Introduction
Mud skippers collected from the wild
Mud skipper (Periophthalmus schlosseri) are widely distributed in coastal zones in the Mekong Delta. Recently, this species was paid more attention primarily as a source of food and for stock enhancement. However, during the rearing and stocking process, severe aggressive behavior and subsequent physiological stress and even high mortality are often observed, and identifying a way to efficiently reduce aggression is now of upmost urgency.
A research team from College of Aquaculture and Fisheries, Can Tho University led by Dr. Tran Nguyen Duy Khoa has worked on the physical enrichment affecting on aggressive behaviour of mud skipper.
The project aimed to evaluate the types and enriched level of substrate on aggressive behaviour and locomotor activity of mud skippers in stocking condition. This study focused to reduce aggressive behaviour and improve fish welfare, survival of mudskipper during the harvesting and stocking of mudskippers.
Background
Environmental enrichment has been proposed to have huge potential for reducing aggressive behaviour and physiological stress and improving the welfare of captive fish. Environmental enrichment can be categorized into several types, for example, physical enrichment, social enrichment, sensory enrichment, and occupational enrichment. Among these, physical enrichment is the most commonly used type. Physical enrichment can be simply defined as introducing objects (e.g., physical structures, plants, and substrates) into the housing environment with the aim of increasing environmental complexity. The obvious effect of introducing physical structures into rearing water is providing shelters for fish. For territorial fish species that require shelters as a part of their instinct, a reduction in aggression among conspecifics may be the primary and direct consequence of physical enrichment (PE). Fish aggressive behaviour is commonly evaluated by parameters related to body/fin condition, such as fin index, fin erosion, and skin scar, due to their ease of handling and simplicity. Numerous studies have demonstrated that PE decreases fin damage in various species. Severe aggression among conspecifics can result in cannibalism and reduced survival rates. PE has been found to effectively reduce fish cannibalism and increase fish survival in mudskippers.
It helps to improve fish welfare during the harvesting and stocking of mudskippers
Reduce aggressive behaviour and improve survival of mudskipper in capacity condition
Maintain mudskippers in good condition and high survival for further purposes
Methodology
Five types of physical structures including (a) PVC pipes, b) pyramidal physical structure c) Gravel substrate d) aquarium cave and e) plastic plant/net were compared to the control conditions (without substrate, mud or sand substrate). The second experiment evaluated the enriched level of physical structures (as no enrichment, 1-level enrichment, 2-level enrichment, 4-level enrichment, 6-level enrichment, and 8-level enrichment) and the stocking density of mud skipper was optimized in physical enriched condition in the last experiment.
Aggressive behaviour was quantified by counting the total amount of chasing, nipping, and biting among fish, and the identification of specific behaviours was based on our previous descriptions. Morphological indicators (including damaged fins, scales, bumps on the head, swelling on the mouth, or eye injury, dulled coloration, a light grey-white covering of mucus on the body of the fish, gill damage, and general weakness) were recorded. Besides, Cortisol hormone and fish performance (growth rate, Coefficient of body weight variation, Condition factor, and survival rate) were also measured.
Results
During the experiment, several injuries that mudskippers often encounter due to chasing and biting each other, leading to damage to fins, skin, and heads were observed.
Mudskippers adapted to captive condition
The artificial fish cave substrate gave the best results in reducing the aggressive behaviour of mudskippers, stress and stabilizing the weight and survival rate of fish. Besides, it may be optimal for fish basal stress level and development of an adaptive capacity to provide a medium-amount (approximately 50% floor space coverage) of mixed enrichment in stocking of mud skippers. The stocking density was suggested at 25 fishes/0.3m2 for mudskippers, but for longer stocking and better performance, it was suggested at 15 fishes/0.3m2.