Fish stock assessment Prof. Dr. Sahar Mehanna National Institute of Oceanography and Fisheries Fish population Dynamics Lab November, November, 2013 ns

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

Unexploited Under Exploited Developing Fully Exploited Over- Exploited Recovering UD 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 boundaries having the same growth and mortality parameters Most 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 -quantitative A method for predicting a fish populations’ response to fishing A 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? العائد 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 fishing Determining the ‘status’ i.e. ‘overexploited’, ‘fully exploited’, ‘under-exploited’

How to do this? Direct and Indirect Measures Direct 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 surveys Indirect methods only allow limited analysis of changes in fish biomass to fishing Methods usually use only data specific to the method – i.e. little data integration but models can overcome

Data requirements Direct 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 Data Fisheries catch sampling - what are the problems? Fisheries-independent surveys – are these really independent? Market sampling – what are the problems? Distinguish between data that is: Inaccurate Biased Selective

Data analysis (Models) Input Output Processes Observation model observation

Direct Methods Measuring input parameters: Growth – Von Bertalanffy growth curve L t =L ∞ (1-exp(-K(t- t 0 ))) Mortality – exponential decay model of N 2 =N 1 *(exp(-Z(t 2 - 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 advice Decision makers Fishery regulations Fishery (Fishery and stocks)

INDIRECT METHODS Surplus production models – based on assumption that CPUE is proportional to biomass

INDIRECT METHODS Surveys: Estimate biomass directly But cannot ‘model’ change in fishing management arrangements such as changing effort, selectivity ‘Data Poor’ methods Uses 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