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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