An international bottom trawl survey in the Mediterranean




Study area

Sampling gear

Target species and biological parameters collected

Work at sea

The MEDITS data

Data analysis





The 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.
The MEDITS survey programme (International bottom trawl survey in the Mediterranean) intends to produce basic information on benthic and demersal species in term of population distribution as well as demographic structure, on the continental shelves and along the upper slopes at a global scale in the Mediterranean Sea, through systematic bottom trawl surveys.
At the beginning of the project in 1993, one of the main challenges was the adoption of a common standardized sampling methodology. The basic protocols have been adopted by the four first partners (Spain, France, Italy and Greece) early in 1994, just before the first international survey. These protocols were defined taking into account the characteristics of the areas covered by the first survey, but also in such a way to easily allow for an enlargement of the programme to other Mediterranean areas. They include the design of the survey, the sampling gear (feature and handling), the information collected, the management of the data as far as the production of common standardized analysis of the data. Before the first survey, all the common protocols were brought together in a “Manual of protocols” agreed by the Steering Committee of the programme, distributed to the participants before the first survey (1994), and published later (Anon. 1998) to ensure its distribution. This manual has been established from different experiences, and particularly from that of the IBTS Group (ICES 1992). The protocols have been amended when necessary for the following surveys, and particularly in 1995 to take into account the experience gained during the first survey. The fifth version of the manual adopted during the 2007 MEDITS co-ordination meeting (MEDITS 2007b) identifies the contribution of ten partner countries. It is available through this MEDITS web site.
The first formal presentation of the MEDITS surveys (Bertrand et al. 2000) has been done during a symposium held in Pisa in 1998 (Bertrand & Relini 2000). This presentation gives the main specifications of the programme. It has been updated in a paper (Bertrand et al. 2002a) presented in a special volume of Scientia Marina edited in 2002 and dedicated to the MEDITS surveys (Abelló et al. 2002).


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1. Study area

The MEDITS programme aims at conducting co-ordinated surveys from bottom trawling in the Mediterranean Sea. The surveys intend to include as much as possible all the trawlable areas over the shelves and the upper slopes from 10 to 800 m depth off the coasts of the partner countries (
Fig. 1). These limits have been adopted to cover at best the distribution areas of the main exploited - or potentially exploitable – demersal species, considering the administrative and technical constraints of the project. The first two surveys (1994 and 1995) had been conducted only along the coast of Spain, France, Italy and Greece. In 1996 the area was enlarged to cover almost all the Adriatic Sea (including Slovenian, Croatian and Albanian waters). The south of the Alboran Sea has been included in the survey programme since 1999 (Moroccan contribution), the waters around Malta are surveyed since 2000, and the ones around Cyprus since 2006. Since 2002, the MEDITS survey is included in the European regulation related to the collection of fishery data (DCR, Anon. 2000). In 2007 Montenegro intends to join the project (first survey foreseen in 2008). So far, at this moment ten Mediterranean countries contribute in the yearly surveys, and contacts exist with countries from the south of the Mediterranean Sea (notably Algeria and Tunisia).
The stations are distributed applying a stratified sampling scheme with random drawing inside each stratum. The stratification parameter adopted is the depth, with the following bathymetric limits: 10, 50, 100, 200, 500 and 800 m. Each position has been selected randomly in small sub-areas defined to get a compromise between the constraints of statistics based on random sampling and those of geostatistics (Green 1979; Hilborn & Walters 1992). The organisation has been adjusted to take into account the geographical sub-areas defined by the GFCM (
Fig. 2) (GFCM 2007).
The target average sampling rate is one station per 60 square nautical miles in all the areas except in the Adriatic Sea where it is laid down to one station per 200 square nautical miles because of the relative monotony of the depth. The same positions are visited every year. A total of about 1100 hauls are carried out during every annual survey. Taking into account the total trawlable surface of the covered area (about 518 000 km²) and the surface swept during each haul, the average ratio of surface sampled by survey is about 0.026 %.

