Biodiversity of Fishes Population Growth and Exploitation Rainer

Biodiversity of Fishes Population Growth and Exploitation Rainer Froese 18. 01. 2018

Thomas Robert Malthus (1766 – 1834, English Economist) •

World Population Growth "The power of population is indefinitely greater than the power in the earth to produce subsistence for man"

Pierre François Verhulst (1804 – 1849, Belgian Mathematician) • (1844)

Bio. Div. Pop. Growth. MSY. xls

Logistic Curve Properties Bio. Div. Pop. Growth. MSY. xls

Milner Baily Schaefer (1912 – 1970, US Fisheries Scientist) •

The Schaefer Model Bio. Div. Pop. Growth. MSY. xls

Surplus Production Implications • Surplus production (Y) is the production of biomass beyond what is needed to maintain current population size • If a fishery only catches the surplus production, then the population size remains • If a fishery catches more, then the population shrinks • If it catches less, then the population grows

Surplus Production Implications • Surplus production has a maximum at about half of unexploited population size B∞ • Keeping a population at 0. 5 B∞ allows catching MSY forever • A population is kept stable if the fishing rate F equals the intrinsic growth rate rt • MSY is reached at ½ B∞ and ½ rmax • Thus, MSY is reached at Fmsy = ½ rmax

The Law of the Sea • The Law of the Sea (UNCLOS 1982) requires all signatories to maintain fish stocks at levels that can produce the maximum sustainable yield MSY • The EU Common Fisheries Policy has implemented the MSY provision in 2014 • Thus, the fishing rate F may not exceed Fmsy in European fisheries management

The MSY Framework CFP 2013 High biomass High profit High catch Overfishing Recovering Outside safe biological limits

Status and Exploitation of European Stocks Presentation of 397 stocks in European Seas in a pressure (F/Fmsy) – state (B/Bmsy) plot. The red area indicates stocks that are being overfished or are outside of safe biological limits. The yellow area indicates recovering stocks. The green area indicates stocks subject to sustainable fishing pressure and of a healthy stock size that can produce high yields close to MSY.

Exploitation of 397 stocks in European Seas in 2013 -2015. Note overlapping of different types of overexploitation, and therefore the numbers do not add up to 100%. Froese et al. 2016

Status of 397 stocks in European Seas. Froes

Analysis of current (2013 -2015) and potential catches for 397 stocks in European Seas. Because of trophic interactions, all stocks cannot support maximum yields simultaneously.

Status and Exploitation of European Stocks Froese et al. 2016

Conclusions • The Common Fisheries Policy sets the correct goal of rebuilding stocks above levels that can produce MSY • An analysis of all European stocks shows that the goal of sustainable exploitation by 2015 has been missed by a large margin • Therefore, there is no visible trend of rebuilding in European fish stocks • However, sustainable catches can be 5 MT (57%) higher if stocks have been rebuilt

Why is Management so Bad? • Depending on the productivity of the species (rmax), about 20% of the biomass can be taken sustainably every year • Therefore, about 80% of the fish have to be left in the water, although they are easily accessible • There is only one problem

GREED

“Heringszaun”, Kappeln, Germany Fish weirs in rivers typically cover 60 -100% of the river width, facing toward the sea, so they catch the fish before they can spawn. And there used to be many of these.

Exercise • Go to www. ices. dk, select Community, Advisory Process, Latest Advice • Search for a stock for which ICES gives Fmsy • Use graphs to discuss trends in biomass and catch as a function of F / Fmsy • What happens if F > rmax? [show example: mackerel]