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Possible reason for the fish deaths
Published on: Thursday, March 19, 2015

Kota Kinabalu: A series of field tests by a combined team from the Department of Environment and Sabah Fisheries Department suggest dissolved oxygen and red tide measuring hundreds of thousands per millilitre water in the Kolombong monsoon drain to be the cause of death of hundreds of thousands of fish last Saturday."Any red tide density of 100,000 per millilitre water and above is considered bad," an aquatic biologist told Daily Express.

But further tests conducted Monday morning showed the dissolved oxygen level read 5-6 parts per million, salinity read about 10 parts per thousand (compared normal ocean salinity of 34-35ppt) while the pH read about 6, according the DOE and Fisheries officers on site.

Red tide is a common name for algae bloom meaning large or dense concentration of singled-celled aquatic micro-organisms called dinoflagellates or phytoplanktons in scientific jargon.

And the level of dissolved oxygen in water in this case plunged to zero last Sunday, is a key parameter used to assess the effect of red tide on water quality and aquatic life.

But algae bloom is usually fuelled by too much nutrients in the water (called eutrophication by aquatic biologists).

Algae or phytoplanktons are microscopic plants which under normal circumstances, oxygenates and enlivens water as a by-product of photosynthesis.

Oxygen also diffuses from the atmosphere into water surfaces where microscopic bubbles of it get mixed in between water molecules and fish, crabs, and worms remove those bubbles of dissolved oxygen and pass it into their blood as the water moves through their gills.

The problem with algae bloom is there won't be enough filter feeders or fish fry to eat up the excess and so much of this high density biomass die and sink to the bottom where decomposition by bacteria uses up lots of dissolved oxygen until little or none is left.

Scientists generally agree that most fish need about 5mg per litre of dissolved oxygen to live and thrive especially spawning migratory fish and eggs need about 6mg/litre although size matters, like clams and worms need just 1 mg/litre , crabs and oysters about 3mg/litre.

But mass mortality of tilapia has been observed to repeat itself in the monsoon drains around Kota Kinabalu during hot and dry spells, like the current one, when hotter water can hold less oxygen, it is believed.

On the other hand, mass tilapia deaths generate little or no public shock or even concern since the water quality of public drains across the capital city is generally so foul and polluted that few people considered them desirable and unsafe to consume.



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