Fish stock assessment

Prof. Dr. Sahar Mehanna

National Institute of Oceanography and FisheriesFish population Dynamics Lab

10-14 November, 2013ns

Stock assessmentWhat it is, why do it and how can it be used?

Unexploited UnderExploited

Developing FullyExploited

Over-Exploited

Recovering

U D F O R

Fishery status

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The idea of a fish ‘stock’Sub-set of a ‘species’ (fish identification

necessary)Usually, but not always, confined by

geographic boundarieshaving the same growth and mortality

parametersMost importantly, it is the unit that is

‘assessed’ and ‘managed’Often deal only with ‘exploitable’ stock

WHAT is fish stock assessment……?A method for mathematically describing the

population dynamics of a fish stock -quantitativeA method for predicting a fish populations’

response to fishingA method for predicting impacts of changed

fishing practices (more boats, change in fishing gear etc)

A method for predicting some aspects of a fish population’s response to change - climate change?

Most of all, its interesting!

………And what it’s not!

Fisheries management – informs management but not the whole story!

Economic, social & equity issues not part of fish stock assessment

However, stock assessment is a key element of fisheries management plans

Why we do stock assessment?

ئدعاال

التكاليف

To provide advice on the optimum exploitation of aquatic living resources

Describing Fish Populations – the Basics  GAINS

LOSSSES

FISH STOCK WEIGHT

Recruitment

Growth

Fishing mortality

Natural mortality

Emigration

Immigration

Assessing ‘stocks’‘Assessing’? Involves 3 key objectives:calculating current biomass

(absolute or relative)determining the response to fishingDetermining the ‘status’ i.e.

‘overexploited’, ‘fully exploited’, ‘under-exploited’

How to do this? Direct and Indirect MeasuresDirect Methods:Measuring the ‘drivers’ of fish stocks (growth, fishing

mortality, natural mortality, recruitment)Direct measures allow analysis of changes in fish

biomass to changes in input parameters, incl. fishing

Indirect Methods:‘Surplus production’ models and surveysIndirect methods only allow limited analysis of

changes in fish biomass to fishingMethods usually use only data specific to the method –

i.e. little data integration but models can overcome

Data requirementsDirect methods:Measure input parameters by (a) Age data:

‘Age-based assessments’ (b) length frequency data – ‘Length-Based assessments (c) possibility for a combination of both

Indirect methods:Catch and fishing effort, biomass data from

surveys, biological based (‘data poor’) methods etc

Source of DataFisheries catch sampling - what are the

problems?Fisheries-independent surveys – are these

really independent?Market sampling – what are the problems?Distinguish between data that is:InaccurateBiasedSelective

Data analysis (Models)

Input OutputProcesses

Observation model observation

Direct MethodsMeasuring input parameters:Growth – Von Bertalanffy growth curve Lt=L∞(1-exp(-K(t-

t0)))

Mortality – exponential decay model of N2=N1*(exp(-Z(t2-t1))

Total mortality for the exploited part of a population consists of natural mortality (M) + fishing mortality (F)

natural mortality (M) often difficult to measure but (a) changes in ‘total mortality ‘with fishing effort (b) environmental parameters

Fishing mortality – (a) fishing effort, (b) changes in ‘total mortality ‘with fishing effort

What is the outcome of stock assessment?

Prediction of yield under different scenarios of fishing

Estimation of the optimum fishing mortality and age at first capture

Obtain a database that includes the abundance, demographic distribution of different population and the potential productivity.

Establish a future management plan for long term sustainability of the fishery

Stock assessment and Management

Data Collection Stock assessment

Scientific adviceDecision makers

Fishery regulations Fishery

(Fishery and stocks)

INDIRECT METHODSSurplus production models – based on

assumption that CPUE is proportional to biomass

0 2 4 6 8 10 12 14 160

5000

10000

15000

20000

25000

30000

35000

40000

catch

INDIRECT METHODSSurveys:Estimate biomass directly But cannot ‘model’ change in fishing

management arrangements such as changing effort , selectivity

‘Data Poor’ methodsUses spawning rate relative to un-fished levels

(spawning per recruit – SPR) rather than biomass trends to determine catch levels that stabilize stocks at management targets. Requires only size studies