increased at an accelerating rate with fish
length. Handling times of immature birds were longer for all fish sizes than
those of adults and the relationships between log handling time and log fish
length were linear (fig. 7.3).
Figure 7.3. Log. handling times (secs.) of haddock and whiting by adult and
immature great black-backed gulls at trawlers around Shetland in relation to
log. fish length (cm).
Table 7.7. Numbers of discard fish swallowed by scavenging seabirds behind
Nephrops trawlers in the Clyde in relation to numbers predicted from the
relative numerical abundance of each species and age class.
Mean Number Relative
Present Fish Expected Number Foraging
Species Age per Boat Swallowed Swallowed* Success
Northern gannet Adult 5.7 53 25.9 2.05
Great black-backed gull All 6.0 35 27.3 1.28
Herring gull Adult 78.5 405 357.9 1.13
Herring gull Immature 162.7 742 742.1 1.00
Lesser black-backed gull All 2.6 11 11.9 0.92
Kittiwake Adult 21.8 27 99.4 0.27
Kittiwake Immature 2.3 1 10.6 0.09
NOTES: Null hypothesis that birds of all species and age have equal
likelihood of obtaining each fish; relative foraging success is calculated
as number of fish swallowed divided by expected number.
*Derived from the relative abundance of the birds.
Foraging Efficiency of Different Species
When offal was discarded from Shetland trawlers virtually all of it was
consumed by northern fulmars. The large numbers and aggressive behavior of
northern fulmars generally precluded other species from entering the area
close beside the boat where offal was discharged (figure 7.4). In areas
where northern fulmars were rarely found (e.g., the Clyde Sea) offal was
taken by gulls in preference to discard fish, as might be expected since it
has a considerably higher calorific value. Northern fulmars have great
difficulty swallowing whole fish and around Shetland they tended to consume
offal but ignore fish. Although northern fulmars outnumbered gulls, skuas,
and northern gannets around inshore Shetland trawlers by a factor of 1.65 to
1 (table 7.6), they swallowed only 88 of the fish experimentally discarded,
compared to the 3,705 swallowed by gulls, skuas, and northern gannets. Thus
northern fulmars represented 62.3 percent of all the birds present but
obtained only 2.3 percent of the discards swallowed.
Comparing the foraging success of adult seabirds taking discards behind
Nephrops trawlers in the Clyde Sea area, northern gannets swallowed all 53
fish they attempted to take (100 percent); lesser black-backed gulls
obtained 9 in 9 attempts (100 percent); great black-backed gulls obtained 21
in 22 attempts (95 percent); herring gulls obtained 405 in 477 attempts (85
percent); and kittiwakes obtained 27 in 32 attempts 184 percent). Apart from
3 fish missed by herring gull adults, the failure to swallow fish was due
either to the fish being dropped or to it being stolen by a kleptoparasite.
Figure 7.4. Fulmars scavenging offal around a trawler in Shetland.
Although kittiwakes obtained 84 percent of fish that they attempted to
take, the species took a much lower proportion of all fish discarded than
would have been predicted from the numbers of each seabird species present
(table 7.7). Numerically, immature herring gulls predominate around boats in
the Clyde and took fish in the proportion predicted by their abundance
relative to all seabirds present. Northern gannets took twice as many fish
as their numbers would have predicted. Great black-backed gulls and adult
herring gulls also obtained more than their numerical abundance would have
predicted. The differences between observed and expected numbers taken by
each species and age class are highly significant (X26 = 98.1, p < 0.001).
One reason for kittiwakes and lesser black-backed gulls only attempting
to take a fraction of the number of fish taken by larger species is that
they are unable to handle and swallow large fish. The mean length of fish
swallowed by each species is less than the mean length of fish dropped, and
increases with seabird size (table 7.8).
The handling times of all seabirds around Shetland increased at an
accelerating rate as fish length increased (figure 7.5). Northern gannets
and great black-backed gulls were considerably quicker at swallowing fish
than herring gulls or great skuas and also managed to swallow larger fish.
Northern fulmars were particularly slow to swallow fish. These differences
in handling times between species were statistically significant.
