Neville William Pankhurst

Effect of holding temperature on ovulation, egg fertility, plasma levels of reproductive hormones and in vitro ovarian steroidogenesis in the rainbow trout Oncorhynchus mykiss

Abstract Sexually mature female rainbow trout were held at temperatures ranging from 9–21 °C for up to 3 months prior to the natural time of ovulation, in experiments conducted over three different spawning seasons. The majority of fish held at 9 and 12 °C ovulated, variable numbers ovulated at 15 and 18 °C and only one fish ovulated at 21 °C. Egg production was similar at 9, 12 and 15 °C, significantly lower at 18 °C and near zero at 21 °C. Egg survival to the eyed stage after incubation at 11 °C was similar at 9, 12 and 15 °C, and nil at 18 and 21 °C. Histological examination of oocytes from fish held at 12, 15 or 18 °C for 1 or 2 months showed no evidence of gonadal atresia. Plasma levels of gonadotropin (GtH) were measured in samples taken 1, 2 and 3 months after introduction to temperatures of 9, 12, 15, 18 or 21 °C, and showed no differences between temperatures at any time. Plasma levels of testosterone (T) and 17β-oestradiol (E 2 ) were similarly unaffected. Repeat measurement of T and E 2 in another spawning season also showed that holding temperature had no effect on plasma steroid levels. In contrast, in vitro basal steroidogenesis by isolated ovarian follicles was generally lower at 18 °C than at 12 or 15 °C. Follicles from fish held at 18 °C for 2 months did not retain responsiveness to stimulation with steroid precursors or GtH, whereas those from fish held at 12 and 15 °C did. The results indicate that elevated autumn holding temperatures have a deleterious effect on ovulation, egg production and fertility but have equivocal effects on endocrine parameters associated with vitellogenesis. This suggests that the effects are exercised on processes associated with final maturation and ovulation rather than vitellogenesis.

Cold and dark or warm and light: variations on the theme of environmental control of reproduction

Seasonality is a common feature of teleost reproduction and is most marked at higher latitudes. Seasonal cycles are principally entrained by photoperiod and temperature but a range of other variables including lunar phase and social interaction may also operate. Photoperiod information is transduced through the light inhibition of pineal melatonin synthesis but the mechanism by which this regulates the reproductive endocrine system is not clear. There is less information on the specific effects of temperature but these are likely to involve effects on expression and activity of regulatory proteins. Environmental regulation of high latitude species appears to operate across a hierarchy of variables with photoperiod then temperature as primary drivers, whereas in tropical habitats, the hierarchy may be inverted.

Global Warming: Temperature effects on the reproductive performance of fish

Introduction Scientists are increasingly being called upon to predict the outcomes of global temperature changes on fish populations. Both reproduction and early development in fish are particularly sensitive to temperature perturbations. Numerous in vitro and in vivo studies have shown that reproductive endocrine homeostasis in fish (Fig. 1) is responsive to changes in temperature. This includes alterations in the secretion and actions of hormones associated with all components of the hypothalamic–pituitary–gonadal axis which controls reproductive processes. By comparison, far fewer studies have considered the longer term consequences of altered temperature profiles on the reproductive cycle or larval development. Even fewer studies have considered the long-term ecological consequences of multigenerational exposures to elevated thermal regimes. Given the existing data and the need for a rapid response to questions on the outcomes of temperature change, our best informed judgement will have to be based, in large part, on information from short-term assays. This leads to the question of whether current methods of assessing the effects of temperature on the molecular and cellular events mediating reproductive processes can be used to predict effects at whole animal and population levels (Fig. 2). This chapter reviews our current understanding of the effects of elevated temperature on reproductive performance in fish. The initial focus is on endocrine homeostasis and the effect of temperature on hormone biosynthesis, metabolism and actions.

Oocyte maturation and changes in plasma steroid levels in snapper Pagrus (= Chrysophrys) auratus (sparidae) following treatment with human chorionic gonadotropin

Abstract Snapper Pagrus (= Chrysophrys) auratus were captured by long-lining from the Hauraki Gulf, NE New Zealand, and returned to the laboratory. Eighteen female fish were injected with either saline or 1000 U kg −1 human chorionic gonadotropin (hCG). Blood samples were taken at the time of injection and subsequently at 1, 6 and 24 h post-injection (pi). Plasma levels of 17β-estradiol (E 2 ), 17,20β-dihydroxy-4-pregnen-3-one (17, 20βP), and testosterone (T) were measured by RIA. Seven out of the eleven hCG-injected fish ovulated within 24 h, whereas no control fish ovulated. In fish injected with hCG, plasma E 2 levels were elevated at 24 h pi, 17,20βP levels at 6 h pi, and T at 6 and 24 h pi, relative to controls. In a second experiment, twelve fish were injected with 1000 U kg −1 hCG as before and bled at 0, 4, 7, 13, 17 and 26 h pi. At each sample time a small piece of ovarian tissue was taken from each fish using a 12 G syringe needle introduced through the body wall. Aspirated follicles were measured and then cleared to determine the position of the germinal vesicle. Follicles from six of the fish underwent a significant increase in diameter, accompanied by germinal vesicle breakdown (GVBD) and oocyte hydration. Plasma E 2 and T levels were elevated in fish with maturing oocytes from 13 h pi. 17,20βP showed a peak at 4 h pi, but only in the group of fish in which oocyte maturation did not occur. There was no difference in plasma cortisol levels between fish that showed oocyte maturation and those that did not, suggesting that proximate stress factors were not responsible for the differential response to hCG treatment observed in this experiment.

