The effects of light, rearing density, and fish size in culturing juvenile Lumpfish

ObjectiveThere is an impetus for increasing Lumpfish Cyclopterus lumpus hatchery production and rearing efficiency due to the high demand for their usefulness as cleaner fish in salmonid ocean farming. One production bottleneck is the loss of juvenile fish due to aggressive behaviors, including cannibalism. Understanding which factors exacerbate Lumpfish aggression and providing mitigation guidelines to aquaculturists are warranted.MethodsTwo experiments were conducted for an 8‐week period in 2022 in 3‐L aquaria with different juvenile size‐classes (4 and 11 g) subjected to three rearing densities (40, 65, and 90 g/L) under different photoperiod regimes (12 h light : 12 h dark [113 lx]; 24 h light : 0 h dark [21 lx]; and 24 h light : 0 h dark [302 lx]), resulting in nine experimental treatments. Survival, growth, and aggression (occurrence and severity of caudal fin nipping) were evaluated.ResultFor 4‐g Lumpfish, light and density significantly affected growth (weight gain and specific growth rate), while an interaction between light and density significantly affected the occurrence of caudal fin nipping but not the degree of fin nipping or survival. None of the tested variables significantly affected 11‐g fish. Generally, a trend toward increased caudal fin nipping occurred in smaller fish (~45% occurrence in 4‐g fish vs. ~20% in 11‐g fish), indicating that cannibalism may be even greater when Lumpfish are smaller than 4 g but abates as the fish grow.ConclusionOur findings indicate that manipulation of lighting at rearing densities of up to 90 g/L can be used to control growth rates in small Lumpfish, often without resulting in increased fish aggression. For small (~4‐g) Lumpfish, constant low light intensity in combination with a low (40‐g/L) rearing density is recommended for reducing growth and caudal fin nipping. However, as Lumpfish grow, these variables are less effective at manipulating fish growth.