15 June 2014

Declining penguin numbers on Marion Island most likely due to rising sea temperatures

SUB-ANTARCTIC - Numbers of penguins and other seabirds on sub-Antarctic Marion Island are declining, Nelson Mandela Metropolitan University (NMMU) researchers have found.

An NMMU team returned from the island in May after a five-week research trip. Marion Island, situated in the Indian Ocean, is a part of South Africa. It is a breeding location for southern rockhopper, macaroni, gentoo and king penguins.

Over the last 20 years, the island's southern rockhopper penguins have decreased by 70% and macaroni and gentoo penguins by 30%. There are currently 40,000 breeding pairs of rockhopper penguins, 290,000 breeding pairs of macaroni penguins, and only 900 breeding pairs of gentoo penguins on Marion Island.

NMMU zoology senior lecturer Dr Pierre Pistorius said, “The most likely explanation for the declining numbers of several of the seabirds breeding on the island is that oceanographic conditions are changing and sea temperatures are rising.

"The Antarctic Polar Front, an important foraging area, is moving further south and away from the island due to ocean warming and this places greater energy demands on foraging seabirds commuting back and forth to provide food for their chicks.

Each year, the rockhopper and macaroni penguins are weighed when they arrive on the island to breed. This gives researchers an indication of prey availability.

“Over the past few years, there has been a substantial drop in the ‘arrival weight’ of macaroni and southern rockhopper penguins, clearly indicating that food is limited, which results in poor breeding success,” said Pistorius.

Inshore-foraging gentoo penguins have experienced a similar decline in numbers. Pistorius said this was probably associated with the virtual disappearance of a species of krill endemic to Marion Island that once formed a major part of their diet.

“There are big changes to both the inshore and offshore environment around Marion Island.”

“Inadequate breeding success has influenced the decrease of macaroni and rockhopper penguins, largely because they are starting the breeding season with lower body weight and energy reserves than before.”

“They are now arriving in poorer body condition than they did 10 to 20 years ago. They are often not finding sufficient food and this leads to high chick mortality and poor subsequent recruitment into the adult population.”

There is not much fishing activity in this area, but Pistorius believes that warmer temperatures are influencing prey availability partly by forcing food sources to move further south.

“These penguins will often swim hundreds of kilometres during a single foraging trip. They go where food is predictable, but the level of predictability has diminished and they are swimming further to find food and bringing less back for their offspring.”

King penguins swim even further – up to about 3000km on a single foraging trip. Among the penguin populations on Marion Island, their numbers are the most stable, but this could also be due to the fact that they dive to great depths to find their prey, where conditions are less likely to change.

During the research trip, Pistorius continued an existing programme to fit GPS devices to king penguins, to get a more accurate picture of their foraging distribution.

“We know little about them, despite Marion Island having one of the largest populations of king penguins in the world.”

He also continued a project started last year to fit passive integrated transponders (microchips) to macaroni penguins. The data collected enables scientists to work out individual survival rates and the durations of their foraging trips.

“A long time spent foraging is an indication of a shortage of food,” he said.

“Seabirds as a group are among the most threatened of any group of animals … The sub-Antarctic regions have globally significant proportions of seabirds.

"We monitor numbers and breeding performance every year as part of a long-term monitoring programme. This gives us an indication of their conservation status as well as any changes in the subantarctic marine ecosystem.”

Pistorius said global change showed its most severe impacts at high latitudes (polar regions).

“By studying top predators like seabirds, you get a window into changes in the ecology of these environments. If foraging or demographic parameters change, it reflects changes in the food chain.”

Source
Declining numbers of seabirds at Marion Island [press release], 5 June 2014, Nelson Mandela Metropolitan University

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