Scientific Treatment Protocol for Biofouling on Larval Rearing Net Curtains of LaiZhou Scallops

I. Industry Background and Problem Overview  
As the nation’s largest scallop hatchery base, Laizhou supplies over 70% of the country’s scallop larvae and serves as a cornerstone of the local aquaculture industry.
During the critical seedling‑raising period in mid‑April each year, as water temperatures gradually rise, a common problem troubling aquaculturists begins to occur frequently: a viscous, “mucus‑like” fouling material appears on the surfaces of netting screens. In Laizhou’s scallop nursery ponds, eutrophication is a recurring issue from April to May. If this problem is not addressed promptly, it can directly lead to oxygen depletion and impaired feeding in juvenile scallops; in severe cases, it may trigger mass mortality, inflicting substantial economic losses on farmers.

Figure 1: A scallop hatchery net curtain showing “mucus-like” fouling, with the mesh clogged by a viscous substance.

II. Analysis of the Causes and Consequences of the Problem  
2.1 Mechanism of Fouling Formation
The essence of this issue is eutrophication of the water body under high-density nursery‑rearing conditions:
As water temperatures gradually rise to 15–18°C in April, feed inputs into the nursery ponds increase, leading to a substantial accumulation of organic matter such as leftover feed, scallop feces, and dead plankton. Upon decomposition, these organic materials stimulate the rapid proliferation of pathogenic bacteria like Vibrio spp. and algae such as Noctiluca scintillans. These microorganisms secrete large amounts of extracellular polymeric substances (EPS); when these viscous compounds mix with organic matter, they form a thick, mucus‑like substance—visible to the naked eye—that adheres to the mesh screens.

Figure 2: Microorganisms and extracellular polymeric substances observed under the microscope in fouling deposits.

2.2 Major Hazards
1. Impeding water exchange: Viscous substances can clog the mesh openings of the netting, hindering water flow between the inside and outside of the cage. As a result, external feed cannot enter the cage, and juvenile shellfish are unable to obtain an adequate food supply.
2. Impact on juvenile scallop physiological functions: Fouling organisms can clog the gills of juvenile scallops, leading to respiratory distress and triggering hypoxic stress; in severe cases, this can result in asphyxiation and death.
3. Deterioration of the aquatic environment: The decomposition of large amounts of organic matter depletes dissolved oxygen in the water and generates harmful substances such as ammonia nitrogen and nitrite, which further promote the proliferation of pathogenic microorganisms and increase the risk of outbreaks of scallop diseases.

Figure 3: A close-up of the soiled mesh curtain, showing viscous material adhering to the mesh filaments.

III. Treatment Principle  
3.1 Photosynthetic Bacteria: Source-Specific Degradation of Organic Matter
Photosynthetic bacteria are a group of microorganisms capable of growing under both aerobic and anaerobic conditions. They can directly utilize small-molecule organic compounds in the water as both carbon and energy sources, rapidly decomposing organic matter such as leftover feed and feces, thereby reducing at the source the precursors for extracellular polymeric substance formation. At the same time, they effectively lower levels of harmful substances in the water, including ammonia nitrogen and COD.

 
Figure 4: Ultra-concentrated photosynthetic bacterial formulation used in the scheme.

3.2 Vibrio: Precise Elimination of Pathogenic Bacteria
Vibrio‑phage bacteria are bacteriophages that specifically parasitize bacterial cells. They can selectively lyse pathogenic aquatic bacteria such as Vibrio and Aeromonas, while remaining harmless to aquaculture species like scallops. These bacteria rapidly eliminate harmful microorganisms from water bodies, reduce extracellular polymeric substances secreted by pathogens, and thereby inhibit the transmission and outbreak of diseases.

Figure 5: The Arcobacter (Arcobacter spp.) formulation used in the study.

IV. Prevention and Control Plan  
4.1 Prevention Plan: Proactive Adjustment to Prevent Fouling
After water temperatures rise in April, implement routine water-quality management ahead of schedule to prevent fouling at the source.
• Dosage: For every 10 net cages, use 1 bottle of Hulijing (Vibrio alginolyticus preparation) plus 1 bottle of ultra-concentrated photosynthetic bacteria, and evenly apply the mixture throughout the entire pond.
• Function: It breaks down accumulated organic matter in the water ahead of time, inhibits the proliferation of pathogenic bacteria, maintains the aquatic micro-ecological balance, and prevents the formation of “mucus‑like” fouling.
4.2 Treatment Plan: Emergency Management and Rapid Recovery
When the biofilter media becomes fouled and the water turns slimy, increase the dosage to rapidly degrade pollutants and eliminate pathogenic microorganisms.
• Dosage: For every 5 net cages, use 1 bottle of Hulijing plus 1 bottle of ultra-concentrated photosynthetic bacteria, applying continuously for 2–3 days, with two applications per day.
• Principle: Once biofouling has already formed, the concentrations of organic matter and pathogenic microorganisms in the water are relatively high, necessitating a higher dosage of microbial agents to achieve rapid efficacy—quickly breaking down the fouling and restoring the aquatic environment.
4.3 Precautions
• When applying the microbial agent, avoid the high-temperature period around noon and instead carry out the application during the cooler morning and evening hours to enhance the agent’s activity.
• After application, monitor the dissolved oxygen levels in the water; if necessary, activate aeration equipment to prevent hypoxia.
• Avoid using disinfectants within 3 days after application to prevent the killing of beneficial microorganisms and ensure optimal treatment efficacy.

V. Application Results and Production Feedback

Figure 6: A scallop nursery net curtain with “mucus‑like” fouling, whose mesh has now regained its permeability.

This article was contributed by Gao Jinghan and Leng Dunpeng.
Editorial Review: Zhang Chuanbao, Lü Zhaoqing, Zang Xueyun