Balanced Harvesting:
Not Supported by Science
Rainer Froese
GEOMAR, Kiel, Germany
Pew Fellows Meeting, Rio Grande
16 October 2015
Early Insights (FishBase Book 1995)
Science 2012, 335:1045-1047
Balanced harvesting “distributes a moderate mortality from fishing
across the widest possible range of species, stocks, and sizes in an ecosystem,
in proportion to their natural productivity,
so that the relative size and species composition is maintained.”
“This study examines the models and the empirical evidence put forward in support of BH.
It finds that the models used unrealistic settings
with regard to life history (peak of cohort biomass at small sizes),
response to fishing (strong compensation of fishing mortality by reduced natural mortality),
and economics (uniform high cost of fishing and same ex-vessel price for all species and sizes),
and that empirical evidence of BH is scarce and questionable.
It concludes that evolutionary theory, population dynamics theory,
ecosystem models with realistic assumptions and settings,
and widespread empirical evidence do not support the BH proposal.”
Claims and Reality I
BH proposal
• “moderate mortality from
fishing…in proportion to
natural productivity”
• “fishing across the widest
possible range of species”
including “all groups
historically fished (including
whaling, sealing, etc.)”
Reality check
• F=M (Law et al. 2013) is not
moderate but maximum
sustainable because M ≈ Fmsy
• Simultaneous MSY from all
species is impossible;
whaling, sealing and
hunting of seabirds is
prohibited in most countries
of the world
Claims and Reality II
BH proposal
• “fishing across the widest
possible range of [] sizes”
• “selective removals will
inevitably alter the
composition of a population
or community and,
consequently, ecosystem
structure and biodiversity”
Reality check
• Doubling mortality at all life
stages severely truncates
size and age structure
• Moderate fishing has been
sustained for thousands of
years (cod North Sea,
bluefin tuna Med., …);
• Moderate fishing at
optimum length has
negligible effect on size
structure
Size matters
Cohort biomass over body length, based on life history data for North Sea cod. Lm indicates
maturity. At Lopt cohort biomass reaches a maximum. The bold curve indicates no fishing. The
three other curves indicate fishing with F=M. The solid curve results if fishing starts at Lc_opt,
resulting in a mean length in the catch equal to Lopt and also in the highest catch. The dash-dot
curve indicates BH fishing of all sizes above 6 cm. Froese et al. in press
You can kill a fish only once…
Survivors over body length, based on life history data for North Sea cod. Lm indicates maturity,
At Lopt cohort biomass reaches a maximum. The bold curve indicates no fishing. The three other
curves indicate fishing with F=M. The solid curve results if fishing starts at Lc_opt = 72 cm,
resulting in a mean length in the catch equal to Lopt and also in the highest catch. The dash-dot
curve indicates BH fishing of all sizes above 6 cm. Froese et al. in press
Claim and Reality III
BH proposal
• “Results from models
suggest that moderating
fishing mortality across a
wide range of species and
sizes maximizes overall
catch summed across
species while better
conserving biodiversity”
Garcia et al. 2012
Reality check
• All Ecosim models used in
Garcia et al. 2012 had a
minimum biomass constraint
set to 40% of unexploited
biomass, i.e., species did not
go extinct because the models
did not allow that. Instead,
realistic modelling of full
exploitation of all species
without such constraint
normally leads to some
extinctions
Claim and Reality IV
BH proposal
• Fishing mortality of
juveniles is compensated by
reduced natural mortality,
because abundance of
larger predators is reduced
by fishing (e.g. Law et al.
2013, 2014; Jacobsen et
al.2014)
Reality check
• Abundance of large fish has
been drastically reduced
over the past 50 years; yet,
no increase in recruits has
been observed, rather the
opposite
• If BH were to be introduced
now, no compensation can
be expected, because large
fish are overfished and
need to be rebuilt
Claim and Reality V
BH proposal
• “..in several African smallscale inland fisheries, the
fish size spectrum has been
maintained under intense
and diverse fishing activities
that cause high mortality
with low selectivity” (Garcia
et al. 2012)
Reality check
• “..loss of high-value target
species due to unregulated
effort and selectivity is a
problem in many African
inland fisheries [..] Many
fisheries have relatively stable
total yields, but catches are
now characterized by lowvalue species and decreased
individual catch and income.”
(Froese et al. 2015)
BH: “…relative size and
species composition is maintained”
Maintaining relative size by proportional removal of biomass means loss of top predators
BH: “…relative size and
species composition is maintained”
Maintaining species composition with massive removal of biomass means
steeper biomass gradients and presumably less ecosystem resilience
“Moderate catch (F ≈ 0.5 M)
of resilient seafood species
at the right size (> 0.5 Lmax)
is still the best ecosystem management”
Thank You. Questions?