Table 7.8. Mean lengths of fish taken by seabirds around Nephrops trawlers in the Clyde.
Fish Swallowed Fish Dropped
Species N mean (cm) s.d. N mean (cm) s.d.
Northern gannet 53 28.3 3.6 0 - -
Great black-backed gull 35 29.0 3.4 2 32.5 0.7
Herring gull 1,147 24.7 3.2 100 27.6 2.7
Lesser black-backed gull 11 21.9 4.9 1 26 -
Kittiwake 27 15.8 2.9 5 16.4 4.6
Figure 7.5. Log. handling times (secs.) of haddock and whiting by
adult northern gannets, great black-backed gulls, great skuas, herring gulls
and northern fulmars at trawlers around Shetland in relation to log. fish
Although great skuas have the reputation of being specialist
kleptoparasites, most stealing of discards in foraging flocks at trawlers
around Shetland was done by great black-backed gulls, and great skuas lost
more fish to kleptoparasitism than they obtained in this way (table 7.9). In
general, larger species of seabirds tended to steal more than smaller
seabirds and this appears to be related to the difficulty that the smaller
species had in swallowing the larger discarded fish. The mean length of fish
stolen from great skuas 128.3-cm haddock and 29.9-cm whiting) were
significantly larger than the mean lengths of these fish swallowed by great
skuas (25.5-cm haddock and 27.4-cm whiting;. t= 7.7 and 7.2 respectively, p
< 0.01). Kleptoparasitism appears to provide an important part of the total
discard consumption of great black-backed gulls (table 7.9) and is also an
important cause of loss of fish for herring gulls and lesser black-backed
In the Clyde, kittiwakes were never, and northern gannets rarely, seen
stealing fish. Herring gulls were less often kleptoparasitic than were
lesser black-backed gulls or great black-backed gulls, although because of
their numerical predominance in the area most fish were stolen by herring
gulls (the proportions of discarded fish swallowed that were obtained by
kleptoparasitism increased as follows: kittiwakes 0.0 percent, northern
gannets 1.9 percent, herring gulls 7.9 percent, lesser black-backed gulls
9.1 percent, great black-backed gulls 12.9 percent: the total was 1,277 fish
As a consequence of size preferences and kleptoparasitic interactions
most discarded fish were consumed by seabirds, although flatfish, gurnards,
and roundfish more than 35 centimeters in length tended to be ignored unless
nothing else was available. Overall about 75 percent of all experimental
discards were taken by seabirds both around Shetland and in the Clyde and
around the Small Isles. This proportion probably applies to the fish
discarded by fishermen too, except where catches include high proportions of
flatfish, which are often ignored by seabirds and also tend to sink faster
The quantities of offal and discarded fish made available to scavenging
seabirds in the sea areas round the British Isles (figure 7.1) are enormous
and could possibly support as many as 2.5 million 1,000-gram seabirds ltable
7.5; allowing for 25 percent of discards being lost to seabirds). Data in
the ICES statistical bulletins indicate that there is little seasonal
variation in total catches of fish. This is probably the case also for
provision of offal and discards, although there is a brief hiatus over the
Christmas and New Year period when most fishermen are at home. We cannot say
at present whether seabird requirements for offal and discards is greater in
summer or winter. Some species are migratory, while the availability of
alternative foods (e.g., surface shoals of sand eels) also varies between
Tab1e 7.9. Kleptoparasitic interactions involving scavenging seabirds at
trawlers around Shetland
Fish Obtained or
Lost as a
Number of Fish Number of Fish Kleptoparasitism *
Obtained by Lost to __________________
Species Kleptoparasitism Kleptoparasitism Obtained Lost
Great black-backed gull
adult) 335 173 12.7 6.5
Great black-backed gull
(immature) 44 36 9.6 7.9
Northern gannet 58 40 10.8 7.5
Great skua 94 135 17.2 24.6
Herring gull 5 58 2.2 25.9
Lesser black-backed gull 0 27 0.0 32.1
Northern fulmar 2 69 1.3 43.4
*(As a percentage of all discarded fish handled by the species.)
