4.1 Fishing-induced life-history variation
Overall, this study provides evidence of serious fishing-induced
life-history variation in L. crocea population and represent a
glimpse of fishery collapse. The observed life-history parameters show
that body size of L. crocea have on average, decreased during the
last five decades (table 1), whereas we would expect an overall decrease
of body size if the heavily fishing was the dominated driver of body
size changes and STE. China’s marine resources have suffered serious
overfishing issues, characterized by the progressive depletion of
economic stocks (Cao et al. 2017). Specifically, indiscriminate fishing
(~50% trawl with 54 mm mesh size) in China can cause
both age or size truncation of target populations, leading to increasing
instability and vulnerability of the population dynamic (Kirby 2004,
Gaichas et al. 2014, Kuparinen et al. 2016). Hence, the extremely high
fishing mortality and exploitation rate have caused systematic decline
in landing catch. Previous studies in the 1970s and 1980s revealed that
the main catch of L. crocea consisted of two or three years old
(400 – 800g) individuals (Yu and Lin 1980), while 95% of catches in
this study were individuals aged zero or one years old. Another major
finding was that ~70% of potential catch area have
disappeared (Supporting
information Fig. S2), with the highest disappearance rate in offshore
areas (122°E – 125°E), which
follows the life-history variation of L. crocea during the last
five decades.
Previous studies have identified continuous fishing pressures and
subsequent fishing-induced life-history variation is likely to have
negative effects on overall population structure and their distribution
patterns in several ways. First, continuous fishing pressure can erode
fish biomass by substantially decreasing the proportion of large, old
individuals. Still, STE-caused decline in maturity size and increase in
mortality, may reduce reproductive output or the size of the breeding
population (Johnston and Temple 2002, Morita et al. 2005, McMahan et al.
2020), and subsequent cause irreversible changes such as recruitment
bottleneck, even extinction, to some populations, such as the collapse
of Eastern blue groper, gemfish and blue-eye trevalla populations,
following intense commercial fishing in Australia (Last et al. 2011).
Second, a decrease in the population size can also reduce variation
(causing inbreeding and genetic abnormalities) and, therefore, restrict
the potential for adaption (Langangen et al. 2019). Thirdly,
fishing-induced life-history variation may constrain species
distribution because the migratory ability of a species is strongly
dependent on dispersal characteristics, such as morphological traits,
like body-size (Hsieh et al. 2009). Consequently, constriction of
geographic distribution, associated with a decline in body size, may
reduce the ability to respond to climatic stress, by limiting movement
(Reusch et al. 2005).
4.2 Climate-induced overwintering habitat
degradation may intensify
the effect of overfishing
The best model (next best model: ∆AIC=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 common 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). It is not uncommon that that heavily exploited species that do
not ‘fill’ their suitable habitat against climate change background. 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. Firstly, our study demonstrated that L. crocea have both
life-history degradation 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 of L. crocea (Jorgensen et al. 2008, Opdal and
Jørgensen 2015, 2016). Ultimately, 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 to L. 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.