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). 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. Still, we indeed find that the maturation proportion of L. crocea is dramatically decline during last five decades (Fig. 2B). It is likely that decrease of average body size, size-truncation and maturation proportion is affected by continuous fishing pressures and indiscriminate fishing (~50% trawl with 54 mm mesh size) in China (Kirby 2004, Gaichas et al. 2014, Kuparinen et al. 2016). Previous studies have identified continuous fishing pressures can erode fish biomass by substantially decreasing the proportion of large individuals, and subsequent fishing-induced life-history variation is likely to have negative effects on overall population structure and recruitment (Johnston and Temple 2002, Morita et al. 2005, McMahan et al. 2020). For instances, the intense commercial fishing in Australia have caused recruitment bottleneck, even extinction to some populations, such as the collapse of Eastern blue groper, gemfish and blue-eye trevalla (Last et al. 2011).
Fishing-induced life-history variation may also constrain species distribution because the migratory ability of a species is strongly dependent on dispersal characteristics, such as morphological traits (Hsieh et al. 2009). To substantiate our finding on potential negative effect of life-history variation to overall population, we compared historical and recent distribution of L.crocea . The result showed 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. 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).