Clearly seabirds cannot consume all of the offal and discards made
available by fishing boats. Some must sink or be taken by fish or marine
mammals. However, our observations in the Clyde, Inner Hebrides, and around
Shetland indicate that very little offal is not obtained by seabirds, and
few discards other than flatfish or roundfish of more than 35 centimeters in
length are not consumed by birds. Probably about 90 percent of offal and 75
percent of discards are taken. Off eastern Canada, fishermen clean whitefish
by cutting off the head and throwing it into the sea with the guts attached.
This rapidly sinks and so seabirds are able to obtain only a small part of
the offal before it has sunk too deep for them to reach it (R. G. B. Brown,
personal communication). This simple difference in gutting method (British
fishermen remove the guts from the slit-open abdominal cavity without taking
the head off the fish) is a major determinant of the ability of scavenging
seabirds to exploit offal. Around the British Isles, offal thrown overboard
by fishermen tends to float, since the liver in particular is buoyed up by
its high lipid content.
Recent estimates of seabird population sizes in northwest Europe (Evans
1984) indicate that there are well over 3 million scavenging seabirds in the
fishery areas around the British Isles. Estimates of the breeding
populations in the British Isles alone are of 600,000 northern fulmars
(breeding individuals), 700,000 herring gulls, 300,000 northern gannets,
50,000 great black-backed gulls, 100,000 lesser black-backed gulls, 1.1
million kittiwakes, and 20,000 great skuas. There is probably an equal
number of immature birds of each of these species. Further, large numbers of
herring gulls breed on the continental coast of Europe. In addition, large
numbers of birds from populations in Norway, Iceland, and the Faeroe Islands
spend the winter around the British Isles and some nonbreeders may remain in
British waters all year. A large proportion of the herring gulls around
Shetland and on the east coast of Britain in winter are from an estimated
200,000 pairs breeding in northern Norway (Coulson et al. 1984),
while the numbers of great black-backed gulls (particularly immature gulls)
that we recorded around Shetland in summer greatly exceeded the number that
could be attributed to British breeding colonies alone, and presumably
originate from the much larger populations of that species found in Norway
(an estimated 60,000 pairs; Evans 1984). Since the Norwegian local fishery
for demersal fish is principally one of longline fishing for cod, which
results in little or no discarding, it is perhaps not surprising that
immature Norwegian gulls may prefer to feed around Shetland where discard
volumes are very large (table 7.2).
Because the estimates of total seabird population sizes (including
nonbreeders) are not verv precise, and knowledae of their movements is
limited, and because the estimates of the total mass of offal and discards
are very crude, we cannot infer that there is not enough offal and discard
fish to satisfy the requirements of scavenging seabird populations, but the
data suggest that seabird numbers considerably exceed the estimated 2.5
million or so that could be supported on fishery waste alone.
Clearly there are differences in both the geographical distributions and
habitats used by each scavenging seabird species. Northern fulmars
predominate around Shetland and northwest Scotland, while herring gulls
predominate in the Irish Sea, the Clyde Sea, and off eastern Scotland. While
northern fulmars represent two-thirds of the seabirds behind trawlers
inshore around Shetland in summer, they represent over 95 percent of the
seabirds behind offshore trawlers. Interestingly, great black-backed gulls
show a greater tendency to utilize the offshore habitat than do herring
gulls (table 7.6), a pattern also found on the Grand Banks and Scotian Shelf
(R. G. B. Brown, personal communication).
Our observations of the feeding behavior of scavenging seabirds at boats
indicate that there is indeed considerable competition for food. There is a
clear dominance hierarchy, with northern fulmars at the apex, able to obtain
the choicest pickings which are clearly fish livers or the entire offal. For
anatomical reasons northern fulmars are unable to swallow whole fish
efficiently unless these are particularly small, so that northern gannets
and adult great black-backed gulls obtain the pick of the discard fish and
generally achieve the highest feeding rate on discards (table 7.7). Immature
great black-backed gulls are less efficient than adults (figure 7.2), a
situation found for herring gulls foraging at garbage dumps (Burger and
Gochfeld 1981; Greig, Coulson, and Monaghan 1983).
