Whilst the data have been extensively checked by both the supplying institutes and ICES there are
inherent flaws in gathering the information. For example:
* In spite of the fact that now all bottom trawl surveys are in the same database and often at least
partially overlap in time period or area covered, there are differences between the surveys
that do not allow the combination of data from different surveys in one analysis unless this
point is specifically addressed.
* There is no guarantee that the gears deployed adequately sample all species (e.g. catchability of flatfish
in beam trawls is markedly higher than that of GOV). It should be realized that each survey gear catches a
gear-dependent subset of the fish community.
* A common property of all surveys is that over time species determination skills improve. As a result new
taxa will emerge in the dataset that were previously reported as part of another closely-related taxon.
* Over time the sampling procedures, gear characteristics, timing of the survey or the area covered may have
changed thereby influencing the catches. Expert advice recommendations are given in order
to prevent bias in the analyses using DATRAS catch data, caused by such factors.
Thus users are urged to treat the data with caution. If the user has any queries on the validity of
the data or the conclusions to be drawn they should contact either the ICES Secretariat or the Chairs of
the relevant Working Group (IBTS, BITS or BEAM)
To guide the users of the trawl data the relevant
working groups have provided the users with an expert advise. Please
read these before using the data: Expert advice