Fish ears can provide data on water contamination

Materials World magazine
31 Jan 2019
Sutton Lake in eastern North Carolina, which serves as cooling ponds for coal-fired power plants. Credit: Jessica Brandt, Duke Univ.

Fish ears provide accurate data for analysing water contamination from coal fired plants, as Ellis Davies reports.


A way to measure the effect of waste streams from coal-fired power stations on water systems using fish ears has been developed by researchers at Duke University, USA. The bones in a fish’s ear, called otoliths, have been studied for many years in environmental science, but have recently been looked at for their potential to reveal water contamination encountered by fish during their lifetime, painting a picture of its development by looking at the strontium isotope ratio (SIR). Researchers say that by measuring the SIR, which is associated with coal ash, contamination in a water system can be accurately assessed, and in the future be a much faster, more conclusive method. 


Coal correlation

Researchers identified the SIR in otoliths using a method called thermal ionisation mass spectrometry, which is a magnet-based system able to distinguish between different isotopes of a given element. Duke's Nicholas School of the Environment PhD graduate and lead author of the paper, Jessica Brandt, told Materials World, ‘To prepare the samples for that kind of analysis, the waters are dried down to the salts, re-diluted and run through stronium resin columns before loading to the instrument. We applied a similar method for the otoliths, preceded by an acid digestion process.’ The team took surface water and pore water – water in sediment – samples to compare with the otolith results.

Four systems were looked at during the study – two that received regular discharge from coal ash ponds, and two in North Carolina that did not receive any. The latter acted as a reference to ensure that the otoliths matched the water samples in an uncontaminated area. ‘We looked at whether the ratios [in water samples], from the same systems, were similar to the otoliths, and also to ratios from water upstream that should not be impacted by the coal ash waste stream as a reference,’ said Brandt. 

In all cases, the study found that the best predictor of otolith chemistry was the pore water, which Brandt believes highlights how important it is to understand the system being worked on. ‘There were two systems where we found different chemistry in the surface water and otoliths, but the pore water did match. We speculate two reasons for this. In one system, associated with coal ash, the surface water did not match the otolith signature because the SIR from the coal ash pond is changing itself over time, which could be because a different combination of coal is being burnt and will have different SIRs. The otolith grows over the lifetime of the fish, so integrates signatures as it's growing, and, if they are changing, what we collect could reflect that variation over time,’ she said. 

In another system, which was a reference system with fast moving water, the team concluded that the fast flow was integrating water from systems upstream with a possible different SIR. ‘In conclusion, pore water is probably a better indicator of the true SIR background of the system. Fish spending their lives in the systems are going to best reflect the pore water,’ said Brandt. 

A step up

The analysis of otoliths is both a superior, and complimentary, method to water testing. This study was meant as a proof of concept, but Brandt and her team look to use more advanced methods in the future. ‘Otoliths grow over time. Here we analysed the entire otolith as a bulk sample. But in the future we plan to use a laser-based method that allows us to detect changes in SIR over the lifetime of the fish. This would allow us to look at how changes to the system have evolved overtime, which is an upgrade because it gives you a time series worth of data,’ Brandt explained. To get the same amount of information from a water sample, you would need to collect samples at regular intervals for a very long period of time, whereas laser-based methods can generate similar data from a single test. 

Brandt concluded, ‘We're pretty excited about bringing this type of research into environmental toxicology. We think using otoliths as tracers of specific sources of contamination is an important advancement in the field.’