Three simple rules for high catches, high profits and healthy ecosystems
Position paper for the round-table discussion Towards a Sustainable Fishery Sector
Block 2, Interactions between Fisheries and Nature, Wednesday 23 June 2021
by Rainer Froese, GEOMAR, Kiel, Germany, rfroese@geomar.de

Rule 1: Do not take out more than is regrown
Taking out more than is regrown is called overfishing and will shrink the fished stock below levels that
can produce the maximum longterm catch (MSY). Despite the legal obligation to end overfishing in 2020,
the total allowed catches (TACs) for about 40% of the stocks with suitable data in the North Sea and
adjacent waters constituted overfishing (1). Overfishing is stupid: with overfishing, more effort = money
is spent to get lower catches than possible at lower value and with unnecessarily high impacts on the
ecosystem. The lower value is caused by overfishing shrinking the mean size of the fish and smaller fish
bring lower price per kg. Impact is unnecessary high because better catches can be obtained from nonoverfished stocks with less gear deployment, therefore less by-catch and less physical impact. To end
overfishing, catches have to be reduced for 1-4 years, depending on current stock size. The loss caused
by such reduced catches is easily regained within a few years after rebuilt stock size allows for
permanent high catches at e.g. 90% of the MSY level. The 90% MSY is needed for stability; with fishing at
MSY, there is a 50% chance of shrinking the stock below the MSY level and thus reducing future catches.

Rule 2: Catch fish at their optimum size
Most commercial fish about double their body weight every year. Maximum growth of body weight is
reached at about 30% of the maximum weight or about 2/3 of maximum length (2). At this size and age,
all fish have reproduced and thus passed on their genes to the next generation, an important
contribution to genetic diversity as key ingredient of resilience against climate change. Instead, current
regulations allow the onset of fishing at about 15% of maximum weight or 1/3 of maximum length. In
other words, with the current minimum conservation reference lengths (MCRLs), about twice as many
fish are killed than are needed to supply the allowed catch. With the current MCRLs, unnecessary
damage is done to the stock, the ecosystem, and the stability of the fishery, to obtain a catch that is of
less value because of the smaller size of the fish.

Rule 3: Let commercial fish fulfill their natural ecosystem roles as prey or predator
Healthy, stable fisheries depend on healthy, stable ecosystems with as much as possible resilience
against climate change. Predators such as cod and prey species such as herring or sandeel are keystone
species that stabilize their ecosystems. Ecosystem reference points for minimum functional population
size have yet to be established, but common sense predicts that the current stock sizes of about only
30% of the natural population size are insufficient to supply the variety of natural predators with food or
to stabilize and prevent outbreaks of a variety of low trophic level species, including jellyfish. Common
sense instead predicts that stock sizes above 60% of natural population size should be able to fulfill these
roles. The good news for fisheries managers is that such stock sizes are the expected result of
implementing rules (1) and (2), and that stock sizes between 60 and 70% of natural size produce the

highest profits (the maximum economic yield, MEY), simply because more and larger fish in the water
mean less effort = cost is required to obtain a certain catch with higher value of the catch. For these
reasons, Australian fishing law prescribes MEY as target stock size that as default has to be 20% larger
than the minimum stock size required to produce MSY.
Conclusion
Rules (1) – (3) are already contained in the reformed Common Fisheries Policy (CFP) in force since 2014.
However, these provisions have been ignored or misapplied in the various regional Multiannual Plans
(MAPs) that were supposed to implement the CFP. Evidence of this failure of the MAPs to properly
implement the CFP is the ongoing overfishing and the continued presence of depleted stocks in EU
waters.
Note: Above text referred to fish for simplicity, but the same rules apply also to invertebrates (e.g. brown
shrimp, Norway lobster, mussels).
References
(1) Froese, R., Tsikliras, A.C., Scarcella, G., Gascuel, D. 2021. Progress towards ending overfishing in the
Northeast Atlantic. Marine Policy, 125C: 104282,
https://www.fishbase.de/rfroese/MarPol_EU_Fishing_2021.pdf
(2) Froese, R., Winker, H., Gascuel, D., Sumaila, U.R., Pauly, D. 2016. Minimizing the impact of fishing.
Fish and Fisheries, 17(3):785-802 https://www.fishbase.de/rfroese/MinimizingImpactfaf.pdf