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Impact du réchauffement climatique sur la distribution spatiale des ressources halieutiques le long du littoral français: observations et scénarios


par Sylvain Lenoir
Université Lille 1 Science - Doctorat 2011
  

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2.5. Discussion

Small planktivorous fish such as sandeel, sprat, anchovy and sardine are an important trophic group in marine ecosystems where they facilitate the transfer of energy from lower trophic levels, the plankton, to higher trophic levels such as large predatory fish and seabirds. Our analysis of sandeels and sprats in the North Sea from 1960 to the end of the 21st century suggests that a continued warming of the North Sea will cause the probability of occurrence of both species to decline as sea temperature exceeds their thermal niches and the fish move northwards and decline in abundance as a consequence. Encouragingly, our analyses of the probability of occurrence of sandeel, sprat and snake pipefish predicted changes in the abundance of these species that have already been observed at the end of the 20th century and the first decade of the 21st century. Our model predicted that the probability of occurrence of both the lesser sandeel and sprat should decreasse in the North Sea in two phases, the first at the end of the 1980s and the second at the end of the 1990s and this coincides with two periods of intensification in warming in the North Sea (Beaugrand et al. 2008). The NPPEN model also predicted a transient increase in snake pipefish in the North Sea during the first decade of the 21st century and this parallels an observed increase in their abundance in the northeastern part of the North Atlantic, which was linked to increased SST (Kirby et al. 2006), and also with reports of their increased presence in the diet of North Sea seabirds during this time (Harris et al. 2007). The transient increase in snake pipefish predicted by NPPEN during 2000-2008 could therefore, arguably reflect the changing thermal regime of the North Sea providing a temporary window of opportunity favouring this species.

The lesser sandeel and sprat are important prey species for several North Sea seabirds and changes in the abundance of these fish are considered to influence the breeding success of birds such as guillemots (Uria aalge, P.), kittiwakes (Rissa tridactyla, L.) and Atlantic puffins (Fratercula arctica, L.) (Lewis et al. 2001; Wanless et al. 2005). During the first half of the 21st century, our model indicates that an increase or sustained probability of occurrence of North Sea sprat could compensate any reduction in North Sea sandeels (figure IV.2b). Furthermore, even by the end of the 21st century the reduction in the probability of occurrence of sprat is more moderate along Scottish coasts of the North Sea, which might help sustain seabird colonies albeit at lower numbers than at present. However, a much more pronounced warming (see Scenario A2, and electronic supplementary material, figure IV.S4) might precipitate the decrease in the probability of sprat occurrence especially over coastal areas. Such a scenario would extend the foraging journeys of adult seabirds, increasing the time that chicks are left unattended at the nest, further affecting seabird breeding success (Wanless et al. 2005). As kittiwakes are surface feeders unlike guillemots, which are pursuit divers, a reduction in coastal sprat may influence kittiwakes especially; guillemots less constrained in their foraging depths may be less likely to encounter food limitation (Wanless et al. 2005).

In the North Sea, the reduction in the biomass of cod has already been related to a decrease in the abundance of suitable plankton prey during the fish-larval stage (Beaugrand & Kirby 2010a,b). During the second half of the 21st century our analyses suggest that the breeding success of North Sea seabirds may be affected similarly by a reduction in prey occurrence. The North Sea supports a breeding population of about 20 seabird species and among these kittiwakes and puffins may be highly sensitive to the disappearance of lesser sandeels that comprise an important component of their diets (Lewis et al. 2001; Poloczanska et al. 2004; Frederiksen et al. 2005; Daunt et al. 2008). As the North Sea warms due to hydroclimatic change it might be expected that warmer water, southern species will colonise suitable habitats in the North Sea (Hiddink & Ter Hofstede 2008). Indeed, colonisation by warm water species newly recorded in the North Sea has already been observed in the both the plankton and the benthos (Beaugrand et al. 2009; Lindley et al. 2010). Bear and colleagues (Beare et al. 2004b) noticed that although anchovies and sardines were very rarely observed during the period 1925-1994 in the northern part of the North Sea, they became more prevalent after 1995. Warmer water fish species have occurred in the North Sea before. For example from 1900 to 1950, there was a commercial fishery in the region for Atlantic bluefin tuna, Thunnus thynnus (MacKenzie and Myers 2007). In the 1930s, anchovies were exploited in the Dutch Wadden Sea (Boddeke & Vingerhood, 1996). However, under neither climate change scenarios A2 nor B2 did the thermal regime of the North Sea encourage an increase in the European sardine or anchovies within the Northern North Sea where the main seabird breeding colonies currently occur. Anchovies might become exploitable by the second part of the Century as it was in the 1930s (Boddeke & Vingerhood, 1996) but our model indicates it is unlikely to become abundant in the northern part of the North Sea if warming follows Scenario A2 and B2. Our results suggest therefore that neither the European anchovy nor the European sardine will compensate for the adverse biological changes affecting the prey of North Sea seabirds until the end of this century. However, it should be noted that if warming becomes more intense, this figure will undoubtedly change.

