Hatchery-reared larvae of the silver therapon Leiopotherapon plumbeus, an endemic Philippine freshwater fish of growing aquaculture interest, exhaust their yolk at 40 hours post-hatch (hph) at 25 °C and reach the point-of-no-return (PNR) — the stage of irreversible starvation — near 50 hph, leaving a mixed-feeding window of only about 12 hours, from complete yolk resorption to exhaustion of the oil globule (approximately 40 to 52 hph at 25 °C). Garcia, Sayco and Aya (2020, Aquaculture Reports 16:100288) established these timings but left three linked questions open: (1) whether survival improves when live prey is provided immediately at complete yolk resorption (40 hph) and under continuous or repeated feeding, rather than at the delayed times actually tested (46–54 hph, each as a single one-hour exposure); (2) whether mortality after delayed feeding is caused mainly by pre-ingestive foraging failure (immature mouth and gut) or by post-ingestive digestive failure (insufficient pancreatic and intestinal enzymes) — never experimentally disentangled; and (3) how the PNR, mixed-feeding duration, and post-feeding survival windows depend on rearing temperature, prey type and density, and feeding regimen, all of which were fixed at 25 °C with particular prey in the source study. These questions remain unanswered.

Background

Silver therapon (Leiopotherapon plumbeus, Kner 1864; Teleostei: Terapontidae), locally called ayungin, is a small freshwater fish endemic to Philippine lakes and rivers, notably Laguna de Bay, where wild stocks have declined; this has motivated hatchery-based seed production research, chiefly at the Southeast Asian Fisheries Development Center Aquaculture Department (SEAFDEC/AQD). Like most fish, therapon larvae hatch with a yolk sac (the endogenous energy reserve, together with an oil globule) that is consumed within a few days, after which the larvae must locate, capture, and digest suitably sized planktonic prey while their feeding apparatus and digestive organs are still immature. The interval during which both endogenous reserves and exogenous food can contribute is the transitional or mixed-feeding period. If first feeding is delayed too long, larvae pass the point-of-no-return (PNR), operationally defined in this literature as the time at which the feeding incidence of previously starved larvae — the percentage with prey in the gut after a standard short exposure to live food — falls below 50% of its observed maximum; beyond the PNR, larvae can no longer resume feeding even when food is supplied, and they die. Earlier work on this species documented diet composition, prey preferences, and mouth morphology of early larvae (Aya, Corpuz and Garcia 2015) and embryonic and larval development (Aya, Nillasca, Garcia and Takagi 2016), and a subsequent study examined rearing-tank substrate, prey selectivity, and the daylight (diel) feeding cycle (Aya, Nillasca, Sayco and Garcia 2019). Garcia, Sayco and Aya (2020) then quantified the starvation clock: at 25 °C, yolk is fully resorbed at 40 hours post-hatch (hph); feeding incidence rises to a peak near 48 hph as the mouth gape (the maximum aperture of the opened jaws, which bounds the size of prey a larva can ingest) increases, while the oil globule is exhausted near 52 hph and feeding ceases by 54 hph; the PNR falls at about 50 hph. In their delayed-feeding experiment, unfed post-yolk-sac larvae received a single one-hour exposure to wild zooplankton at 46, 48, 50, 52, or 54 hph: larvae first fed at 46–48 hph survived about 12–14 hours after that single feed, whereas larvae first fed at 50 hph or later survived no more than about 10 hours. All measurements were made at a single rearing temperature (25 °C), with particular prey (wild zooplankton, or wild zooplankton plus rotifers), and with one-hour rather than continuous exposures.

Problem Statement

Under the hatchery conditions of the source study (25 °C, wild zooplankton prey, single one-hour exposures), silver therapon larvae exhaust their yolk at 40 hours post-hatch (hph), reach the point-of-no-return — the stage beyond which starved larvae can no longer resume feeding even when food is supplied — near 50 hph, and cease feeding by 54 hph; larvae given a single one-hour feed at 46–48 hph survive about 12–14 hours after that feed, while larvae first fed at 50 hph or later survive no more than about 10 hours. The problem is to determine, experimentally, how the timing and conditions of initial live-food provision govern the survival of first-feeding silver therapon larvae, by resolving three linked questions flagged as unresolved by the source study: (1) Survival and rescue — what survivorship is obtained when live prey is provided immediately at complete yolk resorption (40 hph) rather than at the later delays actually tested, and whether continuous or repeated feeding regimens (rather than single one-hour exposures) improve survival or rescue larvae whose first feeding has been delayed? (2) Mechanism — is mortality after delayed feeding attributable primarily to pre-ingestive foraging limitation (incomplete mouth, jaw, and gut development limiting prey capture and ingestion) or to post-ingestive digestive failure (immature liver and pancreas, insufficient pancreatic and intestinal enzymes limiting assimilation of ingested prey), and in what proportion? (3) Condition dependence — how do the measured PNR, the duration of the mixed-feeding period (the interval from complete yolk resorption to exhaustion of the oil globule), and the post-feeding survival window (how long larvae survive after their first feed) change across rearing temperatures, prey types, prey densities, and feeding regimens? The common objective is a mechanistically grounded, condition-validated first-feeding schedule for hatchery production of silver therapon seed: the three questions are linked and jointly establish when prey must first be offered, under which conditions, and why delayed feeding kills, and none of them is answered independently of the others. Resolving the question requires the result named by each sub-question: survivorship outcomes for immediate (40 hph) versus delayed first feeding and for single versus continuous or repeated feeding in (1); an attribution of delayed-feeding mortality between pre-ingestive and post-ingestive causes, with the proportion each accounts for, in (2); and, in (3), a characterization of the PNR, the mixed-feeding duration, and the post-feeding survival window that covers all four named factors — rearing temperature, prey type, prey density, and feeding regimen — with their specific test levels and ranges left to the experimental design.

References

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