Why are there so many fish in tropical rivers?

Good morning folks,

In this post I will be moving outside of the focus of Florida to talk about global river fisheries. In particular, I will be discussing why rivers that occur in the sub-tropical and tropical regions have so many fish.  In these regions, rivers have been vital to the persistence of humans. Rivers provide an easy transportation way, a source of freshwater, and most importantly, provision a large predictable supply of protein in the form of fishes.  Across the globe, protein from river fishes can completely subsidize many human populations. For instance, in the Mekong River basin, 65-75% of the total protein consumed by local populations comes from the Mekong river. Thus, if these river fisheries disappear, famine would ensue for many.

Given that rivers are relatively small in size compared to oceans, big lakes, and other water bodies that sustain humans, it is amazing that rivers are able to do the same.  The reason why such a small little area can produce so much fish biomass has received considerable attention, and  studies have found one surprising similarity across the global tropical rivers.

What research shows is that the rivers are only part of the story. In fact, many of the exploited fishes either come from or rely on food from adjacent wetlands called floodplains. In the rainy season rainfall increases, causing rivers to swell and overflow their banks. When rivers begin to overflow, the areas adjacent to the rivers (i.e. floodplains) become inundated.  Once floodplains flood, one group of fishes move out of the river into these shallow areas where they spend to remainder of the wet season. Fish really like these habitats  because food is plentiful and there are no big predators (including people).  On these floodplains, fishes put on a lot of weight, grow really fast and spawn, creating lots of new fishes.

Diagram showing the relationship between river fish production and floodplain inundation

In the tropical South America and Central America, fishes like arapaima (see Crystal Hartman’s post) use extensive networks of floodplains every wet season. Likewise, catfishes in Africa do the same thing.  Closer to home, largemouth bass in the Florida Everglades also follow this dynamic. 

Arapaima

However, there is another group of fishes that rely on floodplains to sustain their high numbers. But, unlike the first group, these fishes never leave the rivers, and are better known as river monsters!  In Africa, the goliath tiger fish is one of these monsters and in Florida and central America, snook fill this river monster role.  

Goliath tigerfish

 Even though they do not venture into the floodplains, they are very much dependent on on these habitats. The river monster group waits for the dry season when rainfall decreases and floodplains begin to dry. When these habitats desiccate, all of these fish that were living on the floodplains are now forced to move back into rivers.  The river monsters then gorge themselves on these unsuspecting floodplain fish.  The river monsters eat so much of these floodplain fishes that 75% of the biomass they consume per year comes from floodplain fishes. Therefore, without these large food subsidies, the river monsters would be way less abundant.

These highly productive floodplain habitats are one of the main reasons why there are so many great fisheries in tropical and sub-tropical rivers.   Stay tuned for the next post outlining some potential threats to these floodplain habitats and some responses of these fisheries to the losses of floodplains.