Oecologia (1988) 76:155 157 Oecologia �9 Spfinger-Verlag 1988

Short communications

Kelp gulls Larus dominicanus as predators in kelp Macrocystis pyrifera beds P.A.R. Hockey Percy FitzPatrick Institute of African Ornithology, University of Cape Town, Rondebosch 7700, South Africa

Summary. Minimum daily consumption of bivalves Gaimar- dia sp by kelp gulls in a Macrocystis kelp bed at Porvenir, Tierra del Fuego, was calculated to be ca. 525 000 individ- uals, corresponding to an energy removal of ca. 21 k J . m - 2 . d - 1. This is far in excess of energy removal by any other higher-trophic-level predator in southern Chilean Macro- cystis beds, but previous work has ignored kelp gulls as a component of the kelp bed community.

Key words: Kelp gulls - Gaimardia - Kelp - Predation - Energy flow

Birds as predators frequently are ignored in studies of tro- phic pathways in intertidal and nearshore communities (Hockey and Bosman, in press), despite their potential im- portance by virtue of their mobility and high energy de- mand (Edwards et al. 1982). Trophic pathways in kelp Mac- rocystis pyrifera beds in southern Chile have been described (Dayton 1974; Dayton et al. 1973, 1977; Castilla and Mor- eno 1982; Castilla 1985), and, with respect to birds, the presence of the predatory Magellanic flightless steamer duck Tachyeres pteneres has been mentioned (Castilla 1985). However, the potential predatory impact of kelp gulls Larus dominicanus has not been assessed, although Murphy (1936) recognized that they feed in Macrocystis beds.

This report deals with the minimmn daily consumption of food by kelp gulls feeding in nearshore Macrocystis beds in the Straits of Magellan,, and reaffirms the importance of considering birds in trophic studies of such communities.

Study area and methods

Observations of kelp gulls feeding in a Macrocystis bed 1800 m 2 in extent were made in a sheltered bay at Porvenir (53 18 S, 70 22 W), Tierra del Fuego, during 20-23 No- vember 1985. On 22 November, the birds were observed continuously for 9 hours (70% of a full tidal cycle), and their behaviour (feeding or not feeding) was recorded at 10-minute intervals using the instantaneous scantechnique (Altmann 1974). Three age classes were recognised on the basis of plumage characteristics. In addition, feeding rates (number of food items swallowed per minute) were recorded

throughout the study period. To confirm prey identifica- tion, samples of regurgitated pellets were collected from the adjacent shoreline to which birds flew to roost after feeding.

Results

The kelp gulls fed almost exclusively (>99%) on small (< 10 mm) epifaunal bivalves Gaimardia sp which they picked from the upper stipes and floating fronds of the kelp, sometimes by immersing the head and neck. Similar foraging techniques were used by the small numbers of Chi- lean skuas Catharacta chilensis present, which also fed on Gaimardia. Both feeding activity and feeding rates of adults and immatures peaked approximately 2.5 h before low tide (Fig. 1), and the numbers of birds feeding were maximal 1-2 h after the period of maximum feeding rate. The largest number of gulls present during one observation was 96, and the maximum feeding rate recorded was 89 Gaimardia min-1. During the observation period, 525 021 Gaimardia were calculated to be removed, equivalent to approximately 290 individuals m-2 (Table 1). Given that birds were feed- ing at both the start and finish of the observation period,

80

20-

60 mean

% feeding

4O

Immatures

Adul ts

1 0 0 -

LOW TIDE

toloo 12:oo 14100 ~d.oo 18:oo t ime

Fig. 1. Feeding activity of adult and immature kelp gulls in a Mac- rocystis bed at Porvenir

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Table l. Feeding activity and ingestion rates of Kelp Gulls preying on Gaimardia in a Macrocystis bed at Porvenir

Immature Subadult Adult

N feeding Ingestion N Con- N feeding Ingestion N Con- N feeding Ingestion N Con- mean_+ SD rate min 1 obs sump- mean_+ SD rate rain- 1 obs sump- mean_+ SD rate min- 1 obs sump-

