* Northern Gannet (Morus bassanus)

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Northern Gannet. Photo: Ben Lascelles



SUMMARY INFORMATION

GENERAL ECOLOGY: Young birds migrate south, small numbers reaching the equator. Adults disperse less extensively [37]. This species is strictly marine, with movements largely confined to the continental shelf. Individuals nest on cliffs and offshore islands and occasionally on the mainland [37]. Its diet consists primarily of shoaling pelagic fish, mostly caught by plunge diving. Birds can also be seen attending trawlers in large numbers [37]. This is a ground nesting species, usually within large colonies. The nest is built with seaweed, grass and earth stuck together with excreta [37].

FORAGING RANGE: Max 640 km, mean max 308.36 km, mean 140.09 km.

FORAGING DEPTH: Max 34 m, mean 8.8 m.

FORAGING TRIP DURATION: Mean 19.27 h (n=14)

KEY HABITATS: Deep-water depressions, tidal mixing fronts, shelf break, sandbanks, inshore and coastal waters.

KEY PREY ITEMS: Surface schooling fish, fisheries waste; discards important for some colonies and/or in some seasons.

KEY FORAGING ASSOCIATIONS: Often feed in association with cetaceans and large predatory fish species, which herd fish shoals into concentrations near surface where they can easily be taken with rapid, shallow dives; sometimes feed in multi-seabird species feeding assemblages.

DETAILED INFORMATION

General feeding behaviour
Gannets feed chiefly on fish taken by plunging from heights of 10-40 m or sighted when swimming with head immersed and caught following dive from the surface. They may hunt singly, but generally forage communally in large flocks of up to 1,000 over shoals of prey species. Significant differences in the foraging behaviour of males and females have been reported [24]; female gannets were not only more selective than males in the areas where they foraged, but they also made longer, deeper dives and spent more time on the sea surface than males. Fish and offal are also taken while swimming and Northern Gannets will rob other birds feeding at the surface [7].
During foraging trips, birds typically intersperse periods of rapid direct flight with periods of slow sinuous travel [18]. The initiation of such area-restricted search behaviour usually results from detection and pursuit of prey, although birds also make occasional dives without any subsequent deviation from rapid direct flight. GPS tracking of gannets at Grassholm, Wales revealed complex scale-dependent changes in flight speed and tortuosity as a function of differences in sea temperature, primary productivity, copepod abundance and the location of fishing vessels [32].

Foraging habitat – breeding season, migration and wintering period
Gannets generally forage in relatively shallow continental shelf or coastal waters [11, 17]. Studies suggest that gannets focus their activity on bathymetric features that are probably associated with high primary production [14]. One study found that the terminal destinations of foraging trips of satellite-tagged gannets were mainly associated with deep-water depressions in the North Sea (the Farne Deeps, Outer Silver Pit and Buchan Deep), and sandbanks (Dogger Bank, Wee Bankie, Halibut Bank) [14, 15]. In a later study, it was found that birds fitted with GPS loggers at Bass Rock, Scotland concentrated much of their foraging effort around a tidal mixing front offshore from the colony, even though trips encompassed both mixed coastal water and seasonally stratified water east of the tidal mixing front [18]. Nine foraging trips were restricted largely to coastal or frontal water and 6 trips extended well over stratified water beyond the front. By contrast, satellite-tracked gannets from Great Saltee Island, South East Ireland did not exhibit strong association with marine features, and foraged much closer to the colony [15], in the main spawning ground of the Celtic Sea Herring (Clupea harengus) stock [1]. Individual birds at Bass Rock showed a high degree of foraging-area fidelity, with successive trips having very similar bearings and significant repeatability in distance travelled, whilst birds at Great Saltee did not show this behaviour [15].
As an opportunistic feeder, gannets utilize prey species as they become available through the season, as they move in and out of inshore and offshore waters through-out the breeding season [14, 26]. Around the U.K., gannet distribution changes from around the shelf-break in the pre-breeding season, to the inner continental shelf, especially around the colonies, once breeding commences [36]. Considerable numbers are also recorded in some inshore and coastal areas, with moderate densities in the Minches (off the coast of western Scotland) and around the Outer Hebrides, Scotland [36].
A study of Northern Gannets on Bas Rock showed that observed individuals attended the breeding colony on Bass Rock until between 24 September and 16 October. Afterwards, individual birds engaged in different migratory behaviour. Of the 22 birds tracked until at least December, 18% wintered in the North Sea and the English Channel, 27% in the Bay of Biscay and the Celtic Sea, 9% in the Mediterranean Sea and 45% off West Africa. Individual winter home ranges as measured by the 75% kernel density contours varied between 8100 and 308 500 km2 (mean = 134 000 km2). Several Northern Gannets migrated northwards from Bass Rock after leaving the colony for a stay of a few days to a few weeks, independent of whether they migrated to Africa or other southern areas later. Birds wintering off West Africa migrated to their wintering areas mostly within 3 to 5 weeks, usually starting between early and late October. Most of these birds stayed off West Africa for a period of about 3 months, where they remained in a relatively restricted area. Return migration was initiated between the end of January and mid-February, and took about as long as autumn migration [21].
When discards form an important component of the diet, the distribution of gannets is likely to be influenced by that of traditional commercial fishing grounds [2]. For instance, in the North Sea there are a number of traditional commercial fishing grounds for pelagic trawlers for herring and mackerel Scomber scombrus [4, 29] and industrial sandeel Ammodytes sp. fisheries [2], which coincide with major feeding areas for gannets from Bass Rock [14, 15, 17, 35]. However, a separate study found only weak evidence to suggest that the average density of fishing vessels over 4 years influenced foraging behavior [32]. In another study, it was found that gannet density was strongly related to the biomass index high in the water column (8-11 m depth) indicative of shoaling pelagic fish [30].

