Marine Research Findings of the VECTORS Project

This website provides access to the research results of the VECTORS project, which can be used to support marine management decisions, policies and governance as well as future research and investment. VECTORS was a large scale project that brought together more than 200 expert researchers from 16 different countries. It examined the significant changes taking place in European seas, their causes, and the impacts they will have on society.

What are the drivers of common sole population in the Eastern Channel

The common sole is an abundant species in the Eastern Channel and sustains important fisheries (4000-5000 metric tons per year).The common sole has a complex life cycle; after the eggs have hatched the larvae spend several weeks drifting in open water. Those that survive this vulnerable life stage go on to metamorphosise into bottom dwelling fish that live in coastal and estuarine areas, close to the shore. Juveniles of common sole spend the first two years of their life in coastal nurseries before migrating to deeper areas, where they reproduce and are harvested. To investigate the title’s question, the team developed an integrated approach coupling different models and describing the whole life stage of the common sole in the Eastern Channel. This approach allowed them to estimate the different sources of mortality throughout the life cycle.

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Hydroclimate is the main driver of abundance patterns during early stages of the life cycle. As mortality (~1/1000 survival) is especially high at this stage (eggs and larvae), hydroclimate drives the year class strength abundance, without viewable relation to the spawning biomass.

A biophysical individual-based model for eggs and larvae in offshore waters was used to simulate the abundance of larvae supplied to coastal and estuarine nursery grounds from deeper spawning grounds. Using this type of model allowed the role of climate-driven changes in water circulation patterns on the magnitude and timing of larval ingress to be examined. Simulations spawning three decades suggested that the supply of larvae to coastal nursery areas was more sensitive to factors affecting individuals along their drift routes from offshore spawning grounds compared to either changes in phenology of reproduction or the biomass of adults at those spawning grounds. Only about 1 in 1000 individuals was expected to survive and be successfully transported to  nursery grounds. Thus, bottom-up (physical) processes are critical drivers of year class strength which, importantly, appears unrelated to the productivity of the adult stock as measured via spawning stock biomass1.

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Flatfish juvenile dependent on restricted coastal and estuarine nursery grounds. Juvenile mortality on nursery grounds is lower than during previous life stage but still high (~1/100 survival) and also strongly limits the population size. Nursery habitat degradation leads to important losses to the sole population in the Eastern Channel.

Using a meta-analytical approach, we show that flatfish display a lower interannual variance in the recruitment success compared to other exploited orders of marine fishes and that the variance of recruitment success decreases with increasing stock level2. This is consistent with the hypothesis of density-dependent process during the juvenile phase of the life cycle, linked to their dependence to restricted nursery grounds. Essential fish habitat suitability models and geographic information system were combined to quantitatively describe nursery habitats for Solea solea. Juveniles strongly depend on shallow, soft-bottom, sheltered coastal and estuarine habitats. These habitats host a large proportion of total juvenile common sole, suggesting that these restricted areas should be considered essential nursery habitats for S. solea3. As a consequence, coastal and estuarine habitat loss and degradation, particularly in the highly populated Seine estuary, has led to a decrease of the nursery capacity, impacting population renewal. 

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Multiple stressors. Realistic restoration of the Seine estuary lead to a 2/3 increase in biomass and catches for the adjacent sub-population of sole. Fishing remains the main source of population depletion on the Eastern channel adapting fishing mortality to maximum sustainable yields would increase biomass (x2) and catches (1/3).

An integrated framework was developed for the spatially-explicit modelling of the life cycle of sole. The approach combined: (1) the model for larval drift that provided estimates of the dispersion from spawning grounds to settlement in nurseries1; (2) the nursery habitat suitability model that estimated juvenile densities and surface areas of suitable juvenile habitat in each nursery sector3; (3) a spatialised catch-at-age model for the estimation of the numbers-at-age and the fishing mortality on sub-adults and adults. The model aimed at developing scenarios to disentangle the effects of multiple interacting stressor (e.g., estuarine and coastal nursery habitat degradation, fishing pressure) on population renewal45. Results emphasise the importance of nursery habitat availability and quality for these species. Realistic restoration scenarios of the Seine estuary lead up to a two-third increase in catch potential for the adjacent sub-population. Fishing however remains the main source of population depletion in our simulations when considering the whole Eastern channel system. Indeed, adapting fishing mortality to local MSY levels leads to substantial increases in biomass (x2) and catches (one-third increase) at the Eastern channel scale. We also show how overfishing increases the sensitivity to unfavourable hydroclimatic conditions. Our results pave the way toward the assessment of spatial management schemes, in the context of marine spatial planning for sustainable management of fish resources6.

Relevance for Policy:
  • Common Fisheries Policy
  • Directive on Maritime Spatial Planning and Integrated Coastal Management (forthcoming)
  • Marine Strategy Framework Directive

References

Lead Author:

Olivier Le Pape
(olivienospamr.le.pape@agrocampus-ouest.fr)
Agrocampus Ouest (AGRO)
Date of research: November 2014

Related articles:

Cod, recruitment, temperature and zooplankton

Impact of environmental changes on North Sea herring

Population dynamics of sprat in the Baltic Sea 

Processes impacting overwintering larval survival 

Changes in the upper trophic level: impact on fish

Connectivity: plaice spawning and nursery areas 

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This project has received funding from the European Union’s Seventh Framework Programme for research, technological development and demonstration under grant agreement no 266445
© Vectors 2015. Coordinated by Plymouth Marine Laboratory.