Maintenance at elevated temperature delays the steroidogenic and ovulatory responsiveness of rainbow trout Oncorhynchus mykiss to luteinizing hormone releasing hormone analogue

Abstract Vitellogenic rainbow trout were held at 12 or 18°C for 83 days; then injected with either saline or luteinizing hormone releasing hormone analogue (LHRHA). Repeat blood samples were taken over the 96 h following injection, and fish were monitored for ovulation for up to 44 days post-treatment. Most fish held at 12°C ovulated in response to LHRHA within 8 days of injection, whereas controls began to ovulate 19 days after injection and all had undergone ovulation by day 44. In contrast, no control or LHRHA-injected fish held at 18°C underwent ovulation for 23 days after injection. Fish held at 18°C were reinjected with saline or LHRHA 25 days after first treatment, and were now responsive to LHRHA, with six out of eight fish ovulating within 11 days of injection. Spontaneous ovulation also occurred in about half of control fish at 18°C over the 20 days following the second injection. Eggs from all ovulations were incubated at the same holding temperature as the adults. Eggs held at 12°C showed high survival to neural streak (>90%) and eyed stages (>80%), whereas incubation at 18°C resulted in low survival to equivalent stages ( 2 ) at 72 and 96 h post-injection (pi), a transient increase in plasma testosterone (T) at 24 h pi, and a marked elevation in 17, 20β-dihydroxy-4-pregnen-3-one (17,20βP) at 48, 72 and 96 h pi relative to controls. At 18°C (first injection) LHRHA-injected fish had elevated plasma T at 48 h pi, but no changes in plasma levels of E 2 or 17,20βP. In contrast, the plasma steroid profile of fish at 18°C after the second injection of LHRHA was very similar to that shown by fish at 12°C at first injection. Ovarian follicles from fish held at 12°C were unresponsive to in vitro treatment with human chorionic gonadotropin (hCG), whereas follicles from fish held at 18°C produced increased amounts of E 2 in response to hCG. This was consistent with the low aromatase activity seen in vivo in fish held at 12°C. The results show that maintenance at elevated temperature retards steroidogenic responsiveness to LHRHA, either by delaying pituitary responsiveness to LHRHA, or the shift in ovarian steroid secretion from E 2 to 17,20βP.

The effects of common laboratory and husbandry practices on the stress response of greenback flounder Rhombosolea tapirina (Günther, 1862)

Abstract This study examined the stress response of the greenback flounder Rhombosolea tapirina to common laboratory and aquaculture practices. Plasma levels of cortisol in wild fish sampled within 2 min of capture were comparable to plasma cortisol values in other species captured from the wild and sampled immediately. Plasma levels of cortisol were significantly higher in wild fish sampled after capture, confinement and transport. The latency of the plasma cortisol response to stress was approximately 10 min. Cultured greenback flounder, exposed to normal husbandry conditions, had low plasma cortisol levels; however, 3 h of crowding combined with 5 min chasing (simulated grading) resulted in significantly elevated cortisol levels for up to 48 h. Plasma cortisol was significantly higher in fish held at medium and high stocking density than at low density. The plasma cortisol stress response of greenback flounder is similar to that shown by other marine teleosts. Plasma lactate levels in wild fish sampled after capture, confinement and transport, were considerably higher than levels in fish sampled within 3 min of capture, or exposed to 30-min exercise. No significant changes in muscle lactate were observed in response to exercise; however, there were significant increases in plasma lactate, and muscle [H + ] and a significant decrease in plasma [H + ] following exercise, indicating that muscle and blood physiology of greenback flounder do change in response to exercise. Unlike other flatfish, there was little evidence for in situ glycogenesis within white muscle tissue after exercise and there was some indication that greenback flounder have higher aerobic scope than other flatfish studied to date. This study showed that some routine husbandry practices have the capacity to stress greenback flounder.

Blood oxygen transport, rheology and haematological responses to confinement stress in diploid and triploid Atlantic salmon, Salmo salar.

Abstract Diploid and triploid all-female Atlantic salmon (Salmo salar) smolts were challenged with 2.5 h of confinement stress in aerated seawater. Subsequent stress responses were quantified by plasma cortisol, glucose, and lactate, and by the haematological parameters haematocrit (Hct), red blood cell count (RBCC), mean cell volume (MCV), blood haemoglobin concentration (Hb), mean cell haemoglobin (MCH), mean cell haemoglobin concentration (MCHC), total protein and erythrocyte adenosine triphosphate (ATP). The magnitude of stress responses was similar between diploid and triploid smolts. Triploid smolts displayed a higher MCH, but lower Hb than diploid smolts. The blood oxygen affinity of diploid and triploid fish was similar at 15°C over the pH range 6.76–7.99 and the isohaemoglobin components were identical. The enlarged triploid erythrocytes showed reduced shear-dependence on blood viscosity at constant Hct and are therefore unlikely to contribute to greater peripheral vascular resistance. The results show that despite having fewer, larger erythrocytes, triploids have very similar oxygen carrying capacity and haematological response to stress as diploids. This suggests that the higher farm mortality reported for triploids in response to stress is not generated by their failure to show respiratory homeostasis in the face of stress.

