WWW.WAS.ORG • WORLD AQUACULTURE • DECEMBER 2017 65 (CONTINUED ON PAGE 66) deformities, and mortality (Utting and Doyou 1992). Depending on biochemical composition, the nutritional content of broodstock diets can be readjusted to increase energy reserves in eggs. Simple characteristics of gametes such as buoyancy, appearance, and motility are quality characteristics that are difficult to evaluate. Thus, the development of quality criteria has focused on fertilized eggs. Morphology, transparency, and blastomere symmetry are more reliable ways to evaluate quality. Generally, eggs with damaged fertilization membranes, delayed development, asymmetric early cleavage and irregular morphology are considered to be of bad quality (Primavera and Posadas 1981). Such eggs should be discarded because there is a high likelihood of natural mortality. The focus in the hatchery should be on eggs with normal appearance that can potentially result in high survival. Protocols to assess functionality and viability of sperm, oocytes, and embryos need further investigation. Nutritional requirements, reproductive biology, and egg quality should be investigated together because they are closely linked in broodstock management. Multidisciplinary studies combining different facets of broodstock conditioning are sometimes hard to conduct and most studies still focus on only one of these topics. Thus, the combined effects of diets and reproductive cycles on egg quality remains unclear for many invertebrate species. Future Concerns Water pollution and climate change The increasing pollution of coastal areas has been of a great concern. Many marine invertebrates are particularly susceptible to contaminated seawater. Organic pesticides, for example, can inhibit gametogenesis in echinoderms (Sugni et al. 2007) and lead to masculinization in female abalones (Horiguchi et al. 2005). Stress on the immune system caused by pollutants can also reduce energy allocation to reproduction with consequences on gonad growth, fecundity and offspring quality. An increase in seawater temperature can affect immune functionality, osmoregulation, pathogen life cycle, reproductive cues, diet digestibility, gene expression and behaviour (Matozzo et al. 2012). However, temperature is well controlled in many hatcheries where domesticated broodstock are conditioned year-round. The effect of temperature is more challenging in areas where adults are still collected from natural populations and induced to spawn in captivity without an acclimation period to promote gonad maturation. Ocean acidification might be a more serious problem to broodstock management because most marine hatcheries do not have the capacity to adjust seawater pH. Invertebrates conditioned at pH ~7.6 show discrepancies in oocyte morphology and buoyancy, reduced spermatozoa motility, low fertilization rates, irregular embryonic development and high offspring mortality (Morita et al. 2010). To mitigate these problems, hatcheries need to be wellequipped with water filtration and sterilization systems and establish programs to monitor the chemical properties of the incoming water. An alternative option is the incorporation of specific climate change adaptive traits into broodstock breeding programs to ensure high resistance to diseases or environmental stressors. Disease management As aquaculture expands and new species are introduced around the world, disease outbreaks will present special challenges. The conditioning of adults in captivity is stressful due to handling, crowding, intense light, temperature change, inadequate diet, lack of natural substrate and diseases. Moreover, mortality of broodstock might be expected because reproduction is very stressful, especially for those that need either chemical induction or eyestalk ablation. Consequently, broodstock are always being replaced with new individuals. Although great developments have been made in disease diagnosis and treatments, efforts have focused primarily on juveniles because they account for the bulk of farm production. With the addition of new individuals in the broodstock population each spawning season, disease prevention is the cornerstone to limit pathogens. This can be achieved through culture methods and biosecurity protocols. To minimize the incidence of diseases, many invertebrate hatcheries work with batch culture (all in, all out). At the end of the season, all individuals are moved out and the hatchery is completely disinfected prior to the introduction of the next batch of broodstock and larvae. Biosecurity measures include the maintenance of sanitary conditions, such as disinfection of supplies, frequent health inspections and proper disposal of infected/dead individuals, tank maintenance, including removal of excess feed and faeces, and quarantine of sick or newly arrived adults. Despite careful vigilance, pathogens can still be introduced through both contaminated algae culture and fresh fish. Recently, immunostimulants, probiotics, and vaccinations have been used in finfish aquaculture to provide protection against specific diseases but success with these materials has not been proven in marine invertebrates (Goulden et al. 2003). Domestication and genetic improvement programs Considerable efforts have been made in recent years to select disease resistant strains through breeding programs. In general, domestication of invertebrates has intensified in recent years mainly due to the inconsistent supply of wild-caught adults as broodstock. The domestication of bivalves has allowed genetic manipulation and the establishment of breeding programs (Racotta et al. 2003, Allen 2011). A selective breeding program for the Eastern oyster C. virginica developed lines that are resistant to the disease (MSX) caused by the protozoan Haplosporidium nelsoni. Shrimp farmers have focused on developing families resistant to Taura syndrome virus and whitespot syndrome virus. Although results are promising, shrimp survival is still low (Huang et al. 2011). Where diseases are not the main issue, growth rates, and yields can be improved, reducing culture time and production costs (Allen 2011). Because hatcheries work with a limited number of adults, breeding programs must avoid loss of genetic variation through inbreeding depression. Unplanned inbreeding can ruin a population by decreasing growth rates and increasing abnormalities and mortalities, as documented in oysters (Plough and Hedgecock 2011), shrimp (Doyle 2016), clams (Zhao et al. 2012), and abalones (Kobayashi and Kijima 2010). Low inbreeding can be ensured by ensuring genetic contribution from a large number of individuals. Triploid oysters have become popular because of their faster growth compared to diploids. Ploidy manipulation can be achieved by
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