The Fish Habitats of a Peruvian Amazon River, Part 1 (Full Article)Author: Shannon Loughnan
Being in the Peruvian Amazon can feel remote, uncomfortable, and even lead to a sense of entrapment, but for those intrigued by the natural ecosystem and biodiversity abound in the region, it is a world away from the major development of bustling cities and traffic congestion—a breath of fresh air from the lungs of the earth.
The river port of Pucallpa, one of the major entrances into the Peruvian Amazon, is a lively region of cargo, dock workers, and passengers. Any designated footpaths toward the stationed ferries have long been smothered in thick mud and cut tree logs that need to be hurdled while descending the embankment.
As I first boarded the ferry in Pucallpa, my previous ambitions had finally come to life with the thought of moving deeper into the Amazon. This was the beginning of my journey to Iquitos, with the aim of exploring the underwater environment of a small section of the river, hiring a local guide, and heading into the jungle for a number of days.
Iquitos is the trade hub of the Peruvian Amazon, and some of its major industries include fishing and tourism. There is a small airport, but generally the river is the most common form of transport for people and goods. The streets are dominated by two-stroke motorcycles, the most efficient way to move around town.
As in the majority of Latin American cities, the local square provides a fun atmosphere for families, with many being entertained by musicians and fed by ice cream vendors. There is additionally a steady flow of tourist guides moving throughout the square offering jungle trips to travelers at a range of prices that mostly depend upon your ability to negotiate.
I was fortunate enough to come across a guide at my hotel who happened to be an ornamental fish collector for an exporter prior to his current profession in tourism. Seeking business in Iquitos, he offered jungle trips from a small village approximately three hours away, where his family was located. I was confident that I had found a guide that could not only lead me to productive collecting regions but also aid in the identification of fish species. Being the only participant in the tour I had full control over the expedition, which increased to three as a nephew and close friend of my guide joined the party.
We were heading to a small tributary of the Amazon River known as the Yarapa River, in the Loreto region of the Peruvian Amazon. Yarapa could be argued to be a tributary of the Ucayali River, as there are conflicting reports regarding the location of transition from one river to the other.
From Iquitos a two-hour bus ride in a southwesterly direction was taken to the town of Nauta, where we purchased a crate of supplies mainly consisting of rice and potatoes. All was loaded onboard a leaky wooden motorboat for the final leg of the trip east to the Yarapa River.
We disembarked in the small river town of Puerto Miguel, the home of my guide. The majority of the town was set high on the southwest bank of the river. Built on stilts, the houses were roughly made of local cut timber and corrugated iron. The river provided sustenance to the town, income in a variety of ways, and fed and bathed the locals.
My first impression of the Yarapa River was of grasses and saplings sparsely decorating the slopes. The terrain was very unstable due to its silty nature and lack of many solid areas, and no more than 50 meters (165 feet) in width at any point was so far observed. The majority of Peruvian rivers are classified as whitewater environments (Kullander, 1986) and the Yarapa certainly fit within this category, with its milky appearance that showed it was high in nutrients. It was late September, during the period of falling water just prior to the water level reaching its lowest point, and the river cut deep into the earth leaving behind steep eroding banks.
We were equipped with a homemade seine net made from shade cloth and weighted with lead sinkers, plus fishing poles and a variety of dipnets. Three dugout canoes powered by hand-carved paddles in the shape of teardrops were our transportation. The guides used their paddles with efficiency, creating a gentle slapping rhythm, contacting and exiting the water, which was soothing and almost mesmerizing.
I think you can estimate the amount of time one has spent on the river through their ability to strike the water so cleanly and efficiently with seemingly no effort, for when it was my turn to paddle, my unproductive technique splashed up water like an inexperienced platform diver. My novice splashing aside, it was nice to cruise the river without the sound of a motor, being able to listen to the jungle.
As we moved further from the village, the riparian zone of the banks began to change from steep sides that contained plantations of corn and sugar cane on the ridges to established jungle that grew to the river’s edge. Blue- and red-flecked kingfishers perched on fallen timber branches watched the water current for small fish while predatory kites and hawks spied fish from a higher vantage point in the treetops. Freshwater turtles were also observed lazing on emerged logs, but they scattered when we tried to approach for a closer inspection.
