Island streams
Tutuila Island has about 160 small streams that flow year-round along at least a portion of their main channel. These streams are steep, shallow and short (most are less than a mile). Stream flows are generally low but they can flood quickly in response to heavy downpours. But even with the high rainfall in our mountains (200-300 inches per year), the water drains quickly to sea or percolates into the porous volcanic soil to recharge our groundwater supply of drinking water.
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| Photo by Don Vargo. |
Our streams support surprisingly few species -- there are only 10-12 freshwater fish species and not many more freshwater invertebrates here. The principal species are freshwater eels (tuna), gobies (apofu, mano'o), mountain bass (sesele, inato), shrimp (ulavai) and snails (sisivai). Additional species may enter the lower ends of streams, but they are not restricted to a freshwater environment. Two non-native fish species were also introduced here, probably in the 1970s: mollies (fo-vai) to control mosquitos and tilapia to grow in aquaculture. The impact of these alien species on the native populations is not known. Additionally, we can only wonder about the impact of the alien marine toad (lage) that sometimes has thousands of its young tadpoles swimming in local creeks.
The low number of species in our streams is in stark contrast to the many marine species living in our coastal marine waters (890 fish species and countless invertebrates). Part of the explanation for this difference is simply that our streams are small and offer limited habitat for stream-dwelling organisms. But another more interesting aspect of this low diversity is: how did any freshwater species get to American Samoa in the first place? We are a small island surrounded by hundreds of miles of deep ocean. Freshwater species generally cannot survive in saltwater, so how could these freshwater fish, shrimps and snails cross the ocean barrier to get here? The trick is that they all have a marine stage in their life cycle. After they spawn, their newly hatched larvae wash out of the stream into the ocean where they drift about as marine plankton for a few weeks or months. Some make their way back to a coastline where they seek a stream to live out the rest of their lives. It might be expected that the few freshwater species that got to our remote islands have evolved over thousands of years into unique (endemic) species found nowhere else in the world, but the opposite is true. The marine stage of these species allows a wide dispersal and continual genetic mixing of populations, so the species inhabiting our streams are widely distributed across the South Pacific region.
Because streams drain the valleys we live in, they serve as good indicators of how well we are taking care of the land. Sad to say the message is not good. Our streams once provided food and drinking water, but now they are treated as a place for people to throw rubbish and piggery wastes. And, after a heavy rainfall, some streams turn chocolate brown with the dirt that erodes from the landscape. Much of this soil erosion is due to poor land-use practices such as the farmer's bare-earth clearings for plantations on steep mountain slopes and the run-off from inadequately designed construction sites. In the former case, the farmer not only loses the soil needed to grow his crops, but the eroded dirt fouls our streams and ends up in coastal waters where it degrades our coral reefs. It does not have to be this way. There are better ways to dispose of rubbish and to prevent erosion that can make streams a healthier place for fish as well as for the children who play in the streams.
P. Craig, NPS