This article is a part of the Special Issue on Aquaculture
Light intensity and suppression of nocturnal plasma melatonin in Arctic charr (Salvelinus alpinus)☆

https://doi.org/10.1016/j.cbpa.2018.11.012Get rights and content

Highlights

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    Arctic charr were sensitive to nighttime illumination as low as 0.1–0.3 lx.

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    Light intensity above 50 lx was effective to suppress the nocturnal plasma melatonin.

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    Somatic growth was independent of nighttime lighting.

Abstract

The problem of early sexual maturation among farmed Arctic charr and other salmonids can be effectively reduced by 24 h light overwinter, provided it is bright enough to over-ride interference from the natural daylength cycle. To determine the threshold light intensity to suppress the nocturnal elevation of plasma melatonin, three groups of individually tagged fish (n = 26–28/group ca. 1040 g) were reared on 12 h light: 12 h dark (LD 12:12) and subjected to nighttime light intensities of either 50–65, 0.1–0.3 or 0 (control) lux for five months (November to April). Daytime light intensity was 720–750 lx. Diel plasma melatonin profiles in both November and April were similar; mean daytime levels ranged from 20 to 100 pg/ml, and nighttime levels were inversely proportional to light intensity. In the control group at 0 lx, plasma melatonin increased about four-fold after lights-off, ranging between 320 and 430 pg/ml. Nighttime light intensity of 0.1–0.3 lx halved plasma melatonin levels to 140–220 pg/ml, and 50–65 lx further reduced the levels to one quarter of the control group, 68–108 pg/ml. Among the lit groups, daytime plasma melatonin levels were about 20–30 pg/ml, significantly lower than the nocturnal levels suggesting the diel hormonal rhythm was not completely abolished. Fish grew steadily from about 1100 g to 1600 g between November and April, independent of light intensity (P = .67). Overall, the study demonstrated the sensitivity of pineal melatonin hormone to different light intensities in Arctic charr.

Introduction

In Atlantic Canada, unwanted early sexual maturation has greatly hindered the commercial farming of Arctic charr (Fraser River strain, Labrador stock). Our goal is to devise lighting protocols to minimize this problem (Liu and Duston, 2016, Liu and Duston, 2018). Photoperiod manipulation, specifically, continuous light overwinter, is used extensively to prevent early sexual maturation among farmed Atlantic salmon (Salmo salar; Iversen, et al. 2016). Failure to reduce maturation can occur, particularly in large sea cages, when the intensity of the electric lighting is too low to override the ambient daylength cycle (Hansen et al., 2017). Measurement of the suppression of plasma melatonin is a useful tool to estimate the appropriate lighting requirements (Porter et al., 1999; Skulstad et al., 2013). Here, the objective was to determine the light intensity needed to suppress the nighttime elevation of plasma melatonin in Arctic charr.

Across the vertebrates, the pineal gland transduces light information into a hormonal melatonin signal reflecting the prevailing photoperiod, with circulating titers high during the night and low during the day (Falcón et al., 2011). Arctic charr, the northernmost freshwater fish, is no exception, the oscillation of plasma melatonin conforms to the extreme seasonal change of daylength in the circumpolar regions (Strand et al., 2008). Wild Arctic charr under thick ice in a sub-Arctic lake were able to track daylength despite very small changes in irradiance; melatonin was suppressed by intensities above a threshold of 1–10−2 to 1–10−3 W/m2 (Strand et al., 2008; about 0.25–2.5 lx based on Thimijan and Heins, 1983). The Fraser River Arctic charr also overwinter under thick ice in Labrador (Dempson and Green, 1985; Latitude 56°N). Here, two nighttime light intensities were compared: 50–65 and 0.1–0.3 lx, at the extreme of what is hypothesized to suppress nocturnal melatonin secretion, against a 0 lx ‘black-out’ control group.

Section snippets

Rearing conditions and photoperiod treatments

Arctic charr (Fraser River stock, Labrador strain) were supplied by a pedigreed breeding program at Shippagan, New Brunswick (Valorēs). In our lab, fish were reared under simulated light cycle (Latitude 45°N) in a single tank (1200 L) supplied with well water (10 °C; Oxygen saturation > 80%; total alkalinity 100 mg/L; total hardness 188 mg/L). In late October, two-year old immature fish (n = 83, ca. 1040 g), each identified with a passive integrated transponder (PIT) tag, were randomly stocked

Results

In the control group, mean plasma melatonin increased over four-fold from 98 pg/ml at 18:00 h (daylight) to 430 pg/ml at 22:00 h, 2 h after ‘lights off’ (P < .05; Fig. 3). During the 12 h night period at 0 lx, the mean plasma melatonin declined, but the change was not significant. Following lights-on at 08:00 h, mean plasma melatonin at 10:00 h was 99 pg/ml, a significant decrease from nighttime levels (P < .05; Fig. 3). Both nighttime illumination treatments significantly reduced melatonin

Discussion

The light intensity necessary to suppress a melatonin response in Arctic charr is very low. Illumination above 50 lx at the mid-water level is recommended to ensure the effectiveness of photoperiod manipulation and deserves to be further tested in a commercial setting. Nighttime illumination suppressed the plasma melatonin in Arctic charr dependent on light intensity, confirming previous studies on salmonids (Porter et al., 2001; Taylor et al., 2006; Vera et al., 2010).

The significant

Acknowledgements

This work was part of the project ‘Aquaculture Development and Profitable Commercialization of Arctic charr in Canada’ funded by the Atlantic Innovation Fund (199444) through the Valorēs, Shippagan, New Brunswick (Coastal Zones Research Institute). We would like to thank Marcia Versloot for assistance in melatonin analysis performed at the New Brunswick Research and Productivity Council.

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    This article is part of a special issue entitled: Aquaculture, edited by: Dr. Tillmann Benfey, Dr. Inna Sokolova and Dr. Mike Hedrick.

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