Finally, we identify limitations for the broader application of endocrinology in fish conservation and opportunities for fish endocrinologists to make meaningful contributions to the most urgent conservation challenges of our time. We also examine how non-invasive sampling techniques, profiling of steroid hormones, and the integration of endocrinology with emerging fields such as ecotoxicogenomics and host-microbiome interactions will have impacts on future conservation efforts. Specifically, we provide examples of how endocrine signals are used to guide the development of conservation hatcheries, to reveal how exposure to environmental stressors can affect development and growth, to enable assisted reproduction, to mitigate the impacts of climate change and endocrine-disrupting chemicals on fish reproduction, and to facilitate the management of invasive species. In this review, after a brief overview of the endocrine systems involved in regulating stress, growth, and reproduction, we examine how fish endocrinologists are developing and applying new tools to monitor, conserve, and assist threatened and endangered wild fish populations. Decreasing melatonin is expected to result in an inhibition of DA and thereby stimulate sexual maturationĮndocrine systems are regulators of physiological responses to environmental conditions, acting as key transmitters of external and internal cues, and can therefore provide valuable insights to help address pressing issues in fish conservation biology. It is expected that MEL affects the KiSS/GPR54 system (KiSS) and/or GnRH, directly or indirectly via gonadotropin inhibiting hormone (GnIH), and thereby inducing puberty (Dufour et al. 17β-estradiol, 11-ketotestosterone, testosterone) exert positive and negative feedbacks on different levels in the BPG axis and liver. DA inhibits gonadotropin releasing hormone (GnRH) and synthesis and release of gonadotropins follicle-stimulating hormone (FSH) and luteinizing hormone (LH), which regulate gonadal activity (steroidogenesis and gametogenesis) (Dufour et al. Subsequently, MEL stimulates dopamine (DA) synthesis (Sébert et al. External cues, such as photoperiod, temperature and salinity, regulate the daily synthesis and release of melatonin (MEL), which transmits the external cues on all levels of the brain–pituitary–gonad (BPG) axis. Schematic representation of the proposed neurohormonal control of puberty in eels. In this review, we provide an overview of the current knowledge of studied natural cues possibly affecting reproductive function and the plausible role of melatonin in the regulation of puberty in eels. Thus far, its mode of action is still an area which needs to be explored. In eels, we hypothesize that melatonin is an important key player in the regulation of sexual maturation. The time-keeping hormone melatonin is a well-known transmitter of external cues, and influences various physiological processes, including reproduction. Still, the natural triggers involved in the gametogenesis of eels are poorly understood. In many fish species, changes in external environmental cues, such as photoperiod and temperature, are crucial to induce gonadal recrudescence and development. In order to improve current protocols it is clear that a different approach towards stimulating sexual maturation is required. Artificial reproduction of eels has been pursued for almost 80 years, and maturation protocols have changed little. However, the success rates are still far too low to support a sustainable farming industry, due to low gamete quality and low survival rates of larvae. Eel propagation is possible to some extent by applying extensive, expensive, unnatural hormonal treatments. Unfortunately, eels in captivity do not reproduce spontaneously as they remain in a (pre)-pubertal state resulting from a strong neural blockage. Worldwide, there is a dramatic decline in freshwater eel populations (Anguilla spp.), resulting in an urgent need to improve eel management and artificial reproduction protocols.
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