tion tion tion n.h-1 n.h-1 n.h-1

0940-1030 3.7__+2.5 10.0+ 3.8 7 2220 0.3-t-0.5 no data 12.2_+ 6.1 24.1-t- 9.8 16 17641 1040-1130 0.8_+1.2 nodata 0 - 9.5+ 8.3 22.3_+ 9.0 26 12711 1140-1230 2.0+1.3 nodata 1.0_+0.9 30.6-t-8.7 5 1836 17.6-+10.9 41.1-+12.1 31 43402 1240-1330 3.5-t-0.7 50.5_+ 9.0 6 10605 3.5_+0.7 no data 37.5-t- 0.7 50.0 2 112500 1340-1430 7.3_+0.8 17.9_+ 7.0 17 7840 1 .3_+0.5 15.3-t-2.5 3 1193 51.2-t- 6.2 20.5-t- 6.7 10 62976 1440-1530 14.4___2.0 17.7_+10.1 9 15293 4.0_+0.6 no data 53.0-1- 4.0 19.7+_10.0 12 62646 1540-1630 12.2_+3.4 15.3_+ 4.8 7 11200 3.5-t-1.0 no data 50.7_+12.6 14.9_+ 9.7 14 45326 1640-1730 14.3+1.8 23.8+ 8.9 16 20420 5.0-+2.1 no data 45.0-+ 4.0 18.2-+ 6.9 19 49140 1740-1830 11.0_+2.4 14.5_+ 7.0 15 9570 1.7_+1.2 no data 31.0___ 7.8 20.7_+10.6 15 38502

TotM 77148 3029 444844

actual p reda tory removal of Gaimardia by gulls p robab ly was > 300 ind iv idua l s -m-2 , d -1 . Adul t gulls, by virtue of their abundance rather than greater feeding rate, accounted for ca. 85% of the removal (Table 1).

Discussion

The maximum density recorded for a single species of high- er-trophic-level p reda tor in a Macrocystis communi ty in the Beagle Channel (55 05 S, 67 04 W) was 9.7 individuals per 100 m 2 for the starfish Comasterias lurida, which preys on Gaimardia to a limited extent (Castil la 1985). Blankley (1985) repor ted mean starfish Anasterias rupicola densities of 17.5 per m 2 in Macrocystis beds and intert idal areas at Mar ion Is land (46 54 S, 37 45 E). The maximum avian preda tor density recorded by Castil la (1985) was 0.001 birds per 100 m 2 for Tachyeres pteneres. In contrast , the maxi- mum density of kelp gulls recorded in this s tudy was 5.3 birds per 100 m 2. Using da ta in Blankley (1985) and the equations of Walsberg (1983), calculated density of kelp gulls at Mar ion Is land is ca. 0.1 birds per 100 na 2 of Macro- cystis bed.

Kelp gulls are able to retrieve prey only from the upper few centimetres of the water column, and thus prey avail- abili ty in Macrocystis beds is temporal ly restricted by t idal rise and fall. The peak in foraging efficiency before low tide may indicate that there is appreciable prey deplet ion on a daily basis. Despite the above, the consumpt ion by gulls of Gaimardia at Porvenir, measured either as numbers of prey or amount of energy, p robab ly is greater than that of any other higher-trophic-level predators present. Such intense predat ion, assuming that it is not an episodic event, p robab ly depends on very high product ivi ty of Gaimardia and rapid growth of Macrocystis plants, making available to the gulls Gaimardia which have initially settled too low on the plant stipe to be accessible to the birds.

A t Mar ion Is land kelp gulls prey on relatively large (15-30 ram) Gaimardia trapezina. Blankley (1982) repor ted low densities o f G. trapezina in the surface layer o f Macro- cystis compared with densities in deeper strata, and at t r ib- uted this difference to preda t ion by gulls. Assuming that the Gaimardia at Porvenir are of comparable size and ener- gy content to the ' i m m a t u r e ' G. trapezina of Blankley

(1985), the minimum daily energy removal by gulls of Gai- mardia in this s tudy was ca. 21 kJ. m - 2 d - 1 , compared with 0.42 kJ. m - Zd- 1 at Mar ion Island. Ext rapola ted to the area of the kelp bed, this is equivalent to the D E E of 47 kelp gulls (Walsberg 1983). This ext rapola t ion is suppor ted by the mean hourly number o f foraging gulls in the study area (N=44) .

This short study serves to highlight the potent ial impor- tance of mobile predators with high energy demands, in this case birds, in the t rophic and energy pathways o f inter- t idal and nearshore communities. Energy consumpt ion per unit area was greater than that of any preda tor studied at Mar ion Is land (Blankley 1985), and was far in excess of the p reda tor consumption rates that may be deduced from da ta presented by Castil la (1985) for Maeroeystis beds in the Beagle Channel.

Acknowledgements. This work was supported financially by the South African CSIR and the University of Cape Town, and formed part of the Percy FitzPatrick Institute's 25th Anniversary expedi- tion to Chile.

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Received September 20, 1987