3.2 Overwintering habitat suitability of L. crocea in the
ECS
We used both fitting-based and regression-based methods to construct SI
models of each environmental variable and employed both the arithmetic
mean model and the geometric mean model under different environmental
variable combinations to calculate HSI values. Fitting-based arithmetic
mean model with two variables (e.g. depth and SST) yielded the maximum\(R^{2}\) and the minimum AIC value (Supporting information), thus was
selected as the final HSI model. The statistical analysis of
fitting-based SI models (Supporting information) shows they were all
significant (P < 0.05). As shown by the SI curves (Supporting
information, Fig. S1), the optimal range for depth, SST and SSS during
winter in our study area was 36 – 72 m, 18.2 – 20.5℃ and 33.89 –
34.27, respectively.
The recent five decades’ cooling trend in winter is remarkable, the
reduced average SST (–0.028℃/year, R2=0.31,
P<0.05) between 1982 and 2019 in the mid-southern ECS in
winter (Fig. 4). Consisted with the cooling trend of SST, the results of
HSI models show the mean habitat suitability of the 1970s
(1971 – 1980), 1980s (1981 –
1990), 1990s (1991 – 2000), 2000s (2001 – 2010) and 2010s (2011 –
2019) shifting in our study area. Fig. 5A shows that there was no
significant change (P > 0.05) in the common and
optimal habitat area from the 1970s to the 1990s. However, the
percentage of optimal habitat decreased significantly (P< 0.05) from 13%, 12% and 13% in 1970s, 1980s and 1990s to
4% and 5% in 2000s and 2010s. Fig. 5B shows that the spatial
distribution of habitat suitability also changed: optimal area has moved
toward a southeast direction, with suitable habitats became
offshore-oriented.