Increased sea temperatures behind May fish kills in Gulf of Aden, say scientists
Posted by Zawya
Yemen Times
25 July 2010


SANA'A -- In a recent report on fish kills along the coast of the Gulf of Aden in May 2010, scientists concluded that the death of the fish was due to an increase in water temperature.

Some types of fish die when the water is too hot, reported Professors Mohammed Abubakr, Mohammed Al-Saafani and Hisham Nagi of the department of earth and environment at the University of Sana'a.

"The investigation showed strong evidence of [a] sudden increase [in] sea water temperature, which lowered the amount of dissolved oxygen in sea water and increased fish metabolism," they wrote. "This eventually [lead] to the death of the [blue triggerfish] which cannot tolerate such sudden increase[s] in temperature. Other possible factors [such as] depletion of oxygen due to algal growth, could have also contributed, to some extent to the stress on [fish]."

The study indicated that this happens naturally in the Gulf of Aden during the transition period between winter and summer. The same warming of the water has been reported in the past for different natural reasons, and most probably, it could happen again during the forthcoming southeast monsoon, depending on the strength of the wind and upwellings of seawater.

Yemen Liquid Natural Gas (YLNG) reported the fish kill in May 2010 in Balhaf on the coast in Shabwa governorate, where they have a plant to liquefy Yemeni natural gas prior to its export by sea.

On May 26, the team from Sana'a University travelled to the reported sites to investigate. The study was made upon the request of the company to investigate the possible causes of the incidence.

Besides visiting the YLNG plant, the scientists' field visit covered more than 150 km of the coastline, from Ain Ba Maba'ad west of Balhaf to the city of Mukalla in the east.

During their field visit, the scientists interviewed fishermen and studied site findings to examine all possible causes of such occurrences in the area. They also used meteorological data provided by satellite images for the northern Arabian Sea at the time of the fish kills.

Field observations and interviews with fishermen and local communities confirmed the death of fish along the 150 km of coastline between Al-Mukalla and Balhaf, and it turned out that the blue triggerfish was the most affected species, comprising over 95 percent of the dead fish.

According to locals, the fish kill incident started on the 8th and 9th of May 2010 and lasted for a few days. Fishermen have described this kind of fish kill as a naturally occurring phenomenon which happens regularly, according to the study.

However, the scientist couldn't find samples of dead fish to collect because most of dead fish had already been washed out to sea. It was also too late to measure other environmental parameters such as pH, dissolved oxygen, toxic materials, and trace metals, so they used data from the YLNG weekly monitoring reports for Balhaf in their study.

"By examining the common causes of death, dissolved oxygen jumps out as the most probable reason for fish mortality. The ability to tolerate low dissolved oxygen levels depends on the species and their size," scientists wrote.

The study pointed out that there were unconfirmed reports of fish kills, mainly of blue triggerfish, in the Arabian Sea in November 2007.

According to the Yemeni study, a similar event was reported from the island of Reunion in the Maldives in 2002 and 2001, where groups of triggerfish were found dead. The results from this event reported the presence of a species of bacteria known to be associated with fish deaths in the spleens of the dead fish.

By Khaled Al-Hilaly

© Yemen Times 2010

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