Authors show guano birds declined because food source was depleted by 85 percent
In the late 19th and early 20th centuries, the tens of millions of marine birds living in the "Bird Islands of Peru" became famous around the world. This was due to their appeal as a visual spectacle and because they became economically important as high producers of guano, droppings that the country mined and exported around the world for fertilizer.
A new study published in the current issue of the journal Fisheries Oceanography says the populations of these famous birds have declined dramatically in the last 40 years, largely because the availability of their main food supply, anchovies, once plentiful, has been severely reduced by the Peruvian fishery.
The authors of the study are Jaime Jahncke and George Hunt of the University of California, Irvine, and David Checkley of Scripps Institution of Oceanography at the University of California, San Diego.
The results are the product of a new scientific model developed by Jahncke while studying with Checkley at Scripps Institution. The model characterizes the Peruvian marine ecosystem by cross-referencing natural factors such as wind strength with human influences through fishing activity.
"We saw a very different system prior to and after fishing was increased," said Jahncke. "This model gives us a very clear example of how physical processes in the ocean and human influences can limit the populations of these birds, changing them from one state to another."
The Peruvian anchovy fishery, the largest single-species fishery in the world, significantly increased production in the 1950s and '60s, exporting millions of metric tons of anchovies per year.
The study shows that wind forces in the early part of the 20th century caused a significant rise in the nutrient supply off the Peruvian coast and thereby led to a boom in the anchovy population. With more food available, the numbers of guano-producing seabirds, including cormorants, boobies and pelicans, similarly increased from 1925 to 1955. But in the decades that followed, the seabird populations declined significantly. "The decrease," the authors note, "appears to be due to the depletion of their food by the fishery, which grew to catch about 85 percent of the prey otherwise available to the seabirds."
All told, the latter half of the 20th century saw a dramatic decline--from about 20 million seabirds to about five million, according to the paper. Today, many of the so-called bird islands of Peru are largely devoid of seabirds. "Birds are very visible manifestations of the health of an ecosystem," said Checkley, a professor in the Integrative Oceanography Division at Scripps. "This study demonstrates how fishing can profoundly affect an ecosystem. That's important because we are coming to a time in the United States and elsewhere in the world when people recognize that fisheries have to be managed not only based upon the dynamics of the individual populations, but on the ecosystem in which they reside. In order to do that we have to understand how fisheries affect the ecosystem and vice versa."
The authors say their study would not have been possible without the prominent leadership and high quality of long-term scientific data provided by the Peruvian fisheries agency, called the Instituto del Mar del Peru (IMARPE). The authors are aware of IMARPE's effort to change from single-species management to an ecosystem-based approach, and to consider a more reasonable anchovy harvest.
Checkley notes that although the fishery has profoundly impacted the abundance of these birds, a reversal is certainly possible.
"The important point is that we achieve an understanding of the causes of these population changes over time and manage them accordingly," said Checkley. "Once we have the understanding I think it's really important that management and policy decisions are based on that understanding."
Source: Eurekalert & othersLast reviewed: By John M. Grohol, Psy.D. on 21 Feb 2009
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