*3.4. State of Behavior*

Through daily observations, the behaviors of the large yellow croaker during swimming and feeding on the ship during the experiment were reported (Figure 5). Large yellow croaker can adapt to the ship environment, actively feed, and exhibit no irregular swimming behavior. Large yellow croaker were usually distributed in the 3–4 m water layer and swam in the reverse current. When feeding, all the large yellow croaker moved upstream to the surface of the water, in a relatively disorderly swimming.

#### *3.5. Survival Rate and Growth Performance*

Through almost two months of on-site culturing, the change in the survival of the large yellow croaker was well documented (Figure 6). The result showed that no largescale death events and serious disease conditions of the fish occurred in the two groups. However, the mortality rate of large yellow croaker in the cage group was relatively high, with the highest single-day mortality rate of 0.23%, which was higher than the highest single-day mortality rate of 0.05% in the ship group.

**Figure 5.** Swimming behavior of the large yellow croaker in the tank of the ship. (**A**,**B**) represent pictures and schematic images of the large yellow croaker colonies swimming against the current. (**C**,**D**) represent pictures and schematic diagrams of large yellow croakers swimming loosely while feeding, respectively.

**Figure 6.** Daily mortality in ranched large yellow croaker in the nearshore cages (denoted in orange) and offshore ship (denoted in blue).

During the culture period, it was found that large yellow croakers in three tanks on the ship grew and developed well through the measured changes in body weight, body length, CF, and VSI (Figure 7). The body weight of large yellow croakers increased from 325.67 g to 460.71 g, with an average monthly weight gain of 67.52 g (Figure 7A). The body length of the large yellow croaker increased rapidly from 25.23 cm to 29.14 cm (Figure 7B). At the beginning of the experiment, the CF and VSI of the large yellow croaker were the highest. With the increase in culturing time, the VSI and CF decreased rapidly in the first 4 weeks and remained stable in the last few weeks (Figure 7C,D).

**Figure 7.** Body weight (**A**), body length (**B**), condition factor (**C**), and visceral index (**D**) in ranched large yellow croaker in the different tanks in an offshore aquaculture ship throughout the experiment period (n = 30); T1, T2, and T3 are represented as the three different tank.

At the end of the experiment, the survival rate (SR), weight gain rate (WGR), feed factor (FCR), condition factor (CF), and visceral index (VSI) of the large yellow croaker were significantly different under different aquaculture modes (Table 1). The SR, WGR, and CF of large yellow croakers in the ship group were significantly higher than in the cage group (*p* < 0.05). The FCR and VSI of the ship group were significantly lower than those of the cage group (*p* < 0.05). During 8 weeks of culturing, the density of the ship group increased rapidly from 12.1 kg/m3 to 15.74 kg/m3, while that of the cage group decreased to only 5.02 kg/m3.

**Table 1.** Survival rate (SR, %), weight gain rate (WGR, %), feed conversion ratio (FCR, %), condition factor (CF, g/cm3), visceral index(VSI, %), and stocking density (SD, kg/m3) of the large yellow croaker that were reared under the different farming environments.


Note: Values represent the mean ± SD. Significant differences (*p* < 0.05, n = 30) among treatments were indicated by different letters.

#### *3.6. Muscle Composition Analysis*

The nutrient composition of the muscle of the large yellow croaker was significantly affected by the culture mode (Table 2). The contents of crude protein and ash in the ship group were significantly higher than those in the cage group (*p* < 0.05), and the ether extract and moisture contents were significantly lower than those of the cage group (*p* < 0.05). In conclusion, the muscle nutrient composition of the large yellow croaker in the ship group was better than in the cage group.

**Table 2.** Nutrient composition in the muscle tissues of the large yellow croaker that were reared under the different experimental groups.


Note: Values represent mean ± SD. Significant differences (*p* < 0.05, n = 10) among treatments were indicated by different letters.

#### **4. Discussion**

#### *4.1. Comparison of Nearshore and Offshore Aquaculture*

In China, mariculture is mainly concentrated nearshore and provides an important economic source for local fishermen [24]. However, with the general deterioration of the nearshore environment, the Chinese government has begun to reduce the area that is used for nearshore aquaculture [19]. On the other hand, there are nearly 70,000 km<sup>2</sup> of almost undeveloped offshore waters in China. The lack of shelter, such as islands in the offshore area, means more severe sea conditions, such as stronger winds, higher waves, and faster currents. This poses a greater challenge to the aquaculture system [25]. This study shows that the ship-platform can resist the more severe offshore sea conditions during the experiment without adverse effects on aquaculture and could ensure offshore aquaculture. In addition, the ship can avoid events such as typhoons, red tides, and other special disaster situations, through the way of autonomous navigation to escape. This also reduces the operational risk for producers. Open cage culture was vulnerable to changes in aquaculture environmental conditions that were caused by climate change, especially water temperature and dissolved oxygen, which have a significant impact on the production of cultured fish [26,27]. For example, due to the influence of high and low temperatures, the optimal growth time of large yellow croaker was only about half a year. In addition, the ship also realized the rapid growth of the large yellow croaker in the summer hightemperature period by transferring the culture in the sea area between Fujian, Zhejiang, and Shandong, which verified the feasibility of shortening the culture cycle. In conclusion, by taking advantage of the stability of the ship platform, the aquaculture ship can expand the space of offshore and move independently to avoid disaster weather and obtain an appropriate water temperature, thus increasing the stability of production.

Traditional nearshore cages use little mechanical equipment, the labor intensity of fishermen was large, and the risk of the marine operation was also high [28]. In contrast, although the cost of large machinery equipment was higher, it can reduce the unit cost of culture through large-scale operation and reduce the number of staff and work intensity. For example, the aquaculture ship uses mechanized feeding and sucking fish instead of manual operation, greatly reducing the use of labor. It was estimated that the production of large yellow croaker could reach 3000 tons per year by the 100,000-ton aquaculture ship, and the culture staff only needs 9–10 people.