Important foraging associations with other species
Northern Gannets often feed in association with cetaceans and large predatory fish species such as Bluefish Pomatomus saltatrix, which herd fish shoals into “frenzied” concentrations near surface where they can easily be taken with rapid, shallow dives [4, 6]. Gannets sometimes feed in multi-species feeding assemblages (often ruining the feeding opportunities for other species).

Diet
The Northern Gannet is a voracious, opportunistic, generalist predator that feeds on a wide variety of species and sizes of prey [14, 25, 32], mostly surface schooling fish and squid, with significant geographic, annual and seasonal variation in diet. Gannets are also among the dominant scavengers for discards from trawlers [1, 5, 7, 19, 25]. The amount of fishery discards consumed varies among individuals, with evidence of specialization [32]. Differences in diet between adults and young, if any, are quite small [20]. Recent evidence from stable isotopes indicates sex-specific differences in gannet foraging behavior (Votier et al. unpublished).
In the north-eastern Atlantic region, the main fish species taken include mackerel and herring, although in some localities gannets depend heavily on other species, such as Capelin Mallotus villosus off Norway, Saithe Pollachius virens off Iceland, and cod Gadus morhua in North Sea, as well as whiting Merlangius merlangus, haddock Melanogrammus aeglefinus, sprat Sprattus sprattus, Pilchard Sardina pilchardus, and Garfish Belone belone. Gannets from Bass Rock preyed extensively on sandeels, mainly Ammodytes marinus in 2003 [17], which are concentrated in the region of an offshore tidal mixing front. In North American waters, the most important prey species taken during the breeding season, are mackerel, Short-finned Squid Illex illecebrosus, Capelin and herring [28]. Gannets also take fish from fishing nets near surface which include species normally out of diving range, such as Blue Whiting (Micromesistius poutassou), Dolphin Fish (Coryphaena spp.), Blue Ling (Molva dipterygia), Lemon Sole (Microstomus kitt), Common Dab (Limanda limanda), and Long Rough Dab (Hippoglossoides platessoides).
Actual prey selection appears to be a compromise between preferred larger (more than 30 cm), more energy-rich (7.0 kJ/g) species [14, 26], and prey availability within foraging distance of breeding colonies [27], consequently there are often large differences in diet between locations, years and seasons. For instance, parental food loads at the Norwegian colonies were lighter and contained less energy than those measured in Newfoundland, Canada [20] and Scotland [34]. Gannets at Bass Rock took a significantly higher proportion of mackerel than birds from Great Saltee, Ireland and although there was no significant difference between colonies in the proportion of sandeels, clupeids (Clupeidae) and gadoids (Gadidae) taken, birds from Great Saltee took a significantly higher proportion of other species than birds from the Bass Rock [25]. On St Kilda, Scotland in 1987, sandeels comprised 48% by weight, mackerel 21%, herring 10%, and whitefish (probably discards) 6% [22]. Gannets also readily alter their diet in response to changes in food availability. For example, gannets at Hermaness, Scotland fed primarily on sandeel in 1981, but switched to a diet comprising mackerel, herring and gadoids when sandeel availability crashed in 1987-88 [26]. However, there was no variation in reproduction, suggesting Northern Gannets are able to switch prey without affecting breeding success [26]. Similarly, mackerel formed the majority of the diet (c. 70% by weight) of gannet chicks on Ailsa Craig, Scotland in 1975-76 and 1981; however, in 1983 over 75% of fish were sandeels and just 12% mackerel [34]. The wide variation in species and sizes of prey utilized by gannets and ability to switch prey suggests that they are better buffered against reductions in food supply than most other seabird species.

