An international bottom trawl survey in the Mediterranean
Jacques BERTRAND, Luis GIL DE SOLA, Costas PAPACONSTANTINOU, Giulio RELINI and Arnauld SOUPLET
Mediterranean Sea is a complex bio-geographic system including a large variety
of specific areas. Due to the small extend of the continental shelf along most
of the coasts, benthic and demersal resources are mainly exploited close to the
shore. As most of the benthic and demersal fish stocks are defined and exploited
at a national level, and as exploitation is strongly distributed in a wide
number of units (type of vessels, fishing techniques, landing ports, species
caught, etc.), very few global information is available on the status of these
resources from the fisheries.
beginning of the project, the same sampling gear is used throughout the survey
series by all teams. The standard device is a bottom trawl, including all the
material and its rigging from the doors to the codend of the net. The net (GOC
73) is a bottom trawl (Fiorentini et al. 1999) designed for experimental fishing
with scientific purpose. It achieves a compromise between different constraints.
In particular, the characteristics of this gear make it usable over all the
depth range and in the various conditions encountered in the whole survey area.
To increase the catch of demersal species, it has a vertical opening slightly
superior to the most common professional gears used in the area. Its codend mesh
size is 20 mm (stretched mesh).
A list of thirty common target species (including fish, molluscs and crustaceans) was established at the beginning of the project with reference to their commercial production, their accessibility by a bottom trawl and their potential interest as biological indicator in the different areas. It has been enlarged to thirty-height species during the following years (Table 1), particularly to include species of interest in areas newly integrated in the MEDITS survey, and taking into account the removal of one species (Sparus pagrus) from the list in 1996 due to its very rare occurrence in the samples. Observations on these species are the total number of individuals, length frequency distribution, sex (including sexual maturity stage) and total weight. The characteristics of each kind of observation are specified in the common manual of protocols. For all the other sampled species of fish, crustacean and mollusc, the total number and total weight are reported for each haul. During each annual survey, a total of approximately 150 species are identified aboard each vessel.
1994, one survey has been carried out every year, during the spring and the
beginning of summer. To reduce the whole duration of each survey, several boats
(8 to 11 vessels according to the year) are working at the same time. Each of
these vessels works at sea during about one month per year. Research vessels and
chartered fishing vessels are used, depending on local possibilities. As much as
possible, the same vessel is used every year in each area.
are put in computer files by the teams in charge of the survey. Five standard
exchange formats (in ASCII) including normalized coding are defined (Table 2). A specific software has been written (Souplet 1996b) for an automatic checking of
the data by each of the partners for its own data before their regrouping. Up to
2001, after a second validation in the regrouping place (IFREMER-Sète), copies
of the total set of data files were deposited on CD-ROM at the national
co-ordinators and EC-DG Fish offices. The process was managed such as to make
the data available a few months after the end of each survey (usually in October).
Since 2002, as all the other data collected under the European regulation
related to the data fishery collection (DCR), the MEDITS data collected by the
European country memberships are managed by the relevant national authorities.
the first years of the project, common analyses focused on the reference list
species were conducted by the MEDITS partners at the end of each survey. They
were based on the production of biomass and relative abundance indices (in kg/km²
and in number of individuals (No/km²) as well as length frequency distributions
by species and strata. These analyses were made using statistical methods
approved by the MEDITS Steering Committee and included in a specific software (Souplet
1996a). The results obtained by these working groups were distributed in annual
survey reports. This approach has allowed for a second level of data validation
by cross analysis of the data.
year, the MEDITS surveys are carried out thanks to a very intense collaboration
between a lot of regional teams who have the best knowledge on the biology of
the demersal species in the different areas. The quality of the data collected
is strongly linked with their watchfulness in ensuring the full respect of the
protocols, and the quality of the biological information included. The quality
of the quantitative data is directly linked with precision in handling the
sampling gear and reporting information related to its filtering power and
motions. The present experience shows that this goal may be obtained only from a
strong collaboration between biologists and technologists. More generally, the
success of such a long-term survey programme requires a permanent investment in
quality approach to ensure the quality in data collection and the best level of
consistency in the series, and an intense co-ordination between the partners.
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