How Catla and Mrigal Stocking Changed Native Fish in Indian Reservoirs

Indian reservoir fisheries management through the mid-twentieth century operated on a straightforward premise: large, fast-growing species convert plankton and detritus into harvestable protein faster than most native fish. Catla catla, the surface-feeding column dweller, and Cirrhinus mrigala, the bottom-grazing detritivore, were selected precisely because they occupied different feeding zones and did not obviously compete with each other. Rohu, Labeo rohita, completed the trinity. Stocking these three across the Hirakud, Nagarjunasagar, Tungabhadra, and dozens of smaller impoundments was framed as productivity enhancement, not ecological intervention.

What the framing missed was that a reservoir is not a pond. Impoundments created by large dams hold remnant river communities, species that evolved in lotic, flowing-water conditions and were already stressed by the loss of their natal habitat. Introducing millions of carp fingerlings annually into that already-disrupted system was a second pressure arriving before the first had been measured.

Catla catla is a filter feeder in its juvenile phase, switching to a mixed zooplankton and phytoplankton diet as it grows. In high stocking densities, this creates measurable zooplankton suppression in the upper water column. Zooplankton are the primary food source for larvae of most native cyprinids and for small pelagic fish like Amblypharyngodon mola, the mola carplet, which historically formed the base of subsistence fisheries in many central Indian reservoirs.

Cirrhinus mrigala disturbs the sediment-water interface while foraging, releasing nutrients that accelerate phytoplankton growth and reduce water clarity. Reduced transparency shrinks the photic zone, which compresses the habitat available to visual predators and to species that depend on submerged macrophytes for spawning. Several native hill-stream species that persist in reservoir tributaries require clear, structured littoral zones. Mrigal-driven turbidity degrades exactly those zones.

Studies from reservoirs in Madhya Pradesh and Andhra Pradesh tracked in ICAR-CIFRI research programmes have documented declining catch-per-unit-effort for native minor carps and predatory species, Wallago attu, Sperata seenghala, and Notopterus notopterus, over stocking-intensive decades, while total biomass yield figures remained stable or rose. The aggregate numbers looked like success. The species composition underneath them told a different story.

Catla and Mrigal do not predate native fish directly. Their impact is subtler and harder to reverse: they outcompete larvae and juveniles for food at the stage when native species are most vulnerable. A Catla fingerling stocked at two to five grams is already large enough to suppress zooplankton patches that a native cyprinid larva depends on for its first weeks. Because stocking happens seasonally and in large pulses, the suppression is not chronic, it is timed almost exactly to overlap with the spawning and larval recruitment windows of native species.

This is the mechanism behind what reservoir ecologists describe as recruitment failure without visible adult mortality. Native fish adults persist in the catch for years after the ecological shift, because adults are long-lived. The collapse is happening in the cohorts that are not recruiting into the adult population. By the time it shows in catch data, a decade of recruitment has already been lost.

Introduced carp also alter the competitive landscape for benthic invertebrates. Mrigal and Rohu foraging reduces chironomid and oligochaete densities in soft-sediment zones. These invertebrates are the primary food of juvenile Mystus species and of Channa juveniles before they transition to piscivory. Reducing invertebrate prey density during the juvenile phase of native predators creates a secondary suppression effect that compounds the direct zooplankton competition.

Pre-impoundment fish surveys from rivers that were later dammed, the Mahanadi before Hirakud, the Krishna before Nagarjunasagar, recorded species assemblages that included twenty to forty cyprinid species, multiple catfish families, the featherbacks, spiny eels, and climbing perches that characterise the Deccan and central Indian ichthyofauna. Many of these species are laterally compressed, cryptic, or small-bodied: they were never the target of commercial fisheries, but they structured the food web from below.

Post-impoundment surveys conducted thirty and forty years later, collated in assessments by the Zoological Survey of India and ICAR-CIFRI, show consistent patterns: large-bodied native predators reduced in abundance, small-bodied native species absent or rare in reservoir catches, and the catch dominated by the three stocked species plus common carp, Cyprinus carpio, which escaped or was introduced separately and now reproduces independently in most major reservoirs. The diversity loss is not uniform, deeper, larger reservoirs with more complex littoral structure retain more native species, but the directional trend across managed systems is consistent.

ICAR-CIFRI and several state fisheries departments have begun trialling modified stocking protocols in select reservoirs: lower stocking densities, staggered timing to reduce overlap with native spawning peaks, and the inclusion of native predatory species like Tor tor and Labeo calbasu in stocking mixes to maintain trophic structure. The Gobind Sagar reservoir in Himachal Pradesh and parts of the Tungabhadra system have been sites for some of this work.

The harder problem is economic. Catla and Mrigal produce high yields per hectare and are what fishing cooperatives and state lease systems are built around. A stocking protocol that reduces carp density to protect native recruitment reduces short-term yield, and the fishers absorbing that reduction are not the ones who made the stocking decisions. Any ecologically informed revision of reservoir management has to account for that asymmetry, or it will not hold past the first lease renewal.

The native fish communities in Indian reservoirs are not gone. They are suppressed, and in some systems they recover measurably in years when stocking is reduced or delayed. That partial reversibility is the most important biological fact in this story, and the one that makes the management question genuinely open rather than already settled.