Foraging range
Gannets are capable of traveling >1000 km on a single round-trip to obtain food [11, 17, 32]. The range of foraging trips from breeding colonies varies with colony location, stage of breeding cycle, and distribution patterns of the primary prey species [20, 29].
Gannets from the Bass Rock were frequently recorded feeding at the Wee Bankie (approximately 33 km away) or off Fife Ness around 20 km away [35]. Later work at Bass Rock (field work undertaken in July and August 2003) found the mean distance covered per trip by gannets fitted with a GPS logger and external depth and temperature probe was 439.8 ± 234.4 km (range=159-984 km) and a mean foraging range of 155.2 ± 65.3 km (range=68–276 km) [18]. Ship-based transects around St Kilda, Scotland found large feeding flocks of gannets within a few km of Boreray and several smaller groups were seen over the Whale Rock Bank, c.40 km away [22]. Adult birds were also seen at the extremities of the survey tracklines 90 km from the island. One study found that the highest concentrations of gannets were within 100 km of the nearest breeding colony [31]. The maximum foraging range of birds from Noss, U.K. appeared to be 150 km, but with the majority seen within 37 km. There was also a seasonal trend in gannet distribution around Noss, with birds apparently travelling further from the colony in July [31]. Gannets from Rouzic, France were found to travel extensive distances when at sea, with an average foraging path length of 480 km (max. 856 km), but with a mean foraging range of 100 km (max. 176 km, min. 40 km; [13]).
Foraging range, as inferred from trip duration, also increases significantly with increasing population size, probably as a result of interference competition between birds within a colony and, to a lesser extent, from neighbouring colonies [23]. For instance, a series of U.K. based studies found that the foraging range of satellite-tracked gannets breeding at the Bass Rock averaged 232 km (±95 km, range 39 km to 540 km) and destinations covered a wide area of ocean encompassing >200,000 km2 within the northwest, west and central North Sea [14, 15, 16]. In contrast, satellite-tracked gannets from the much smaller population at Great Saltee Island, Ireland foraged much closer to the colony - mean 89 km (±49 km, range 14 km to 293 km); and covered about 45,000 km2 between the coasts of northwest Wales, southwest England and southern Ireland, about a quarter of the area covered by the Bass Rock birds [15, 16].
Complete tracks obtained for 11 foraging trips from 7 Northern Gannets, equipped with GPS and TD loggers, from the colony on Funk Island, Newfoundland, Canada [12] showed that dives were also not spread through the whole foraging area but occurred relatively close to the coast, at distances of 32–70 km (mean 51 ±8 km), mostly in an area southwest of Funk Island. Another study also recorded that most dives (67%) by birds from the Bass Rock, Scotland were on the outward phase of the trip prior to reaching the maximum foraging range, with relatively few dives on the return leg [18].
One study found significant positive relationships between foraging trip duration and maximum foraging range, and between foraging trip duration and foraging path length of Northern Gannets breeding at a colony at Rouzic [13]. Birds from this colony were fitted with GPS data loggers (some with combined GPS data logger and time depth recorder), and the results showed the positive relationships with trip duration according to the equations: Maximum foraging range (km) = 2.88 x foraging trip duration (h) + 49.2; and Foraging path length (km) = 21.06 x foraging trip duration (h) + 66.0. Another study found a highly significant relationship between trip duration and the maximum radial distance from the colony for satellite tagged gannets, indicating that distance travelled could be predicted with a high degree of accuracy from time spent at sea, and following the equation: Maximum distance from the colony while foraging (km) = 7.05 (SE ±0.22) x trip duration (h) [14]. In a separate study, foraging ranges at Hermaness were determined by using time-activity recorders that logged the times birds were flying, resting on the water and feeding [8]. The times between the departure from the colony and first ingestion of prey and between take-off after the last ingestion of prey and arrival at the colony were logged. The duration of these movements was multiplied by a flight-speed of 14.9 m/s to estimate a maximum foraging range of 128 km.
Fishing trips usually last 7–13 h, but may extend to 2–3 days [7, 29]. One study suggested a range of 320 to 480 km from a colony for breeding birds, based on observed trip durations and assuming continuous flight at 65 to 80 km/h [29]. However, a separate study series found that whilst birds attained maximum flight speeds of 80 km/h or more, the average speed of travel over entire foraging trips was only 14.1 ± 0.4 km/h at Bass Rock (mean travel speed for individual birds of 15.7 ±2.5km/h) and 13.8 ± 0.8 km/h at Great Saltee (mean travel speed for individual birds of 15.0 ±2.3 km/h) [14, 15, 16], and so the other estimate [29] of foraging range is likely to be too high. Indeed, using an overall rate of radial travel of 14 km/h, BirdLife International calculated the foraging range for other colonies at which trip durations have been recorded [2]. At Bass Rock in the 1960s the average range would have been 70 km, with 94% within 135 km [29]; at Ailsa Craig the average range would have been 87.5 km, with 84% within 135 km; at Bempton Cliffs, England the average would have been 40 km with 90% within 91 km [29]; at St. Kilda the average range would have been approximately 148 km with a maximum 170 km [33]; and at Hermaness the average range was calculated at 92 km and the maximum 190 km [8].
A study at Bass Rock found the mean duration of foraging trips was 21.5 ± 6.7 h (range= 11.1–34.9 h, n=15 trips from 13 birds), and on average, birds spent 41.6 ± 11.7% (range=22.0–60.4%) of each foraging trip in flight, with an average flight speed of 44.7 ± 7.2 km/h (range = 33.4–60.2 km/h) [18]. In other studies, flight speed over ground has been recorded between 53.6km/h and 58.4km/h [12]. However, another study found that average flight speed was only around 49 km/h but birds regularly reached horizontal speeds of over 100 km/h (max. 129 km/h) [13]. Chick-rearing adults on Funk Island equipped with different types of data loggers had foraging trip durations ranging from 3.0–39.2 h (median 13.5 h) and showed a bimodal distribution, coinciding with a partitioning of foraging trips into 1- and 2-day trips [10].




