| As an important part of marine ecosystem, fish plays a significant role in meeting the market demands and ensuring food security, and provides a large amount of high-quality protein for human being. With the serious effects of anthropogenic activities (e.g. overfishing) and environment changes on marine ecosystem, biological characteristics and population dynamics of marine fishes have been greatly changed, which further threaten the service function of marine ecosystem. In the present study, interannual changes in biological characteristics and relative stock densities in three high trophic level demersal fishes in the Bohai Sea and Yellow Sea ecosystem were analyzed, involved in small yellow croaker Larimichthys polyactis, Pacific cod Gadus macrocephalus and anglerfish Lophius litulon. The data for this study were from the bottom trawl data in the Bohai Sea and Yellow Sea (data for small yellow croaker, Pacific cod and anglerfish were 1960-2010, 1999-2009 and 1985-2009, respectively). This study will provide useful information for management of marine living resources and modeling population dynamics.The allometric factors (b) of body weight-length relationship in the small yellow croaker were not significant differences (P>0.05) between sexes in most years from 1960 to 2010; but significantly negative correlations (P<0.01) were found between condition factor (a) and b in two small yellow croaker stocks, the northern Yellow Sea-Bohai Sea stock (NYBS) and the southern Yellow Sea stock (SYS). (lga)/b ratio was constant during the decades, and connected with the fish body density. The isometric growth was found in the NYBS from 1960 to 2004, and b increased during the stock regression stage (1982-1983) and the early recovery stage (1993). The negative-allometric growth was found in the SYS from 1960 to 2010, b showed a decreasing trend during the stock regression stage (1986) and the early recovery stage (1994), increased since the late 1990s. The condition factors of male and female individuals in the NYBS decreased from 1960 to 2004; the condition factors of male and female individuals in the SYS in 1986 were significantly smaller than those in 1960 (P<0.05), Since 1986, the condition factors slightly increased, but no significant differences were found between surveys. These changes were closely related to the variations in the proportion of sexual maturity. The b of male individuals showed better coordination relation with fishing stress, increased with the increase of stock density. The decrease of condition factors of small yellow croaker were mainly caused by the declines in age at maturity from 1960 to the middle 1980s, since the 1990s, the variations in condition factors in both stocks mainly attributed to the changes of sea surface temperature (SST), and the condition factor increased with the increase of SST.As depicted in maturity ogives, body length and age at 50% maturity were studied for female small yellow croaker in the Bohai Sea and the Yellow Sea during the 1960-2010 spawning seasons. Body length (L50) and age at median sexual maturity (A50) were estimated by an arcsin-square-root (ASR) transformative logistic model and an inverse von Bertalanffy growth function, respectively. The results show that L50 decreased from 152.8 mm to 105.3 mm between 1960 and 2003-2005 in the NYBS and from 184.4 mm to 110.1 mm between 1960 and 2010 in the SYS. Over the same period, A50 decreased from approximately 1.5 years in the NYBS and 2.4 years in the SYS to about 1 year in both stocks. Significant intrastock changes (P<0.01) in length-maturation curves were found in both stocks over long time periods (≥4 years); however, there were no significant changes (P>0.05) over short time intervals (<4 years). Significant interstock changes (P<0.01) were observed in length-maturation curves in corresponding sampling years. Significant positive correlations were found between the instantaneous rate of maturation (δ) and the growth potential index (ω) in the NYBS and between L50 and asymptotic body length (L∞) in the SYS. Significant negative correlations were found between L50 and the sea surface temperature (SST) daily rise rate in the NYBS and betweenδand the mean monthly SST in the SYS. These correlations suggest that declines in length and age at maturation primarily reflect changes in growth associated with overfishing and rising SST. Additionally, a fisheries-induced evolutionary response has contributed to changes in maturation schedules in both stocks. The principal pressures are the stress of continuously higher fishing intensity and an increasing proportion of yearling fish in the catches over time.The relative stock density of the NYBS were significantly increased since 2000 when compared with that in 1985, while the relative stock density in the SYS decreased from 1985 to 2010. The highest relative stock density was generally in autumn in both stocks due to the recruitment of yearling fish. However, the highest relative stock density in SYS was found in spring in some El Ni?os year, which might be caused by migration to the north of spawning population for the increase of SST in spawning ground in the northern East China Sea. Generally, the relative stock density and yield were both increased in those years and in 1-2 years after an El Ni?os event.The relative stock density of Pacific cod increased from 1.0 kg/h to 11.7 kg/h, and dominant age increased from 1 year old to 3 years old over 1999-2002 to 2007-2009. Moreover, proportions of females at sexual maturity (maturity degree at IV- VI) were increased from 21.9% to 67.3%. Body weight-body length relationships were W=4.564×10-3 L3.333 in 1999-2002 and W=1.550×10-2 L2.994 in 2007-2009, and no significant differences were found between male and female individuals (P>0.05). The changes of body weight-body length relationships indicated Pacific cod population was from positive-allometric growth to isometric growth. Stomach contents analysis showed that crustaceans were dominant diets of Pacific cod in both periods. The diversity of diet species in Pacific cod increased from 1.9 to 1.0 for the changes of its distribution area, and increased its niche width. The age structure of Pacific cod in the present study was simpler than that in the 1980s, and Pacific cod stock was sensitive to the climate changes and human activities.Anglerfish in the southern Yellow Sea were increasingly characterized by simpler age structure and smaller size from 1985 to 2009.. Body weight and body length were from negative-allometric growth to isometric growth. Male individuals were dominated in the population for the differences of growth speed between sexes. A few individuals reached to sexual maturity in autumn, which indicated the changes in spawning time of anglerfish. Food composition of anglerfish was closely connected with spatio-temporal heterogeneity of diet species, so anglerfish showed low selectivity for prey items and strong adaption to the changes in food-web. The niche width of anglerfish continuously decreased from 1985-1987 to 2009, which also indirectly improved community structure changes of fishery species in southern Yellow Sea. Since 2000, the relative stock density of anglerfish significantly increased, and its yield in year n was extremely positive correlation to SST in year n-2 (P<0.01), however, high SST negatively impacted its relative stock density and catch.
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