The photoperiod control of coho salmon smoltification

Aquaculture, 28 (1982) 105-111 Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands

THE PHOTOPERIOD

CONTROL

105

OF COHO SALMON SMOLTIFICATION

EDWARD P. BRAUER Domsea Farms, Inc., 4398 West Old Belfair Highway,

Bremerton,

WA 98310

(U.S.A.)

(Accepted 15 January 1982)

ABSTRACT Brauer, E.P., 1982. The photoperiod control of coho salmon smoltification. 28: 105-111.

Aquaculture,

Photoperiod has been implicated as an effective mediator of coho salmon smoltification. To investigate the possibility of manipulating photoperiod to control the occurrence of this event, a study on coho salmon (1.7 g initial weight) was conducted over a 24-week period. Of the treatments tested: natural; rate extended; sustained maximum; and phase adjusted; the latter resulted in the greatest level of growth and smoltification in terms of osmoregulatory ability. This suggested that not only was the appropriate photoperiod important to smoltification, but so were certain growth characteristics such as absolute weight and growth rate.

INTRODUCTION

During the parr- smolt transformation in fresh water, some species of juvenile salmonids experience a series of morphological (Johnston and Eales, 1966), behavioral (Mason, 1975; Fried et al., 1978) and physiological (Hoar, 1976) alterations which, in nature, are expressed as the seaward migration of silvery fish with the osmoregulatory capacity for ocean survival. For hatchery-reared coho salmon (Oncorhynchus hisutch) the first opportunity to undergo this metamorphosis occurs in June, following the first 6 months of growth post hatching, provided an accelerated growth response has been induced (Brannon et al., 1975). For the salmon culturist, failure to produce such fish, commonly referred to as zero-age smolts, forces an additional 12 months of freshwater rearing. Because the carrying capacity of any freshwater culture system is finite, and holdover, or yearling smolts, are of a larger size, production, expressed as numbers of fish, is significantly reduced. These two factors -- longer culture time and reduced throughput - combined with various other inefficiences related to seasonal production, provide an incentive to identify practical techniques for smoltification control. Of the environmental factors that have been implicated as smoltification mediators; temperature (Zaugg et al., 1972; Adams et al., 1975), salinity

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(Otto, 1971) and photoperiod (Saunders and Henderson, 1970; Wagner, 1974; Komourdjian et al., 1976), the latter has been characterized as the primary stimulus. To investigate the potential of controlling the timing of coho salmon smoltification through photoperiod manipulation, the experiment described as follows was conducted. METHODS

AND MATERIALS

Test fish The coho salmon used in this study were collected as eggs from the Washington State Department of Fisheries Skagit Fish Hatchery in early December 1977. Following culture at 10°C through the eyed stage at Domsea Farms’ Gorst, Washington facility, the eggs were air shipped to the Union Carbide, Tarrytown Technical Center at Tarrytown, New York, for final incubation at 12°C. In March 1978, approximately 2 months postponding, 65 fish, averaging 1.7 g, were randomly distributed into each of 24 test vessels. Treatments The four photoperiod treatments tested were designated as follows: (1) natural; (2) phase adjusted; (3) rate extended; (4) sustained maximum. These treatments were maintained by weekly adjustments of timer-controlled broad spectrum fluorescent lamps (Duro brand Vita-lites) to provide the daylength exposures listed in Table I. The natural treatment was designed to simulate a spring photoperiod at 50”N latitude during the actual study period of 22 March through 6 September. This treatment served as the control. The phase adjusted treatment was designed to delay the day of longest daylength 5 weeks. This was accomplished by maintaining the 22 March day length regime for 6 weeks, and then repeating those adjustments previously used in the natural treatment. The rate extended treatment maintained the normal linearly increasing daylength phase of the natural treatment for an additional 5 weeks after 17 May. Subsequent weekly changes in daylength were computed to equal those differences that would have occurred had the natural treatment been followed. During the first 14 weeks of the sustained maximum treatment, the weekly daylength adjustment schedule of the natural treatment was followed. Thereafter, in contrast to the declining photoperiod of the natural exposure, this treatment continued its longest daylength dosage until termination of the study.

