River diversion affects fish growth

commercial fisheries are often held responsible for the decline of fish population worldwide. A study in the us has found that changes in the flow of a river are also a major cause. It affects growth. The study by researchers of University of Arizona says diversion of river water for multi-purpose projects such as dams ends up reducing water flow downstream of the river and in estuaries.
EndangeredCommonly called totoaba (Totoaba macdonaldi) and the first commercially important fish in the Gulf of California, the endangered fish variety was the subject of the Arziona researchers' study. Totoaba is a large, endemic species that spawns only in the Colorado river's estuary. Commercial totoaba fishery, however, crashed in 1975 and overfishing was blamed for the fish's decline. The study, however, says that since 1960, diversion of Colorado river's water for other uses, including the Hoover dam on the river, reduced the amount of fresh water that reached the Gulf of California. This increased salinity of the estuary, which affected breeding of totoaba and other marine organisms in the estuary.

To understand how diversion of water affected the fish population, the researchers compared the growth of otoliths--sensory organs made of calcium carbonate that grow in layers in the totoaba--before and after the construction of the Hoover dam. The otoliths of pre-dam totoaba were obtained from aboriginal shell middens (remains of shellfish) in the upper Gulf of California. The researchers compared oxygen isotopes (O¹⁸ and O¹⁶) in pre- and post-dam otoliths to find out the kind of habitats the fish species used. O¹⁶ values indicate estuarine habitat and O¹⁸ values indicate habitats in the ocean. The researchers found more O¹⁸ values in the post-dam period, which indicated reduction in flow of fresh water from the Colorado river.

They also found that the pre-dam fish population grew twice as fast and matured 1-5 years earlier than the fish that grew after the dam came up. Pre-dam totoaba reached sexual maturity by the second year, whereas post-dam fish did not attain maturity until their seventh year. Growth rates were also high during the first year in pre-dam otoliths compared to those in post-dam otoliths.

"Both reduced river flow and over-fishing have resulted in getting the totoaba in the endangered category, yet recovery efforts have only focused on reducing fishing," says Kirsten Rowell, researcher at the Department of Geosciences, University of Arizona, and the lead author of the paper. Recovery of the totoaba, Rowell says, will depend on enforcement of fishing ban and resumption of seasonally appropriate river inflow. Such restoration effort will also benefit other commercial species such as shrimps, Rowell adds.

Recognizing and documenting the importance of rivers and relating these to the productivity of estuarine nursery habitat are critical for responsible management of the world's large rivers and their adjacent marine habitats, the researchers wrote in the paper published in Biological Conservation (Vol 141, No 4, April 2008).

In India, although there are no studies to link change in river flow with fish growth, its effect on deltas have been documented (see box Shrinking deltas).

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