Stock Assessment Workshop 25th-30th June 2007 SPC Headquarters Noumea New Caledonia

Day 5 Session 1 Biological reference points

Overview Review of reference points Why calculate them? Reference points considered by WCPFC Reference points calculated by OFP How are they calculated? What can you do with RPs? Target or Limit reference points? The precautionary approach and reference points Reference points used by other RFMOs

Review of reference points (RPs) Reference points are used to judge the status of a stock They are also used to provide advice to management from the outputs of stock assessments In age-structured models, information on the SRR, growth and mortality are required

Why calculate RPs? How else can you assess the condition of a stock? How else can you assess the impacts of fishing and/or fisheries on a stock? How else can you evaluate the performance of fishery management?

What RPs does the WCPFC use? The WCPFC convention lists several reference points Maintain or restore stocks at levels capable of producing MSY [Article 5(b)]: i.e. Maintain Bcurrent  BMSY Eliminate overfishing and excess fishing capacity [Article 5(g)]: i.e. Maintain Fcurrent  FMSY Assess the impacts of fishing on target and non-target stocks and associated species [Article 5(d)] i.e. 1 - Bcurrent / B0 ;

What RPs does the WCPFC use? The WCPFC convention also lists several other points that may potentially develop into RPs; Long-term sustainability of highly migratory species [Article 5(a)] Minimise wastes, discards and catches of non-target species, in-particular protected species [Article 5(e)] Protect biodiversity and marine environment [Article 5(f)] e.g. Maintain total discards (mt) below 2005 levels?

What RPs does the WCPFC use? In addition; develop stock-specific reference points [Article 6.1(a)] ensure that, when reference points are approached, they will not be exceeded [Article 6.3] adopt … conservation and management measures and recommendations for non-target species and species dependent on or associated with the target stocks, with a view to maintaining or restoring populations … above levels at which their reproduction may become seriously threatened [Article 10.1(c)]: Maintain SSBcurrent  SSBMSY?

RPs calculated by OFP

RPs calculated by OFP MSY - The maximum average catch that can be removed under given environmental conditions over an indefinite period, without reducing a stock’s ability to reproduce at MSY, Catches are maximised (for the fishery specific combinations of F and selectivity) Catches and natural mortality are balanced by recruitment and growth

Bt+1=Bt+R+G-M-C Death (Natural mortality) Recruitment (+) Whole population (-) Catch (Fishing mortality) (-) Growth (+) Movement

RPs calculated by OFP FMSY – the fishing mortality (or effort) at MSY. This can be thought of as the level of fishing effort needed to achieve MSY FMSY is influenced by the selectivities of different gears and method fisheries (depending on the component of the stock [e.g. adults, juveniles] each gear or fishery targets) BMSY – the biomass (adult, juvenile or total) of the stock at MSY

WCPO example: Skipjack (SA WP-4, SC-1) The horizontal line at 1.0 indicates the overfished state reference point

How are reference points calculated? General Estimates of various quantities are generated from the outputs of a stock assessment Ratios are constructed to provide information about the level of fishing, the biomass of the stocks and other values, relative to these parameters at MSY e.g. Fishing effort Fcurrent/FMSY : provides a ratio of the current level of fishing effort compared to the level required to achieve MSY If > 1, overfishing is occurring IF<1, overfishing is not occurring

Information required for constructing RPs from age-structured models Growth: how fast do fish grow and contribute to the biomass of the stock? [i.e. contribution of additional biomass at age of new and existing fish, weight–at-age]

Information required for constructing RPs from age-structured models Mortality: what is the rate of mortality, both M and F? [i.e. how fast is biomass removed from the population, mortality rate (Z= M+F)-at-age] F M

Information required for constructing RPs from age-structured models Maturity: What is the proportion of mature fish–at-age?

Information required for constructing RPs from age-structured models SRR: how many new recruits will be produced at a range of adult stock sizes? [i.e. how many new fish will contributed to the population at different levels of adult biomass] SRR

How are reference points calculated? Natural mortality Fmulti Fishing mortality Further analyses Impact, Historical Time-series Growth BMSY SBMSY MSY Maturity ogive SRR

How are reference points calculated? 1. An age-specific vector of fishing mortality (F) is calculated for the entire model region (i.e. a series of F estimates for each age class) This is really the F-at-age estimates from the model for a recent period of time. Typically it is an average of F for each age class averaged over a useful time period for fishery purposes. In the WCPO, this is typically over the previous 3 years [Can also omit the most recent year, as often the end of a time-series is very influential] [This is often referred to as “current” or “recent” fishing mortality in SA reports].