Figure IV.4 : Correlation (Spearman's rank correlation) between the probabilities of occurrence modelled by NPPEN with the percentage of the number of actual presence data points in the North Sea for 2000-2008 in the North Sea for (a) lesser sandeel, (b) European sprat and (c) snake pipefish. Only half of the data, not incorporated to the model, were used for validation.

If the preferred fish prey of North Sea seabirds declines in abundance due to a northward movement constrained by their thermal niche, it is worthwhile speculating as to what may happen to current seabird populations. For example, climate-driven changes in the distribution of European sardine have been the cause of latitudinal expansion of the Balearic shearwater, from the French Biscay coast to southern UK coasts (Yésou 2003; Wynn et al. 2007). While it seems unlikely that European anchovy, as well as European sardine will occur in sufficient quantity in the North Sea to become a prevalent prey at more northern latitudes for black-legged kittiwakes and common guillemots, some northern seabird species could expand their range southwards to take advantage of any increase in abundance of these two fish. Southern movements of some species previously common in the Northern North Sea have already been observed in response to North Sea warming. This hypothesis implicates a large plasticity for other key environmental factors such as temperature that can become a source of physiological stress even for these endotherm species. How quickly North Sea seabirds may acclimate to a new diet is uncertain since most seabirds have a highly specialised diet and it is unclear whether they have sufficient plasticity to forage on alternative prey species (Grémillet & Boulinier 2009).

There is strong evidence that the North Sea is a tightly coupled ecosystem controlled by the hydroclimatic environment and influenced by trophic interactions (Kirby et al. 2009; Beaugrand & Kirby 2010a, b). Previously, we have shown how climate-induced changes in species composition in the benthos, plankton and among fish have altered trophic interactions in the pelagic food web to drive the North Sea towards a new dynamic regime favouring jellyfish in the plankton and decapods and detritivores in the benthos over commercial fisheries. The results we report here extend the influence of hydroclimatic change to include the putative consequences for the small pelagic fishes of the North Sea. In other marine ecosystems where changes in top predators due to overfishing have freed the small planktivorous pelagic fishes from top-down control, the subsequent increase in their abundance has comprised trophic cascades that have influenced plankton abundance and fish recruitment (Frank et al. 2005; Daskalov et al. 2007). The decline in abundance of planktivorous fish in the North Sea predicted by our model may therefore reinforce further the trophic amplification of a climate signal already witnessed in the North Sea (Kirby & Beaugrand 2009), and also extend this to the avian fauna providing the most comprehensive example yet of the effects of climate-induced ecosystem change. We remind that our model and projections are highly dependent on the intensity of warming (Beaugrand et al. In press).

Figure IV.5 : Comparison (a) and correlation analysis (Spearman's rank correlation) (b) of long term changes of lesser sandeel (two years old) spawning-stock biomasses (SSB, ICES 2010) and the probabilities of occurrences modelled by the NPPEN in area IV for the period 1983-2006.

We used in this paper probable scenarios of SST change (Intergovernmental Panel on Climate Change 2007b). More intense warming, comparable to Scenario A1FI (IPCC 2007), could accelerate the biogeographic movements and the trophic amplification in the North Sea ecosystems.

Acknowledgements

R.R.Kirby is a Royal Society Research Fellow. We thank the Centre National de la Recherche Scientifique (CNRS) for financial support. We thank Dr Maud Moison for helpful comments on the figures, and the IFREMER of Boulogne-sur-Mer for helpful advice concerning IBTS surveys.

Supporting Information

Figure IV.S1 : Salinity preferendum estimated for (a) lesser sandeel, (b) European sprat, (c) snake pipefish, (d) European anchovy and (e) European sardine.

Figure IV.S2 : Spatial distribution of the bottom-sediment type used for the estimation of lesser sandeel spatial distribution.

Figure IV.S3 : Estimated probability of occurrence using NPPEN for the time periods 1960-1969 (a), 2000-2008 (b), 2050-2059 (c) and 2090-2099 (d) in the North Sea (bounding box) and adjacent seas for lesser sandeel.

Figure IV.S4 : Estimated probability of occurrence using NPPEN for the time periods 1960-1969 (a), 2000-2008 (b), 2050-2059 (c) and 2090-2099 (d) in the North Sea (bounding box) and adjacent seas for European sprat

Figure IV.S5 : Estimated probability of occurrence using NPPEN for the time periods 1960-1969 (a), 2000-2008 (b), 2050-2059 (c) and 2090-2099 (d) in the North Sea (bounding box) and adjacent seas for snake pipefish.

Figure IV.S6 : Estimated probability of occurrence using NPPEN for the time periods 1960-1969 (a), 2000-2008 (b), 2050-2059 (c) and 2090-2099 (d) in the North Sea (bounding box) and adjacent seas for European anchovy.

Figure IV.S7 : Estimated probability of occurrence using NPPEN for the time periods 1960-1969 (a), 2000-2008 (b), 2050-2059 (c) and 2090-2099 (d) in the North Sea (bounding box) and adjacent seas for European sardine.

CHAPITRE V

Réchauffement climatique, Calanus finmarchicus et la morue de l'Atlantique

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