4.2 Climate-induced overwintering habitat degradation may
intensify the effect of overfishing
The best HSI model (next best model: ∆AICc=2, for other
models, Supporting information Table S2) to explain catch patterns under
unexploited status over time included SST and depth. The result of SI
suggested that the optimal overwintering temperature range (18.2 –
20.5℃) and depth (36 – 72m) of L. crocea mirror previous
lab-based observation of optimal growth temperature (17℃– 24℃) and
empirical observation of optimal overwintering depth (50 – 60 m) (Xu
and Chen 2011, Liu 2013). It is worth noting that SST in the
mid-southern ECS have decreased by an average of 1℃ between the 1980s
and 2010s with annual decrease SST rate – 0.028℃/year (Fig. 4). HSI
variation results in the last five decades suggests that cooling trend
of SST in the ECS has significantly reduced the proportion of optimal
and average overwintering habitats for L. crocea (Fig. 5B).
Consequently, in response to the SST decrease in winter, migratory
species, like L. crocea are expected to respond in two ways as
follows. Generally, marine organisms respond to climate change through
shifts in distribution (Guisan and Thuiller 2005). For instances, in the
North Sea, both exploited and un-exploited fish species have shifted to
higher latitudes and deeper water between 1977 and 2001 in response to
rising sea temperatures (Perry et al. 2005); in the Eastern Tropical
Pacific, demersal species were projected to move into shallow water by
the mid-21st century in response to high greenhouse
gas emissions (Representative Concentration Pathways, RCP8.5) and strong
migration (RCP2.6) scenarios (Clarke et al. 2020). Alternatively, it is
also commonly observed that species stay in poor habitat against climate
change but suffered climate-induced life-history variation.
Particularly, it occurs when marine fishes are living in temperature
outside their physiology optima: this results in reduced aerobic scope,
which negatively affects their growth and reproduction (Pearson and
Dawson 2003, Pörtner and Knust 2007, Toresen et al. 2019). Hence,
species like L. crocea must either migrate to remain within
suitable habitat or suffer the consequences (Bell et al. 2015).
Interesting, L. crocea overwintering distribution pattern did not
shift alongside a decrease in winter SST, which is a good indicator that
temperature per se did not explain the overall shift of L. croceadistribution. Such absence of a clear systematic impact of temperature
may be due to the life-history parameters degradation, which could
constrain the hedging capacity against climate change (Thorson et al.
2017). For example, STE-caused a change in the length-age structure is
the main diver of inter-annual shifts in summer flounder distribution,
while temperature had little influence on the change in distribution
(Bell et al. 2015).
More broadly, the ‘match/mismatch hypotheses’ may explain the combined
effects of heavily fishing and climate change on decrease of overall
population (Cushing 1990, Edwards and Richardson 2004). Based on our
study, we suggest that fishing-induced life-history variation leading to
the ‘mismatch’ of L. crocea optimal overwintering habitat.
Specifically, our study demonstrated that L. crocea have both
life-history variation and size truncation compared with 1980s, with
significantly smaller body size and advanced maturation
(Fig. 2B). This truncation in
overall size-structure can significantly affect swimming ability, such
as reducing the sustained swimming time and average swimming speed,
namely size dependent, consequently reducing the distribution range ofL. crocea (Jorgensen et al. 2008, Opdal and Jørgensen 2015,
2016). Given the climate-induced changes in overwintering habitat
suitability that occurs in the mid-southern ECS, the fishing-induced
life-history population variation that constrains dispersal capability
could pose a significant ‘mismatch’ of optimal overwintering habitat toL. crocea -like migratory species (Fig. 6). Such applicability of
‘mismatch hypotheses’ to the specific long-lived migratory fish exposed
to fishing and climate change had rarely been demonstrated.