Enhancing growth and disease resistance in offshore seabream farming through rabbitfish integration in polyculture systems

Authors : Nabil Mansour, Ahmed G. Ismail , Sabhah Alhmoudi and Fouad Lamghari

Abstract Introduction: White-spotted rabbitfish (Siganus canaliculatus), an herbivorous species that grazes on algae and seaweed, contributes significantly to ecosystem balance by controlling algal blooms and enhancing water quality. This ecological function makes it a valuable companion species in polyculture with carnivorous gilthead seabream (Sparus aurata). Accordingly, the present study investigated the integration of rabbitfish into offshore seabream farming systems under tropical aquaculture conditions, with an emphasis on growth performance, biofouling mitigation, and resilience against Enteromyxum leei infection. Methods: Experiments were conducted over two full production cycles in open-sea cages in the United Arab Emirates. Six cages were assigned to seabream monoculture and six to polyculture with rabbitfish. Key performance indicators including growth rate (GR), specific growth rate (SGR), feed conversion ratio (FCR), mortality, biofouling biomass, annelid abundance, seabream biting behavior, and E. leei infection prevalence and severity were assessed using area under the curve (AUC) analysis. Results: Polyculture significantly improved seabream performance compared to monoculture. Mortality was lower (4.7% vs. 8.4%), FCR and feeding rate were reduced, and both GR and SGR were higher in polyculture. Net fouling biomass (220.3 g/m2 vs. 260.2 g/m2 ), annelid abundance (74.4 vs. 90.8 individuals/m3 ), and biting frequency (34.1 vs. 42.6 bites/100 fish/h) were also reduced in polyculture. E. leei prevalence and severity peaked at 29.4% and degree 2.5 in polyculture compared to 49.8% and degree 4.0 in monoculture. Infections were undetected in both systems from July to October, aligning with seawater temperatures exceeding 31.8°C. Net replacement intervals were nearly doubled in polyculture (344 vs. 196 days). Conclusions: Integrating rabbitfish into gilthead seabream cages can improve growth efficiency, reduce parasite load, minimize net fouling, and enhance overall system sustainability, making it a promising strategy for tropical offshore aquaculture Keywords: polyculture, gilthead seabream, rabbitfish, enteromyxosis, biofouling control, offshore aquaculture

Overview

This pioneering study investigates the integration of white-spotted rabbitfish (Siganus canaliculatus) into offshore gilthead seabream (Sparus aurata) farming systems under tropical aquaculture conditions in the United Arab Emirates. The research assesses how polyculture influences fish growth, feed efficiency, net biofouling, and disease resistance against Enteromyxum leei—a major parasitic threat in seabream aquaculture.

Methodology

Experiments were conducted over two full production cycles (2023–2024) at Omega Fish Farm, Dadna, Fujairah, using twelve open-sea cages (six monoculture and six polyculture).

• Each cage held 100,000 seabream juveniles (2.6 g avg. weight).

• 3,000 rabbitfish fingerlings (5 g avg.) were introduced into each polyculture cage.

• Growth rate (GR), specific growth rate (SGR), feed conversion ratio (FCR), mortality, net fouling biomass, annelid abundance, and infection severity were evaluated using area under the curve (AUC) analysis.

Key Findings

Polyculture with rabbitfish significantly enhanced seabream performance compared to monoculture systems:

Parameter	Polyculture	Monoculture
Mortality Rate	4.7%	8.4%
Feed Conversion Ratio (FCR)	338.7	490.3
Growth Rate (GR)	31,183	15,084
Specific Growth Rate (SGR)	781.7	384.0
Net Fouling Biomass	220 g/m²	260 g/m²
E. leei Infection (Peak)	29.4%	49.8%
Net Replacement Interval	344 days	196 days

Rabbitfish grazing reduced algal and invertebrate fouling, lowered annelid density (potential E. leei hosts), and minimized seabream net-biting behavior. As a result, cages remained cleaner, fish welfare improved, and maintenance intervals nearly doubled.

Ecological and Health Insights

• Temperature Dependency: E. leei infections disappeared when seawater temperatures exceeded 31.8°C, indicating an upper thermal threshold for disease transmission.

• Behavioral Benefits: The presence of rabbitfish reduced seabream stress and aggression, promoting better feed utilization and health.

• Biofouling Control: Rabbitfish feeding naturally limited macroalgae and annelid proliferation—key factors in disease spread.

Conclusions

Integrating rabbitfish into seabream offshore cages is a sustainable and practical innovation that:

• Boosts growth performance and feed efficiency

• Reduces disease prevalence and biofouling

• Extends net service life and operational efficiency

• Enhances environmental sustainability in tropical marine aquaculture

Acknowledgments

The authors thank H.H. Sheikh Mohammed bin Hamad Al-Sharqi, Crown Prince of Fujairah, for his continuous support of scientific research and sustainable aquaculture development.