Pelagic trawl, on an ecosystem cruise in the Barents Sea. Malin Lie Gulbrandsen with diet. Photographer: Erlend A. Lorentzen/HI

REDUS has led to a better estimate in the stock advisory

NORWAY
Monday, November 30, 2020, 18:00 (GMT + 9)

The following is an excerpt from an article published by the Havforskningsinstituttet:

The researchers have gone through most things: from cruises, trawling methods and sampling to calculations and management strategies. They have found new methods and tools. After five years with REDUS, the advice to HI has become safer.

How much fish there is in the sea, and how much can be safely discarded without destroying the fish stocks, is a key issue for both Norwegian fisheries and marine research.

"Fish live in the large, deep sea and are much more difficult to count than animals on land. Therefore, we marine scientists must use indirect ways to be able to say something about how large and solid a fish stock is", explains project leader for REDUS, research manager Erik Olsen.

Specifically, the researchers went on scientific voyages, and they collect catch data from the fishery. This is further analyzed through advanced mathematical models. This is how the researchers find an estimate of how many fish there are in the sea today, and how the stock will develop in the coming years.

The goal: Make stock advice and quota advice safer

But an estimate is never one hundred percent accurate.

"There is always a certain uncertainty associated with an estimate, and it is frustrating for both the fishing industry and management when it sometimes happens that the estimate does not agree with what one later observes at sea", says Olsen.

This was the background for the REDUS project, which started in 2016. REDUS stands for Reduced uncertainty in stock assessments. When the project is now to be completed after five years, it has led to several changes in the processes leading up to a quota council.

Herring samples at the fish lab with Jostein Røttingen. Photo: Erlend A. Lorentzen / HI

New tools will be used on all large stocks

The researchers in the REDUS project worked broadly and examined all stages of the process: from sample collection on cruises to the methods for trawling, acoustics, stock calculations and management strategies.

"We have developed a "virtual workbench" for analysis of data from cruises and catches that makes it possible to test how different models, user settings and selection of data affect a stock estimate. This tool enables us to measure how great the uncertainty is in every single part of the process, from the catch one takes on the cruise and all the way to a finished piece of advice", says the project leader.

Herring and cod are our "test rabbits" for the new tool, but the results are so good that the tool will now be used on a much larger scale.

"From 2021 onwards, the tool will be used on all the large fish stocks we at HI can afford."

More precise - and cheaper - cruise estimate

Developing new methods has been central to the REDUS project.

- Among other things, we have developed new methods for the way one conducts cruises, which both provides a better cruise estimate and at the same time saves time on cruises, something which in turn makes cruises cheaper, says Erik Olsen.

- We have made sampling on trawls with trawls more standardized, something that has led to less variation in results than when we started in 2016. Furthermore, we have increased the precision of the biological sampling when we estimate the age structure of the stock, which we need to be able to create good age-structured population models.

HI has also collaborated with the Norwegian Computing Center to further develop the models that calculate how large the stocks are.

"In addition, we have developed a new model to cover the 'hola' in our voyages, which occurs when ice, storms or accidents mean that there are parts of an area that we are not allowed to investigate in some years."

Sonar: Not as easy to use as a hope

Many of the cruises to HI are so-called acoustic cruises, where the researchers use data from echo sounders to measure the amount of fish in the water masses.

In the REDUS project, an attempt has been made to develop the acoustic methods, and in particular to investigate whether and how sonar can be used to estimate the amount of fish in the water masses.

"This work we are not getting to the finish line yet, because it turns out not to be as straightforward to use sonar to count fish, as we first thought", Olsen said.

Photo: Åse Husebø / Institute of Marine Research (2020)

"Herring lottery" provides more representative samples of catch

In order to find out how old (or young) the fish that are being fished up are, it is important that the samples from the catches in the fishery are as representative as possible. It is best ensured that it is a random sample that is sampled.

Therefore, the researchers in REDUS, together with the fishing industry, developed a "lottery" to be able to take random samples of catches. The "lottery" started with herring, but has now been expanded to apply to other pelagic stocks.

"Together with the Norwegian Computing Center, we have also developed a statistical tool that calculates catch per age, and shows the uncertainty in the estimate, it is important data to get into our stock models", says Erik Olsen.

Simulates the outcome of different strategies

An important tool for fisheries management in Norway and the other countries around the North Atlantic is to find good harvesting strategies. Such a strategy should allow one to fish as much as one can, without a stock being in danger of collapsing.

"To find the most optimal harvesting strategies for a fish stock, one must test thousands of potential strategies with computer simulations. In REDUS, we have been able to develop such a tool, where one can both simulate strategies for a single population, or for an entire ecosystem. With the latter, we can see how a strategy for a fish stock can again have an impact on other species."

Source: Havforskningsinstituttet / Norwegian Institute of Marine Research (translated from original in norwegian)

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