Broodstock spawning capacity and juvenile fish quality are among the most critical aspects of any aquaculture operation, whether for stock enhancement or commercial production. California halibut (Paralichthys californicus), an emerging candidate species for both applications, is the subject of ongoing research at Hubbs-SeaWorld Research Institute. The spawning characteristics of a broodstock population held under an experimental thermal regime have been monitored since 2019. Volitional spawning occurs during seasons that last up to 10 months (more frequently and for longer than separate broodstock populations exposed to ambient water temperature conditions); this reliable source of high-quality eggs has facilitated sustained research on key areas of juvenile quality, specifically sex determination, pigmentation, and malformations.
Supported by evidence of temperature-dependent sex determination in related paralichthids, California halibut were reared at three distinct water temperatures (15, 19, and 23°C) through the presumed critical development window. Visual sex identification of fish >100 mm TL indicated that complete masculinization occurred at the highest temperature, with females apparent at the lower temperatures. Ongoing development of molecular biomarkers, to identify phenotypic sex as early as possible, may facilitate more complex (and shorter duration) experimental designs. Exceptionally high rates of pseudoalbinism (>95%) and ambicoloration (100%) have been observed in prior production cohorts of California halibut. A stepwise series of replicated rearing experiments have been undertaken involving manipulations of rotifer enrichment, dietary timing, and water temperature. These resulted in incremental reductions of malpigmentation (up to 46% normally-pigmented juveniles), with additional modifications of key nutritional and environmental parameters planned to reduce this further. Lastly, preliminary work has also begun to apply an external malformation surveillance protocol (originally developed for a white seabass stocking program) to California halibut. Cultured and wild California halibut have been used to refine existing malformation categories and establish a baseline reference for this species.
This research has been undertaken on intensively cultured California halibut primarily in the context of a pilot stock enhancement program. Thus, the focus has been on achieving morphologically, physiologically and behaviorally wild-like fish. As the program develops, we are continuing to explore the synergistic development of commercial production and the optimization of quality attributes more suited to that purpose (e.g., faster-growing all-female cohorts).