Photo
by Don Vargo.
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 generally true. The
marine stage of these species allows a wide dispersal and continual genetic mixing
of populations, so most of the species inhabiting our streams are widely distributed
across the South Pacific.
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.
There have even been deaths in American Samoa due to leptospirosis, a bacteria
from piggeries and other animals that pollutes many streams. 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 harms 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.
Tutuila
Island has about 141 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 long). 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.
Our
streams support surprisingly few species -- there are only about 8-12 freshwater
fish species, and not many more freshwater invertebrates. 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 stream environment. Three non-native fish species were also introduced
here, probably in the 1970s: mollies (fo-vai; Poecilia mexicana) and mosquitofish
(Gambusia affinis) to control mosquitoes, and tilapia (Oreochromis mossambicus)
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.
25. Island streams