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Table 1: Showing average value (km) of foraging range in different country/sea areas.


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Figure 1: Showing cumulative frequency (with standard deviation) and proportion of birds found foraging at different distances from colony. Source: Birdlife Seabird Foraging Range Database


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Figure 2: Morus bassanus. Foraging ranges and destinations of foraging trips by gannets from Bass Rock, SE Scotland, and Great Saltee, SE Ireland. White dots = Locations of adults at sea; Black dots = destination bearings of foraging trips. Source Hamer et al (2001)


Diving and depth association
Northern Gannets plunge dive to 12–15 m [28] but birds can go as deep as 19-20 m [12, 18, 24], and occasionally even deeper [3, 9]. They feed diurnally and gannets do not dive until after they have located prey [29].
One study reported a bimodal frequency distribution of dive depths with two peaks in depth usage at 0.3–1 m and 6–7 m, indicating a clear separation between shallow dives (<2 m) and deep dives (≥2 m) [18]. The mean proportion of shallow dives per trip was nine times higher in mixed coastal water (81.6 ± 26.5%) than in stratified water beyond the tidal mixing front (9.0 ± 8.7%). Using underwater data-logging systems attached to adult gannets at Funk Island, another study distinguished two different dive types: relatively short, shallow, V-shaped dives and relatively long, deep, U-shaped dives [9]. Most dives made were relatively shallow; only 10% of all dives (n=336) were >10 m deep, and the mean depth of all dives was 5.2 m (range <1.0–22.0 m). The maximum depth of V-shaped dives was 8.7 m, and of U-shaped dives 22 m; U-shaped dives occurred at all depths, but were most numerous at depths >8.0 m. On basis of food collected from telemetered birds, U-shaped dives were directed at deep schools of capelin [9]. A later study found dives were mostly relatively shallow, with about 50% of them reaching up to 3.8 m and 10% of the dives reaching as far deep as 9.3 m [12]. Mean dive depth was 4.7 m, and on the deepest dive a Northern Gannet reached 19.1 m. In one study, it was found that females consistently made deeper and thus longer dives than males [24]. The deepest and longest dives recorded were 11 m and 28 s for males, compared with 18 m and 37 s for females.
A study based on a colony in France found that birds fitted with a data logger dived to a maximum depth of 12 m, but most dives were much shallower, with principal modes around 2 and 6 m [13].

References
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Compiled by: Ben Lascelles, Nigel Varty, Kate Tanner, Rory McCann