MSFD Indicators and Reference Points for Data-Limited Stocks

Rainer FroeseGEOMAR

ICES MSFD Workshop, Copenhagen13 January 2014

Overview

• General proxies for unknown Fmsy and Bmsy• DATRAS: millions of records of survey data• SMALK: LWR, growth & maturity• Length at 90% of female maturity as reference

point for size/age structure• CPUE: abundance and relative stock status• Combining survey data with catch gives stock

status and exploitation rate

M as Proxy for Unknown Fmsy

A fishing mortality rate F that equals the natural mortality rate M effectively- doubles the mortality rate that the species has

evolved to withstand- reduces life expectancy and duration of

reproductive phase by half- reduces the number of old, large, fecund fish

M as Proxy for Unknown Fmsy

There is a broad, longstanding agreement in fisheries science that sustainable rates of fishing F may not exceed Fmsy and shall not exceed M.Gulland 1971, Shepherd 1981, Beddington and Cooke 1983, Clark et al. 1985, Beverton 1990, Patterson 1992, Thompson 1993, Walters and Martell 2002, 2004, MacCall 2009, Pikitch et al. 2012.

Proposed secure sustainable fishing rates vary from 0.5 to 0.8 Fmsy or M.

Thus, M can be used as a proxy for Fmsy

2 * Bpa as Proxy for Unknown Bmsy

• Over the past two decades the border to safe biological limits Bpa was used as reference point in European fisheries

• Recently, a new reference point BmsyTrigger has been introduced: it marks the lowest possible value for Bmsy and is often set equal to Bpa.

• With the same reasoning, Bpa and BmsyTrigger mark the lowest possible value for a good stock status.

• In fisheries textbooks, half of Bmsy is typically assumed as the borderline to unsafe stock status.

• It follows that 2*Bpa or 2*BmsyTrigger are reasonable proxies for Bmsy

DATRAS: Millions of Survey Recordshttp://datras.ices.dk

SMALK: Length-WeightNeeded to turn length into weight and biomass

North Sea flounder: 25,609 observations of length and weight; 42 outliers were removed. Weight = 0.007 * Length3.16 [SMALK_Analysis_25.r]

SMALK: GrowthNeeded to turn length into age

North Sea flounder: 25,501 observations of length and age. Length-at-age = 48 * (1 – e-0.11 * (age – 2.8)) [SMALK_Analysis_25.r]

SMALK: Length & Age at MaturityNeeded for proxy recruits and spawners

North Sea flounder: 10,814 observations of length, age, and state of maturity. Female length where 90% are mature is 23 cm, age is 2-3 years. [SMALK_Analysis_25.r]

Length at 90% Maturity as Reference Point for Size/Age Structure

North Sea cod: fishing mortality F and the mean length Lmean targeted by fisheries , with Indication of natural mortality M and length where 90% reach maturity Lm90 or where fishing would have the least impact on the stock Lopt. [MSFD_8.pdf]

Discards included???

Average catch in numbers of North Sea cod resulting from one hour of trawling with standardized survey gears. The bold line represents a three-year moving average to reduce scatter and facilitate interpretation of trends. The dotted line indicates the mean catch of the time series. [MSFD_8.pdf]

CPUE in Area 6

CPUE by Length

North Sea dab. a)shows the raw data. The red line indicates individuals above 50% maturity;b) shows immature individuals as proxy for recruits; d) shows length converted to biomass of mature (red) and all fish (black); c) shows total mortality Z over the respective previous two years and scaled exploitation rate u+M, both as a proxies for F, with indication of M [MSFD_8.pdf].

a) b)

c)d)

CPUE by Length

North Sea dab: preliminary stock-recruitment relationship based on CPUE in weight as proxy biomass of mature fish and number of young fish (= proxy-recruits) two years later, derived from length composition in standardized survey catches. A hockey stick was fitted by a rule-based procedure. The dotted vertical line indicates proxy SSBpa as the precautionary borderline to potentially compromised recruitment. [MSFD_8.pdf]

CPUE by Length

North Sea dab. a)shows the raw data. The red line indicates individuals above 50% maturity;b) shows immature individuals as proxy for recruits; d) shows length converted to biomass of mature (red) and all fish (black); c) shows total mortality Z over the respective previous two years and scaled exploitation rate u+M, both as a proxies for F, with indication of M [MSFD_8.pdf].

a) b)

c)d)

Catch-MSY RevisitedMartell & Froese 2013

Output of the Catch-MSY method showing “viable” pairs surplus production rate r and unexploited biomass k for North Sea cod. The red line indicates all r-k pairs that would result in the same estimate of MSY; the red circle indicates the geometric. While the estimate of MSY is robust, the geometric mean r = 0.24 depends on the lower bound for r.

Catch-MSY Improved

Minimizing uncertainty in r and k leads to an objective lower bound of r (blue line) and to an objective estimate of the maximum rate of population increase r = 0.42 and the corresponding unexploited biomass k.

Stock Status for Data-limited Stocks

North Sea cod: The black line shows the biomass predicted by the CMSY-method. The red line shows observed total biomass. The vertical blue lines show the prior biomass windows used asfilters by the CMSY-method. The upper dotted line represents the unexploited biomass k and the lower dotted line represents Bmsy, as estimated by CMSY.

k

Bmsy

Icelandic cod: The black line shows the biomass predicted by the CMSY-method. The red line shows observed total biomass. The vertical blue lines show the prior biomass windows used asfilters by the CMSY-method. The upper dashed line represents the unexploited biomass k, the middle dotted line represents Bmsy, and the lower dashed line represents Bpa.

k

Bmsy

Bpa

Exploitation Rate for Data-limited Stocks

Icelandic cod: exploitation rate catch/biomass as predicted by the CMSY method(black line) and as observed (red line). The dotted line represents a proxy for MSY-compatible exploitation.

umsy

Conclusions

• M is a good proxy for Fmsy and 2*Bpa is a good proxy for Bmsy

• For most data-limited stocks in the ICES area, DATRAS contains sufficient information to estimate LWR, growth, maturity and relative abundance

• Combining survey results with catch data leads to reasonable proxies for stock status and exploitation pressure

• Length where 90% of the females reach maturity seems to be a reasonable lower limit reference point for mean length in commercial catches.