Scientists at the Alaska Fisheries Science Center (AFSC) have substantially increased the number of fish tested for their fat content, using a new streamlined method for sampling Pacific salmon, Pacific herring, cod pollock and capelin.
Lipids, or fat, have long been regarded as the most important energy reserve for animals in marine ecosystem, the Alaska Fisheries Science Center said in a Sept. 24 news release on the new technique. These lipids are more calorie-dense than proteins and sugars and are used to fuel growth. The total lipid content of fish is widely used as a metric to understand the overall health and condition of a population. Moreover, scientists said prey species with higher lipid content contribute to the overall health of the predators that eat them.
Using traditional gravimetric analysis for analyzing lipids, a technician can produce results from 15 samples in an eight-hour day. Using the new method, a technician can produce results from 69 samples in the same eight-hour day. The new method drastically increases productivity while maintaining very high accuracy and precision. More data generated on a faster turnaround is important for decision-makers to inform fish stock assessments and fisheries management on tighter time scales, they said.
Cody Pinger, analytical chemist and lead author of the article at the AFSC noted that good data about fish stocks requires large sample sizes. In his experience, Pinger said, laboratory methods are usually slow and tedious.
“For nearly a decade, our group has been working to optimize the sulfo-phospho-vanillin assay, and I’m thrilled to say that we have increased our productivity many-fold while maintaining good accuracy and precision,” he said.
The sulfo-phospho-vanillin (SPV) assay has been studied for a while as a rapid alternative to traditional methods for lipid analysis.
For some fish species, the entire fish is blended into a uniform mixture, while for others only the muscle tissue is processed. Samples are extracted into organic solvent. Heat and concentrated sulfuric acid are used, followed by a reaction with vanillin in the presence of phosphoric acid. (Hence the name sulfo-phospho-vanillin.) Vanillin is a fragrant compound that is the essential component of vanilla. The process generates a final product that is a clear liquid with a pink tint. The darker the pink equates to more lipid in the sample.
From there the sample is passed through a spectrophotometer, which measures the amount of light absorbed by the sample. This fine-tuned instrument can quantify the amount of lipid in each sample.
AFSC researchers said the chemical reaction between lipids, acid and vanillin is dependent on the specific molecular composition of the fat, which varies with the species. In a perfect world all samples could be calibrated using a widely available fish oil. The AFSC tea led by Pinger used mehaden oil, but noted that this does introduce some error when analyzing many different species with different lipid. The team developed calibration models for each species.
