*4.2. Effects of Aquaculture Mode on Survival and Growth Performance of Large Yellow Croaker*

Promoting the healthy growth of aquaculture animals is one of the most important objectives of aquaculture practitioners. Generally, it could be comprehensively evaluated through body weight indexes, body length indexes, key organ indexes, feed efficiency, etc. [29]. In this study, it was found that the large yellow croaker in the ship group gained more weight and died less during the test period. The survival and growth indexes were significantly better than those in the nearshore cage group at the end of the experiment. Similarly, Nicole et al. showed that compared with the traditional nearshore culture areas, the mortality rate of bluefin tuna in offshore culture areas was lower, and the prevalence of parasites was reduced by 30% [9]. Pang et al. showed that large, mechanized platforms offshore were more suitable for medium- and large-size abalone farming, and their growth rate was significantly faster than that of nearshore cages [30].

Previous studies have shown that the difference in water quality parameters that are caused by different farming modes was an important reason affecting the survival and growth of fish [31,32]. In this test, the water quality parameters of the ship and cage were the same except for temperature and dissolved oxygen. When large yellow croaker are exposed to high temperature for a long time, it causes an oxidative stress reaction, adversely affecting digestion, metabolism, and the immune system, and reduce its resistance to pathogens [33]. This study showed that the area of the cage was affected by high-temperature in summer, resulting in rising water temperature, which was higher than the optimal 22–26 ◦C water temperature for large yellow croaker for a long time [34]. However, the ship can keep the fish at optimum temperatures most of the time by moving. Therefore, in this study, high temperature may be an important reason for the high mortality and slow growth rate of the large yellow croaker in the cage group. In addition, studies have shown that very low dissolved oxygen (<2 mg/L) will cause large area death of large yellow crocks in a short time, and long-term low levels of dissolved oxygen will restrict aerobic metabolism, resulting in the slow growth of fish [35]. In this study, it was found that the water dissolved oxygen level was at a low level for a long time due to the high temperature around the cage. At the same time, the ship culture system could maintain the dissolved oxygen level above 8 mg/L. Therefore, low dissolved oxygen may also affect the survival and growth of cage large yellow croaker.
