The legendary hump-backed mahseer Tor sp. of India’s River Cauvery: an endemic fish swimming towards extinction?

Abstract

The Western Ghats region of India is an area of exceptional freshwater biodiversity and endemism. Mahseer of the genus Tor are considered prized sport fishes of great cultural significance; nevertheless, they are threatened as a result of increasing anthropogenic stressors. In the River Cauvery, the mahseer community comprises a 'blue-finned' and an orange-finned, 'hump-backed' fish. Whilst it is not yet known whether these are distinct species or 2 different phenotypes, evidence suggests that the hump-backed phenotype is endemic to the river, whereas the blue-finned phenotype was introduced in the 1980s. Angler-catch data from a managed fishery on the River Cauvery, gathered between 1998 and 2012 and comprising 23 620 h of fishing effort, revealed that captured individuals ranged in size from 0.45 to 46.8 kg, with the blue-finned phenotype comprising 95% of all captured fish and the remainder being hump-backed. The catch per unit effort (CPUE) of the blue-finned phenotype significantly increased over the study period, while the mean weight of individual fish significantly declined. By contrast, the CPUE of the hump-backed phenotype declined significantly over the period, with individual mean weights significantly increasing. These data suggest a recent recruitment collapse in the hump-backed phenotype resulting in an ageing population that may be headed towards extinction. The introduced blue-finned phenotype, however, continues to recruit strongly, suggesting that the mahseer community of the River Cauvery has undergone considerable shifts in the last 30 yr.

Disappearing giants: a review of threats to freshwater megafauna

“…In the last few decades, managed relocation, of species to areas outside of the native ranges has become a common practice for reducing their risk of extinction; however, it can also lead to unintended negative effects. For instance, the Hump Backed Mahseer ( Hypselobarbus mussullah ), an endemic species in the Western Ghats of India, has been pushed to the edge of extinction due to the successful establishment and spread of the nonnative Blue‐finned Mahseer ( Tor khudree ), which was introduced for conservation purposes . Likewise, the invasive monogenean gill fluke, carried by Stellate Sturgeon ( Acipenser stellatus ) and was inadvertently introduced from the Caspian Sea and played a significant role in the extirpation of Ship Sturgeon ( Acipenser nudiventris ) in the Aral Sea …”

Status of Recreational Fisheries in India: Development, Issues, and Opportunities

“…Today, recreational angling in India is permitted only on river reaches outside PAs, and this is where the majority of foreign and Indian recreational anglers now concentrate their efforts. Dominant species targeted by recreational anglers in India include Tor sp (previously known as Tor mussullah or the hump-backed mahseer; see Pinder et al, 2015), T. putitora (golden mahseer), T. khudree (Deccan mahseer), Neolissochilus hexagonolepis (copper or chocolate mahseer), and Gibelion catla (Indian major carp/ catla; also see Table 2). Occasionally caught other Indian native fish species include Channa diplogramma (giant snakehead), C. marulius (bullseye snakehead), C. punctata (spotted snakehead), C. striata (snakehead murrel), Wallago attu (wallago), Hemibagrus maydelli (Krishna red tail Catfish), Bagarius bagarius (goonch), Labeo rohita (rohu), Cirrhinus cirrhosus (mrigal), Hypophthalmichthys molitrix (silver carp), Cyprinus carpio (common carp), Ctenopharyngodon idella (grass carp), and Hypophthalmichthys nobilis (big head carp).…”

“…Uncertainties also exist surrounding the actual number of mahseer species found in India and their exact distribution (Pinder and Raghavan, 2013). The most popular mahseer species targeted by anglers in India, the "Cauvery humpbacked mahseer," awaits the recognition of a scientific name (see Pinder et al, 2015), and other species such as T. putitora, T. tor, and T. khudree, currently known to have a wide range of distribution, could in fact be "species complexes" comprised of several range-restricted species, many of which would need formal taxonomic recognition (see Table 2). Although numerous studies are available on the natural history of some mahseers (for a review see Nautiyal, 2014), the ambiguities surrounding species taxonomy and distribution make these of little value for practical conservation planning and action.…”

“…Large-scale stock replenishment of various "species" of mahseer has been carried out in the Western Ghats region, particularly in the Cauvery River (see Ogale, 2002), which has resulted in the reported proliferation of hybrids and the suspected decline of native lineages (Dinesh et al, 2010;Pinder and Raghavan, 2013;Pinder et al, 2015). It is known that the Tata Electric Company in Lonavala, the source of most stocked fingerlings in the Cauvery, experimentally hybridized mahseer species (Ogale and Kulkarni, 1987) and have provided fingerlings of various mahseer species including T. mussullah (now understood to represent a distinct genus; see Knight et al, 2014) to different angling associations in India (Ogale, 2002).…”

Effective monitoring of freshwater fish

“…For example, in the River Trent, England, angler catch statistics monitored changes in the fish assemblage in relation to improvements in water quality (Cooper & Wheatley, ; Cowx & Broughton, ). More recently, catch statistics from individual anglers were used to assess the population status of mahseer fishes ( Tor spp., Cyprinidae) in the River Cauvery, India (Pinder, Raghavan, & Britton, ,). An issue with angler‐based data is that they tend to be biased for specific species and size ranges (Amat Trigo, Gutmann Roberts, & Britton, ).…”