The Fatter the Better?

New research shows changes in fish fat content thanks to industry data

It is well known that pelagic fish, particularly herring and mackerel, are rich in essential omega-3 fatty acids. In most European countries, it is recommended to eat at least one portion of these oily fishes per week. Fat fish therefore result in valuable products for fishers and fish processors whilst also contributing to a healthy diet for consumers.

But why are pelagic fish fat? And why might their fat content change over time? These questions interest fisheries scientists because the fat content of a fish provides important information about the overall condition, or health, of a fish and the stock which it came from. Hence, data on fish fat contains a snapshot of knowledge about feeding conditions, migrations, and even potential spawning success.

Understanding changes in the condition of commercially important fishes has historically been difficult due to a lack of scientific data. This was particularly true for species such as herring and mackerel because scientific surveys either did not measure fat content or the data did not cover more than a few weeks of the year.

However, a few years ago it was realised that this knowledge gap could be successfully filled using valuable data collected by Scottish and Dutch pelagic fishing industries. Processing factories have measured the fat content of herring and mackerel catches for quality control and marketing purposes for many years (Fig. 1). These data were unrivalled in terms of quantity as well as the long time periods and large areas that they covered.

To gain scientific insights from this unique data source, the Scottish Pelagic Fishermen’s Association co-funded a 4-year PhD project in collaboration with the University of Aberdeen and the Pelagic Freezer-Trawler Association. The project aimed to showcase the scientific value of fat content data collected by the fishing industry by investigating changes in herring and mackerel condition due to multiple factors. Working with quality control personnel at over 20 processing factories, 16 years of data were compiled and analysed.

The project was completed in December 2022 with several key findings. The fat content of both species was found to reflect their activities throughout the year, including feeding, spawning, and migration (Fig. 2). Using additional data on the fat content of ‘maatjes’ herring (i.e. young, virgin herring) the study also found that this species requires large amounts of fat and thus needs to be in good condition to spawn for the first time. The temperature of the sea and the availability of food can also affect the fat content of ‘maatjes’ herring.

A particularly striking result is the sharp decrease in mackerel fat content since around 2010 (Fig. 3). It is thought that the rapid decline may be caused by intensified competition for food due to the increase in mackerel stock size between 2006 and 2015. While the decline in mackerel fat content has been noticed by processors, Helena Bedić (Quality Manager at Northbay Pelagic Limited) says that the trend “has not been flagged as a negative, as the products are still within the range required by customers”. Mackerel of varying fattiness can be used to produce different products. While the decline in mackerel fat content has had minimal impact on its value so far, the study demonstrates that long-term monitoring of fish condition using data collected by industry is key to understanding wider impacts on stocks.

First results of the study, led by Dr. Susan Kenyon, have been published in the ICES Journal of Marine Science (https://doi.org/10.1093/icesjms/fsab244) and further results will be published in full soon.

Fig. 1. Measurement of fat in herring fillets using water evaporation. Photo by Samanta Risovanaite.

Fig. 2. Changes in North Sea herring fat content throughout the year.

Fig. 3. Average decline in fat content of mackerel across sizes from 2004-2021.