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2.  Sampling gear


Since the 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).
The design of the gear has been drawn up by fishery technologists (P.Y. Dremière, IFREMER-Sète) from specifications defined by the biologists. The gear has been tested from a model in a flume tank, then in real conditions at sea before its production for the first survey. Then, specific studies have been conducted to complete the knowledge about the efficiency of the gear (Fiorentini et al. 1996; Fiorentini & Dremière 1996; Dremière et al. 1999; Fiorentini et al. 1999). When necessary the sampling gear and its handling have been slightly modified to improve its performances, especially to better stabilize it when contacting the bottom. The modifications have been decided to limit as much as possible their effects on the series consistency, but improving their quality. The main improvements have been applied just before the 1995 survey (Fiorentini et al. 1999). At that time, the results of the first survey had shown that the contact of the gear with the bottom was not fully satisfactory, especially along the slopes (depth of over 200 m). To improve this contact, buoyancy at the headline was reduced, links between the net and footrope were shortened, the sweeps were reduced, and rules for hauling were modified. As a secondary consequence, the vertical opening of the gear has been reduced (from an average of 3 to 2.5 m), and the average wide opening enlarged consequently. Further, the tickler chain has been removed in 1995 over the slopes, and everywhere in 1996.
A device to follow the geometry of the gear is systematically used only aboard some of the sampling vessels since the beginning of the survey series. Aboard the other vessels, only preliminary tests were conducted with this kind of device at the beginning of the first surveys. Since the 1998's survey, an autonomous recorder is systematically added along the headline aboard all the vessels to record water temperature at the beginning and end of hauls. The device used (Vemco Minilog) also records depth all during the hauls. A first analysis of the gear trajectory described from these recordings (Bertrand et al. 2002b) has shown that some progress had yet to be done to better standardize the distance really swept by the gear at the deepest locations. Finally, an algorithm taking into account the warp length and the depth has been adopted to standardize the estimation of the area swept during the MEDITS surveys (Lembo 2002).


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3.  Target species and biological parameters collected


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.


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4.  Work at sea


Since 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.
The duration of the hauls is fixed to 30 minutes on depths less than 200 m and 60 minutes on more important depths.
The organisation of the work at sea mainly depends on the facilities given aboard the vessels. In some cases, the samples are only taken and preserved on board and all the biological analyses are carried out in the laboratories. On the contrary in other situations, particularly aboard the research vessels, the whole biological analyses of the samples, including the data input in computer files, are conducted on board.


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5.  The MEDITS data


The data 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.
Since the beginning of the project, a specific chart defines the rules for distribution of the MEDITS data. From this chart, the full access to the raw MEDITS data is guaranteed to the European Commission (for the data produced by the European countries) and the laboratories participating in the surveys. This access is managed through the concerned national co-ordinators of the programme. For the other users, due to the specificity of the data produced by the trawl surveys, the MEDITS group promotes a free access to elaborated information, and the MEDITS partners give a special attention to the production and spreading of such data. For the European partners, the data access is organized referring to the rules defined by the European regulation related to the fishery data collection (DCR).


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 Data analysis


During 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.
Since the beginning of the project, the MEDITS data are extensively used for in a large scope of topics related to the biology and ecology of the demersal populations in the Mediterranean (e.g. population biology, population diagnostics, habitat, biodiversity, population and community indicators). Indeed, more than 130 scientific papers have been published using the MEDITS data. Considering the activity done at a large scale, we may identify among others the international symposium held in Pisa in 1998 (Bertrand & Relini 2000), a special volume of Scientia marina dedicated to the demersal resources in the Mediterranean (Abelló et al. 2002), the SAMED project related to stock assessment in the Mediterranean (Lembo 2002), a synthesis of the knowledge on the bottom fishery resources in the central Mediterranean Sea (Relini et al. 1999) and the elaboration of population and community indicators (MEDITS 2007a).


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7.  Conclusion


Every 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.
The utility of such a survey programme as support for fishery management is strongly related to its ability in producing indices able to characterize the diversity of situations in the whole area and their variations and trends in time. So far, its first goal is the production of relative abundance indices and demographic structure of the main exploited species. Further, the programme offers a fantastic opportunity for complementary observations and studies related to biology and ecology of the demersal species and communities.

Acknowledgements: The MEDITS programme is conducted thanks to the incitement and financial support of the European Commission (Directorate for Fisheries) jointly with the contribution of the partner countries.


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Abelló P., J. A. Bertrand, L. Gil de Sola, C. Papaconstantinou, G. Relini & A. Souplet eds, 2002. Marine demersal resources of the Mediterranean: the MEDITS International trawl survey (1994-1999). Sci. Mar. 66 (Suppl. 2): 280 p.

Anon., 1998. Campagne internationale de chalutage démersal en Méditerranée (MEDITS) : manuel des protocoles. Biol. Mar. Medit. 5 (2): 515-572.

Anon., 2000. Council Regulation (EC) No 1543/2000 of 29 June 2000 establishing a Community framework for the collection and management of the data needed to conduct the common fisheries policy. Official Journal of the European Communities (L 176/1 of 15.7.2000): 1-16.