Great skuas, although often attempting to steal fish by kleptoparasitism,
are unable to swallow many of the large fish taken by great black-backed
gulls and as a result they are robbed of large fish more often than they
obtain fish by robbery (table 7.9). Herring gulls are outcompeted by great
skuas, great black-backed gulls, and northern gannets around Shetland. In
the Clyde and Irish seas herring gulls predominate numerically and do not
have to compete with large numbers of the larger seabirds. Fish discarded
from Norway lobster trawlers are smaller and so can be utilized by herring
gulls and to some extent by kittiwakes, which are generally unable to obtain
food at boats around Shetland. However, kittiwakes have difficulty competing
with herring gulls and their foraging success at boats is low (table 7 7),
partly because they drop more fish than do larger birds (table 7.8).
Kleptoparasitism tends to result in unusually large fish being stolen,
but it is not clear whether kleptoparasites select victims on the basis of
the size of fish they carry (as suggested by Brockman and Bamard 1979), or
whether the longer handling times of birds with large fish make them more
vulnerable to kleptoparasitism. Although great skuas lost more fish to
kleptoparasites than they stole (table 7.9) they were the most
kleptoparasitic of the species around boats in Shetland, obtaining 17.2
percent of their fish by kleptoparasitism compared to 12.7 percent stolen by
adult great black-backed gulls and 9.6 percent stolen by immature great
blackbacked gulls. This is consistent with the idea that great skuas are
more specialized kleptoparasites than gulls (Furness in press). The lower
foraging success of great skuas reflects both the problems they have
handling large fish and the fact that they are greatly outnumbered by great
black-backed gulls around Shetland boats.
The fact that offal availability is very high in the north and central
North Sea and northwest Scotland by comparison with other areas is
interesting since it is in these areas that northern fulmar numbers are
highest. The low availability of offal further southwest might have been one
factor inhibiting northern fulmars from colonizing these regions in large
numbers. However, the extent to which discards and offal may influence
population dynamics is unclear. Breeding northern fulmars in Shetland fed
extensively on sand eels when these were abundant (Furness and Todd 1984).
In the last three summers, industrial catches, and apparently recruitment,
of sand eels around Shetland have been declining, with the 1985 year class
considered to be the poorest since industrial fishing for sand eels began
around Shetland in 1974, and so the poorest cohort on record (Gauld, McKay,
and Bailey 1986). Sand eels have not been prominent in northern fulmar diets
(Hudson 1986), suggesting that the extent to which northern fulmars make use
of offal may depend in part on the availability of other foods. Study of
great skua diets between 1973 and 1977 indicated that discards were taken as
the principal food of nonbreeders, but that breeders took a higher
proportion of sand eels and fed the chicks predominantly on sand eels
(Furness and Hislop 1981). Since 1981 great skua diets have contained a much
lower proportion of sand eels, which also reflects the reduction in sand eel
recruitment, and probably in stock size. In addition, the tendency for
nonbreeders to make more extensive use of fishery waste appears to be a
general one. Obtaining fish from boats probably requires more time but less
effort than catching shoaling fish such as sand eels. Since gliding along
behind a fishing boat is probably not energetically costly for a gull or a
skua (perhaps costing about 3 BMR; Furness and Monaghan 1987), while flying
rapidly out to a shoal of sand eels and competing for position in a feeding
flock above the shoal will cost as much as or more than the cost of
sustained flapping flight (about 10 BMR; Furness and Monaghan 1987), then
the energetic cost of following fishing boats to await an opportunity to
obtain discards and offal may be no more than existence costs and may result
in a better reward to nonbreeders than attempting, at high energy cost, to
compete with adults at sand eel shoals. Adults may prefer to expend this
extra energy in order to minimize the time spent foraging so that they can
feed their chicks rapidly and spend more time guarding them. Although
numbers of all scavenging seabirds in the British Isles have increased over
this century (Cramp, Bourne, and Saunders 1974), numbers of kittiwakes,
lesser black-backed gulls, and herring gulls breeding in many Shetland
colonies have decreased over the last ten years, while numbers of great
skuas have almost stopped increasing. Northern fulmar and northern gannet
numbers have continued to increase, while trends in great black-backed gull
numbers are uncertain. These trends generally reflect the competitive
abilities of these species at fishing boats. It is tempting to suggest that
the changes in numbers may be due to the consequences of competitive
abilities of these species at fishing boats but it would be unwise to assert
this until more is known about the importance of discards and offal in
determining the survival rates and breeding success of scavenging seabirds.
