Factories play vital part in scientific study to establish mackerel numbers

Mackerel is a hugely important fish stock for Scotland – representing our highest volume and value fishery, and accounting for around 30 per cent of the total value of fish landings. It is, therefore, very important that the stock is managed carefully.

This can only be achieved through the best possible science to determine the stock and composition of the different age classes from year-to-year, which in turn provides informed advice for setting sustainable catch limits and other management measures.

Such information gathering is done in a number of ways, including sampling surveys of the adult mackerel stock by research vessels and surveys of the abundance of their free-floating eggs.

Tagging the fish is also extremely important. From 1968-2010 Norwegian scientists tagged thousands of mackerel on an annual basis at spawning grounds to the west of Ireland and Scotland. It is an intricate process where the mackerel are caught on jigging hooks from a survey vessel and then a tiny marked metal tag is injected into the abdomen of each fish, before being released back into the sea.

These tags were recovered by special metal detector systems installed inside fish factories, with the information gained providing valuable evidence on the stock and its migrations and age structure.

In effect, fishing vessels are sampling the stock – and by using the tried and tested “mark and recapture” method commonly used by ecologists, it is then also possible to estimate the size of the population.

However, the problem with the metal tags was that once a metal detector in a processing factory had identified a tagged mackerel somewhere in its flow system, it was time-consuming for processing operatives to find the actual fish and then recover the tag.

But all this changed in 2011, when mackerel were tagged for the first time with Radio Frequency Identification (RFID) tags based on initiative between Norwegian scientists and the Norwegian fishing industry. This special electronic detection system automatically identifies a tagged mackerel and updates a common database, without the need for retrieving the fish and dissecting it.

It was a big leap forward and between 2011 and 2014 around 160,000 mackerel were tagged in this way. Crucially, this paved the way for more factories across Europe to install the RFID detection system. Today, virtually every Scottish mackerel processing factory is equipped with these RFID scanners, which means we are also playing our own vital role in participating in this international research.

Scanners have also been installed at factories in Iceland and at the Faroes, and at the moment as much as 300,000 tonnes of mackerel is scanned for tags annually.

In 2016, more than 800 tagged mackerel were captured from the various release years, a large proportion of which were detected at Scottish factories. This information is now used in the official International Council for the Exploration of the Sea (ICES) stock assessment, which is then used for setting catch limits and other management measures.

According to Aril Slotte from the Institute of Marine Research in Bergen, the lead scientist in this RFID-tagging programme, the project has been a great success.

He says: “The project would not have been possible without the economic support and willingness of the Norwegian, Scottish, Faroese and Icelandic fishing industries to work together. This proves the fishing industry can take an active part in developing and maintaining time-series research that can be used to assess commercially important fish stocks.”

Although the mackerel stock tends to fluctuate slightly from year-to-year for a whole variety of natural factors, including spawning success and juvenile survival rates, the good news is that the mackerel population is in generally healthy shape.

This is backed up by the fishery holding the prestigious Marine Stewardship Council ecolabel  certification, which is an independent and internationally recognised rubber stamp of the sustainability of the stock and the sound environmental practices adopted by our fishing fleet.

And thanks to the participation of our fishermen, processors and scientists in the stock assessment process, we can look forward to a sustainable and profitable future for this economically important fishery.

Dr Steven Mackinson, chief scientific officer, Scottish Pelagic  Sustainability Group