The common murre chicks in the Baltic Sea are far more sensitive to high nutrional level in their prat than previously though. Sprat with high nutrient content are important to the chicks, since parents can only feed them a limited number of fish. Photo: F. Wilde/Azote

More not merrier for the murre

Quality food more important than quantity for seabirds in the Baltic

Seabirds are increasingly used as indicators of changes in marine ecosystems. One example is the fish-eating seabird the common murre (Uria aalge) in the Baltic Sea, which has been shown to switch to a 'junk food' diet, because of overfishing, eutrophication and changes in climate.

In a new paper published in the Marine Ecology Progress Series, centre researchers Martina Kadin, Henrik Österblom, Jonas Hentati-Sundberg and Olof Olsson dig even deeper into these large-scale changes in the Baltic Sea ecosystem.

Their paper provides a detailed study of the influence of food quantity and quality for the common murres in the region.

"Seabirds have become well-known symbols of coasts and oceans and their struggles with the consequences of human actions provide an illustration of the need to properly govern marine resources," explains Martina Kadin, first author of the study. She recently defended her Licentiate thesis on the effects of fisheries and fish stock dynamics on seabirds as well as the governance of marine resources.

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Good food or no food at all
In the Baltic Sea, the common murres raise their chicks on a herring like fish called sprat (Sprattus sprattus). Kadin and her colleagues investigated the effects of sprat quality versus the abundance of sprat during the chick-rearing period.

Their study, conducted in the period 2005-2009, was then compared to observations from the 1970s and 1990s. This comparison showed that food quality (measured as sprat weight-at-age), not quantity (sprat abundance), influenced the murre's fledging success.

In the 1990s, local sprat became unusually abundant after their main predator, the cod, collapsed because of overfishing and unfavourable reproductive conditions. Warmer winters since the end of the 1980s also helped boost sprat reproduction.

But as sprats became more abundant they were forced to compete between each other for limited supplies of their own food, zooplankton, and ended up as leaner and less-nutritious food for the murre chicks.

Ignorance is not always bliss
Contrary to previous claims that the birds would adapt and spend more time foraging, Kadin and her colleagues show that despite increased abundance of sprat, the chicks grew poorly. One explanation is that the parents did not compromise their own survival for the provisioning of the chicks. Put differently, they did not spend more time looking for qualitatively good food.

But before you report the bad parent murres to the social services, there might be a different explanation. The adult murre is simply not able to recognise the decrease in nutrition from the sprat.

"Lower mass of individual prey may not influence adult birds, provided food is abundant. Chicks, however, with a smaller digestive capacity, could become satiated without receiving sufficient energy," Kadin explains.

Important beyond the Baltic Sea
The new findings are important beyond the Baltic Sea. Other marine top predators around the world are impacted by changes in their food abundance and food quality, due to overfishing and climate change.

"Our findings can aid the selection and interpretation of indicators for ecosystem approaches to Baltic Sea fisheries management, and further be used to communicate the need for such approaches elsewhere, “Kadin says.

Source: Kadin M, Österblom H, Hentati-Sundberg J, Olsson O (2012) Contrasting effects of food quality and quantity on a marine top predator. Mar Ecol Prog Ser 444:239-249.

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Kadin M, Österblom H, Hentati-Sundberg J, Olsson O (2012) Contrasting effects of food quality and quantity on a marine top predator. Mar Ecol Prog Ser 444:239-249.

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