The data from the PELMED surveys are used for both expertise and research purposes.
1. Expertise
The PELMED abundance and biomass indices are used to conduct stock assessments for sardine and anchovy. A reference trawl is assigned for each ESDU (Elementary Sampling Distance Units) of the cruise track based on distance, stratum (surface, pelagic or near bottom), bottom depth and finally the shape of the echotrace. This allows the partitioning of the total echo-integrals between species using the proportions by species recorded in the specific 'reference hauls'. Acoustic backscatters are then transformed into biomass by taking into account fish size. These biomasses associated with some biological parameters collected during the cruise (size and age structure, condition) as well as a descriptive analysis of fisheries (in terms of catch and effort) allow the assessment of sardine (using a 2-stage biomass model) and anchovy stocks (using a statistical catch-at-age model) by the General Fisheries Commission for the Mediterranean (GFCM).
Figure 3: Biomass time series of the 3 main small pelagic fish in the Gulf of Lion.
2. Research
From 2012 to 2016, the EcoPelGol project (Pelagic Ecosystems of the Gulf of Lion, 2012-2016, funded by France Filière Pêche) aimed to improve our understanding of the fluctuations of small pelagic populations in the Gulf of Lion. This project relied mainly on the analysis of data acquired during the PELMED cruises. Master and PhD students have thus benefited from this large database to carry out their studies. We have chosen here to present some examples of results achieved during this project.
- Demographic and biological changes in small pelagic populations over the past 20 years
While the biomass of anchovies and sardines has dropped considerably in recent years, leading to a crisis in the fishery, their abundance (i.e. the number of fish) has been maintained or even increased for the sardine. Thus, we have as many or more fish but less biomass. At the same time, samples taken from trawls during the PELMED surveys have shown a significant change in size structure in recent years, with sardines and anchovies being much smaller since 2008 than before (Van Beveren et al. 2014). This decrease in size results for sardines from both a decrease in growth and a loss of older individuals (loss of age classes >2 years), while only a decrease in growth seems to be involved for anchovy (Van Beveren et al. 2014).
Figure 4. Proportion of the sardine population in each age class from 1993 to 2012. The lighter the shade the younger the individuals (Van Berven et al. 2014).
In parallel to these changes, a strong decrease in body condition (reserves of the individual, often associated with its fat content) has been observed in both species (Van Beveren et al. 2014). The fish although numerous are smaller and less fatty than before, explaining in part the decrease in fishing effort (number of boats or fishing days) and catches. Finally, we were able to show that the individuals most affected by this drop in condition were the oldest individuals, providing a first step to explain the observed demographic imbalance (Brosset et al. 2015).
Figure 5. body condition indices for the 3 species. Adapted from Van Berven etal 2014.
Figure 6. Body condition as a function of age in 3 periods (poor, average, and good condition period). The current period corresponds to the poor condition period (Brosset et al 2015).
These first studies have highlighted a possible over-mortality of adults in sardines and a potential environmental change, source of a lower growth and a poorer condition. Thus, the samples taken during the PELMED cruises since the 1990s have allowed a study combining stomach contents and stable isotopes to compare the current diet of sardines, anchovies and sprats with their diet of years before the decline. This study showed an increase in trophic niche overlap between species (i.e. possible stronger competition), as well as a decrease in the size of consumed prey (Brosset et al. 2016). This supports the hypothesis of a lower energy intake in the diet of these species, explaining at least in part the decrease in lipid reserves of these species. Despite this decrease in available energy, sardines and anchovies appear to have maintained (or even increased) their reproductive investment by decreasing size at first maturity and increasing the gonadosomatic index (Brosset et al. 2016). In other words, fish begin reproducing at younger ages and develop consistently large gonads in proportion to their size. Thus, these 2 species seem to favor their reproduction over other life history traits. This could be at the expense of their growth and survival. In order to study further the high adult mortality in sardines, we assessed over one year and more than 1000 sardines the presence of pathogens that could weaken or even kill them. No virus could be detected (neither by PCR nor by viral culture), nor any macro-parasite. Regarding bacteria, only vibrios and Tenacibaculum were observed but in low prevalence and without any associated organ lesions. Finally, a study designed to quantify the predation pressure exerted by bluefin tuna on small pelagic species showed that bluefin tuna consumed a very small part of the populations (< 2%) in the Gulf of Lions, without any particular size selectivity patterns, and therefore cannot be held responsible for the drop in biomass or the disappearance of older individuals (Van Berven et al 2017).
