Partial Ovarian Development Without Atrophy in Farmed Eels Following One Year of Daily Cold-Water Cycling
Summary of the Experiment
This result may not be groundbreaking, but it is, to our knowledge, the first documented case in which farmed eels subjected to one year of daily cold-water cycling — without any hormonal treatment — showed partial ovarian development with no evidence of atrophy.
Farmed eels were surgically sexed, and a portion of ovarian tissue was sampled at the outset to establish baseline (pre-treatment) ovarian developmental status. The eels then received no pharmacological treatment and were maintained under light-shielded conditions with circulating seawater. The key manipulation was a daily cooling protocol: water temperature was lowered toward 0°C for a target duration of 8 hours per day, with the eels actually held at 0°C for approximately 6 hours; this cycle was repeated every 24 hours. The regimen was applied continuously for one year, from June 2025 to June 2026, a duration constrained by limited resources and staffing.
After one year, the eels were sacrificed and ovarian tissue was collected. Oocyte diameter was compared with the pre-treatment baseline. The ovaries showed partial development with no sign of atrophy; in some individuals, oocyte diameter reached 0.3 mm, exceeding the diameter generally considered the threshold for successful hormonal induction of final maturation (approximately 0.2 mm). This finding may offer a new avenue for broodstock management.
Discussion
Precedent for cold-induced partial ovarian development. The observation that cooling alone can drive partial ovarian development is not without precedent in the literature. The most directly comparable study is that of Fujiwara et al. (Fisheries Science, 2011), who reared Japanese eels under a gradually decreasing temperature regime (25°C lowered to 15°C over 39 days). The treatment group showed a marked increase in oocyte oil-droplet accumulation and a significant rise in circulating 11-ketotestosterone (11-KT) — the androgen considered pivotal for initiating vitellogenesis in eels — yet, intriguingly, pituitary FSHβ and LHβ mRNA expression were reduced relative to controls. The authors concluded that a temperature decrease alone can induce an early stage of ovarian development but is insufficient to drive eels to full maturity, and that additional stimuli (typically hormonal) are required to fully engage the gonadotropic axis. This aligns closely with the present result — partial development without progression to a pre-ovulatory state — and raises the possibility that cooling-induced ovarian development may not act primarily through enhanced pituitary gonadotropin output, but rather through a direct local effect on the gonad itself (steroidogenesis, lipid metabolism). This distinction is central to interpreting why ovarian growth occurred here in the complete absence of any hormonal treatment.
The oocyte diameter threshold. The 0.2 mm figure cited as the maturation-induction threshold is also supported in the literature. Yamamoto et al. (Fisheries Science) reported that Japanese eel oocytes must exceed a diameter of approximately 180 μm (0.18 mm) before hormonal treatment (e.g., salmon pituitary homogenate, SPH) can effectively drive them toward final maturation. In other words, the 0.18–0.20 mm range marks roughly the physiological watershed between the primary growth stage and a stage responsive to maturation-inducing treatment. Reaching an oocyte diameter of 0.3 mm in the present study therefore represents a meaningful advance beyond this threshold. This is arguably the most practically significant aspect of the result: the cold-cycling protocol appears to have carried the ovaries past the point that would otherwise require hormonal intervention to achieve.
Temperature as an established regulatory variable in eel oocyte development. Temperature is already employed as a tool in eel maturation protocols, albeit typically at a later developmental stage. A further Japanese eel study (Fisheries Science, 2021) used water temperature directly to synchronize the timing of final oocyte maturation: oocytes matured faster at 20°C than at 15°C, and this temperature-dependent rate was exploited to align females at different starting stages before administration of maturation-inducing steroids. Although that study addressed final maturation rather than the initiation of ovarian development addressed here, it establishes that temperature is a validated regulatory variable at multiple stages of eel oocyte development, not a one-off phenomenon specific to the present protocol. Notably, a closely related study — Mikawa (2019, Aquaculture Research), "A preliminary experiment regarding the natural induction of gonadal development in female Japanese eels without hormone treatment" — appears, from its title, to pursue essentially the same objective as the present work: inducing gonadal development through environmental manipulation rather than pharmacological treatment. Full-text access was not available (paywalled), but if the author can obtain this paper, it would be the single most relevant reference for direct comparison and citation.
