Niall Bromage

Broodstock management, fecundity, egg quality and the timing of egg production in the rainbow trout (Oncorhynchus mykiss)

Abstract The full potential of rainbow trout hatcheries can be attained only if the hatcheries can provide producers with regular supplies of high quality eggs and fry every week of the year. This review assesses aspects of brookstock management central to determining total number of eggs produced, quality of eggs, and timing of maturation and spawning. The effectiveness of hatchery programmes in supporting an expanding rainbow trout market is profoundly affected by the conditions under which the broodfish are maintained, their husbandry, and the stock selected for production. The paper examines factors that can critically affect fecundity, egg production, egg quality, spawning time, and maintenance of egg supplies. The discussion also outlines methods of manipulating spawning and areas requiring further advancement of knowledge if progress is to continue in optimising egg and fry production.

Development of broodstock diets for the European Sea Bass (Dicentrarchus labrax) with special emphasis on the importance of n−3 and n−6 highly unsaturated fatty acid to reproductive performance

Commercially fabricated diets allow greater control over the composition of biochemical components and reduce the risks of disease introduction, which are significant concerns when using the wet fish diets commonly used for most farmed marine broodstocks. However, satisfying the dietary lipid requirements of marine broodstock using artificial diets has proved difficult, particularly with respect to their highly unsaturated fatty acid (HUFA) composition. Two groups of mature sea bass, each divided between three replicated tanks, were fed two dry pelleted diets over a 2-year period, encompassing two spawning seasons. The first diet contained a good quality Northern Hemisphere meal and oil; the second differed only in the source of oil, which was substituted with tuna orbital oil (TOO). The use of TOO in the dry pelleted formulation allowed the manipulation of n−3 and n−6 HUFA in the resulting eggs, specifically arachidonic acid (20:4 n−6; AA), eicosapentaenoic acid (20:5 n−3; EPA) and docosahexaenoic acid (22:6 n−3; DHA). The results showed that dietary manipulation of these HUFA could improve levels and ratios of AA, EPA and DHA which were transferred to the resulting eggs with improvements in early survival and hatching success repeated over successive spawning seasons. The dry diet containing TOO facilitated comparable reproductive performance to the wet fish diet (Boops boops) which has previously been considered the most effective broodstock diet. The improvements in reproductive performance are discussed in relation to the proportion of these HUFA with respect to each other in total egg lipid and the phospholipid classes phosphatidylcholine (PC), phosphatidylinositol (PI) and phosphatidylethanolamine (PE) and to their potential impact on eicosanoid formation. Finally, this study has shown that a commercially fabricated diet can be successfully used as sensitive investigative tool for aquaculture research.

The timing of ovulation and stripping and their effects on the rates of fertilization and survival to eying, hatch and swim-up in the rainbow trout (Salmo gairdneri R.)

Abstract The effects of retention of rainbow trout eggs within the abdominal cavity after ovulation on subsequent survival of eggs and fry was examined. The time of ovulation for rainbow trout was confirmed by measurement of 17α-hydroxy-20β-dihydroprogesterone and 17α-hydroxyprogesterone in serum samples taken every other day for 2 weeks before, and 2 weeks after, ovulation. The results indicated that maximum egg and fry survivals are achieved if the eggs are stripped 4–6 days post-ovulation. Survival to the developmental stages considered was closely correlated with fertilization percentage. Low fertilization was followed by reduced success at each subsequent developmental stage. Determination of percentage fertilization is proposed as a management tool to predict subsequent egg and fry performance.

Blastomere morphology as a predictive measure of fish egg viability

Abstract For most fish species, blastomere size and shape are uniform during normal cleavage. However, abnormal blastomere morphology has been observed in a variety of studies, either induced directly by chemical or physical Stressors, or observed in eggs collected from plankton or broodstock. It has been proposed that abnormalities are indicative of low egg viability and may provide a useful predictive tool for egg assessment. However, there is little direct evidence relating blastomere morphology to egg survival. This paper reviews the literature on abnormal embryonic development in fish, and presents new findings on the significance of blastomere morphology in the Atlantic halibut, Hippoglossus hippoglossus. Several studies have reported qualitative relationships between egg batch survival and the proportion of normal embryos for a variety of fish species. In order to investigate more closely the relationship between blastomere morphology and fish egg survival, incubation experiments where carried out with eggs of Atlantic halibut. Eggs were individually assessed at the 8-cell stage for 5 blastomere characteristics and incubated to hatch in multiwell microtitre plates. For data pooled from 13 egg batches, the mean score for each blastomere characteristic showed a significant positive linear regression with the number of eggs hatched. Multiple regression analysis, incorporating all five blastomere characteristics, demonstrated a high degree of correlation between the independent variables. While the experimental procedure that was used for halibut eggs is too laborious for routine hatchery application, it is proposed that subjective observations of blastomere morphology could be readily adopted by commercial hatcheries to provide early assessment of egg viability.

