Enhancing larval survival and re-introduction techniques for Tridacnidae (giant clams): Reversing a path toward extinction

Endangered species Fisheries Threatened species Pacific Area
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Mariculture interest in the Tridacnidae family of giant clams has increased in response to economic and nutritional opportunities, but also in response to increasing reports of local extinctions as a result of over-harvesting, pollution, and habitat degradation. Key impediments to culturing giant clams for stock restoration initiatives include larval mortality, typically 96%, and a lack of methods for re-introducing juveniles to natural habitats. This thesis studied how nutritional conditions during early development influenced giant clam survival and growth, and how survival can be increased when juveniles are recruited directly to coral substrata. Information about the nutritional condition of pre-veliger tridacnids is scarce. A review of pre-veliger nutrition revealed that broodstock feeding, water temperature, oocyte maturity, and dissolved organic matter (DOM) loading in culture water influences larval viability. An experiment to measure the effects of parental conditioning on egg size and biochemical composition showed that feeding Tridacna maxima parents a high-lipid, high-protein diet for 5 weeks prior to spawning increased egg volumes by >13%, and that survival compared to unfed cohorts nearly doubled. However, the effects of parental feeding on egg lipid classes and volumes were inconclusive. Increased bivalve egg size is known to influence larval survival. A protocol was tested for selecting T. maxima eggs by size to increase survival. Results showed that mean egg sizes for two T. maxima was 101.3 μm ± 0.17 (n = 284). However, segregating eggs by size above and below 100 μm proved futile since egg jelly coats ranging from 160 μm to 202 μm interfered with the sieving procedure. Tridacnids take up nutrition in the form of DOM and amino acids during veliger stages of development, but when this process begins was unknown. Experiments conducted on T. maxima and Tridacna squamosa in filtered seawater compared to artificial, nutrientfree seawater showed that uptake begins much earlier than previously thought. In addition to increased survival, trochophore growth increased 50% and 76% for T. maxima and T. squamosa, respectively in response to the availability of DOM and 0.4 μM and 0.8 μM amino acid concentrations. There is debate among tridacnid hatchery managers as to whether veligers should be fed supplemental nutrients. A review of the literature and protocols currently in place showed that supplemental feeding tends to enhance veliger survival and growth, and that the gap between cost and effect is closing. However, risks such as algal overgrowth, bacterial infection, and increased biochemical oxygen demand remain. A field experiment was conducted to determine if reducing predation pressure and water turbulence on juvenile T. maxima recruited directly to coral substrata increased survival. Protective cages were tested on one-, two- and three-year-old T. maxima juveniles for three weeks. Survival of three- year-olds in triplicate cages was 100%, 96% and 76% compared to 40% in unprotected controls. Survival of one-year-olds was 40%, 54%, and 63% compared to 15% in controls, suggesting that two- and three-year-olds are favoured for survival. This thesis shows for the first time that pre-veliger nutrition is critical to larval viability, and that survival can be increased for two- and three-year-old juveniles recruited directly to lagoon substrata.

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