An ecological and migratory-physiology perspective. This treatment may not be purely an artificial manipulation; it could inadvertently approximate aspects of what wild silver eels experience during their oceanic spawning migration. Migrating silver eels perform diel vertical migration (DVM), descending to deeper, colder water layers during the day and rising at night. A recent satellite-tagging study (Environmental Biology of Fishes, 2025) found that this vertical migration pattern is driven not only by photoperiod but also by water temperature, with migration behavior changing markedly as eels enter colder water masses such as the Kuroshio–Oyashio confluence zone. It has long been hypothesized in eel reproductive physiology that this recurring diurnal temperature cycling may itself serve as an environmental cue driving the transition from the migratory state to final gonadal maturation, analogous to temperature/photoperiod-driven puberty onset in salmonids. Viewed from this angle, the daily ~6-hour exposure to 0°C, alternating with a return to ambient temperature, can be seen as a simplified simulation of the recurring cold–warm cycling experienced during natural migration — arguably a closer approximation to the natural rhythmic structure than a simple progressive temperature ramp-down (as in the Fujiwara protocol), and this may be precisely why the ovaries developed rather than regressed. One caveat deserves emphasis, however: 0°C is an extremely cold exposure, colder than what tracking studies report migrating silver eels actually encountering (those studies describe "gradually decreasing" temperature without approaching freezing). It would therefore be more defensible to characterize this protocol as borrowing the diurnal cold–warm cycling structure of natural migration while exceeding its natural intensity, rather than as a precise recreation of the migratory thermal environment — a distinction worth stating explicitly in any manuscript discussion, to preempt the reasonable reviewer objection that wild eels never encounter 0°C water.
A cross-species comparison worth noting. An interesting parallel can be drawn to the mammalian literature discussed earlier, specifically Dorfman et al. (2003, Biology of Reproduction), which examined chronic intermittent cold stress (4°C, 3 hours/day, 5 days/week, for 3–4 weeks) in rats. That study found that intermittent, repeated cold stimulation — as opposed to sustained continuous cold — activated ovarian sympathetic nerve activity and nerve growth factor (NGF) signaling, and was associated with follicles being maintained or even showing thecal-layer hypertrophy rather than undergoing atresia. Notably, that cold-stress paradigm was deliberately chosen because it does not elevate corticosterone, specifically to dissociate the direct neural/local effects of cold exposure from the systemic effects of a stress hormone. The present eel protocol shares a structurally similar design: a brief, extreme, daily cold exposure repeated cyclically over an extended period, again yielding development rather than regression. Although eel gonadal regulation is governed primarily by the pituitary gonadotropic axis rather than direct sympathetic innervation of the gonad, and the two systems' molecular mechanisms are not necessarily homologous, the phenomenological similarity — that repeated, intermittent cold stimulation tends to favor gonadal maintenance or growth rather than degeneration — may be worth noting as a comparative-physiology talking point when discussing why this unexpected result occurred. This should be flagged explicitly as a phenomenological analogy rather than an established shared mechanism.
Broader significance for broodstock cultivation. Taken together, this result carries real practical significance for broodstock (種鰻) management. The Japanese eel is a resource-constrained species heavily dependent on wild-caught glass eels and on artificial hormonal induction of maturation — a process long criticized for producing eggs and larvae of inconsistent quality. If a simple, injection-free temperature-cycling protocol can carry captive females' ovaries past the maturation-induction threshold on its own, this would represent a meaningful step toward a more natural, lower-cost, and potentially higher-quality artificial propagation pipeline. This is precisely the value of pursuing this unexpected finding further, despite the resource and staffing constraints noted at the outset — it represents a credible starting point for a publication and for securing follow-up funding.
Sources
- Fujiwara et al., Effect of temperature decrease on oocyte development, sex steroids, and gonadotropin β-subunit mRNA expression levels in female Japanese eel Anguilla japonica, Fisheries Science 2011. https://link.springer.com/article/10.1007/s12562-011-0358-3
- Effects of rearing temperature manipulation on oocyte maturation progress in Japanese eel, Fisheries Science 2021. https://link.springer.com/article/10.1007/s12562-021-01531-8
- Yamamoto et al., Pretreatment Reproductive Stage and Oocyte Development Induced by Salmon Pituitary Homogenate in the Japanese Eel Anguilla japonica. https://www.jstage.jst.go.jp/article/fishsci1994/64/4/64_4_531/_article
- Cold environment influences on diel vertical migration of silver-phase Japanese eels in the Kuroshio-Oyashio confluence region, Environmental Biology of Fishes 2025. https://link.springer.com/article/10.1007/s10641-025-01750-3
- Mikawa, A preliminary experiment regarding the natural induction of gonadal development in female Japanese eels without hormone treatment, Aquaculture Research 2019. https://onlinelibrary.wiley.com/doi/abs/10.1111/are.14337
- Dorfman et al., Chronic Intermittent Cold Stress Activates Ovarian Sympathetic Nerves and Modifies Ovarian Follicular Development in the Rat, Biology of Reproduction 2003. https://academic.oup.com/biolreprod/article/68/6/2038/2683472
