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The water's always greener....

The water's always greener....

"Why bother?" This is the question scientists have long asked about the 128 species of fish known to migrate at various times in their life cycles between the ocean and fresh water. Large-scale migrations are generally considered a risky business in biology, with large expenditures of energy required and, in some cases, increased chances of mortality. Moreover, when migrations span fresh-and saltwater environments, fish need to make significant physiological adjustments to cope with the changes in osmotic conditions.

The reason for such persistence, researchers report in the March 11 SCIENCE, may be simple: The fish are hungry.

Scientists at the University of Toronto and at the Ministry of Agriculture and Fisheries in Christchurch, New Zealand, compared data on the global distribution of diadromas fish (fish that migrate between the ocean and fresh water) with patterns of primary productivity in various aquatic environments.

Recent studies have shown that anadromous fish -- those that are born in fresh water, spend adulthood in the ocean and return to fresh water only to spawn -- are more common in temperate and arctic latitudes. Catadromous fish -- those that are born in the ocean, migrate to fresh water and return to the ocean to spawn -- are common in the tropics. The researchers reviewed known measures of aquatic productivity (measured in grams and carbon fixed per square meter per year and considered a good indicator of food availability) and found a strong latitudinal correlation similar to that describing fish migratory patterns. In the tropics, freshwater productivity is much higher than in marine waters, while marine productivity far exceeds that of fresh water in temperate and polar latitudes.

"Such a pattern in aquatic productivity (suggests) that some fishes in temperate latitudes may experience greater foraging opportunities in the oceans than in fresh waters, whereas certain fishes in tropical latitudes have greater foraging opportunities in freshwater habitats," the researchers report. They suggest that the benefits of finding more food, which has been shown to correlate with increased growth and higher egg production, outweight the risks of the long commute.
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Title Annotation:research on migration of diadromas fish
Publication:Science News
Date:Mar 26, 1988
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