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

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par Sylvain Lenoir
Université Lille 1 Science - Doctorat 2011
  

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2.4. Results

Figure IV.1 shows the thermal niche of lesser sandeel, sprat, snake pipefish, European anchovy and European sardine superimposed upon the mean annual thermal regime of the North Sea for the four time periods we considered. The thermal niches of the snake pipefish and the sprat range between 7 to 14°C and 7 to 17, respectively. In contrast, the lesser sandeel has a colder and narrower thermal niche between 8 and 12°C. The warmer thermal niches of the European anchovy and European sardine lie between 9 and 28°C and 13 and 23°C, respectively. Our results suggest that the thermal regime of the North Sea may soon exceed the thermal niche of the lesser sandeel (figure IV.1a) and may become too warm for the snake pipefish during the second half of the century under both scenarios A2 and B2 (figure IV.1c). In contrast, while the thermal niche of sprat continues to match the thermal regime of the North Sea throughout the 21st century, the warming of the North Sea may lead to a reduction in their probability of occurrence (figure IV.1b). Interestingly, the extent of North Sea warming appears to be insufficient to lead to a significant increase in abundance of either the European anchovy or the European sardine by the end of the 21st century (figure IV.1d,e).

The long-term changing spatial probabilities of fish occurrence modelled by NPPEN for the periods 1960-1969, 2000-2008, 2050-2059 and 2090-2099 (Scenario B2) revealed a northward movement of all species by 2090-2099 (figure IV.2). In the North Sea, the probability of occurrence of lesser sandeel decreases progressively from the 1960s onwards and, while the model predicts a transient increase in the probability of occurrence of sprat and snake pipefish around the beginning of the 21st century, both sprat and snake pipefish show a decline in abundance after 2050 in the central North Sea, although the probability of sprat occurrence remains moderate along northern Scottish coasts. With respect to changes in the probability of occurrence of the two warm water southern fish species, we found that neither species achieve a high level of abundance during the modelled period. The European sardine appears to not increase in its probability of occurrence in the North Sea even by the end of the 21st century, despite an increase along the European shelf-edge, and while the probability of occurrence of anchovy in the North Sea is predicted to increase by the middle of this century, the increase is only moderate (figure IV.2). We found similar results when Scenario A2 was utilised (figures IV.S3?IV.S7).

Figure IV.1 : Estimated thermal niche for (a) lesser sandeel, (b) European sprat, (c) snake pipefish, European anchovy (d) and European sardine (e). Mean SST for the North Sea were added for time periods 1960-1969, 2000-2008 and both A2 and B2 scenarios for the time periods 2050-2059 and 2090-2099.

The examination of the long-term change in the probability of fish occurrence in the North Sea revealed a strong increase in two phases at the end of the 1980s and the 1990s for snake pipefish, sprat, and a pronounced decrease in the probability of occurrence of lesser sandeel at the end of the 1990s (figure IV.3a). The probability of the occurrence of European anchovy increased slightly at the end of the 1980s. No alteration in the probability of sardine occurrence was detected for the period 1960-2008. When we applied the same model to the period 2009-2100 we found that the occurrence probability of lesser sandeel decreased steadily from 1990 to 2100 (figure IV.3b). In contrast, the probability of occurrence of sprat and snake pipefish levelled off during the first half of the century, following an initial increase at the beginning of the century, to be followed by a decline in their probability of occurrence from 2050 to 2100. The probability of occurrence of the European anchovy increased from 0.18 at the beginning of the 21st century to 0.40 by its end (figure IV.3b). The European sardine would not reach value of 0.05 in occurrence probability before the end of the century.

We found a strong correlation (ranging from r=0.80 to r=0.83) between the observed and modelled occurrence for lesser sandeel, sprat and snake pipefish in the North Sea (figure IV.4); the coefficient of correlation ranged from r=0.80 to r=0.83. Our comparison between temporal changes in Spawning Stock Biomass (SSB) of the lesser sandeel (1981-2006) and modelled data also revealed a strong correlation between the long-term changes in SSB and the probability of occurrence modelled by NPPEN (figure IV.5). Unfortunately, it was impossible to provide a similar comparative analysis for the European anchovy or the European sardine due to the very low abundance of these species in the North Sea during our study period

Figure IV.2 : Estimated probability of occurrence using NPPEN for the time periods 1960-1969 (a), 2000-2008 (b), 2050-2059 (c) and 2090-2099 using scenario B2 (d) for, from left to right: lesser sandeel, European sprat, snake pipefish, European anchovy and European sardine.

Figure IV.3 : Long-term changes in the annual mean of the probability of occurrence in the North Sea (bounding box), for snake pipefish (black lines), European sprat (red lines), lesser sandeel (blue lines), European anchovy (green lines) and European sardine (cyan lines) from 1960 to 2008 (a) based on ICOADS temperature database and from 2009 to 2100 (b) based on ECHAM4 temperature scenarios (solid line for scenario A2 and dashed line for scenario B2).

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