How are reference points calculated? 2. A series of “Fmulti” are established. Fmulti area series of multipliers (usually from 0 to 50 in steps of 0.1 ) that are used to generate a wide range of theoretical Fs, both lower (0 – 0.9) and higher (1.1 – 50) than the current level of F (which is Fmulti=1). [Fmultis are scalars of fishing mortality (effort)] e.g. F0.3 = 0.3 * Fcurrent [The range of Fmulti used are usually enough to generate a dome-shaped yield curve – we’ll come to this later!]

How are reference points calculated? 3. The schedule of natural mortality at age is also required. This is the M-at-age series either included in the model (fixed value(s)) or M-at-age estimated from the model. This can also simply be a single fixed value for all age classes

How are reference points calculated? 4. The M-at-age series is added to each Fmulti-at-age series. This gives the total mortality (Z) for each age class for each level of Fmulti.

How are reference points calculated? 4. The abundance at age per recruit function is estimated for each Fmulti-at-age series. This is to provide a proportion surviving to each age for a cohort of fish, under each Fmulti-at-age series. The abundance of fish in age class 1 is defined as 1 (all alive). Then the mortality of all previous age classes are pooled to estimate the remaining proportion of fish within a cohort surviving to each age class, for each Fmulti series

How are reference points calculated? 4. The abundance at age per recruit function is estimated for each Fmulti-at-age series. [Exponent of the Sum of total mortality of all previous age classes]

How are reference points calculated? 5. The biomass at age per recruit function is estimated for each Fmulti-at-age series. This is simply taking the abundance at age per recruit function estimated for each Fmulti-at-age series in the previous step and multiplying it by the weight-at-age (i.e. incorporating growth) This provides the mean biomass of each original recruit at age for each Fmulti series

How are reference points calculated? 6. The spawning biomass at age per recruit function is estimated for each Fmulti-at-age series. taking the abundance at age per recruit function estimated for each Fmulti-at-age series in the previous step, and multiplying it by the weight-at-age (i.e. incorporating growth) and multiplying by the proportion mature- at-age (maturity ogive) This provides the mean spawning biomass of each original recruit at age for each Fmulti series

How are reference points calculated? 7. The sum of the spawning biomass at age per recruit function, estimated for each Fmulti-at-age series, is applied to the SRR to estimate recruitment for each level of Fmulti. Number of recruits =  - /(spawning biomass per recruit) (at each level of Fmulti) This provides an estimate of the number of recruits expected for the spawning biomass for the stock for each level of Fmulti

How are reference points calculated? 8. The equilibrium abundance at age is estimated by applying the number of recruits to the abundance at age per recruit calculated previously. This applies the mortality schedule and moves the estimate number of recruits through each time step, incorporating the mortality schedule to give the total number of each age class at each level of Fmulti.

How are reference points calculated? 9. The equilibrium biomass at age is estimated by multiplying the equilibrium abundance at age by the weight at age. This provides an estimate of biomass at age for each level of Fmulti.

How are reference points calculated? 10. The equilibrium catch (numbers) at age is estimated by applying the ratio of F to Z by the difference in abundance between age class a and a+1, for each level of Fmulti. Catcha = (Fa/(Ma + Fa))* (Numbersa – Numbersa+1) This formula takes into account the F at time and estimated number of fish that survive to the next age class

How are reference points calculated? 11. The equilibrium catch (weight) at age is estimated by multiplying the equilibrium catch-at-age by the weight-at-age. This estimates the catch in weight at age at each level of Fmulti

How are reference points calculated? 12. The sum of the equilibrium catch (weight) at age is estimated for each Fmulti. This estimates the YIELD at each level of Fmulti. This is plotted against Fmulti “Dome-shaped” yield curve

How are reference points calculated? 13. Estimates of Biomass and Spawning Biomass are plotted against Fmulti.

How are reference points calculated? 14. The Maximum yield is the MSY The corresponding level of Fmulti is the estimate of FMSY

How are reference points calculated? 15. The biomass and spawning biomass at FMSY are the estimates of BMSY and SBMSY

How are reference points calculated? 16. A range of ratios and time-series can be calculated from the data and fishery data.

How are reference points calculated? These calculations take into account age-specific estimates of; Mortality (F and Z) Growth Maturity ogive the SRR to estimate recruitment at the resulting levels of spawning biomass Thus, as Fmulti varies, yield also varies [Think of Fmulti as levels of fishing effort relative to the ‘current’ level]

How are reference points calculated? Recent research – Theoretical maximum sustainable yields (for YFT) that could be generated by harvesting the stock at individual age classes and the proportion of the total biomass of that age class that would be harvested to achieve the theoretical MSY. The current MSY (402,000 mt per year) is represented by the dashed horizontal line.