Dietary lipid enhancement of broodstock reproductive performance and egg and larval quality in Atlantic halibut (Hippoglossus hippoglossus)

Abstract Two series of experiments were conducted to assess whether the trash fish-based diet traditionally used by the industry for broodstock halibut could be replaced by a formulated pelleted feed and to investigate the effects of docosahexaenoic acid (DHA), eicosapentaenoic acid (EPA), and arachidonic acid (AA) on halibut fecundity, blastomere morphology and fertilisation and hatching rates. In the first experiment, two formulated broodstock feeds (one containing Krill meal and one supplemented with tuna orbital oil, which provided higher dietary levels of DHA and AA), gave similar performances (Relative fecundity 9557±3462 and 10,551±2166, fertilisation rate 40.0±6.4% and 40.0±9.1%, respectively) to that of a fish fed a trash fish-based diet (Relative fecundity 11,782±1873, fertilisation rate 41.0±4.0%). In a second experiment, the spawning performance and egg quality of two groups of broodstock maintained on two formulated diets, each containing different levels of AA (1.8% and 0.4%) was compared over two successive spawning seasons. The diet with the higher (1.8%) level of AA resulted in significantly higher ( p

Gamete physiology, fertilization and egg activation in teleost fish

The fertilization and activation of fishoocytes are vital, but unfortunatelyoverlooked, processes in fisheries research.This paper sets out to review our presentunderstanding of these important events inteleost fish and, drawing comparisons withmammalian research, to highlight areas in whichresearch effort is urgently required. Presently, the commercial culture of manyimportant freshwater, but especially marine,teleosts is beset by problems associated withfertilization, hatching and early embryonicdevelopment. These problems have beenparticularly acute in certain species leadingto the application of spawning inductiontechnologies in an effort to optimizeproduction. Increased knowledge of theprocesses of egg activation and fertilizationin these groups of fish is likely to makesignificant contribution to commercialaquaculture. Studies of a wide variety ofanimal and plant species has demonstrated thatdevelopment at fertilization is triggered by anincrease in intracellular Ca2+concentration within the egg that occurs aseither a single transient or a series ofdistinctive oscillations depending upon thespecies under investigation. This increase inintracellular Ca2+ activates the egg andalso appears to play an important role in laterembryonic development. Teleost reproductivestrategies and more importantly, teleostoocytes and spermatozoa, exhibit a remarkablevariety of adaptations. Currently, studies ofegg activation in teleosts are confined tolaboratory species such as medaka Oryziaslatipes and zebrafish Brachydanio rerio.Nevertheless, even between these two species,although an increase in intracellular Ca2+appears to be the trigger in both cases, themechanism of Ca2+ release may be quitedifferent. Activation in medaka is initiatedonly through direct contact with conspecificsperm, suggesting the involvement of asperm-specific factor, while zebrafish eggsappear to require only contact with theexternal spawning medium. In view of the highlyvariable fertility rates evident in manycommercially cultured teleosts, it could bevery rewarding to investigate the mechanism ofegg activation in representative teleost groupsusing the findings and theories emerging fromother animal groups as a starting point. Inorder to successfully conduct such aninvestigation, it will be necessary to employ acombination of physiological, molecular andrecombinant approaches.

The effect of modifications in photoperiod on spawning time, ovarian development and egg quality in the sea bass (Dicentrarchus labrax L.)

Abstract Under simulated natural conditions female sea bass ( Dicentrarchus labrax ) in eastern Spain (latitude 40° N and longitude 0°) spawned over a 6–8-week period in February and March, with each female spawning on more than one occasion. Generally, male fish began spermiation up to 2 months before, and continued for at least 1 month after the spawning period of the female fish. Exposure of fish to 1 month of long days (LD 15 9 ) from either 2nd May (Group C), 3rd June (Group D) or 3rd July (Group E) in an otherwise constant short day (LD 9 15 ) photoperiod regime, speeded up the rates of maturation, thus increasing the proportions of oocytes entering exogenous vitellogenesis during October and November, and also brought forward the timings of ovulation and spawning. In contrast, constant long days from 2nd May (Group B) delayed maturation and spawning time by 2–3 months. Fish maintained under constant short days throughout the experiment (Group A) spawned up to 6 weeks in advance of the control fish, suggesting that endogenous timing mechanisms may operate in this species. Spawning occurred naturally in all the experimental and control fish without the necessity for induction with pituitary or hypothalamic hormones. For the control fish, egg quality (defined as the proportion of eggs spawned which were floating and viable), hatching rate and survival to first feeding (both expressed as percentages of the numbers of floating or good quality eggs) averaged 78.3±7.7% (mean±s.e), 84.8±3.3% and 54.6±20% respectively. With the exception of the eggs and fry from Group B, these measures of quality and survival appeared unaffected by the photoperiod treatments. The eggs and fry produced by the fish maintained under long days (Group B) towards the end of their spawning period (i.e. in May), showed much more variable egg quality and survivals through hatch and first-feeding, possibly because of the higher sea water temperatures which prevailed at the time of their delayed spawning. Fecundities expressed as numbers of eggs per kg of post-spawning fish weight ranged from 247–305 000/kg for Groups A, C, D and E and the controls, but were significantly reduced in Group B fish. Egg sizes, which ranged from 1.147–1.176 mm in diameter, were unaffected by the photoperiodic manipulations of spawning time. It is concluded that artificial control of daylength can improve the supply of eggs and fry for commercial on-growing without any adverse effects on the fecundity of the broodstock or the quality and survival of the eggs and fry.