Camp was always set up very quickly—a plastic tarp stretched as a groundcover that could be rapidly transformed into a shelter from rain, which typically arrived without warning. We slept in hammocks with very narrow tarpaulin roofs that were enclosed in mosquito netting. Although not very effective at keeping the rain out, the netting was a necessity for deflecting the insects, which were much more of a bother than the rain.
The habitat had limited substrate but was blanketed with rich top soils and sticky clays—it is a system that is constantly changing. As the seasons pass, the format of the river alters according to the environmental aspects experienced and the degree of impact placed on the system by those conditions, more specifically precipitation and the process of erosion. Steep banks of clay constantly collapse, not unlike the slip of cliff ice into the depths of the Antarctic Ocean, and tree roots can no longer support heavy trunks in the soil and similarly topple.
Rivers become heavily laden with silt and decaying vegetation, all important factors that alter the ecosystem and create niches for a variety of fish species.
I was particularly interested in the relationship between the habitat and fish species and, due to its diversity of environments, the Yarapa provided me with this opportunity of investigation. A total of nine separate habitats were sampled, with submerged vegetation being a popular haunt for a number of species.
Overhanging submerged vegetation lining the banks was prevalent along the majority of the Yarapa River, particularly when the expedition moved into the more remote regions away from the populated village. Shrubs and tall trees created a canopy of shade for an array of birdlife while a variety of fish species underwater sought refuge among the branches and leaves, away from the dangers of the open water.
Fishing in this habitat was relatively simple, only requiring placing the dipnet beneath a section of vegetation while the branches were shaken to frighten fish into the net. Alternatively, the seine net was wrapped around the selected region of vegetation, followed by a combination of shaking the branches and probing with the oars of the canoe to frighten fish out of hiding and into the trap.
Frequently caught in this environment was Monocirrhus polyacanthus, a perfect example of a fish having the ability to blend in with its immediate environment. The body color was dependant upon the shade of the vegetation it was captured in. It used color not only as camouflage for its own protection but also to ambush prey. M. polyacanthus possesses another adaptation in order to assist in trapping prey, a small appendage protruding from the lower lip that unsuspecting victims may find attractive enough to move uncomfortably closer to the region of its jaws.
Always found to be of a dark shade of maroon, Hypselecara temporalis were also common in this habitat, predominantly juveniles seeking protection and foraging among the leaves. Hoplerythrinus unitaeniatus and Hoplias malabaricus were also captured together among the submerged vegetation. Since they are both known piscivores, we assumed they were taking advantage of the selection of prey species in the habitat that were seeking cover.
Tangles of fallen tree trunks and branches were common in the open sections of the river, where an obstruction had created a collection bay for decaying wood to accumulate. Fishing in these regions was difficult due to the accumulation of obstacles that disallowed the use of the seine net.
This forced us to utilize line and hook among the branches, trying to avoid snags and to restrict a hooked fish from darting deeper into the mesh of woody debris. Such habitats were also present in the narrow, shallower sections of the Yarapa where branches choked and restricted our passage while providing optimum cover for a multitude of fish species.
Crenicichla johanna and C. proteus, the latter being endemic to the western Amazon region (Kullander, 1986), were both encountered in a zone of heavy, submerged branches. Both species laid in wait to ambush prey that sought refuge there or were waiting for smaller, unsuspecting individuals to pass by. In addition Tetragonopterus argenteus were also caught via fishing line in this environment, but these were large individuals capable of fleeing the jaws of a sleek Crenicichla.
Habitat 3: Open Water
Open water provides a habitat only to those aquatic species armed with the necessary equipment required to survive. There are attributes such as size and the appropriate dentition, plus body armor in the form of odontodes and denticulations on the scales and rigid spinesthat allow those species to survive there. In addition, many species rely on short bursts of speed to chase down prey or to avoid becoming the victim.
Acestrorhynchus falcatus and A. falcirostris are fine examples of open-water hunters with their long streamlined bodies built for speed, large protruding eyes for aiding visibility in the murky water, and teeth like those of a canine for gripping prey.