It seems that sardine and anchovy populations are affected by a change in the planktonic community, which is made up of less energetic species than before, resulting in a decrease in the energy available to small pelagics. In response to this change, sardines and anchovies would maintain a strong reproductive investment in relation to a weaker growth that may even affect their survival (Saraux et al 2019).
Figure 7. Schematics on the main drivers of small pelagics population dynamics. Significant drivers appear in bold, while less important ones appear in grey and in italics (from Saraux et al 2019).
- Spatio-temporal mapping of small pelagic species
Acoustic data acquired during the PELMED surveys allowed to study the spatial distribution of small pelagics from 2003 to 2012 (Saraux et al. 2014). Thus, we showed that these species were not uniformly distributed in the Gulf and that areas of high density could be detected. Because species are highly mobile, there is a great temporal variability associated with these areas and with the mapping of these species in general. Nevertheless, some trends seem to persist over time. In particular, sardines and sprats are more inshore than anchovies. It was also possible to distinguish different types of areas: recurrent areas where the biomass is important every year, occasional areas where the number of individuals varies greatly from year to year, and finally unfavorable areas where the individuals almost never occur. This offers potentially important information for the spatial management of the Gulf, whether for marine protected areas or spatial management of fisheries. Ongoing studies are also trying to understand which abiotic parameters (temperature, salinity, currents, etc.) explain these habitats and to determine if they are different at the local scale of the Gulf of Lions compared to those determined at the scale of the entire Mediterranean (study conducted on the MEDIAS surveys as a whole including PELMED, Giannoulaki et al. 2011 and 2013).
Figure 8. Classification of different areas (recurrent, occasional, and unfavorable) (Saraux et al. 2014).
- Concomitant changes in the environment and small pelagic fish community
Recently, the PhD thesis of Guillaume Feuilloley (December 2020) allowed to determine whether changes may have taken place in the planktonic community over the last two decades, in terms of density, size or taxonomic composition, which could explain the changes in the diet of sardines and anchovies and their declining condition. This thesis focused on the variability of different environmental key factors for the planktonic community dynamics. Results indicate that environmental conditions have broadly changed in the Gulf of Lion, with a major change in the mid-2000s, affecting Chlorophyll concentration (with a regime shift in 2007), but also SST, upwelling, thermal fronts, Rhone flow (and in particular N and P nutrient inputs) and winter deep convection. These environmental changes may have affected plankton production and, consequently, the small pelagic fish community, which showed similar patterns of variation (Feuilloley et al. 2020).
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Figure 9. Second common trends of the DFAs (Dynamic Factor Analysis) performed on the environmental variables (red) and on the biological variables (black) (Feuilloley et al. 2020).
The DEFIPEL project (DEveloppement d'une approche de gestion intégrée de la Filière petits PELagiques, funded by France Filière Pêche), currently in progress (2019-2023), aims at consolidating the sustainability of the French small pelagic fishery by proposing a set of indicators on the state of the ecosystem, the resource and the fishery, as well as adaptation scenarios co-built with stakeholders. This project is partly based on the processing of hydrological and biological data acquired during the PELMED surveys and aims at filling the knowledge gaps on the habitats and ecology of small pelagic fish in the Mediterranean in order to better understand the impacts of the environment and anthropic activities.
