Threats

Threats to River Dolphins in General:

River Dolphin found entanglement in fishing net
wwf.panda.org

Unfortunately, the areas in which river dolphins inhabit are undergoing rapid human population growth, urbanization, and industrialization (7). Threats to specific species of river dolphins varies slightly geographically, but the general threats include degradation and fragmentation of habitats, decrease in habitat area and quality, modification of habitats, and fisheries by-catch and entanglement (13). 

Dolphins have been found to prefer areas of deep, muddy water and rocky substrates, and unfortunately these are the same areas that are utilized by fishieries. These areas were found to overlap 85% in terms of space, and the overlap in resource (fish) was found to be 75%. (7). Fisheries deplete the amount of prey (fish) available for the dolphins, and dolphins can drown when entangled by fishing nets. Collisions of river dolphins and boats is also a cause of dolphin fatalities. 

Modification of the rivers by building things such as dams and barges restricts the movement of river dolphins. This not only isolates them socially, but has genetic impacts such as limiting gene flow (3). 

Threats to the Amazon River Dolphin:

The Amazon River Dolphin is threatened by river development projects, such as the creation of dams and rivers, that fragment their populations. They are also threatened by pollution and the removal of river water and siltation arising from deforestation (12). The entanglement in seine nets and gillnets that are used by fisheries is also a huge problem. Lampara seine nets causes over 80% of the deaths that are caused by entanglement (6).

Apparent survival rates of Amazon 
River Dolphin before harvesting (1994-2000) 
and after harvesting began (2000-2011) 
(5)

In some areas, these dolphins are illegally harvested and used as bait for catfish (Calophyus marcopterus). In one study that analyzed the effect of this harvesting on Amazon River Dolphin populations in the central Brazilian Amazon, survival rates were shown to be significantly lower in the years after this harvesting began when compared to the years prior. The results showed that mortality of this species more than doubled during this time period. In this area alone, it was estimated that 1,650 of these dolphins are killed every fishing season, and that there has been a 10% decline in their population each year since this harvesting began. This suggests that the harvesting rates that are currently employed could be unsustainable (5). 

Threats to the Ganges River Dolphin:

The Ganges River Dolphins are in a rapid decline due to loss of habitat, habitat fragmentation by dams and barrages that constrict water flow, pollution and water extraction. They are also threatened by factors including fisheries by-catch, entanglement, and poaching. Furthermore, depletion of their food source, by the overharvesting of fish stocks, is also leading to their decline. In the upper Ganges River between Narora and Bijnor, the increase in fishing activities was found to be the biggest threat to these dolphins (3).

Transport of a river dolphin out of a
destructed area in Brazil 
www.cocha-banner.or

Threats to the Indus River Dolphin:

The Indus River Dolphin population is declining due to habitat fragmentation and degradation by the development of irrigation systems in the Indus River, including dams and barrages. This river has been modified for irrigation, industrial use, and hydroelectric power, which also fragments the dolphin populations.

Other threats include extreme pollution of the Indus River, by-catch, and poaching (8).

Franciscanas caught as as by-catch
www.arkive.org

Threats to the Franciscana:

Entanglement by fishery by-catch is the greatest threat to this species, and the fatalities due to this are thought to be unsustainable. Ingestion of debris, pollution, and habitat degradation are also threats to the Franciscana (11). 

What is still unknown:
River dolphins are mysterious creatures, and there is still a great deal that is unknown about them. Population estimates and distribution for the Franciscana are unknown, and there are many gaps in the estimates of fatalities from all threats. Greater understanding of the river dolphins and the factors that threaten them are necessary for the conservation of these species. 

The Baiji was very difficult to study in the wild, and there were only a few individuals observed, so there is little known about their biology.  There were no historical estimates of the numbers of Baiji, and their populations had already been declining once the first surveys were completed in the 1970's. There are still debates regarding  which factors have the biggest impacts on the decline of Baijji. It has been argued that habitat degradation had the biggest impact on what lead their extinction, while others believe that they were effected more by fisheries. However, neither factor was measured, which makes it difficult to know the exact impact that by-catch actually had on their extinction (1). This lack of information and understanding of the Baiji may have contributed to its extinction. 

River dolphin entangled in fishing net
http://whales.org/en/whales-and-dolphins/living-alongside-humans