Key approaches that will inform the development of the rese - PowerPoint Presentation

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Key approaches that will inform the development of the research include Chai (1991), Casey (1996), Harley (2000), Dingsør (2001), Palsson (2003), McBride and Fotland (1996) and Piet et al (2009).

The initial steps include: Analysing the length composition of landings and the relationship between known discarding and other variables, using data published in the ICES Working Group reports on the Celtic Sea Ecosystem. Using trawl survey data to estimate commercial catch composition. For this, it will be necessary to take into account differences in selectivity between survey and commercial gears. The survey gear, designed to assess the abundance of species, is necessarily designed to capture smaller fish than are allowed in commercial fishing.  It is recognised that commercial fishing grounds are not chosen randomly, therefore it can be expected that the catch composition in the commercial catch will differ inherently from survey vessel catch particularly in relation to species size. To address this to some extent, VMS data will be consulted, and statistical rectangles that appear to have been fished will be used as the basis of selecting research vessel data.  The survey data may then be applied to measures of fishing effort in order to obtain estimates of commercial catch. Once obtained, the length composition of the estimated catch will be compared with that in known landings to assess discarding levels, taking into account length-based and quantity-based discarding. Known data about discarding can be used to assess the accuracy of the estimates derived in this way.

Jackie McLeanUniversity of Strathclyde Department of Mathematics & StatisticsJackie.mclean@strath.ac.uk

The graph below is an example of preliminary research; proportions of fish reaching MLS (from survey data) are compared with proportions discarded (from ICES data).

Discarding fish at sea is one of the most controversial aspects of fisheries management. More recently a high profile public campaign has influenced a reform of the EU’s Common Fisheries Policy, such that discarding will become outlawed under a land-all obligation.Although its effects are deemed to be undesirable, discarding is a normal function of fishing activity, particularly in mixed fisheries where it is impossible to be entirely selective. The sorting and discarding process is influenced by market forces and regulation alike. While knowledge of discard mortality is far from complete, it is generally held that most demersal species do not survive. Estimating the number of fish discarded at sea is an important part of fish stock assessment. However, there is a danger that a ban on discarding may compromise the availability of data obtained by direct observation. 

How many fish are discarded at sea?Estimating discards using fisheries-independent data

There currently exists a body of work on estimating unregulated, unreported or illegal fishing mortality around the world, which provides insight into alternative methods of obtaining discard estimates. Broadly, there are two approaches to estimating discards where direct data is limited:

 Statistical modelling, using known relationships between discarding and other variables to estimate discarding where there is missing data but where data on the other variables is available Estimating catch directly and then applying a statistical model of discarding behaviour. This approach would be taken where there is no data available on discarding at all, and is found less frequently in the literature.

This PhD

research, currently in its early stages,

aims to estimate the number of fish discarded at sea, by using fisheries-independent data. It will focus initially on haddock and whiting in ICES Area

VIa

(West of Scotland), with a view to extending the methods developed to other areas and other species

.