“Fish should be caught only after they reach their optimal length”

80 centimetres, not 38 centimetres, is the optimal minimum catching size of cod, writes Axel Wenblad, head of the goverment body, Swedish Board of Fisheries. Here he explains how length-optimizing would increase catches by around 60 percent in the long run, and why the Baltic Sea would be an ideal pilot case for testing this idea.

The Swedish Board of Fisheries has recently published a strategy document “FISKE2020“, which aims to provide a basis for the debate on sustainable fishing and how to achieve healthier marine ecosystems….


…The focus is on ecosystem-based fisheries management with the goal to restore and maintain resilient ecosystems so that they can supply ecosystem services, including sustainable fishing, sustained biodiversity and functioning food webs.

A growing understanding of aquatic ecosystems has shown that the size structure, distribution and genetic variation in large predatory fish can be used as indicators for the status and health of an ecosystem. The reason that the state of health is reflected in the size and quantity of large predatory fish is that predators at the top of the food chain have a significant effect on stocks of prey fish, which, in turn, have effects further down the food chain. Management of aquatic ecosystems must therefore take into account these effects. Large individuals also contribute to reproduction to a greater extent than do smaller ones, and the more age groups there are contributing to reproduction, the more stable the population becomes over time.

To be able to define resilient ecosystems, reference points or base lines need to be chosen. A suitable reference point could be the structure, distribution and genetic variation of large predatory fish back in the 1950s, before the large scale increase in exploitation of the fish stocks. At that time, the biomass and average size of large predatory fish were considerably larger than they are today.

An ecosystem-based management aiming to achieve well-functioning ecosystems, that provide ecosystem services, requires a great deal of detailed knowledge about ecosystem function. Today, that knowledge is fragmentary and is insufficient to be used as the basis for a fully developed ecosystem-based management. A less knowledge-intensive method, as a step towards ecosystem-based management, would be to apply the principle that fish should be caught only after they reach their optimal length. The optimal length (Lopt) is here defined as the body length when an unfished age group reaches its maximum biomass. This concept is discussed in detail by Froese (2008). Below we will se how this strategy can help to rebuild stocks and minimize impact on the ecosystem.

An example – applying an Lopt fishing regime in the  Baltic cod fishery

Let us take Baltic cod stock as an example and compare the current fishing regime with a minimum size limit (Lmin) with fishing at the optimal length (Lopt). Figure 1 shows that the current minimum landing size of 38 centimetres in the fishery results in a population size structure with very few large fish left in the population. On the other hand, if Lopt is applied as the minimum size at catch there will be considerably more large individuals in the population and the size structure will be much more similar to that of an unfished stock.

Figure 1. Schematic illustration of population size in relation to length, based on data of eastern Baltic cod, for two scenarios with different minimum sizes, and one scenario with no fishing (F=0). Lmin = current minimum size limit; Lopt = optimal length according to the principle described. The solid curve represents the size structure of an unfished population (F=0); the long and short dashed line represents the size structure when current minimum size limit (Lmin) and  fishing mortality rate (F=0.3 is applied); the dotted line represents the size structure if the optimal length (Lopt) and  F=0.3 is applied.

The Lopt-principle does not, however, mean that fishing over the optimal length can be unrestrained. If enough large individuals are to survive, fishing mortality rate needs to be adjusted to an appropriate level.

Using the same data that is used for stock assessment by ICES and applying Lopt-fishing in the Baltic eastern cod fishery shows that good management results can be achieved. If selectivity is increased so that only cod over 80 cm in length (Lopt) are caught and fishing mortality rate remains at current level, cod catches will in the long run increase by about 60 percent compared with current practises.

Due to the fact that fishing mortality has recently reached a fairly low level, a recovery of the population is to be expected even with an unchanged minimum limit, but the recovery will be much greater if the fishing selectivity is increased to the optimal length. Thus, with applying an Lopt fishing regime in the Baltic cod fishery we would both increase landings and make the population structure more similar to an unfished stock! We have, however, to be aware of that the Baltic cod fishery is a single-species fisheries and that the cod stock currently is improving. This makes the implementation of Lopt-fishing rather straight-forward by applying step-wise increase of mesh sizes. The Baltic cod fishery would be an ideal pilot case for testing Lopt – fishing.

