FISH KILL: Why do they occur?

Published January 6, 2021, 12:00 AM

by Ellalyn De Vera-Ruiz

  • BFAR defines fish kill as the ‘massive death of fish stocks in a certain area due to unfavorable water quality parameters of a certain aquatic environment that are intolerable or toxic to the fish stocks.’
  • Some of these occur because of human activity, although around 50 percent of all kills around the world are due to natural causes.
  • One of the most common causes is algal blooms and the resulting water quality issues, such as low oxygen or production of toxins.
  • The oversupply of nutrients in Laguna de Bay like nitrate and phosphate makes the lake very fertile. Combined with environmental conditions such as sufficient sunlight, high water temperature, and calm water, the algae can grow at a very fast rate. If these environmental conditions are sustained, algal bloom happens which appears like a thick green or blue green soup.  Algal bloom becomes harmful when they decompose.
  • Recently, Sampaloc Lake, the largest of seven interconnected lakes in San Pablo City, Laguna also saw thousands of fish killed. It is believed that the fish kill was caused by a lake ‘overturn,’ a natural phenomenon in which colder water falls on the surface of the lake, while the water on the bottom rises.

The Philippines has witnessed several occurrences of fish kill in the past years. But why do they even occur?

The Bureau of Fisheries and Aquatic Resources (BFAR) said fish kill is a sudden and significant mortality of either wild or cultured fish.

(UNSPLASH / MANILA BULLETIN)

Also called “fish die-off,” BFAR further defined fish kill as the “massive death of fish stocks in a certain area due to unfavorable water quality parameters of a certain aquatic environment that are intolerable or toxic to the fish stocks.”

This may also be caused by biological diseases, detrimental anthropogenic activities or weather conditions that affect a certain body of water, it said.

BFAR cited that fish kill can be caused by various factors. “Some of these occur because of human activity, although around 50 percent of all kills around the world are due to natural causes,” it said.

Natural causes may be infectious diseases, algal blooms, toxins from run-off water, or part of their natural life cycle.

One of the most common causes that result in fish kills is algal blooms and the resulting water quality issues, such as low oxygen or production of toxins.

According to the Laguna Lake Development Authority (LLDA), algal bloom is a periodic occurrence in Laguna de Bay.

“Algae are microscopic plants in various forms, shapes and colors. Those commonly found in Laguna de Bay are blue-green algae, green algae and diatoms. They are natural food and their abundance in lake is one of the factors attributed to the success of aquaculture business in the lake. Aside from being a source of food for fish and other aquatic life, these algae provides dissolved oxygen through the process of photosynthesis,” the LLDA explained.

“The oversupply of nutrients in Laguna de Bay like nitrate and phosphate makes the lake very eutrophic, or in layman’s term, very fertile. Combined with environmental conditions such as sufficient sunlight, high water temperature, and calm water, the algae can grow at a very fast rate. If these environmental conditions are sustained, algal bloom happens which appears like a thick green or blue green soup,” it further said.

The LLDA said algal bloom becomes harmful when they decompose.

In the process, they use the dissolved oxygen in water. Dissolved oxygen is the volume of oxygen that is contained in the water and is vital in supporting aquatic life.

Oxygen level below 5 milligrams per liter (mg/L) is already stressful to the fish and if it further decreases, fish kill can occur.

Just this month, Sampaloc Lake, the largest of seven interconnected lakes in San Pablo City, Laguna also saw thousands of fish killed.

It is believed that the fish kill was caused by a lake “overturn,” a natural phenomenon in which colder water falls on the surface of the lake, while the water on the bottom rises, LLDA biologist Jonathan Nicolas said in an ABS-CBN report.

Fish kill response protocol

The LLDA urged fish farmers and operators in Laguna Lake to observe proper approach during the occurrence of fish kill.

First step is to record observations – time and date of incident, location of the incident, owner of the affected fish pen or cage, contact number of the reporter/owner, area of the affected fish pen or cage, dead fish species stocks and density, and weather condition.

(UNSPLASH / MANILA BULLETIN)

Second, report the incident and recorded observation to the LLDA.

Third, provide assistance to the LLDA quick response team during the on-site survey and water sample collections.

Lastly, collect the dead fishes as soon as possible and properly dispose at the pre-identified burial area or site, the LLDA advised. 

 
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