going, going, Gannet... Tough times for Benguela seabirds The endemic Cape Gannet breeds at only six colonies, three each in Namibia and South Africa. The Namibian colonies have been decreas- ing for decades, following the collapse of pelagic fish stocks, but the South African colonies have held their own. Recently, however, the two west coast colonies at Lambert’s Bay and Malgas Island have suffered a shortage of food and increased predation pressure, culminating in Lambert’s Bay being aban- doned in 2005 when Cape fur seals entered the colony to kill adult birds. Active management by CapeNature has tackled this problem, but the birds still face a daunting array of threats.
TEXT & PHOTOGRAPHS BY PETER RYAN
This sign warning visitors to Malgas Island not to disturb gannets breeding near the old guano houses is now redundant. A few pairs attempted to breed there in 2006, but these peripheral groups suffered the heaviest predation pressure from gulls and pelicans.
30 gannets africa – birds & birding august/september 2006 etosha national park 31 martin harvey Unfortunately, this happy state of affairs didn’t last. Since the late 1990s, pelagic fish stocks have been moving steadily eastwards from their traditional west coast haunts onto the Agulhas Bank. Quite what is driving this shift is unclear, but the net result is that by 2004 there was little by way of sardines or anchovies left in the areas accessible to gannets breeding at west coast colonies. he Benguela is one of the world’s most productive coastal ecosystems. The The birds have been forced to switch their diet, eating more saury, a warm- prevailing southerly winds and Coriolis force combine to drive surface waters water pelagic fish found further offshore, offshore, allowing clear, nutrient-rich bottom water to well up to the surface. as well as hake scavenged from demersal trawlers. Much of this hake is in the T As this water warms up, large-celled new stable state dominated by species form of offal and discards, and is a lower diatoms flourish, forming dense blooms with little value to seabirds. As a result, quality than their normal prey. As a that provide food for zooplankton and Cape Gannet numbers have fallen dra- result, the gannets’ breeding success has filter-feeding pelagic fish such as sardines matically, decreasing more than 80 per suffered, with some chicks abandoned and anchovies. This predictable, highly cent from some 125 000 pairs in the or starving. efficient food chain supports a host of 1950s to fewer than 20 000 pairs today. top predators, including three endem- The colony at Possession Island is on f this were the only problem facing ic seabirds: African Penguin Spheniscus the verge of extinction, having plum- the gannets, it might not be too demersus, Cape Cormorant Phalacrocorax meted from 15 000 pairs to only 750. serious. The Benguela is a variable capensis and Cape Gannet Morus capen- Off South Africa, anchovies largely system that has undergone signi- sis. These birds used to occur in such replaced sardines. Anchovies are small- ficantI changes historically, with large also can be a major problem, not only Opposite During the 1990s, when large numbers that their guano was er and are less efficient for gannets to fluctuations in fish abundance and from spills of crude oil and ship fuel, but Malgas Island supported the largest Cape mined for use as fertiliser. Now the popu- catch, but the gannet population re- alternating dominance by sardines and also fish oil from processing plants. Gannet colony in the world, gannets lations of all three are decreasing, thanks mained stable or even increased, aided anchovies. Gannets and other predators But perhaps the most worrying issue covered most of the island’s surface. By to human impacts on the system. in part by fishing restrictions that saw have evolved to cope with this variabil- is predation at the breeding colonies. 2006, large gaps were obvious, making it
Lambert’s Bay A purse-seine fishery competes direct- a slow but steady recovery in sardine ity and, given relatively slow changes, Cape fur seals also are suffering from easier for predators to enter the colony. ly with the birds for pelagic fish and, in numbers in the 1980s. Interestingly, they could doubtless adapt. However, a shortage of fish prey, and some indi- the 1970s, overfishing caused sardine it was routine monitoring of gannet gannets face numerous other threats. At viduals have resorted to eating seabirds. Above Some pairs of Kelp Gulls special- south africa stocks to collapse. Off Namibia, this led diet that first recorded this recovery, as sea, some are killed when they become This has happened from time to time ise in raiding gannet nests for eggs and Saldanha Malgas Is. Western Cape to a vast increase in the abundance of seabirds are more efficient and selective entangled in fishing gear or on long- in the past, but the numbers of seals chicks. Their impact can be estimated gelatinous animals such as jellyfish and predators than purse-seine boats. By lines. A recent study estimated that some involved (and hence the numbers of from the middens of broken eggs that Dassen Is. salps. These indiscriminate filter-feeders the mid-1990s, sardine numbers had 2 000 gannets are killed each year off birds killed) has increased significantly accumulate near each nest, with pairs
CAPE TOWN now proliferate in Namibian waters, recovered to the point where a sub- South Africa alone. Mitigation measures in recent years. The most dramatic adding five to 10 new eggs every day. Mossel Bay Bird Is. efficiently catching not only zooplank- stantial fishery was once again possi- have now been implemented by the incident occurred at Lambert’s Bay in Gansbaai ton but also the eggs and larvae of the ble. Gannet numbers peaked at around trawl fishery, but the problem is likely to 2005, when seals killed several hundred
tl a nt i c oce n Dyer Is. Cape Agulhas few remaining pelagic fish, effectively 11 000 pairs at Lambert’s Bay and 58 000 get worse as more gannets are forced to adult gannets in the colony (see Africa a Indian ocean trapping the pelagic ecosystem in a pairs at Malgas Island. scavenge from vessels. Oil pollution can – Birds & Birding, April/May 2006).
