Some Comments on SSB msy and Indicators of Healthy Size Structure Rainer Froese, GEOMAR WKGMSFD D3 Copenhagen, 4-5 September 2014

The CFP is Binding Law According to CFP Article 2, stocks have to be rebuilt to and maintained “above levels that can produce the maximum sustainable yield” (SSBmsy) While the MSFD allowed SSBpa to be used if SSBmsy is not available, this is not an option under the CFP

Estimating SSBmsy is Possible There are a variety of methods available to estimate SSBmsy with confidence limits These estimates can thus be compared and evaluated The criticism of SSBmsy also applies to SSBlim, SSBpa In comparison, the methods to estimate SSBpa or MSY Btrigger are less rigorous (weak scientific basis) than those for SSBmsy

Single-Species = Multi-Species & Environment Single species stock assessment includes – predators through natural mortality – prey through growth (= weight at age) – temperature and oxygen through growth – wind, weather, currents, plankton etc through recruitment For stocks at/near SSBmsy (e.g. plaice, sprat, NS herring, dab, …), single-species SSBmsy = multi-species SSBmsy

MSY B trigger <> SSBmsy MSY B trigger is defined as the lowest possible value that SSB msy could take on; thus, proper estimation of MSY B trigger requires estimation of SSB msy with confidence limits In practice, SSB pa is used for MSY B trigger, which is often the lowest biomass on record plus 40%

SSBmsy ≈ 2 * SSBpa Reduced recruitment is expected below 0.2 B0; in production models, SSBmsy = 3.7 – 0.5 B0; it follows that 2 SSBpa is a reasonable proxy for SSBmsy An analysis (submitted) of 31 non-European stocks confirms a median SSBmsy/SSBpa ratio of 2.2

Recommendation for D3.2 ICES shall start providing estimates of SSBmsy for those stocks that are near it For stocks that are still well below current estimates of SSBmsy, chose 2 * SSBpa as proxy value

Indicators and Reference Points for Healthy Size Structure For every Fmsy, there is a corresponding length-at-first-capture (Lc) that maximizes catches and achieves true MSY; it also results in larger SSBmsy (B&H 1957) Current Lc and legal landing sizes are too small and can not reach the maximum sustainable yield (and the corresponding large stock size)

New Research For every Fmsy, there is a corresponding length-at-first-capture that maximizes cohort biomass and CPUE and minimizes impact on size structure (Froese et al. submitted)

Yield for different Lc Yield relative to maximum possible yield as a function of fishing mortality, for different lengths at first capture: Lc_max maximizes catch, Lc_opt minimizes impact, Ll is current legal

Biomass for different Lc Relative biomass as a function of fishing mortality, for different lengths at first capture: Lc_max maximizes catch, Lc_opt minimizes impact, Ll is the legal minimum landing length

D3.3 Implementation D3.3.1 suggests length at first maturity as reference point; Froese et al. (in press; on share point) suggest mean length in exploited part of the stock as corresponding indicator, i.e. mean length in catch shall be larger than length where 90% have reached maturity Length (Lopt ≈ 0.66 Linf) where cohort biomass is maximum can be used as target for mean length; this is achieved when Lc = Lc_opt ≈ 0.5 Linf

Recommendation for D3.3 Keep D3.3 Use existing D3.3.1 as lower limit for mean length in catch Use Lopt as target for mean length in catch