Stock assessment

The CGFS survey fulfills a public service mission for Ifremer by monitoring and observing fishery-related activities. In the English Channel, it remains the only tool providing scientific data that is independent from commercial fishing at this scale.

The data collected meet the requirements of the European Data Collection Framework (DCMAP). They are used by numerous ICES international working groups to assess the status of key exploited fish stocks and to provide catch recommendations to the European Commission.

To do this, scientists analyze several factors influencing fish populations, including recruitment, growth, fishing mortality, and natural mortality. Scientific surveys also allow for annual monitoring of juvenile abundance, which is often inaccessible to commercial fisheries or difficult to estimate from commercial data. These insights are essential for forecasting exploitable biomass in the near future.

In this context, the CGFS contributes to the production of abundance indices for species such as red mullet (Mullus surmuletus, Fig. 1), plaice (Pleuronectes platessa, Fig. 2), horse mackerel (Trachurus trachurus, Fig. 3), and cephalopods such as cuttlefish (Sepia officinalis, Fig. 4). It also collects maturity stage data for key commercial species such as whiting (Merlangius merlangus) and cod (Gadus morhua), to estimate the proportion of mature individuals at each age.

Figure 1: Excerpt from the latest 2024 WGNSSK group report (https://www.ices.dk/community/groups/Pages/WGNSSK.aspx) Red mullet in subregion 4 and divisions 7.d and 3.a. Exploitable biomass index from Q3¿Q4 surveys using the "leave-one-out" approach. The figure shows a comparison of trends and uncertainties relative to the combined Q3¿Q4 IBTS, BTS, and CGFS index (yellow line). Dashed lines represent index uncertainties.

Figure 2: Excerpt from the latest 2024 WGNSSK group report summarizing the main stock assessment model results for plaice in the English Channel. Data from the CGFS survey are shown in red.

Figure 3: Excerpt from the latest report by the WGWIDE group (https://doi.org/10.17895/ices.pub.24025482) on Atlantic horse mackerel. CGFS survey data are shown in grey.

Figure 4: Excerpt from the 2020 report of the Working Group on Cephalopod Fisheries and Life History (WGCEPH) (https://www.ices.dk/community/groups/Pages/WGCEPH.aspx).

In addition, the CGFS provides information on the spatial distribution and size structure of small pelagics such as sprat (Sprattus sprattus, Fig. 5) and herring (Clupea harengus), and contributes data for European seabass (Dicentrarchus labrax). These data are critical for estimating spawning stock biomass (SSB), one of the main indicators of stock health, and for helping define stock boundaries.

Figure 5: Excerpt from the latest report by the Herring Assessment Working Group for the Area South of 62°N (HAWG) (https://doi.org/10.17895/ices.pub.25305532), showing the spatial distribution of sprat from 2018 to 2023 based on CGFS survey data from the Eastern and Western English Channel.

Abundance indices generated by scientific surveys can be incorporated into models such as SURBAR, to estimate biomass, mortality, and recruitment trends. In 2020, the WGNSSK working group tested these models for the first time using CGFS data on red mullet, to compare outputs from different stock assessment methods.

Moreover, CGFS data make it possible to analyze the demographic structure by size across all measured species and track interannual variations in key population indicators such as abundance, biomass, mean size, and mean weight. Finally, the survey provides valuable insights into potential spatial and temporal changes within the ecosystem.

Focus ¿ Sprat in areas 7d/7e:
CGFS data indicate that sprat abundance in the Eastern Channel should not be overlooked, contrary to current assumptions used by ICES. The survey shows high sprat densities near the Baie de Seine, along with a decline in average sprat size on both sides of the Channel. Given the low fishing pressure on this stock over the past decade, this change may be the result of an ecological process, possibly linked to climate change.

Ecosystem approach

Sampling the entire ecosystem makes it possible to respond to the demands of the Marine Strategy Framework Directive (MSFD, Figure 5), both for the search for a definition of good ecological status, but also for monitoring of the various descriptors characterizing it, and in particular the descriptors:

1: Biological diversity, habitat quality, distribution and abundance of species

2: Level of presence of non-native species

5: Human-induced eutrophication

4: Food web, diversity and abundance of its elements

6: Seabed integrity, structure and function of benthic ecosystems

High frequency study of planktonic communities (M. Huret - IFREMER)

During the CGFS survey, the physico-chemical parameters of the water are analyzed at each haul using standard oceanographic protocols, to characterize the environment of the organisms studied and identify relationships between the environment and the distribution of these organisms. In addition to these abiotic parameters, the first links in the food chain, i.e. phytoplankton and zooplankton, are sampled and analyzed to know both their specific composition and their size structure. It is thus possible to identify a spatial structuring of planktonic communities, both from a taxonomic and functional point of view. The use of specific gear (bongo net and egg pump) makes it possible to locate and estimate the importance of spawning grounds and larval transitions areas, key habitats for fish populations. The samples collected are generally analyzed with a zoocam before being fixed and stored in vials. An image analysis is thus carried out on board in order to estimate the abundance of eggs and of the large zooplankton groups encountered before the end of the campaign (Figure 9).

Figure 6: Sardine spawning habitat, by compiling data from CUFES + ZooCAM from CGFS and EVHOE survey 

The RECCRU program (RECrutement CRUstacés) ( Morgane Guillam - Station Biologique de Roscoff)

RECCRU aims to set up recruitment indices for the main species of crustaceans of strong commercial interest from the Channel  coasts such as lobster (Homarus gammarus), oil cake (Cancer pagurus), spider crab (Maja brachydactylus) and the red lobster (Palinarus elephas), in order to predict the evolution of their stocks and to put in place appropriate management measures. One of the axes of this program coordinated by the Roscoff Biological Station aims to acquire data on the larval phase of these commercial species with a particular interest for lobster. The characteristics of the early stages of planktonic life of these large crustaceans, such as the lifespan of larvae, their periods of presence, their spatial distribution as well as the potential links between larval abundances and benthic recruitment and connectivity between stocks, are the main aspects studied during this project.

In this context, as part of the CGFS surveys, a series of sampling has been carried out in the Western Channel for 3 years using a MIK plankton net in order to establish the spatial distribution of the larvae of the lobster Palinurus elephas in the Western Channel. To date, a network of 17 to 23 stations, i.e. a total of 61 stations, has been sampled since 2018. No lobster larvae have so far been collected during these campaigns. On the other hand, the preliminary results show the presence of phyllosoma larvae of sea cicadas (Figure 11) (morphologically similar to those of lobsters) mainly in the westernmost part of the Western Channel, west of the Ouessant front (Figure 12). We also note the presence of lobster and spider crab zoes. The year 2021 should end this spatio-temporal monitoring of crustacean larvae in the western Channel within the RECCRU project.

Figure 7: Phyllosome larva of Scyllarus arctus. The total length of the individual is 1.3cm.

Figure 8: Spatial distribution of the phyllosoma larvae of Scyllarus arctus in the Western Channel, observed during the CGFS 2018 survey. The red dots indicate the presence of phyllosomas; larval abundance is expressed in larvae. 10000m-3. The blue crosses indicate the stations where no larvae were observed.