The Fish and the threats

The Amboaboa River is home to unique but highly threatened fish species

The focal species
Ptychochromis insolitus (Joba mena)

Paretroplus nourissati (Lamena)

Paretroplus gymnopreopercularis (Malemisisika)

Rheocles derhami

Pachypanchax sp. nov. “Sofia” (Zono)

The threats they are under
Deforestation and sedimentation of the river bed

The sedimentation of the river bed has been observed before each dam, creating shallow sandy sections of the river. Sedimentation of the river bed, resulting from deforestation, has been recognised as a source of spawning site loss for fish species (Waters, 1995). Indeed, observations in outdoor ponds and aquariums showed that P. insolitus and P. nourissati lay their eggs on rocks or inside small caves at the end of the dry season. Such shallow sandy habitats are therefore not suitable for these species to spawn. In addition, none of the focal species were captured or observed in these degraded habitats during our sampling session in October 2017.

Deforestation on the outskirts of the Special Reserve of Marotandrano (2018)
Sedimentation of the Amboaboa River bed

Dams and water diversion

There are five dams on the first quarter of the Amboaboa River, from the special reserve to the village of Marotandrano. The first dam is located in the head waters and supplies drinking water to downstream villages. Then, a succession of four dams diverts most of the water to extensive rice paddies along the Amboaboa River. It is difficult to put a figure on how much water is abstracted from the river for rice farming. In-depth studies on the dams’ impact and management along the Amboaboa River could answer this question and help to get a better understanding of the extent of their effect on the aquatic habitat and fish populations. Such studies would be beneficial and contribute to a long-term conservation management plan.

Dam along the Amboaboa river to divert water for rice farming
One of the many irrigation channels diverting water from the Amboaboa River to the rice fields
Alien species

There are a few alien fish species present in the Amboaboa River such as the tilapia and the common carp. Previous studies have established strong correlations between the introduction of alien species and the decline of native fish in Madagascar freshwater ecosystems (Reinthal and Stiassny, 1991; Leveque, 1997; Benstead et al., 2003; Sparks and Stiassny, 2003). Native species are suffering from competition from or predation by introduced species (Benstead et al. 2000). No snakeheads (Channa maculata) were captured or observed in the Amboaboa River despite being largely spread in the Island. However, locals reported their presence in the lower section of the river where it meets the Magarahara.  The reason for the absence of the snakehead in the high part of the river is unknown and would be worth investigating.

Tilapia species captured in the Amboaboa River
Fishing activity

Fishing activity on the Amboaboa River does not seem to be regulated or controlled. Any fish species of any size is being caught. Fishing pressure varies along the river. High fishing pressure using unselective technique were observed a few kilometres downstream after Marotandrano. The whole width of the river is blocked using barriers made out of branches and leaves with a trap in the middle to collect the fish. Casting nets and mosquito nets are also used by locals along the river.  Poison fishing using poisonous plants were observed too.

Intensive fishing pressure using barriers and traps downstream of Marotandrno

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