Bertrand J. A., L. Gil de Sola, C. Papaconstantinou, G. Relini & A. Souplet, 2000. An international bottom trawl survey in the Mediterranean: the Medits programme. In Demersal resources in the Mediterranean. Proceedings of the symposium held in Pisa, 18-21 March 1998. J.A. Bertrand & G. Relini eds. Ifremer, Plouzané. Actes de Colloques 26: 76-93.

Bertrand J. A., L. Gil de Sola, C. Papaconstantinou, G. Relini & A. Souplet, 2002a. The general specifications of the Medits surveys. In Mediterranean Marine Demersal Resources: The MEDITS International Trawl Survey (1994-1999). P. Abelló, J. Bertrand, L. Gil de Sola, C. Papaconstantinou, G. Relini & A. Souplet eds. Sc. Mar. 66 (Suppl. 2) (Suppl. 2): 9-17.

Bertrand J. A., I. Leonori, P. Y. Dremière & G. Cosimi, 2002b. Depth trajectory and performance of a trawl used for an international bottom trawl survey in the Mediterranean. In Mediterranean Marine Demersal Resources: The MEDITS International Trawl Survey (1994-1999). P. Abelló, J. Bertrand, L. Gil de Sola, C. Papaconstantinou, G. Relini & A. Souplet eds. Sci. Mar. 66 (Suppl. 2): 169-182.

Bertrand J. A. & G. Relini eds, 2000. Demersal resources in the Mediterranean. Proceedings of the symposium held in Pisa, 18-21 March 1998. Actes de Colloques. Ifremer, Plouzané 26: 238 p.

Dremière P.-Y., L. Fiorentini, G. Cosimi, I. Leonori, A. Sala & A. Spagnolo, 1999. Escapement from the main body of the bottom trawl used for the Mediterranean international trawl survey (MEDITS). Aquat. Living Resour. 12 (3): 207-217.

Fiorentini L., G. Cosimi, A. Sala, V. Palumbo & I. Leonori, 1996. Intercalibration des campagnes internationales de chalutage démersal en Méditerranée centrale. IRPEM, Final report Contract CE Med/93/015. 59 p.

Fiorentini L., P.-Y. Dremière, I. Leonori, A. Sala & V. Palumbo, 1999. Efficiency of the bottom trawl used for the Mediterranean international trawl survey (MEDITS). Aquat. Living Resour. 12 (3): 187-205.

Fiorentini L. & P. Y. Dremière, 1996. Efficacy and selectivity of the trawl used for the MEDITS project. IRPEM, Interim report. Contract IRPEM-EC, Study 95/29. 17 p.

GFCM, 2007. Report of the thirty-first session. Rome, Italy, 9–12 January 2007. GFCM Report 31. 87 p.

Green R. H., 1979. Sampling design and statistical methods for biologists. John Wiley & Sons: 257 p.

Hilborn R. & C. J. Walters, 1992. Quantitative Fisheries Stock Assessment: Choice, Dynamics Uncertainty. Chapman & Hall: 570 p.

ICES, 1992. Manual for the International Bottom trawl Surveys. Revision IV. ICES/CIEM. 47 p.
Lembo G. ed 2002. SAMED Stock assessment in the Mediterranean. EC project n° 99/047. COISPA, Italy, CD Rom.

Medits, 2007a. Assessment of indicator trends related to exploited demersal fish populations and communities in the Mediterranean. DCR Medits Working group. Nantes (France), 15-18 March 2005 and Kavala (Greece), 2-6 April 2006. Available at 168 p.

MEDITS, 2007b. International bottom trawl survey in the Mediterranean (Medits). Instruction manual. Version 5. Ifremer, Nantes. 60 p.

Relini G., J. Bertrand & A. Zamboni eds, 1999. Sintesi delle conoscenze sulle risorce da pesca dei fondi del Mediterraneo centrale (Italia e Corsica). Synthesis of the knowledge on bottom fishery resources in central Mediterranean (Italy and Corsica). Biol. Mar. Medit. 6: 868 p.

Souplet A., 1996a. Calculation of abundance indices and length frequencies in the Medits survey. In Campagne internationale de chalutage démersal en Méditerranée (Medits). Campagne 1995. J. Bertrand ed. Rapport final de contrat CEE-Ifremer-IEO-SIBM-NCMR (MED/93/020,018,006,004) III: 7-9.

Souplet A., 1996b. Checkmed: a software to check the MEDITS data files. In Campagne internationale de chalutage démersal en Méditerranée (Medits). Campagne 1995. J. Bertrand ed. Rapport final de contrat CEE-Ifremer-IEO-SIBM-NCMR (MED/93/020,018,006,004) I: 21-26.



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