Effects of Stocking Density on the Survival, Growth, and Stress Levels of the Juvenile Lined Seahorse (Hippocampus erectus) in Recirculating Aquaculture Systems

Abstract

Seahorses are increasingly regarded as a promising farming object suitable for recirculating aquaculture systems (RASs) due to their high economic value. However, reports on the large-scale farming of seahorses in RASs are rare, and some key parameters, such as stocking densities, are still unclear. In the present study, we employed the lined seahorse (Hippocampus erectus), for which large-scale farming has been achieved, to determine the suitable stocking density for three different-sized juveniles in RASs. The three different-sized juveniles had body heights of 4.0, 7.0, and 9.0 cm, and their test density gradients were 1.0, 0.8, 0.6, and 0.4 inds/L; 0.6, 0.5, 0.4, and 0.3 inds/L; and 0.4, 0.3, 0.2, and 0.1 inds/L, respectively. The juveniles were cultivated for one month, and then their survival, growth, and plasma cortisol and brain serotonin contents (two stress-related indicators) were analyzed. The results show that, regardless of the size of the juveniles, a high density can inhibit growth and trigger stress responses. In addition, for small- (4.0 cm) and medium-sized (7.0 cm) juveniles, a high density can also exacerbate size heterogeneity and cause death. Taking into account the welfare and yield of farmed seahorses, the present study suggests that the suitable stocking densities for 4.0, 7.0, and 9.0 cm juveniles in RASs are 0.6, 0.4, and 0.2 inds/L, respectively.

Keywords: recirculating aquaculture system; seahorse Hippocampus erectus; size heterogeneity; stocking density.

Figure 1

Frame of the recirculating aquaculture system used in the present study.

Figure 2

Panoramic view of the experimental design with test stocking densities.

Figure 3

The final body height and wet weight of the juveniles with an initial body height of 4.0 cm (real body height: 4.02 ± 0.14 cm; real wet weight: 0.210 ± 0.031 g) after one month of rearing. Each solid blue dot represents a seahorse juvenile. The red and green circles represent populations of different body sizes.

Figure 4

The plasma cortisol (A) and brain 5-HT (B) contents in the juveniles with an initial body height of 4.0 cm (real body height: 4.02 ± 0.14 cm; real wet weight: 0.210 ± 0.031 g) after one month of rearing. The lowercase letters (a, b, and c) on each density bar chart indicate the results of multiple comparisons, and the absence of shared letters between two densities indicates significant differences between them.

Figure 5

The final body height and wet weight of the juveniles with an initial body height of 7.0 cm (real body height: 6.98 ± 0.14 cm; real wet weight: 1.153 ± 0.099 g) after one month of rearing. Each solid blue dot represents a seahorse juvenile. The red and green circles represent populations of different body sizes.

Figure 6

The plasma cortisol (A) and brain 5-HT (B) contents in the juveniles with an initial body height of 7.0 cm (real body height: 4.02 ± 0.14 cm; real wet weight: 0.210 ± 0.031 g) after one month of rearing. The lowercase letters (a and b) on each density bar chart indicate the results of multiple comparisons, and the absence of shared letters between two densities indicates significant differences between them.

Figure 7

The final body height and wet weight of the juveniles with an initial body height of 9.0 cm (real body height: 8.92 ± 0.14 cm; real wet weight: 3.030 ± 0.175 g) after one month of rearing. Each solid blue dot represents a seahorse juvenile. The green circle represents a population of one body size.

Figure 8

The plasma cortisol (A) and brain 5-HT (B) contents in the juveniles with an initial body height of 9.0 cm (real body height: 8.92 ± 0.14 cm; real wet weight: 3.030 ± 0.175 g) after one month of rearing. The lowercase letters (a and b) on each density bar chart indicate the results of multiple comparisons, and the absence of shared letters between two densities indicates significant differences between them.