32 gannets africa – birds & birding february/march 2007 gannets 33 This caused the entire colony to aban- feeding on seabird chicks on the island. mining in 1843–’45. Then, however, don the island, to the consternation of By 2005 they had spread their attacks to they faced just the single threat – today, the local tourism industry. CapeNature, the guano islands around Saldanha Bay, Cape Gannets are being challenged from who manages the island, deployed gan- including Malgas Island. Every day, up all sides. net decoys to attract the birds back to 120 pelicans descend on the gannet The only Cape Gannet colony that is in 2006, and have installed a resident colony, each wolfing down a couple of still thriving is at Bird Island in Algoa marksman who has permission to shoot chicks. Combined with the impacts of Bay. If anything, these birds have bene- any seals observed killing birds. The Kelp Gulls, relatively few chicks survive fited from the eastward shift of pelagic good news is that the gannets have to fledge and run the gauntlet of seals fish, and they appear to have fewer returned and are currently breeding, patrolling the waters off the island. problems with predators. One option albeit a little later than usual. Seal attacks on land also occurred ape Gannets are not the only at Malgas Island in 2005, but most species affected by these chang- seal predation takes place at sea. Newi es. Cape Cormorants have been Makhado is studying seal attacks at especially hard hit by pelicans. Malgas Island, and estimates that up to CResearcher Marta de Ponte estimates 6 000 gannets are killed in waters just that in 2006 fewer than 20 per cent of off the island each year. Most of these cormorant chicks escaped pelican pre- are newly fledged birds, representing as dation at islands around Saldanha Bay. much as 80 per cent of chick production When researchers were present on from the colony. Clearly such losses are Malgas Island they prevented pelicans not sustainable and, indeed, estimates from attacking cormorant nests close of the population at Malgas Island made to the houses at the northern side of by Marine and Coastal Management the island. However, during a hiatus indicate a steady decrease since the late of three days when no one was on the 1990s. Similar attacks occur regularly at island, the number of Cape Cormorant Lambert’s Bay, although now the seals nests fell from more than 1 200 to involved are being shot. fewer than 300, with only those nesting Seals are not the only predators target- under bushes or inside buildings surviv- ing breeding gannets. Kelp Gulls Larus ing the attention of the pelicans. Cape dominicanus are versatile predators, using Cormorants face additional threats. At for the west coast gannets would be Opposite The only Cape Cormorant a range of foraging techniques to obtain their main stronghold, Dyer Island, off to head east to join the colony at Bird nests to survive the pelican onslaught on a great diversity of prey. At gannet colo- the south coast near Gansbaai, they Island. However, established breeders Malgas Island are either inside buildings, nies, some gulls specialise in stealing have suffered repeated outbreaks of move colonies only under exceptional such as this old guano-scraper dormitory eggs and chicks. They cannot displace avian cholera, which has killed large circumstances. It is likely to be up to the (top) or out of reach on boulders or the larger gannets from their nests, but numbers of birds. young adults, returning to breed for the under bushes. In open areas (bottom), they either strike opportunistically when African Penguins also are in trouble first time, to shift allegiance eastwards. all the nests are abandoned, except for an incubating bird is squabbling with a on the west coast. They are the least Another option would be to establish one nest on the side of a boulder. This neighbour, or a pair team up, with one mobile of the three species that special- a colony on Dyer Island, close to the nest contained five chicks huddled distracting the gannet while its partner ise on pelagic fish. While feeding small centre of current pelagic fish distribu- together, probably including some grabs the egg or chick. At Malgas Island chicks, they require a reliable source of tion. Cape Gannets bred on the island refugees from adjacent nests. at least 10 pairs of gulls target gannets, food within a 20- to 30-kilometre radi- in the 19th century and attempted to each taking five to 10 eggs a day. Over us of their breeding islands. The virtual breed there in the 1980s. They could be Above An adult Cape Cormorant the course of a breeding season, this disappearance of pelagic fish from the encouraged to settle on the island using valiantly defends its nest against a gang amounts to several thousand eggs and west coast has reversed the gains made decoys similar to those used to entice of marauding pelicans on the island. chicks, and the impact of these losses during the 1990s, with penguin popu- birds back to Lambert’s Bay last year. Such birds have to temper bravado with becomes increasingly serious as the col- lations once more decreasing. But moving east may be only a tem- caution, however, because researchers ony gets smaller. At the same time, the This is not the first time that Benguela porary solution. As fish stocks disap- have observed at least two adult cormor gulls find it easier to get into the colony seabirds have faced severe challenges. pear along the west coast, the fishery ants being eaten. as it shrinks, with gaps forming among During the initial guano rush in the 19th is responding accordingly and heading the once solid mass of gannets. century, breeding islands were divided east. Purse-seiners traditionally fishing The other significant predator is a into cake-like segments among as many out of west coast ports such as Lambert’s new arrival on the scene. The Western as 20 ships, each with a gang of dig- Bay, St Helena Bay and Saldanha have Cape population of Great White Pelicans gers. This doubtless prevented all breed- moved to Gansbaai and Mossel Bay. Pelecanus onocrotalus has increased rapid- ing activity during the initial few years Unless a long-term solution can be found ly in recent years, fuelled in part by their of exploitation. Off Namibia, gannet that allows fisheries and other predators scavenging chicken offal from farmyards. colonies on Possession and Hollamsbird to coexist, we risk the spectre of a similar The pelicans breed on Dassen Island and, islands probably formed when birds from problem occurring on the south coast in as their population grew, they began Ichaboe Island were displaced by guano a few years’ time.
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