What is a primary reason that very few fish inhabit forty of Malaysia's major rivers?

The primary reason that very few fish inhabit forty of Malaysia's major rivers is due to sewage. Urbanization and industrialization in Malaysia have led to significant increases in sewage and wastewater discharge into rivers. This contamination decreases water quality by introducing harmful pathogens, excess nutrients, and organic pollutants, all of which can adversely affect the aquatic environment and fish populations. As a result, the biodiversity of fish species is severely impacted, leading to reduced populations or total absence in heavily affected waterways. Sewage pollution not only harms fish but also disrupts the entire aquatic ecosystem, making it increasingly difficult for fish to thrive.

In contrast, while overfishing can certainly deplete fish stocks, it does not explain the overall lack of fish across the many rivers mentioned effectively. Similarly, although water temperature can influence fish habitats, it does not account for the widespread impacts observed due to sewage contamination. Pollution from agriculture, while also a concern, is selective in its sources and effects compared to pervasive sewage pollution, which is a more direct and immediate threat to fish inhabitation.