What is fishkill?

• Fish kills or mass mortalities of fish can be defined as a sudden and significant death of fish. 
• This is characterized by a large number of fish dying over a short period of time within the defined area. 

Causes

Natural causes

• Globally fish kills are most frequently linked to natural causes such as ecological hypoxia (low dissolved oxygen) or anoxia (no or zero dissolved oxygen), harmful algal blooms (toxic and non-toxic freshwater cyanobacteria, marine dinoflagellates), diseases, extreme or abrupt changes of temperature, salinity, floods, upwelling of the oceans.
• Minor and occasional natural causes of fish kills are volcanic eruptions, earthquakes, and meteorites. 

Anthropogenic causes

• Human activities are also responsible for a number of fish kills, for example, accidental spills (e.g. oil), runoff, and drainage discharge of pesticides and herbicides from agriculture farmlands into water bodies. 
• In addition, mass killing in the name of recreational fishing may also be responsible for significant fish kills in some countries.

Pollution Indicators

• Fish kills event is an indicator of environmental stress, a decline in aquatic ecosystem health, and water quality problems. 
• Fish kills events provide useful information on the distributions of pollutants (e.g.nutrients, pesticides, trace metals, oils) in aquatic environments. 

Why in news ?

• Fishkills were witnessed in Vietnam due to increasing heat waves.