Also well equipped for this habitat is Calophysus macropterus, a benthic dweller that was caught on line and hook in the river’s center. It utilizes long sensitive barbels for foraging through the muddy substrate. Representing the piranha group was Pygocentrus nattereri, Serrasalmus rhombeus, and S. sanchezi, which were always found in open water, occasionally in deep eddies waiting patiently for the current to bring prey within reach.
Piranha fishing was not only fun but reasonably undemanding. Our fishing poles consisted of specially selected sapling branches that were hard and flexible, approximately 15 mm in diameter and stripped of bark with line attached to the tips. Piranhas are instinctively attracted to struggling prey (Carvalho et al., 2007) and, although they do not necessarily hunt in packs, their high density in the river results in large numbers being drawn to the same lure. Slapping the fishing pole on top of the water before allowing the bait to gently settle beneath the surface attracted a bite almost instantly. The most difficult part was being able to fasten the fish before the bait was torn away from the barbs of the hook.
Backwater pools can be small intermittent water reservoirs or large permanent water bodies that sustain a fish population year-round, sometimes with no permanent connection with the main flow of the river. They typically fill during the wet season when the forests begin to flood only to recede in the drier months, concentrating a diversity of fish species. These are reasonably clear lentic habitats perfect for fish to spawn and hunt, not unlike the mangrove regions of coastal inlets, providing a protected nursery for fry and juveniles.
One specific pool sampled was approximately 50 meters (165 feet) from the bank of the Yarapa River. It was a depression in the terrain that had been holding water throughout countless wet seasons and provided a protected watering hole for terrestrial life that would normally avoid the open banks of the main river. Hidden from view and only known to the guides, a steep ascent was required to climb from the waterline of the Yarapa to the flat ground above and leading to the pool. It was such a startling indication of the great rise in water level required during the wet season to initiate the flooding of the forest in this region.
The brief stretch of terrain consisted of scraggly saplings and bushy undergrowth, and not until the ground became swampy and the vegetation was enveloped in water did the pool come into view. The L-shaped pool was no larger than two tennis courts and no deeper than 3 feet in any one spot, with a muddy substrate covered with leaf litter. It teased the unsuspecting collector into the water, which then clouded, reducing the visibility.
The only obstructions were submerged branches that were long dead but also small saplings shooting spasmodically from below the surface of the water throughout the pool area, an indication of its intermittency. After the initial inspection of the pool, the crew lugged the smallest of the three canoes up the bank and through the scrub. With the craft at our disposal we were able to survey the pool more thoroughly and set the seine net more efficiently.
The pool was a hive of activity, containing numerous Chalceus erythrurus constantly striking insects at the surface and Laetacara thayeri keeping to the shallower sections of the pool, out of reach of some of the bigger predators. Cichla monoculus were present in pairs displaying in their territories, possibly protecting young and constantly chasing away intruders. Being a large predatory cichlid, individuals may have entered the pool during high water months only to remain to feed and spawn, more than likely escaping its confines during the next series of downpours.
Overall, the backwater pool was a highly populated water body that may have been at its maximum holding capacity and ability to feed all its residents. Such environments of slowly evaporating water generally follow a typical pattern of events, from the population explosion of numerous fish species to their eventual decline due to the lack of food. Another major contributor is the change in water parameters, such as a reduction in dissolved oxygen levels, slowly eliminating those species that are more sensitive to anoxic conditions.
As the water begins to recede, opportunistic species such as those from the piranha group devour any moving organism, forming a great school of desperation and death. Reptiles such as caiman and a variety of scavenging birds eventually arrive as the clear water turns to mud, feasting on the remnants of the predatory fish. This is a cycle that is generally repeated within intermittent closed water bodies the following season.
Carvalho, L. N., R. Arruda, J. Raizer, and K. Del-Claro. 2007. “Feeding habits and habitat use of three sympatric piranha species in the Pantanal wetland of Brazil.” Ichthyological Exploration of Freshwaters 18(2):109–116.
Kullander, S. O. 1986. Cichlid fishes of the Amazon River drainage of Peru. Swedish Museum of Natural History. Stockholm. 431 pp.
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