Acclimation Cage Conditioning for Flatfish Stock Enhancement: Examining Feeding-Related Behaviors of Two Flounder Species Reared on Different Hatchery Feeds

Spawning and rearing flatfish in captivity and releasing the young (i.e. stock enhancement) may help augment natural populations and stabilize fisheries landings. Mostly attributed to the unnatural characteristics of the hatchery environment, reared flatfish often exhibit irregular behavioral patterns compared with wild conspecifics, and these "deficits" are assumed to translate to increased predation risk. Initially releasing fish in predator-free acclimation cages can help flatfish adjust to the wild environment, establish burial skills, begin pigment change, recover from transport stress and experience natural (live) food sources before full release into the wild. The impact of this short-term caging release strategy has yet to be fully evaluated for flatfish, nor has its effectiveness been gauged in regards to species or size of fish at release. In addition, we know little about the potential benefit when combined with hatchery rearing strategies, particularly hatchery feed.

Japanese flounder (Paralichthys olivaceus) and marbled flounder (Pseudopleuronectes yokohamae) are two currently cultured and stocked species in Japan that exhibit very different natural feeding strategies. In the hatchery, after the traditional rotifer/Artemia larval feeding regime, Japanese flounder (fast growing, large-mouthed, cannibalistic benthi/piscivores) are reared onto pellets, while marbled flounder (slow growing, small-mouthed, non-cannibalistic benthivores) are weaned onto chopped frozen krill. We examined the influence of cage conditioning on burying, feeding (on live mysids), and predator/threat avoidance (using a model predator) behavior of these two species to compare the benefits of the cage conditioning experience. Each set of trials for each size class (30 mm, 60 mm) of each species consisted of 7 days in an acclimation cage along the shallow coast followed by 3 days of continuous laboratory video. While in the acclimation cages, fish were fed their respective hatchery feed once per day.

Although both Japanese and marbled flounder were feeding in the acclimation cages, Japanese flounder mostly consumed the provided formulated pellets while marbled flounder ate a combination of provided frozen krill and wild prey.  At the end of the 3-day video trials, stomach content indexes revealed no significant differences in mysid consumption for any cage-conditioned and nonconditioned flounder except for 30 mm Japanese flounder, in which conditioned fish consumed more than nonconditioned fish (p < 0.05).

Active stocking efforts are currently being established in the US for southern flounder (Paralichthys lethostigma) and winter flounder (Pseudopleuronectes americanus). Any direct technology transfer of well-established, successful, Japanese fish stocking protocols will benefit US efforts.