Surface sediment samples were collected from 19 sites throughout Daya Bay, China to study the concentrations, spatial distributions, potential ecological risk, and possible sources of heavy, including metals copper (Cu), zinc (Zn), arsenic (As), cadmium (Cd), nickel (Ni), lead (Pb), mercury (Hg), and chromium (Cr). The mean concentrations of the eight heavy metals were 24, 109, 6.5, 0.09, 35.3, 26.8, 0.07, and 109 µg g⁻¹, respectively. The concentrations of most heavy metals were within range of those recorded in previous years. The spatial distribution pattern of most heavy metals were similar, with lowest values recorded along the southeast coast and the open sea area; the highest values were recorded in the northern Daya Bay, especially the northwest. Cu, Zn, As, Cd, Pb, and Hg were classified as Class I, and Ni and Cr were classified as Class II according to the Sediment Quality Guidelines (SQGs) of China. The potential ecological risk (Eⁱ_f) indices of Cu, Zn, As, Pb, Ni, and Cr specify that these metals pose low risk to the ecosystem of the Bay, whereas Cd and Hg pose a very high risk in some sites. The geoaccumulation indices (I_geo) of Cu, Zn, As, Ni, and Cr specify weak or no pollution in Daya Bay, whereas those of Pb, Cd, and Hg in some sites indicate moderate or even high pollution. Spatial distribution, carbon/nitrogen analysis, Pearson correlation, and principal components analysis indicated that Cu, Zn, As, Pb, Ni, Cr, total organic carbon (TOC), and total nitrogen (TN) originated from the same sources. Ballast water or sewage from the cargo ships that park at the harbors or anchor in the Bay were the important sources for Cu, Zn, As, Pb, Ni, Cr, TOC, and TN. Other anthropogenic sources, such as agricultural runoff and aquaculture, might also be responsible, whereas Hg and Cd originated from other point sources.

The effects of diets fermented with compound probiotics, namely Lactobacillus acidophilus, Limosilactobacillus reuteri and Lactiplantibacillus plantarum, on the growth performance, physiological and biochemical indexes, fatty acid composition and intestinal health of juvenile largemouth bass (Micropterus salmoides) were investigated. Three hundred healthy juvenile M. salmoides (5.29 ± 0.02 g) were selected and randomly divided into two groups with triplicates for each. The basic diet was set as the control group (CON), and fermentation of the basic diet with a mixed bacterial solution (1.8 × 10⁹ cfu/mL, L. acidophilus:L. reuteri:L. plantarum = 1:1:1) was set as the fermentation group (FER). Fish were hand fed to satiation for 56 days and two-thirds of the culture water was renewed every 3 days. The results showed that feed intake of fish in the FER group was significantly lowered, thereby increasing feed efficiency (FE) and protein efficiency (PER) (p < 0.05). Serum alanine aminotransferase (ALT) activity was significantly decreased, and catalase (CAT) activity was significantly increased in the FER group (p < 0.05). The liver superoxide dismutase activity (SOD) was significantly enhanced, and intestinal trypsin was significantly increased in the FER group (p < 0.05). Being fed with the fermentation diet significantly increased the content of n-3 polyunsaturated fatty acids (PUFA), docosahexaenoic acid (DHA) and the n-3/n-6 PUFAs ratios in the liver (p < 0.05). Intestinal histology showed that villus height and width of the intestine and the number of goblet cells were significantly increased in the FER group (p < 0.05). Those fed with fermentation diets had limited diversity of gut microbiota. Compared to the CON group, the relative abundance of Aeromonas decreased significantly (p < 0.05), while the relative abundance of Fusobacteria, Cetobacteria and Lactobacillusis in FER increased greatly in the gut microbiota of the FER group. In conclusion, fermented feed with the three probiotics effectively improved the feed utilization and antioxidant capacity, promoted digestion and absorption of dietary protein, improved the ability of synthesize DHA and n-3 PUFAs in the liver and reduced the abundance of pathogenic bacteria in the gut. Therefore, the present research provided a new way of co-fermented feed with three probiotics for the aquaculture of M. salmoides.

Indoor and outdoor experiments were carried out simultaneously to evaluate the effects of dietary phosphorus levels on growth performance and body composition of crucian carp. Monocalcium was used to formulate extruded feed containing available phosphorus (AP) of 1.5 (D1), 4.7 (D2), 6.3 (D3) and 9.0 (D4) g kg⁻¹. Twelve 500-L recirculating tanks were assigned to four triplicate groups with each tank stocked with 15 juveniles (39.88 ± 0.14 g) for indoor experiment. Twelve polyethylene enclosures were assigned to four groups with each one stocked with 120 juveniles (44.08 ± 1.36 g). After 8 weeks of feeding, survival ranged from 96% to 100%. Weight gain, special growth rate, protein efficiency ratio and feed conversion rate did not show significant differences among dietary treatments in both experiments. Fish cultured in the enclosures grew better, although D1 and D2 treatments did not reach significant level. Crude protein and phosphorus content of the fish were not significantly affected by dietary phosphorus level. Moisture, crude lipid, crude ash and vertebrae phosphorus were significantly affected by different phosphorus levels. Based on vertebrae phosphorus, crucian carp cultured in the tanks and in the enclosures showed phosphorus deficiency under 4.7 g kg⁻¹ and 6.3 g kg⁻¹ AP, respectively.