Effect of Two Shark-Repelling Methods on the Seawater Quality and Acute Toxicity of Seawater Fish

In this study, two kinds of shark repellents, chemical shark repellent and electric pulse shark-repelling device were selected to test their effects on seawater quality as well as the physiology and survival of fish using simulated ecology experiments. Three marine fish species, Japanese flounder (Paralichthys olivaceus), black rockfish (Sebastes schlegelii), and spotted grouper (Oplegnathus punctatus) were selected as test subjects. The results of water quality indicators showed that the chemical shark repellent could significantly reduce the water transparency and pH (P < 0.05), while the electric pulse shark-repelling device had no significant effect. The mortality rate of the fishes under the chemical shark repellent was 100% within 15 min, while it was 0% under both the electric pulse shark-repelling device and the blank control. While investigating the effect of the two repellent methods on the survival and physical activities, the gills and fins of juvenile fish in the chemical shark repellent group were rapidly dyed blue reaching a mortality rate of 100% within 15 min, while the juvenile fish in the electric pulse shark-repelling device group resumed their natural activities rapidly except for exhibiting avoidance behavior when the device was started. The blood indices of the species did not show any significant differences in the chemical shark repellent group because of the acute mortality caused by poisoning. However, in the electric pulse shark-repelling device group, the number of erythrocytes and leukocytes significantly increased in all three fishes. In terms of blood biochemical indices, the chemical shark-repellents caused a significant increase in chloride content, blood glucose, and alkaline phosphatase (AKP) enzyme activity in the juvenile fishes, while the blood albumin content reduced significantly in all three fishes. The use of electric pulse shark-repelling device caused a significant increase in blood glucose and glutamate transaminase activity only in the Japanese flounder. Histopathological results showed that chemical shark repellents caused tissue damage to the gill, liver, spleen, and heart in juvenile fishes. It can be seen that compared to chemical shark repellents, the electric pulse shark-repelling device is a more environmentally friendly way to repel sharks as it has less impact on seawater quality as well as less toxic effects on major marine fishes. These results would provide a scientific basis for the selection of shark repellent methods in ocean exploration activities.