Swimming against the Current: the Fish of the Main Ridge Reserve

In this week’s column Amy Deacon, Lecturer in the Department of Life Sciences at The University of the West Indies, St Augustine and Secretary of the Trinidad and Tobago Field Naturalists’ Club, returns to the Main Ridge. This time, we dip our nets into the cool, glistening streams that run through the reserve and see what we might catch! This feature was first published in the Tobago Newsday on March 16, 2017.

Thirteen species of freshwater fish have been recorded in Tobago, but only a handful of these have successfully conquered the Main Ridge. While we humans can easily drive along the Roxborough – Parlatuvier road to access the reserve, fish must initially colonise inland from the sea. In North East Tobago, fish in the lower reaches moving upstream soon reach one of many waterfall barriers at the edge of the Ridge. Often these are tens of metres high, much like those at Argyll. Most fish will never manage to traverse these barriers, forever restricted to the lowland rivers. Only the most intrepid make it through to dominate the uplands.

Typical aquatic habitat in the upper reaches of the Main Ridge Reserve. Photo courtesy Amy Deacon

Without a doubt, the king of the Main Ridge rivers is the Jumping Guabine, Aneblepsoides hartii. Ordinarily, this species reaches around 10cm from head to tail. However, the Main Ridge individuals are decidedly larger, as with fewer fish predators around they live longer. As the name suggests, this fish has the ability to launch itself completely out of the water. The ability to beach yourself may not seem like much of an evolutionary advantage, but the jumping guabine has another trick up its sleeve– it can breathe atmospheric air through its tail, which is covered in capillaries. As such, it can ‘jump’ out of water onto land and survive for a considerable time, as long as it does not dry out entirely.

This allows it to travel short distances over land to new bodies of water, and it is not unusual to find a few individuals in isolated puddles, often some distance from permanent water. Travelling outside of water is also the reason that they are often the only fish found in the highest stretches of rivers in both the Main Ridge and the Northern Range: Jumping guabine can simply make multiple jumps to scale the sheer rock face of a waterfall! Another advantage of jumping is that it allows hungry individuals to hunt prey, such as ants, that may be otherwise out of reach. Incredibly, they have been recorded jumping as high as 14cm, more than their total body length, to catch prey on overhanging vegetation or at the water’s edge. While in the water, they are ravenous predators of aquatic invertebrates, small fish and tadpoles.

Jumping guabine can travel for some distance on land and even up waterfalls. Photo courtesy Amy Deacon

A second species that has managed to breach at least some of the barriers and make its home in the Main Ridge streams is the Guppy or Millions-fish, Poecilia reticulata. Like the jumping guabine, these tiny (2-3cm) fish are also found almost everywhere in both Trinidad and Tobago and are usually seen in shoaling in large numbers in shallow water. Males are smaller and display vivid patches of orange, green and yellow, while the larger females are plain. Even from the river bank it is possible to watch males perform their distinctive ‘sigmoid’ courtship dance, which involves arching into an s-shape, raising their dorsal fin and intensifying their colouration in an attempt to seduce a female.
Although they are not quite as adept at scaling waterfalls, their special trick is that once a single female guppy manages to find herself in a new stream or pool, she can colonise it single-handedly by using stored sperm to produce vast numbers of live babies in a matter of weeks. As well as being prolific breeders they also mature extremely quickly, completing as many as 3-4 generations in a single year. Therefore, one chance colonisation above a waterfall can lead to a permanent population.
Many do not realise that the guppy, now world-famous in the aquarium trade and as a mosquito control agent, is named after a Trinidadian. Robert John Lechmere Guppy was an avid naturalist, and in 1866 sent samples of these pretty little fish to the British Museum in London where the species was initially named after him: Girardinus guppii. The name was later changed when it was discovered that the same species had already been described from Venezuela, but the ‘guppy’ part stuck.
The guppy is also famous in the world of science. Biologists from all over the world regularly travel to Trinidad and Tobago (including the Main Ridge) to study these fish and how they adapt to different environments. To date, local and international scientists have published hundreds of studies into ecology, evolution and animal behaviour on our humble guppy. It may seem small and insignificant, but it is a fish to be proud of!

Guppies are excellent colonisers and thrive in the shallow streams of the Main Ridge. Photo courtesy Sean Earnshaw

A third fish species that has conquered the Main Ridge boundaries has done so not by jumping or by reproducing fast after a chance introduction, but by actually climbing the cliff! Sicydium punctatum, also known as the Rock-climbing Goby or Tri Tri, reaches around 10cm long and feeds on algae in the streams of many of the Caribbean islands, as well as coastal regions of mainland South and Central America. Unlike the other fish of the Main Ridge, newly hatched Rock-climbing Gobies are washed downstream by the currents to grow and develop in the ocean. After several months, when
fully formed, they migrate back inland by climbing vertical barriers along the way. They do this using a technique known as ‘power-bursting’, in which they use a combination of synchronised fin movements and body twists to move upwards, and a special sucker to hold onto the surface of the rock while resting between bursts of movement. While Main Ridge guppies and jumping guabine only had to colonise the uplands once in their history, every individual goby in the reserve will have made that obstacle-filled journey during its lifetime – incredible!

Although the Main Ridge supports only a few fish species, I’m sure you’ll agree that in this case it is a matter of quality over quantity, as the adaptations of those who manage to live in these streams are truly astonishing!