The relationship of ocular morphology to feeding modes and activity periods in shallow marine teleosts from New Zealand

SynopsisThirty one species of shallow water teleosts were captured from the NE coast of New Zealand. Ocular morphology was assessed in terms of eye size, pupil shape, theoretical sensitivity and acuity based on retinal morphology, and regional distribution of photoreceptors within the retina. Eye size was relatively or absolutely larger in carnivores than herbivores. Diurnal planktivores and nocturnal species of small body size maximise vision by having relatively large eyes. Anterior aphakic spaces were present in most of the species examined, and 25% of the species also had posterior aphakic spaces. Theoretical sensitivity was generally higher among nocturnal than diurnal species, however, a number of benthic and pelagic carnivores showed retinal specialization for enhanced sensitivity. Diurnal species displayed high spatial acuity, with maximum acuity occurring in carnivorous species. Crepuscular species had either high or low acuity, whereas that of nocturnal species was generally lower than in diurnal species. Ten species displayed regional variation in rod density, with crepuscular and nocturnal species showing streaks of high rod density in the retina. Eleven species of carnivores displayed regional variation in cone density, with highest density usually occurring in the caudal part of the retina. In most of the species with areas of high cone density, there was a forward visual axis that coincided with the location of the aphakic space, suggestive of accomodation along that axis.

Evidence of daily spawning in natural populations of the New Zealand snapper Pagrus auratus (Sparidae)

SynopsisNew Zealand snapper, Pagrus auratus, were captured by trawling from NE New Zealand over two successive spawning seasons, and examined for acute temporal changes in gonad condition. Fish with oocytes completing final oocyte maturation predominated during the morning, with a peak in ovulated fish occurring just after midday. Afternoon catches were dominated by fish in which the most advanced oocytes had yet to begin final maturation. This suggests that ovulation is synchronised to occur soon after midday, and the high proportion (up to 100% of the catch) of fish with particular gonad stages captured at any one time indicates that daily spawning involves most of the population. Diurnal changes in oocyte diameter support a daily spawning rhythm, with numbers of large hydrated oocytes peaking in the late morning, followed by the disappearance of these oocyte stages in the afternoon. Snapper captured alive by longlining were returned to the laboratory to examine the relationship between ovulation and probable time of spawning. Unovulated fish generally ovulated close to midday on the day of capture (morning captures), or the following day (afternoon captures). The viability of ovulated eggs (proportion undergoing division after fertilization) decreased markedly after oviduct residence times of over 8 hours. This suggests that natural spawning occurs before the late evening. The results of this study are consistent with anecdotal evidence suggesting that spawning occurs every day during the late afternoon or early evening, and is similar to the reproductive patterns displayed by a number of closely related sparids.

The effects of salinity on reproductive development, plasma steroid levels, fertilisation and egg survival in black bream Acanthopagrus butcheri

Abstract The effects of salinity of holding water of 5‰, 20‰ or 35‰ on seasonal reproductive development, plasma steroid levels, the efficacy of luteinizing hormone releasing hormone ethylamide (LHRHa) to stimulate ovulation, sperm motility, and egg fertility and development to hatching were investigated. Fish were captured from the wild from December to February, placed into salinity regimes in May and held until the normal times of spawning the following November. Blood samples were taken in August, September and November. Female fish were injected with saline or LHRHa (50 μg kg−1) in November and bled and checked for ovulation for 5 days. Gonadal maturation was unaffected by salinity in both sexes. In females, seasonal plasma steroid levels were unaffected by salinity, whereas in males, plasma levels of 17,20β-dihydroxy-4-pregnen-3-one (17,20βP) and 11-ketotestosterone were higher in fish held at 35‰ than in fish held at 5‰ in September, and in fish held at 5‰ and 20‰ in November, respectively. Plasma estradiol (E2) and testosterone (T) levels in saline-injected fish, either remained low or were significantly suppressed. LHRHa treatment resulted in the short-term elevation of plasma E2 and T levels at all salinities, whereas plasma 17,20βP levels were elevated over controls on days 1 and 2 post-injection in fish held in 20‰, but remained unchanged in fish held in 35‰ or 5‰. All but one fish ovulated in response to LHRHa, however, the number of ovulations and egg volumes was lowest in fish held at 5‰. Both fertilisation and sperm motility were significantly reduced at 5‰. Naturally fertilised eggs (35‰; 2–8 cell stage) were incubated to hatching at salinities of 0‰, 5‰, 10‰, 15‰, 20‰, 25‰, 30‰ or 35‰. Eggs hatched in all salinities except 0‰, but with lowered survival at 5‰ and 10‰. Larvae showed high levels of deformity at salinities below 15‰.