How are reference points calculated? Recent research - Average age class of fish harvested by each fishery. The dashed vertical line represents the “critical age” (11 quarters); the critical age is the age of capture that generates the maximum yield from the stock.

MSY for different species (from SC-1) BET: 65,000 mt YFT: 260,000 mt

Fmulti and subsequent yields: SA1 WP – 4 Albacore MSY FMSY (about 20 times current F) Fmulti

Fmulti and subsequent biomass estimates: SA1 WP – 4, Albacore FMSY BMSY BsMSY Fmulti

WCPO example: Skipjack (SA WP-4, SC-1) B/BMSY and F/FMSY are calculated over the entire time-series of fishery data, (Bs/BsMSY also)

How are reference points used? Changes in RPs through time are due to; Changes in gear use and subsequent changes in selectivities, fishing mortality, impacts on the growth of the stock (via the distribution of size at age of fish) and catches (e.g. compare LL to PS catches for BET and YFT). Increasing catches through time and subsequent impacts on estimates of biomass due to the SRR

e.g. YFT in WCPO Change in MSY due to changes in proportion of catches by method fisheries, due to differences in selectivity and therefore F

What can you do with RPs? Impact analysis: how much has the stock been reduced from unfished (virgin) levels by fishing? 1- Bcurrent/Btime 0 ~ 65% reduction since fishing commenced Most reductions since 1975

What can you do with RPs? 2. Investigate the impacts of individual fisheries (or all fisheries) on the biomass levels (e.g. what would the biomass be if there was no fishing?; Only LL fishing? etc) Reduction in biomass due to fishing. Significant impacts after 1980. Biomass

What can you do with RPs? 3. Estimate how much of the reductions in biomass can be attributable to different method fisheries % Impact

What can you do with RPs? 4. Examine historical trends in significant reference points F/FMSY

What can you do with RPs? 4. Examine historical trends in significant reference points

5. Assess RPs against management objectives What can you do with RPs? 5. Assess RPs against management objectives Maintain or restore stocks at levels capable of producing MSY [Article 5(b)]: Maintain B  BMSY Eliminate overfishing and excess fishing capacity [Article 5(g)]: Maintain F  FMSY Assess the impacts of fishing on target and non-target stocks and associated species [Article 5(d)] Assess B / B0

Impacts Management objectives

What reference points are used elsewhere? Who What Notes UN FMSY Minimum standard ISC BMSY F0.1 SSBmin Suggested; none formally adopted although reported in some cases IATTC MSY AMSY SBRX% FAMSY None formally adopted although reported in some cases

Other considerations in regard to RPs Trend to more conservative RPs ISC: 30% to 40% SSB ISC: Buffer reference points Ecosystem considerations: ‘leave more biomass in the system’. BMSY reduced from B/2 to B/2.5 ? RPs as limits, not targets. Maintain F<FMSY, not aim for F = FMSY. This may provide some insurance if variables beyond fisheries change, affecting stock productivity and reducing estimated MSY (e.g. unfavourable oceanographic conditions)

Limit & Buffer RPs e.g. The NAFO Scientific Council Flimit = FMSY Fbuffer as "a fishing mortality rate below Flimit that acts as a buffer to ensure that there is a high probability that Flimit is not reached Flimit Blimit Bbuffer Fbuffer Yield  Biomass 

RPs for different species? The characteristics of some species may make some RPs more or less useful Protected species Maintaining MSY may be (socially) inappropriate. Reduction and minimisation of catches may be a better objective A more reasonable RP may be to maintain the catch of these species below a certain level e.g. Maintain catch rates of seabirds < 0.05 birds per thousand hooks [Seabird target bycatch level set by the Australian government for L fisheries within the EEZ]

RPs for different species? Discard species Reduction and minimisation RP – may be to stay below a certain level of discards per time period (e.g. less than X kg per set?) This RP is likely to be more about social acceptability than truly about stock status for discard species

Summary RPs are a way of summarising the outputs of a stock assessment They allow; the status of a stock to be assessed The impacts of fishing on a stock to be estimated The performance of the stock against management objectives to be assessed RPs vary through time due to changes in fisheries, catches and the incorporation of new information RPs are not management tools, but provide scientific advice to management to develop harvest strategies