Effects of egg size on early growth and survival in rainbow trout (Salmo gairdneri Richardson)

Abstract The effects of egg size on the subsequent growth and survival of developing eggs and fry from rainbow trout were examined. Eggs, whose size varied from 3.36 to 5.63 mm in diameter, were stripped from six 2-year-old and seven 3-year-old ripe females, selected at random from a large group of commercially-maintained broodstock, and after fertilization the survival rates to eying, hatch and swim-up, and the growth and survival of the fry determined. At the time of first feeding (8 weeks post-fertilization) there was a positive correlation between the size of the water-hardened egg and that of the fry, with the largest eggs from both ages of broodstock producing the largest fry. Four weeks after first feeding the relationship between egg size and fry weight was lost, suggesting that genetic and environmental influences exert greater effects on growth after this time. This was also supported by the finding that the specific growth rates of the fry up to 26 weeks post-fertilization were not correlated with the initial size of the eggs irrespective of whether they had been derived from 2- or 3-year-old parental fish. Survival rates to eying, hatch and swim-up and as 3-month-fed fry also showed no correlation with initial egg size, indicating that under the generally favourable conditions found in commercial hatcheries egg size has no direct implications as far as overall egg quality and fry survival are concerned.

Photoperiodic mechanisms and rhythms of reproduction in the female rainbow trout

The present work investigates the importance of circadian and circannual rhythms in the photoperiodic control of reproduction in the rainbow trout. Maintenance of groups of 20–30 female trout under continuous light (LL), constant long (18L:6D) or short (6L:18D) days and conditions of constant temperature (8.5–9.0°C) and feeding rates (0.5% body weight, day−1), starting in February, produced markedly different spawning periodicities during the first and subsequent years of treatment. At the end of the first year, spawning was advanced by up to 2 months in the fish under LL and 18L:6D and delayed by up to 5 months in those under 6L:18D when compared with the December spawning of control fish under ambient light. Continued exposure of the fish to the same photoperiod regimes produced cycles of spawning and peak levels of testosterone, oestradiol-17β and calcium (as an index of vitellogenin) at intervals of 150–170 days under LL and 18L:6D and 320–420 days under 6L:18D. The functional importance of these spawning cycles and their relationship to circannual rhythms and the control of reproduction is discussed.Exposure of fish to skeleton (6L:4D:2L:12D, 6L:6D:2L:10D and 6L:8D:2L:8D) and resonance (6L:42D, 6L:48D and 6L:54D) procedures produced ranges of spawning times up to two months in advance of control fish. Results with the resonance regimes, where fish received only a half the light-dark cycles and a quarter of the total daylight hours of those on ambient light cycles show that trout do not measure daylength and time by counting daily cycles or by hour-glass mechanisms. Collectively, these data support the proposition that rhythmic processes of photosensitivity are involved in the modulation of reproductive development in the rainbow trout and that circadian and circannual rhythms cooperate in the timing and entrainment of this cycle.

Relationships between serum levels of gonadotropin, oestradiol-17β, and vitellogenin in the control of ovarian development in the rainbow trout: II. The effects of alterations in environmental photoperiod

Abstract The present study investigates the effects of seasonally changing photoperiod regimes on the levels of serum gonadotropin, oestradiol-17β, and vitellogenin (as indicated by phosphoprotein phosphorus and calcium measurements) on the time of spawning of female rainbow trout ( Salmo gairdneri ). This included a control regime and also seasonal cycles in which the increasing and decreasing components of the photoperiod were maintained but compressed into 6- and 9-month periods. Spawning in the control fish occurred in mid-January at the same time as fish in outside ponds under ambient conditions, whereas under the 6- and 9-month cycles it was advanced by approximately 12 and 6 weeks, respectively. Under both the control and experimental regimes, the primary change was an increase in serum levels of gonadotropin during the early stages of ovarian development, after which levels were reduced. This was followed by increased oestradiol-17β levels which became particularly pronounced during the period of exogenous vitellogenesis. These changes in oestradiol-17β appeared to stimulate vitellogenin production and release by the liver as evidenced by increases in serum calcium and phosphoprotein phosphorus levels. Subsequently, vitellogenin levels remained high until after spawning was complete. During the later stages of ovarian development oestradiol levels fell, thus signalling, probably by a negative feedback mechanism, a further and larger increase in gonadotropin levels presumably required for the control of final oocyte maturation and ovulation. This sequence of endocrine changes was similar under the shortened photoperiod cycles, although modifications were apparent in both their timing and duration, resulting in earlier spawning times.