1. Diagnosis of the state of the King scallop (Pecten maximus) stock in the Bay of Seine.
The Bay of Seine is divided into two main administrative areas, each with its own fisheries management and regulations: the area known as "Extérieur baie de Seine" lies beyond the limit of French territorial waters and is therefore subject to European regulations only for all European fishing fleets (including the British fleets which have been fishing this area since the early 2010s), and to the annual fishing season set by Ministerial Decree for French fishermen. The area known as the "Baie de Seine" corresponds to the classified seabed located within French territorial waters: it is closed to fishing by European fishers other than French (with the exception of historical fishing rights for Belgium and the Netherlands, which are not claimed). The regulations imposed by the French administration, in agreement with the professional organisations, are much stricter, both in terms of access to fishing zones (limitation of fishing effort), and in terms of the characteristics of authorised vessels and the gear deployed.
By way of illustration, we present below some of the diagnostic work carried out on the classified seabed in the Bay of Seine during the most recent COMOR survey in July 2023, together with an analysis of the long-term trend in the state of the stock in the Bay of Seine.
For almost 10 years (2014), the pre-recruitment indices (age 1) in the Seine Bay have been excellent. This trend was confirmed during the COMOR 2023 survey, since the index for juveniles (class 2022) is the highest (1091.69) ever observed in the entire historical series (Figure 6).
Figure 6 : Trend of abondance indexes by age in the Bay of Seine from 2000 to 2023.
Similarly, the abundance indices for the other age classes, both recruitment (2-year-old scallops) and fishing residuals (adult scallops aged 3 years and over that have already been fished at least one full fishing season), are also very good, confirming the good health of the stock. The total available biomass (Figure 7) has been estimated at 72614 tonnes in 2023. The current level of this biomass in the Bay of Seine is without equivalent in the historical series, although this species is still heavily exploited by the industry fleets. It is almost 2 times higher than the average for the period 2013-2022 (39867 tonnes), and 7.5 times higher than the average calculated between 1998 and 2012 (9608 tonnes).
Figure 7 : Exploitable biomass for the classified deposit in the Bay of Seine from 2000 to 2023
In terms of density, the average density in the Bay of the Seine for all ages combined is 0.58 scallops/m², with the highest density observed being 2.26 scallops/m². This represents 359 million adult individuals for a total of 727 million shells, all ages combined.
In July 2023, the distribution of this biomass (Figures 8 and 9) on the sea ground was relatively homogeneous between the 3 zones (West, Central and East with respectively 30175 t, 20167 t and 22272 t from west to east), 42% of this biomass being however concentrated in the western part of the bay.
Figure 8 : Adult King scallop abondance indexes location in the Bay of Seine estimated after COMOR2023.
Figure 9 : Geographical distribution of exploitable biomass in the Bay of Seine estimated in July 2023.
The use of the CMSY++ stock assessment model, which uses as input data a series of annual catches calibrated by the series of abundance indices from the COMOR surveys, shows that the Bay of Seine King scallop stock is in good ecological condition, with both a 2023 biomass higher than the Bmsy reference biomass, and an overall fishing mortality lower than the MSY Fmsy fishing mortality (Figure 10).
Figure 10: Kobe matrix calculated by CMSY++ for the Bay of Seine King scallop stock. The trajectory of the stock status shows that we have gone from a heavily overexploited stock in 2000 to a stock in good condition today.
