DATRAS

Untitled Page

Expert advice

The descriptions of the various surveys indicate that over time the sampling procedures, gear characteristics, timing of the survey or the area covered may have changed to a more or lesser extent. This probably affects the catches and hence may bias the analyses and cause erroneous conclusions to be drawn from these analyses. In order to prevent this some recommendations are provided in this chapter that can be used as guidelines using these survey data. As the survey/area combinations provide a certain level of consistency within them, these recommendations are given for each survey/area combination separately. In providing the guidelines two aspects of the data were considered:

• Time-series
• Spatial coverage

As some surveys or countries within surveys initially only sampled a suite of commercial or "standard" species there is also information included on the year all participants of the survey started sampling all species. This is particularly relevant for studies on biodiversity or the entire fish community. For extractions of data for analyses on anything other than the community there is a chance that the results are biased if only those hauls in which that species or suite of species is caught, are used. Hauls with 0 catches should be included in analyses unless there is a good reason not to.

IBTS North Sea
NE Atlantic, Quarter 1, Scottish West Coast Surveys
NE Atlantic, EVHOE survey
Baltic Sea, BITS
North Sea, Beam trawl survey

IBTS North Sea
Time-series The IBTS North Sea is structured by quarter and therefore the data can only be analysed on a quarterly basis. The first quarter survey represents the longest time-series but has also been subject to the biggest changes over time. All of these changes had taken place before the third quarter survey started (1997) and any type of analyses can be performed on the whole dataset. The 2nd and 4th quarter were only sampled as part of a series of quarterly surveys in the period 1991-1996. This period can therefore be used when exploring seasonal patterns. As the changes that may cause bias only occurred during the 1st quarter survey, expert advice is only provided for this quarter.

Gear - Prior to 1977 a 78' Dutch herring trawl was recommended as the standard trawl but in 1977 this recommendation was altered in favour of the GOV. The latter became the standard gear operated by all nations from 1983 onwards. Haul Duration - Initially the hauls were of one hour duration but in the early 1970's this recommendation was changed to 30 minutes. In practice this did not become the standard until 1999. Catches are always reported as numbers per hour trawling. Age Determination - From 1980 onwards age length keys (ALKs) were expanded to include ages 1 to 6. Prior to 1980 only the younger ages were reported. Sex/Maturity - From 1981 onwards sex and maturity (using the 4 stage ICES scale) were reported by all nations.
Spatial coverage The whole North Sea, the Skagerrak and the Kattegat was covered from 1974 onwards. The stratification of the survey has always been based on the grid of ICES statistical rectangles. Each rectangle is usually fished by the ships of two different countries so that at least two hauls are normally made per rectangle. Within rectangles vessels are free to choose fishing positions but because most nations choose from a limited list of trawl tracks the procedure is defined as semi-random.
Community Records on all species exist for all countries from 1988 onwards.
NE Atlantic, Quarter 1, Scottish West Coast Surveys
Time-series The whole area was sampled consistently from 1996 onwards.
Age Determination - Ages are reported from 0 to the maximum obtained.
Sex/Maturity - Sex and Maturity (ICES 4-stage scale) are reported.
Spatial coverage The Scottish west coast, within the 250 metre contour line, from west of the Orkneys to Northern Ireland and the Clyde. In 1996 the survey area was extended to the northern Irish Sea. The stratification of the survey has always been based on the grid of ICES statistical rectangles. Each rectangle is usually fished at least once except for the Irish Sea where multiple hauls are made in each of the rectangles surveyed.
Community Records on all species exist for the complete time-series.
NE Atlantic, EVHOE survey
Time-series From the start of the survey in 1997 there have been no major changes in the survey. In 1999, however, the survey was conducted a month later in the year.
Ages are reported for: Lophius budegass, Lophius piscatorius, Merluccius merluccius, Gadus morhua, Merlangius merlangius, Lepidorhombus whiffiagonis.
Spatial coverage Celtic Sea and Bay of Biscay covered. In 1999 part of the Bay of Biscay could not be covered due to the oil spill from the ERIKA.
Community

From the beginning of the survey all fish species as well as three cephalopods : Loliga vulgaris, Loligo forbesi and Illex coindetti have been sampled for length composition.
Baltic Sea, BITS
Time-series The survey is a quarterly survey with surveys in the first and fourth quarter. However, as there is the risk of ice in the north eastern part of the Baltic Sea some part of the first quarter survey is carried out in April. These data should be considered part of the first quarter. Standard gear is applied from 2001 onwards. Cod data before that are converted from the old gears into the new gear (TV3) using conversion factors. For all other species these gear differences may cause a bias.
Spatial coverage The Baltic Sea has not been covered very well before 2001. After 2001 the entire Baltic Sea can be considered covered from a cod assessment point of view (ICES sub-division 22-28). This implies that areas where cod does not occur are not necessarily covered. The use of different gears in the period before 2001 may also cause bias in the spatial distribution patterns of species.
Community For all years all species are delivered from all countries expect from Poland where only cod and flounder are provided.
North Sea, Beam trawl survey
Time-series The longest time-series that is consistently sampled from the beginning of the survey (1985) is that of RV "Isis" using a BT8 gear. Age and sex and maturity are only consistently reported for Plaice (Pleuronectes platessa) and Sole (Solea vulgaris).
Spatial coverage RV "Isis" using a BT8 gear has covered the south-eastern North Sea consistently ever since the start of the survey. In 1996 the area was expanded using RV "Tridens" and now also covers the south-western and central North Sea. As these vessels use different types of gear the two can not be combined without at least a conversion factor.
Community Records on all species exist for the complete time-series.