Interannual variations in the biological characteristics, distribution and stock density of anglerfish Lophius litulon in the central and southern Yellow Sea
In the Yellow Sea and East China Sea regions, the anglerfish Lophius litulon has become an important fisheries species in recent years. In this paper, we examined interannual variations in the biological characteristics, stock distribution and stock density of this fish based on bottom-trawl survey data collected in the central and southern Yellow Sea during the autumn of 1985, 2000, 2005 and 2009. Results showed that the average body length of the female was larger than that of males in all sampling years, and the differences were significant in 1985 and 2005 (P<0.05). In addition, average body length in both sexes decreased markedly over the past two decades, with individual miniaturization and the dominant frequency distribution of body length being reduced from double peaks to a single peak,indicating clear simplification of the population structure. The relationship between body length and body weight showed negative allometric growth in 1985 and 2000, and the allometric growth parameter b was significant smaller than 3 (P<0.05) in these two years, while growth became isometric in 2005 and 2009. The sex ratios were significantly smaller than 1 (P<0.05) in the sampling years 1985, 2005 and 2009, and males were dominant in the samples, but was insignificant from 1 (P>0.05) in the 2000 sampling year. Moreover, sex ratios in both the <20 cm and 40-50 cm body-length groups were not significantly different from 1 in both 1985 and 2000 (P>0.05),but male dominated in the <20 cm body-length group (P<0.05) and all individuals were female in the 4050 cm body-length group in 2005 and 2009. Gonad development was mainly stage II in all sampling years,while a very small proportion of sexually mature individuals were found in 2005 and 2009. Bottom water temperature within the high density distribution area ranged from 9.3 to 10.8 ℃. , with salinity in the range 32.433.2. The relative biomass and percentage of anglerfish in the total catch showed a clear increasing trend from 2.94 kg/h, 3.0% (1985) and 1.33 kg/h, 1.2% (2000) to 5.86 kg/h, 10.5% (2005) and 5.62 kg/h, 9.8% (2009). The stock distribution area shifted towards the northern Yellow Sea over the past two decades. An effective index was the changes in stock density and occurrence frequency in the Yellow Sea above 36° latitude north:the relative stock density and occurrence frequency increased from 0 in 1985 to (1.01.4) kg/h and 57.9% in 2000,and then to (6.99.8) kg/h and 83.3% in 2009 in the same sea area. Regression analysis showed that the annual catch of anglerfish (Y_n) was highly and significantly,positively correlated with sea surface temperature (SST) of the 2 years previously (r= 0.61, P = 0.004) , which may be due to the influence of SST on the early life history of this species. However, the yields in 19972001 and the corresponding SST in 19951999 were excluded during the regression process, as the especial high SST in 19951999 may have interfered with anglerfish stocks and resulted in abnormally low catches of anglerfish during 19972001. Accordingly, the changes in biological characteristics, stock distribution and relative stock density of anglerfish may be related to pressures originating from increased fishing intensity and climate change.