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In Other Words

June 03
Biological reference points

​Colm Lordan, ​chair of ICES Working Group for the Celtic Seas Ecoregion​ (WGCSE)​​

colmcropped2.jpg 

 'Biological reference points are a key element of how ICES formulate fisheries management advice. This advice is based on assessments of the state of over 260 fish and shellfish stocks; assessment helps us to understand the long-term dynamics of populations and their response to historical exploitation rates.

The first step is estimating each stock’s spawning stock biomass, which is the mature fraction of the population. Recruitment, or the number of juvenile fish reaching a size or age to be caught, is also estimated particularly with scientific surveys. The relationship between the spawning stock size and recruitment for fish and shellfish tends to be very complex due to many ecosystem factors that can have a big impact on recruitment success. For many stocks we have observed reduced levels of recruitment produced at low levels of spawning stock biomass. 

In order to guide fisheries managers and other stakeholders in their decisions, we define stock specific biomass reference points such as Blim, Bpa and MSY Btrigger. Below Blim (or biomass limit) we think future recruitment will be impaired. It is not advisable to fish stocks that low because recovery will be very slow. We have seen this for several cod, herring and whiting stocks. In order to avoid approaching Blim we define another reference point Bpa (where pa means precautionary approach). This is set to have a low risk of the stock falling below Blim and takes into account uncertainties in our stock assessments. 

More recently ICES has adopted an MSY (Maximum Sustainable Yield) approach. This involves giving advice on optimum exploitation levels that will deliver long-term maximum sustainable yields from fisheries. ICES estimates stock specific Fmsy points or ranges that take into account growth rates, historic stock and recruitment relationships, fishing and natural mortality patterns. For many stocks an MSY Btrigger has also been defined. This is a biomass level which is normally equal to or higher than Bpa. When biomass goes below the MSY Btrigger, ICES advises fishing less than Fmsy. The aim of this approach is to keep stocks sizes at a level that will support MSY for future generations.' 

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