Flow tolerance of juvenile Lateolabrax maculatus

Sea cage aquaculture is an important industry for China to obtain marine fish. With the compression of offshore cage culture space and the requirements of ecological and environmental protection, the development of deep-sea cage culture is an inevitable trend to promote the high-quality development of mariculture industry. Flow velocity has an important impact on fish behavior, physiology and growth. Studying the swimming performance of fish is of great significance for the optimal stocking specifications of deep-sea aquaculture species, improving the design and construction of aquaculture facilities, and promoting the high-quality and orderly development of deep-sea aquaculture. Lateolabrax maculatus is an important aquaculture species in offshore and deep-sea cages in northern China. However, due to its own swimming ability research and the influence of flow on its swimming behavior is not clear, it is impossible to provide scientific basis for the site selection, breeding equipment design and stocking specifications of deep-sea cages for the fish. To evaluate the flow tolerance of juvenile Lateolabrax maculatus, this study investigated critical swimming speed (U_crit), burst swimming speed (U_burst), endurance swimming time, and their correlations with 14 morphological traits using a fish behavioral experimental circulating flume with specimens of (9.61±0.74) cm body length. Through ethological research methods, we analyzed tail beat frequency, swimming states, and swimming trajectories under varying velocities. Results demonstrated that absolute U_crit and U_burst were (0.69±0.05) m/s and (0.86±0.05) m/s respectively, while relative values reached (7.12±0.4) BL/s and (9.09±0.48) BL/s (BL represents the body length of experimental fish, BL/s indicates the swimming speed in body lengths per second). The endurance swimming time measured (200.83±45.73) min at 0.6 m/s, but showed dramatic decline at 0.7 m/s. Significant positive correlations (P<0.05) were identified between swimming performance (absolute U_crit and U_burst) and most morphological traits, except for first dorsal fin length, second dorsal fin length, ventral fin length, and caudal fin length (P>0.05). Tail beat frequency exhibited significant positive correlation with velocity (P<0.05). When the flow velocity was ≥ 4 BL/s, the relative swimming distance of Lateolabrax maculatus decreased significantly, the proportion of countercurrent forward swimming time decreased significantly, the proportion of countercurrent static swimming time increased, and the proportion of downstream backward time decreased with the increase of flow velocity. The swimming area of the experimental fish was reduced, and it could only move in the corner of the flume at high flow velocity (6 BL/s). Based on swimming performance analysis, we recommend considering partial morphological traits (tail length, head length, and trunk length) alongside body length for stock selection. For juvenile L. maculatus aquaculture facilities, maximum flow velocity should not exceed 6.24 BL/s, while behavioral considerations suggest maintaining velocities below 4 BL/s. The results of this study can provide a reference for the site selection of sea cage aquaculture, the design and construction of breeding facilities and the determination of stocking specifications of L.maculatus farming.