H. Thorarensen

Photoperiod and temperature affect plasma growth hormone levels, growth, condition factor and hypoosmoregulatory ability of juvenile Atlantic salmon (Salmo salar) during parr-smolt transformation

Abstract Nine groups of juvenile Atlantic salmon (Salmo salar) were subjected to one of three different daylengths (2L:22D, 8L:16D and 14L:10D) for the duration of 1, 2 or 5 months, and subsequently to 24L:0D. Four groups on natural photoperiod were exposed to a combination of two different temperature regimes (6 and 11°C) with or without night illumination (5 lux), and one group was kept on 24L:0D. Plasma growth hormone (GH) levels, growth rate and hypoosmoregulatory ability increased in response to increased daylength, independent of season. A smoltification-associated decrease in condition factor was absent in fish on 24L:0D and in fish exposed to only 1 month of reduced daylength. Is is concluded that GH has direct roles in stimulating growth and increasing hypoosmoregulatory ability during parr-smolt transformation. Increasing daylength is of overriding importance as a “zeitgeber” for increases in GH levels, growth and hypoosmoregulatory ability during smoltification. Different physiological changes associated with smoltification may be put out of phase by the manipulation of environmental cues, and thus possibly cause an incomplete smoltification.

Gastrointestinal blood flow in the red Irish lord, Hemilepidotus hemilepidotus: long-term effects of feeding and adrenergic control

Abstract Cardiac output, blood flow to the coeliac and mesenteric arteries, dorsal aortic blood pressure and heart rate were recorded simultaneously at rest and postprandial for 6u2009days in a teleost, the red Irish lord (Hemilepidotus hemilepidotus). We anticipated that gastrointestinal blood flow would increase postprandially, supported by an increase in cardiac output. However, we had no predictions for either the exact time-course of this response, or for the regional distribution of blood flow between to the two major arteries comprising the splanchnic circulation. In resting, unfed animals, blood flow to the coeliac artery and mesenteric artery was 4.1u2009±u20090.6u2009mlu2009min−1u2009kg−1 and 4.9u2009± 1.3u2009mlu2009min−1u2009kg−1, respectively (meanu2009±u2009SEM, n=7), which together represented 34% of cardiac output. Feeding increased blood flow to the coeliac and mesenteric arteries in a time-dependent manner. The increase in coeliac artery blood flow preceded that in the mesenteric artery, a finding that is consistent with the coeliac artery supplying blood to the liver and stomach, while the mesenteric artery supplies blood to the stomach and intestine. Coeliac blood flow had increased by 84u2009±u200918% after 1u2009day and had a peak increase of 112u2009±u200940% at dayu20094 postprandial. Mesenteric blood flow was not significantly elevated at dayu20091, but had increased by 94u2009±u200919% at dayu20094 postprandial. Cardiac output also increased progressively, increasing by a maximum of 90u2009±u200930% at dayu20094. Because the increase in cardiac output was adequate to meet the postprandial increase in gut blood flow, the postprandial decreases in vascular resistance for the coeliac and mesenteric circulations mirrored the increases in blood flow. Intra-arterial injections of adrenaline and noradrenaline into resting fish more than doubled coeliac and mesenteric vascular resistances, and blood flow decreased proportionately. This adrenergic vasoconstriction was totally abolished by pretreatment with the α-adrenoceptor antagonist phentolamine, which in itself approximately halved coeliac and mesenteric vascular resistances. These observations indicate a significant α-adrenergic tone in the gastrointestinal circulation of the red Irish lord, the loss of which could not entirely account for the postprandial increase in gastrointestinal blood flow. Other control mechanisms are suggested.

CO2 transport and excretion in rainbow trout (Oncorhynchus mykiss) during graded sustained exercise

A quantitative analysis of CO2 transport and excretion was conducted in seawater acclimated rainbow trout (Oncorhynchus mykiss) swimming at different sustained swimming velocities. CO2 excretion increased linearly with cardiac output during exercise but arterial P(CO2) (Pa(CO2)) and total CO2 levels also increased indicating a diffusion limitation to CO2 excretion. The elevated Pa(CO2) was not accompanied by a decrease in pH, indicating that the acid-base compensation was rapid. Mixed-venous P(CO2) increased to a greater extent than Pa(CO2) resulting in a large increase in the venous arterial difference in P(CO2) (Pv(CO2) - Pa(CO2)). The Pv(CO2) - Pa(CO2) difference was used to calculate the proportion of total CO2 excreted comprised of dissolved CO2 which accounted for less than 1% of total CO2 excreted in fish swimming at 11 cm sec(-1) but increased to about 9% at the greatest swimming velocity indicating that the pattern of CO2 excretion changes during exercise. There was no effect of exercise on the proportion of CO2 excreted which was dependent upon HCO3-/Cl- exchange (54%) or that which was dependent upon the dehydration of HCO3- that resided within the red cell prior to gill blood entry (42%). The large proportion of total CO2 excreted that was dependent upon HCO3-/Cl- exchange is significant because this is thought to be the rate limiting step in CO2 excretion.

The interaction between O2 and CO2 exchange in rainbow trout during graded sustained exercise.

