fish stock assessment
DESCRIPTION
Fish stock assessment. Prof. Dr. Sahar Mehanna National Institute of Oceanography and Fisheries Fish population Dynamics Lab 10-14 November, 2013 ns. Stock assessment. What it is, why do it and how can it be used?. Fishery status. R. O. F. U. D. - PowerPoint PPT PresentationTRANSCRIPT
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
Collapse
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