However, in a parallel situation at garbage dumps in New Jersey, Burger
(1981) found that laughing gulls (larus atricilla) were unable to
successfully compete with herring gulls, and she speculated that this
competitive disadvantage prevented the laughing gull population from
increasing where herring gulls were abundant and exploited the dumps to
their full potential. It is proposed that the net mesh size used by
whitefish trawlers around the British Isles should be increased to 90
millimeters from January 1987, and possibly further increased at successive
stages in the future, in order to reduce fishing effort on smaller-size
classes of demersal fish. This would have important and interesting
implications for scavenging seabirds, since we have shown that the smaller
species cannot utilize the largest discard sizes effectively. An increase in
net mesh size will presumably have two immediate effects. First, it will
reduce the total quantity of fish discarded. Second, it will increase the
mean size of discarded fish. Both of these trends seem likely to reduce food
availability to scavenging seabirds, but particularly to herring gulls,
lesser black-backed gulls, and great skuas, the species that presently make
most use of the smallest discards. We might expect a shift in the balance
between species, with northern gannets and great black-backed gulls little
affected by the changes but the smaller species finding it increasingly
difficult to compete with the larger birds for discarded fish.
Scavenging seabirds can obtain food in the form of fish offal and
discarded whole fish from the activities of fishing fleets around the
British Isles. The amounts of food made available depend on the fishing
method. Whitefish and Norway lobster boats provide most of the food
available to scavenging birds at sea. Most discarded whole fish are small
(10-25 cm) gadoids from Norway lobster boats or 25-30-centimeter gadoids
from whitefish boats. Quantities of offal discarded represent about 11
percent of the mass of gadoids and 6.5 percent of the mass of flatfish
processed for market. Crude calculations suggest that offal and discards
around the British Isles could support up to 2.5 million l,000-gram
seabirds. Northern fulmars predominate at fishing boats in north and
northwest Britain and offshore, with herring gulls predominant further
south. Great black-backed gulls are also numerous at fishing boats inshore
in the north and northwest, and kittiwakes mainly attend boats further south
or east. Northern gannets, great skuas, and lesser black-backed gulls also
exploit fishery waste. Adult scavenging seabirds are more efficient at
feeding than are juveniles or immature birds. Northern fulmars dominated
other species and so obtained almost all of the offal in northern Britain.
Northern gannets had a higher foraging success on discards than did other
species, and foraging success tended to decline with decreasing body size.
Handling time increased with fish size, and larger seabirds had shorter
handling times for a given length of fish. Larger species tended to steal
fish from smaller species, although northern gannets were not
kleptoparasitic. Flatfish and fish difficult to swallow due to bony
exteriors were generally avoided. We estimated that 90 percent of offal and
about 75 percent of discarded whole fish were consumed by seabirds. The
implications of competitive interactions for food at fishing boats, of net
mesh size, and of discarding practices on seabird population dynamics are
This study was supported by a grant from the Natural Environment Research
Council. The assistance of the Department of Agriculture and Fisheries for
Scotland and the University Marine Research Station at Millport is
gratefully acknowledged. Jim Atkinson, Roger Bailey, John R. G. Hislop,
Geoff Moore, and Stan Jermyn provided advice, logistical assistance, and
comments on an earlier draft. Numerous skippers and crew of fishing boats
in Shetland, the Clyde, and Mallaig allowed us to make observations while
they were working and gave us generous hospitality on board. Cathy McClaggan
kindly assisted with bomb calorimetry. We are also grateful to Joanna Burger
and Dick Brown for helpful comments on the submitted manuscript, and thank
Dick Brown and Tom Wassenberg for providing useful comparisons with our work
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