Data collection takes place in the Seine estuary sector extending from Ouistreham (Coordinates in projection world geodesic system 1984 or WGS84, 49°17′N 0°16′W) to Antifer (49°40′20″N 0°11′21″E) and from the Pont de Normandie (49°26′09″N 0°16′28″E) to roughly 20 meter-depth offshore to the west (Fig. 1). This 20 meter-depth limit delimitates the area considered as part of the nursery grounds23. The survey follows a fixed stratified sampling design. The stratification is based on bathymetry and distance to the mouth estuary. In total, 47 hauls are distributed across 12 sectors. Haul positions are randomly drawn in each sector. Due to rocky outcrop and the presence of many shipwrecks in the area, hauls’ locations are later assessed based on recommendations from professional fishers operating in the area and adjusted where needed. Morin and Schlaich23 provided a standardized sampling protocol for nursery zones from 1995 to 2017. In 2018, the protocol was updated in order to obtain a standardized sampling protocol on a national scale and to comply with the French Marine Strategy Framework Directive (MSFD) survey plan19. Differences in the two protocols for this particular survey are highlighted where needed. Sampling occurred once a year from 1995 to 2002, then from 2008 to 2010 and from 2017 to 2019. The two first periods strictly follow the first protocol. Only the last years are susceptible to changes due to protocol updates.
Geographical extent and sectors of the NOURSEINE survey displaying the position of hauls performed across all years. Sectors are originally established from the distance to the estuary and the bathymetry.
Sampling is carried out with a 20 mm mesh size beam trawl of 2 or 3 m wide depending on the sectors, with a 0.50 m vertical opening. The beam trawl is equipped with ground chains. Each haul lasts 15 minutes and is done against the tide at speed between 2.5 to 2.8 knots. From 2018 onward, a length of 7 minutes for the 2 m beam trawl was applied, in line with the updated national protocol. Shooting and hauling coordinates, times and depths of each haul are systematically noted. Using two different fishing gear may cause differences in the catchability of individuals, leading to differences in population characteristics estimates. An intercalibration exercise was implemented and results are presented in Riou’s work24. Data on flounder and sole captures were used to draw the comparison. Briefly, they showed that there were no differences in the mean density nor in the size structure for these two species. Therefore, the density values are considered comparable no matter the gear used in this protocol.
The period of reference for sampling is at the end of summer or beginning of autumn. Sampling dates scope from August 25 to September 30 over the time series. The juvenile stages here regroup individuals of age 1 and age 0. The latter corresponds to individuals who settled in the estuary on the year of the survey. Fish from age 0 group had their first period of growth over the summer. Sampling in late summer or early autumn ensures good catchability by the 20 mm mesh size beam trawl providing an accurate image of the fish distribution and abundance. Each survey day, 12 to 15 trawl stations are performed. In total, 40 to 47 stations are sampled each year. In 1996, 63 stations were surveyed as replicates were done. Hauls of a given station locate themselves relatively close to each other throughout the surveys.
After each haul, the content of the trawl is emptied on deck, and a total or partial sorting is carried out depending on the volume and homogeneity of the capture. All taxa, both fish and benthos, are sorted, identified, counted and weighted. Fishes of commercial value and all others flatfish are measured. Otoliths are collected on the main commercial fish species (sole, plaice, flounder, dab, pouting, large whiting and European bass) for later age group determination in the laboratory. In 1999, the sampling was incomplete and only commercial fish and invertebrates (King scallop and lobster) were sampled. The year was kept in the dataset to ensure continuity for these taxa.
Sorting the capture can be separated into three different steps (Fig. 2):
- 1.
Total capture weighting: when the hauls are emptied on the deck, the whole capture is distributed in several baskets/box in order to weight it.
- 2.
Fish and large taxa sorting: All fish and large taxa of invertebrates easily identified (edible crab, common spider crab, large cephalopods) are sorted, identified, numbered, measured (for fish) and weighted (total weight per taxa). Depending on the size of the capture, subsampling might be necessary. Operations are performed on the subsample in such a case. If visual identification is too difficult (for instance due to a large mud proportion), the capture is washed using a 5 mm sieve. The weight ratio between the total capture and the subsample form a “division” variable that allows the calculation of density. Another subsampling may be needed if a taxon has a high abundance. In that case, for practical reasons, only a subsample of the individuals are numbered, measured and weighted.
- 3.
Benthic fauna sorting: What is left from the second step is weighted before the sorting operation. All taxa constituting benthic fauna are sorted, identified, numbered and weighted (total weight per taxa). Some taxa may be measured (whelk, scallop). As for step 2, a subsample might be necessary before sorting according to the quantity of benthic fauna.
Sketch of the capture sorting process used during the NOURSEINE surveys (adapted from19).
All observations are manually recorded on fieldwork paper books before being checked and registered in the NOURSEINE database.
The NOURSEINE database consists of all information on fish and benthic taxa collected in a given haul, together with haul and survey information. Throughout the survey period, some changes on the level of identification are observed: while all fish taxa were normally considered and processed, sampling was reduced to commercial taxa solely during the 1999 survey. Changes in human operators may lead to mis-identifications and irregular records of a same taxa through the dataset. To provide a readily exploitable dataset, taxa clustering was applied to keep a homogenous record in the time series. Changes were mostly applied to benthic taxa (Table 1). In hauls where several taxa were clustered, abundance and weight were summed to calculate taxa density accordingly. Out of the 161 taxa initially recorded in the database, 138 remained after clustering.
As only raw abundance is available first-hand, taxa densities are calculated based on trawled surface but also takes into account if the haul has been partially sorted or not. The formula to calculate the density of individuals per surface unit is:
$$Density=frac{(Raw,abundanceast Divisionast Coefficient)}{Trawled,surface}$$
where Division is a factor used to elevate the abundance if the whole haul was not sorted. The same formula with abundance replaced by the capture’s weight gives the captured weight per surface unit.
The database is reworked and corrected in an R script before being provided here. The coordinates of each haul are given at the beginning and the end of the fishing operation in degrees, minutes and seconds. They are converted in decimal degrees. It is in this R script that the taxa density is calculated, along with the mean weight of the capture, and that taxa clustering happens.
Efforts have been made to detect and correct any typos that potentially slipped through the first correction when data are entered in the database.
Source: Ecology - nature.com
