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: 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 ≈ F msy 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. L m indicates maturity. At L opt 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 L c_opt, resulting in a mean length in the catch equal to L opt 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. L m indicates maturity, At L opt 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 L c_opt = 72 cm, resulting in a mean length in the catch equal to L opt 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 Reality check All Ecosim models used in Garcia et al 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 small- scale 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 low- value 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 L max ) is still the best ecosystem management”

Thank You. Questions?