Overfishing linked to algal blooms

Nitrogenous fertilizers and detergents have long been known to cause algal blooms that block sunlight and strangle ecosystems, but a study now reveals that overfishing of large predatory fish is also playing a key part.

Britas Klemens Eriksson at the University of Groningen in the Netherlands noticed that populations of predatory fish in the Baltic Sea seemed to be declining in areas where algal blooms subsequently tended to form. Curious as to whether there was a connection, Eriksson and a team of colleagues from the Swedish Board of Fisheries in Öregrund set up an investigation.

The team reviewed a year's worth of field data on predatory pike (Esox lucius) and perch (Perca fluviatilis) populations from nine areas covering 700 kilometres of coastline in the Baltic Sea. They then compared this information with information collected during the same period on smaller fish and algal populations in the region. They found some intriguing patterns.

"In areas where there were strong declines in perch and pike there were massive increases in smaller fish and large blooms of algae," comments Eriksson. Where perch and pike populations were intact, the surrounding waters had a 10% chance of experiencing an algal bloom; in areas where their populations had been substantially reduced, the chances of an algal bloom were 50%.

Intrigued by these trends, the researchers ran small-scale field experiments for 2 years in unpolluted waters to investigate the forces responsible for controlling algal growth. They manipulated the environmental conditions in these experiments by: sometimes excluding large predatory fish through the use of cages; sometimes adding nitrogenous fertilizer pellets; sometimes applying both techniques; and sometimes leaving areas as untouched controls.

As expected, the nitrogenous pellets increased algal growth. But surprisingly, when predatory fish were prevented from accessing a given area, algae in that area became much more prevalent. The effect even proved to be true when nitrogenous pellets were not added to the system.

"This is the first study to show that top predators are linked to the formation of macroalgal blooms," says marine biologist Heike Lotze, at Dalhousie University in Nova Scotia, Canada.

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