In 2018, fishery scientist Asim Nath, with the help of the local fisher community, netted the hilsa fish along upstream and downstream locations on the Hooghly river on India’s east coast. A year earlier, on the other side of the country in the Gulf of Kutch, on India’s west coast, another set of researchers, aided by the fisher community, caught catfish (Arius sona).

The focus of their aquatic endeavours spanning east-west coasts was on otoliths or fish ear stones – pearly white, pea-sized hard structures found inside the heads of bony fishes that help them with hearing and balance. These calcium carbonate structures grow over time throughout a fish’s life, similar to the way tree rings grow. And just as tree rings are packed with information about trees and the environment, otoliths record growth patterns in sync with changes in the water’s condition. For example, otoliths show seasonal changes in their environment as alternating opaque and translucent rings.

Researchers in India are listening in to these clues by tapping into otoliths from live-caught fish and dead fish in fossil records, to mine and analyse a wealth of growth data that advances paleoclimate science and can be plugged into climate models. Combined with other sources of data, otolith-based data also informs current practices in fisheries and their management amid human-caused pressures.

A modern otolith. Photo credit: Anindya Sarkar and team

After extracting the otoliths and polishing them off of dirt and debris, they look at carbon, oxygen and other atoms in the otolith layers and analyse the composition of stable isotopes – a version of an atom – in the otoliths, using sophisticated tools and techniques.

“What we are trying to do in addition to unpacking how the climate was like during the fish’s lifetime, we are also unraveling the variations in climatic factors during the lifetime of the fish. So, that is the high-resolution analysis that we are working on which gives us monthly data of temperature,” explained Torsa Sengupta, a PhD student at IIT Kharagpur and the lead author of the paper documenting a novel laser system to investigate seasonal change in Sea Surface Temperature recorded in otoliths.

Climate information

The isotopes of oxygen in these otoliths depend on the temperature of the water in which the fish grew and record continuous snapshots of past temperatures during the lifetime of the fish for years, said lead investigator Anindya Sarkar at Indian Institutes of Technology, Kharagpur’s Department of Geology and Geophysics.

Sarkar and his team devised a technique for more precision analysis of otoliths – one can examine a few millimetre-sized otoliths by a carbon dioxide laser at a few micron scale intervals to pinpoint their oxygen isotopic compositions. The conventional method using acid-digestion of micro-milled samples is a “multi-step time-consuming process”. The technique is a “fast method”, they write in a recent paper.

“So if a fish is living from summer to winter and when the summer temperature is high and the winter temperature is low, we will be getting these cycles of summer-winter temperatures of the water in which the fish lived,” Sarkar said. “Secondly, because the otolith material composition is calcium carbonate, we also are able to analyse carbon isotopes, which talk about the carbon cycle in the water – for example, the productivity levels in the aquatic system.”

“Taken together, the temperature and carbon data points give you a piece of very comprehensive climate information at any given time over the time the fish had been living,” Sarkar told Mongabay-India.

The high-resolution climate information, like sea surface temperature, can be plugged into the climate models making them much more robust.

Additionally, analysis of five-thousand-year-old fish otoliths from Indus valley sites also sheds light on the waxing and waning of past civilisations that had fish as part of their diets. “We are studying how the Indus Valley civilisation evolved and declined and if we are able to detect or trace the seasonal variation through the time it will help the past climate modelers in a major way to understand the dynamics of these civilisations,” added Sarkar.

Dholavira otolith from the Indus Valley civilisation. It is about 4,500 years old. Photo credit: Anindya Sarkar and team

Collaborator and study co-author, archaeo-zoologist Arati Deshpande Mukherjee of Deccan College, who has extensively studied the otolith assemblages and other biological remains at Dholavira, the southern centre of the Harappan civilisation on the Rann of Kutch in Gujarat, said, fish otoliths are becoming increasingly important in reconstructing the past climate and understanding climate sensitivity to seasonal variations apart from their use in fishery management and conservation.

In a 2019 paper, Deshpande Mukherjee and colleagues at the Indian Institutes of Technology, Kharagpur and Physical Research Laboratory, Ahmedabad, pointed to climate evidence coming from high-resolution oxygen isotopes in snail shells Terebralia palustris which typically grow in mangroves and were a source of food for the Dholavirans. They linked the decline of Harappan city Dholavira to the disappearance of a Himalayan snow-fed river which once flowed in the Rann of Kutch.

“We will need more collaborations going forward to advance interdisciplinary research using otoliths and other similar records that can also help us understand future risks of extreme events linked to climate change,” she added.

Understanding hilsa migration

Prosenjit Ghosh at the Centre of Earth Sciences at the Indian Institute of Science, who worked with Asim Nath on understanding hilsa migration patterns in the Bay of Bengal, said India has a very limited number of studies based on chemical signatures of the otoliths (less than 1% according to Web of Science, a research citation database).

“There are only a few researchers in India which work on fish otoliths,” Ghosh told Mongabay-India. “Most otolith-based studies explore the utility of otoliths to identify fish stocks and to report fish age and growth. The major limitation of otolith based studies is its extraction from the fish head which requires fish sacrifice and limits its use for the threatened fishes. Another limitation is that it contains limited amount of organic matter which restricts its use in food web relationship studies.”

In the study on hilsa’s migration, documented in a recently published paper, the scientists used stable isotopes to design a technique to observe the favoured habitats for hilsa individuals in Bay of Bengal waters. Hilsa is a highly sought-after, silvery fish that travels between the sweet river water and saline seawater in the Bay of Bengal. Hilsa, a migratory fish between India and Bangladesh waters, is imperiled by over-exploitation. It swims from the Bay of Bengal to the rivers to spawn (release eggs).

The otoliths of hilsa fish reveal that they prefer marine water to freshwater. Photo credit: World Fish/ Flickr

The oxygen stable isotope composition of the otolith carbonates has a strong relationship with the progressive distance from the coastal area to inland water. “The hilsa shad otoliths revealed that the marine environment was enriched in oxygen isotope as compared to freshwater which indirectly suggests that the marine water is the favoured place for the maximum growth of the hilsa shad or for probable aquaculture,” said Ghosh, a corresponding author of the paper.

The researchers found that smaller hilsa-shad individuals prefer to stay in freshwater for some time after birth (a few weeks to months) before moving downstream of the Hooghly river or to the Bay of Bengal. They start moving downstream and near-shore coastal waters as they grow bigger.

The adult hilsa spend about 20% of its time in freshwater mainly for feeding purposes. The mature hilsa shad is a resident of the mouth of the estuary where mating partners and food availability are abundant.

Asim Nath said otoliths are a reliable source of information to understand their migration patterns in response to various pressures. While tagging hilsa with anchor tags is another option, it is difficult to track the fish over long distances.

Combined with the fish otolith-based analysis and other experiments, co-author Asim Nath said the results point to the fishing practice where fishers use nets of smaller mesh size to trap small-sized hilsa (350 grams-400 grams) that results in their spawning. “A mature hilsa weighs 800 grams-900 grams but we get the maturity at 350 grams-400 grams due to the operation of small size nets. Juveniles spawn on getting caught.”

“Hilsa migrate up to 80 km from marine to the riverine zone for spawning,” explained Nath. “Not beyond. We can tell from the results that 20 km to 25 km from the mouth of the river is the zone of maximum growth. Beyond that, there is excessive pressure from the fisheries.”

This article first appeared on Mongabay.