Most fisheries in the North Sea, the Skagerrak and the Kattegat are multi-species fisheries with substantial by-catches that require additional solutions to target individual species as separately as possible. This requires further development of species-selective gear, temporal regulations, such as closed seasons, and spatial regulations like closed areas and other types of zoning. Beneficial effects could also be achieved by implementing management measures aimed at the largest or most sensitive species, allowing the others to” free ride”.

The conditions and methods used for achieving the Lopt-principle will consequently shift between different sea and water areas. Ecosystem plans are therefore needed as a basis for long-term decision making. These plans must in detail specify a realistic rate for implementation as well as appropriate measures.

Adaptive management beyond maximum sustainable yield (MSY)

The Lopt-principle demonstrates how we, with today’s understanding and means, can take a big step towards ecosystem-based management without drastically reducing the size of catches. It is important to evaluate the results of the Lopt-principle from an ecosystem perspective, which is best done in the form of adaptive management in which management effects on the fish populations and the ecosystems are evaluated and management measures gradually adjusted.

Most of the fish stocks in European waters currently indicate low population biomass, a skewed length distribution and therefore a dominance of smaller individuals. Length-optimised management is a strategy that favours a stable size structure and at the same time allowing exploitation of the fish stocks in accordance with ecosystem based fisheries management. At the same time Lopt-strategy takes advantage of the maximum production capacity of the fish stocks, which means that the impact on the population can be low at the same time as providing high yield.

The current efforts to implement an MSY management regime in European waters is a step in the right direction. Extending this work to include Lopt-fishing would be a further leap forward for both the ecosystem and the fishery.

Further reading:

Froese R., A. Stern-Pirlot, H.Winker, D. Gascuel. (2008) Size matters: How single-species management can contribute to ecosystem-based fisheries management. Fisheries Research 92.

Swedish Board of Fisheries. (2010) Fiske 2020 – Towards an Ecosystem-Based Fisheries Management in Sweden. (www.fiskeriverket.se).

Axel Wenblad

Director General, Swedish Board of Fisheries

Personal facts

Name: Axel Wenblad
Position: Director General, Swedish Board of Fisheries
Family: Married, two grown-up sons and four grandchildren age 9 years to 4 months
Other: Degree in limnology and chemistry. Background in government, FAO, and business, Volvo and Skanska (a global construction company). Ex-board member of  Millennium Ecosystem Assessment that delivered a report on the status of the worlds ecosystems to UN Secretary General Kofi Annan in 2005. Board member of World Fish Centre.

What are you reading at the moment?
I am rereading Roadsigns by Dag Hammarskjöld, as Swedish national who was the second Secretary General of the UN. It is believed that he was assassinated in Africa in 1961 when his plane was crashed during a mission to negotiate peace in Congo. The book is a series of thoughts and words of wisdom that he wrote down over several years. They are useful as a help when you sometimes feel that you loose your orientation and I believe that he used them for that purpose as well.
The second book is a collection of short stories with the title The Boat by Nam Le who is the son of a boat refugee from Vietnam. It is fascinating set of short stories on different human fates from different parts of the world.

What is your strongest memory of fish?
My first memory dates back to when I was probably 6 or 7 years old fishing with my father. It was in lake where we were angling for perch and the excitement when I caught my first fish, however of very moderate size. In the beginning I was too eager as children are, but my father had told me to be patient in order to allow the fish to really eat the worm that we used as bait.
My second memory is from about the same time when my father and I were fishing freshwater bream with gillnet. We got so much fish that my father, who was a very rational person, decided that this we could eat the whole week when he took care of me. Anyone who has eaten freshwater bream knows that it is full of bones, which is not a childs favourite, so you could imagine how much I longed for my mother when she came for the weekend.

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