2. Use of COMOR survey data for research: connectivity between stocks and larval dispersal.
Work on connectivity between different stocks of King scallops in the Channel was part of action 4 of the ANR-COMANCHE project. It was largely based on data from the historical series of COMOR and COSB surveys. The aim of this research project was to acquire detailed knowledge of dispersion patterns and larval exchanges between the different adult scallop stocks in the Channel, which is an essential prerequisite for any spatialised management of the fishery. Initially, a coupled biological-physical dispersion model was developed in order to establish the model's sensitivity to the various biological parameters taken into account, in particular the date of spawning, the larval lifespan, the migratory behaviour of the larvae and the spatial heterogeneity of the distribution of the spawning stock. It was then applied to the example of the two main stocks of the species, in the Bay of Seine and the Bay of Saint-Brieuc (Nicolle et al., 2013[1]). Depending on the stock under consideration, the intra- and inter-annual variability of larval transport depends on the relative role of tidal circulation, which is major in the Bay of Saint Brieuc, and wind-induced circulation, which is dominant in the Bay of Seine. Temperature plays a non-negligible role, controlling both the date of spawning and the life span of the larvae, and therefore partly the average dispersion distance. At the level of each stock, the key sectors for the local persistence of populations have been identified. Their role is explained by the local specificities of hydrodynamics and their contribution to the reproductive effort. In the Bay of Saint Brieuc, for example, this is the north-east sector of the bay off Erquy.
Secondly, the Lagrangian model developed was applied to the 18 King scallop stocks identified in the Celtic Sea and Channel from different sources (i.e. VMS data, scientific surveys, expert opinion) (Thiébaut et al., 2015[2]; Nicolle et al., 2016[3]). The contribution of each stock to the regional spawning effort was estimated using landing data provided by ICES. At this scale, the relative importance of local processes (i.e. local larval retention) and regional processes (i.e. allochthonous flows) in maintaining populations is extremely variable from one stock to another depending on their size and hydrodynamics. Three major functional units, which are subject to large-scale and regular larval exchanges, have been identified: (i) the eastern Channel with the key stock in the Bay of Seine, (ii) the Normandy-Breton Gulf and the north Brittany coast, and (iii) the south-west coast of England (Figure 7). The Bay of Brest appears to be isolated from the other stocks. A small stock north of Cherbourg provides a link between the western and eastern basins of the Channel along the French coast.
Figure 11 : Average connectivity map between different King scallop seabeds in the Channel, which allows to define three potential management units.
These results were an essential first step towards understanding the regional dynamics of the species and taking this dimension into account in stock management. In addition, the model developed will make it possible, where appropriate, to answer various management questions such as the development of fishing grounds, popular in Great Britain, the effectiveness of seeding in strengthening stocks or interactions with MRE sites. Finally, it is an opportunity to understand the dynamics of stocks on a pan-European scale, including in particular the British Isles and Ireland.
Finally, it should be noted that this distribution of Pecten maximus scallop populations into three relatively distinct entities in the Channel was subsequently confirmed by genetic studies carried out on the different seabeds (Handal et al., 2020 [4]).
[1] Nicolle A., Dumas F., Foveau A., Foucher E. and E. Thiébaut, 2013. Modelling larval dispersal of the king scallop (Pecten maximus) in the English Channel: examples from the bay of Saint-Brieuc and the bay of Seine. Ocean Dynamics, Volume 63, Issue 6, 661-678. http://link.springer.com/article/10.1007%2Fs10236-013-0617-1
[2] Thiébaut E., Nicolle A., Ogor J., Dumas F. and Foucher E., 2015. Modelling larval dispersal of Pecten maximus in the English Channel: a tool for spatial management of stocks. Book of abstracts, 20th International Pectinid Workshop, 22nd - 28th April 2015, Galway (Ireland), ISBN 978-0-9932622-0-3, 133-134.
[3] Nicolle A., Moitié R., Ogor J., Dumas F., Foveau A., Foucher E. and Thiébaut E., 2016. Modelling larval dispersal of Pecten maximus in the English Channel: a tool for the spatial management of the stocks. ICES Journal of Marine Science, doi:10.1093/icesjms/fsw207.
[4] Handal W., Szostek C., Hold N., Andrello M., Thiébaut E., Harney E., Lefebvre G., Borcier E., Jolivet A., Nicolle A., Boyé A., Foucher E., Boudry P., Charrier G., 2020. New insights on the population genetic structure of the great scallop (Pecten maximus) in the English Channel, coupling microsatellite data and demogenetic simulations. Aquatic Conservation-marine And Freshwater Ecosystems, 30(10), 1841-1853. Publisher's official version : https://doi.org/10.1002/aqc.3316 , Open Access version : https://archimer.ifremer.fr/doc/00648/76025/
