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 AICc 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). The cooling trend in our study area may influenced by
the Kurushio extension, specifically, in the latest IPCC report (2019),
the Kurushio extension exhibit long-term cooling, which is consisted
with our result. Also, another study also revealed the cooling trend
along China and Japan coast (−0.69± 0.44 °C/decade), opposing to the
overarching global warming trend, especially in winter season due to the
extreme cold events (Bindoff et al., 2019; Liao et al., 2015). Consisted
with the cooling trend of SST during overwintering phase of L
crocea , the results of HSI models show the mean habitat overwintering
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
average 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.
Unfortunately,
regarding data availability, the HSI models conducted in our study may
biased because we used catch data during 1971-1982 as a measure of
abundance (e.g. highly dependent on the effort). Hence, abundance data
obtained from scientific cruise is more convincing than using catch data
as abundance data and should be encouraged in future study.