Stocking density effects on aggressive and cannibalistic behaviors in larval hatchery-reared spotted seatrout, Cynoscion nebulosus

Cannibalism and aggression are major sources of mortality in the larviculture of the spotted seatrout, Cynoscion nebulosus. High stocking density can either increase cannibalism by increasing the likelihood of encounters between aggressors and prey or decrease cannibalism by interfering with normal territorial or aggressive behaviors. The goal of this study was to assess the effect of stocking density on cannibalism and aggressive behaviors in hatchery-reared spotted seatrout. Seven-day-old larvae were stocked randomly into three replicates of three different densities (15 (233 total fish), 30 (465 total fish), and 60 (930 total fish) L− 1) in 15.5-L aquaria. Feeding was conducted every 8 h based on residual rotifer or Artemia counts. Growth was determined 6 days poststocking at the conclusion of the experiment. To quantify cannibalism and aggressive behaviors, three cameras filmed three tanks for 9 h each day. Recorded behaviors were quantified for three selected 30-minute segments per tank per day: 1 h, 4 h, and 7 h post-feeding. Aggressive acts were scored as: nip (aggressor strikes prey causing prey to dart), chase (aggressor moves more than one body length toward prey), and capture (predator captures and holds prey but unable to consume). In all stocking densities there was a significant increase in aggression and cannibalism with time since feeding. Growth was significantly higher in the lower density treatment. An observed density threshold existed at a stocking density of 30 fish L− 1, beyond which the intensity of aggressive behaviors did not increase. Based on the results of this study, aggression in early stage hatchery-reared spotted seatrout might be alleviated with increased feeding frequency. Further, spotted seatrout could possibly be cultured at densities higher than the current protocol allows.