A quantitative analysis of O2 and CO2 transport was conducted in resting and exercising rainbow trout, and these data were used to quantify the magnitude of coupling between O2 and CO2 exchange, in vivo. The release of Bohr protons during haemoglobin-oxygenation was non-linear over the Hb-O2 equilibrium curve used in trout subjected to different levels of sustained exercise. At low swimming speeds, when venous blood O2 content (CvO2) was high, there was a small acidosis as blood passed through the gills, indicating more protons were released during oxygenation of Hb than were consumed during HCO3- dehydration. At higher swimming speeds, when CvO2 was low, there was a significant alkalosis in arterial relative to venous blood, indicating that fewer protons were released upon oxygenation than HCO3- ions were dehydrated to CO2. Haldane coefficients (moles of protons released per mole of O2 which binds to Hb), calculated from steady state arterial and mixed-venous parameters, revealed that under resting conditions all blood CO2 removed from the blood during gill transit was stoichiometrically related to O2 uptake through the release of Bohr protons during Hb oxygenation. The magnitude of coupling between CO2 excretion and O2 uptake decreased from 100% to less than 40% at the maximal swimming velocity when the largest region of the Hb-O2 equilibrium curve was used for gas exchange. The non-linear release of Bohr protons over the range of Hb-O2 saturation in the blood reduces HCO3- dehydration at the gills during greater work loads elevating arterial P(CO2) levels, leading to an increase in HCO3- buffer capacity of the blood and tissues.

Energy Turnover in the Normoxic and Anoxic Turtle Heart

We examined the possibility that the heart of the tuttle Chrysemys scripta is an exceptional anaerobic performer, by measuring myocardial power output, lactate output, and estimated ATP turnover in perfused heart preparations. Over a range of myocardial power outputs at 5 and 15 degrees C we find that turtle hearts perfused with anoxic saline do not show a particularly outstanding ability to produce ATP anaerobically. Furthermore, at 15 degrees C anoxia reduced the ATP turnover rate to 50% of the normoxic rate. At 5 degrees C the anoxia-induced depression of ATP turnover was even more pronounced, being 4-fold lower than the normoxic rate. In addition, anoxia at 5 degrees C reduced the basal metabolic rate of the tuttle heart. We conclude that long-term cardiac tolerance of hypoxia in this species is more likely related to metabolic depression rather than to an exceptional anaerobic performance.

Smoltification induced by a ‘skeleton’ photoperiod in underyearling coho salmon (Oncorhynchus kisutch)

Underyearling coho salmon fry were subjected to three initial photoperiod treatments (6L∶18D, 10L∶14D, 14L∶10D) for two months and subsequently to three final treatments (16L∶8D, 9L∶6D∶1L∶8D, 10L∶14D) in a factorial design. Growth rates and seawater adaptability were monitored regularly. The groups that were exposed initially to 6L∶18D or 10L∶14D and then to 16L∶8D grew faster and had lower plasma sodium ion levels after seawater challenge tests than any of the other groups. Fish which were initially exposed to 6 L or 10 L daylength and then to a 9L∶6D∶1L∶8D skeleton photoperiod, showed a slightly lower growth rate and seawater adaptability than those given the corresponding complete 16L∶8D photoperiod. However fish maintained on skeleton photoperiods had significantly greater growth rates and seawater adaptability than those kept on the 10L∶14D photoperiod. This indicates that it is not the accumulated number of hours of exposure to light that initiates smolting, but rather the time during the day when light is experienced. Fish exposed initially to 14L∶10D showed little or no response to subsequent changes in photoperiod, suggesting that responsiveness to inductive photoperiods depends on the initial photoperiod treatment.

Effect of photoperiod and various intensities of night illumination on growth and seawater adaptability of juvenile coho salmon (Oncorhynchus kisutch)

Abstract Newly emerged coho salmon fry were exposed to a short-day (10L:14D) priming period for 12 weeks and then to inductive photoperiods (9L:9D:1L:5D or 24L:0D) for 10 weeks. During part of the priming period and all the inductive period some groups were exposed to different levels of night illumination ranging from 0.0001 lux to 0.05 lux. Controls were in total darkness at night. All the groups that were exposed to night illumination showed lower growth rate, silvering index, and poorer seawater adaptability than the controls, which responded as typical smolts. The threshold level for the inhibition may be close to 0.0001 lux. The threshold is lower than the light levels that the fish can be expected to experience at night in their natural environment, which may delay smolting for 1 year or more. Transmission of light at 500 nm by the skin and the skull over the pineal was 25% in fry compared with 10% in yearling presmolts.

INFERENCE ON SEGMENTED POLYNOMIAL MODELS

Segmented polynomial regression models with unknown change-points are used in a wide variety of biological settings. The application that stimulated this work uses a segmented polynomial model to examine the optimal hematocrit hypothesis. We discuss problems in the fitting of these models and compare, by simulation, two methods of inference in these models: that based on the chi-squared approximation to the distribution of the likelihood ratio statistic and that based on the asymptotic normality of the least-squares estimates. The results show that, of the two, only the likelihood ratio statistic produces reliable inference concerning the change-point.