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TABLE1 Experimentalphotoperiod conditionsexpressed ashoursof daylight Photoperiodtreatmentdaylength exposures Week No. Date

Natural (h)

Rateextended Sus.max. (h) (h)

Phaseadjusted (h)

Initial 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

12.27 13.38 13.13 13.55 13.98 15.07 15.45 15.15 15.50 15.80 16.02 16.87 17.00 17.03 17.00 16.88 16.05 16.48 15.53 15.20 15.50 15.13 14.05 13.63 13.20

12.27 13.38 13.13 13.55 13.98 15.07 15.45 15.15 15.50 16.65 17.07 17.48 17.22 17.63 18.60 18.82 19.00 19.13 19.17 19.10 18.98 18.15 18.58 17.63 17.33

12.27 12.27 12.27 12.27 12.27 12.27 13.38 13.13 13.55 13.98 15.07 15.45 15.15 15.50 15.80 16.02 16.87 17.00 17.03 17.00 16.88 16.05 16.48 15.53 15.20

3122 3127 41 5 4112 4119 4126 51 3 5/10 5/17 5124 5131 61 7 6114 6121 6128 71 5 7112 7119 7126 81 2 81 9 S/l6 S/23 8130 91 6

12.27 13.38 13.13 13.55 13.98 15.07 15.45 15.15 15.50 15.80 16.02 16.87 17.00 17.03 17.03 17.03 17.03 17.03 17.03 17.03 17.03 17.03 17.03 17.03 17.03

Facilities Four light proof boxes were constructed around arrays of six 19.0 1 circular containers to maintain treatment integrity. Each of these replicate containers was equipped with its own supply of 12.O”C well water at a constant flow rate of 3.8 l/min. To provide uniform illumination, a single 46-cm fluorescent tube was suspended 20 cm above the test vessels which were equipped with individual clear plexiglass covers. Using a spectral radiometer, the spectral intensity from 380 to 750 nm, at lo-nm intervals, was measured immediately above the water’s surface and was determined to be equivalent in each chamber. Culture A common dry diet was fed throughout the experiment at dosage rates that varied with body size (Bardach et al., 1972). A proximate analysis of

108

this diet is presented in Table II. Individual daily rations were distributed over four to five feedings in a 7-h period, 5 days/week. On the other 2 days, a normal single day’s ration was divided over a similar number of presentations. Every 2 weeks the fish in each container were counted and weighed (total wet weight). Feed rates were then readjusted accordingly. An additional adjustment was made on the seventh day following a weighing, by assuming a 50% food conversion on a dry to wet weight basis. TABLE II Proximate analysis of experimental feed Parameter

Value

Protein Moisture Ash Fat Fiber Carbohydrates Calories NFE

47.5% 6.8% 11.0% 10.4% 2.7% 21.6% 37O/lOOg 21.6%

Data collection and analysis Average weight gains (calculated from regressions), specific growth rates and food conversions were subjected to one-way analysis of variance and if significant (P
Growth During the first 18 weeks of the study no significant differences in average weights of test groups were detected. During this period the average weights for all treatments increased from 1.7 to 18.8 g. However, during the next 6 weeks the phase adjusted group achieved a significantly larger average weight resulting in a final weight of 34.7 g versus 23.7 g for the other groups. A complete analysis of the average 2-week gain/fish, specific growth rate and

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TABLE III Average 14day gain/fish, specific growth rate and food conversion rate observed during a 26-week exposure to natural, rate extended, sustained maximum and phase adjusted photoperiods, Underlined values indicate significant (P >0.05) similarity Photoperiod

treatments

Natural

Rate extended

Sustained max.

Phase adj.

1.1

2.1

2.1

2.8

Specific growth rate (% gain/day)

1.6

1.6

1.7

1.8

Conversion (feed g/weight gain g)

2.2

2.0

1.9

1.6

Gain/fish (g)

TABLE IV Average weights and specific growth rates during the week of longest daylength of fish exposed to natural, rate extended, sustained maximum and phase adjusted photoperiods Treatment

Week No.

Average weight (9)

Specific growth rate (% gain/day)

Natural Rate extended Sustained maximum Phase adjusted

13 18 13 18

11.6 17.3 12.4 20.7

1.05 1.28 1.09 1.54

food conversion for all treatments is presented in Table III. Additionally, the average weight and specific growth rate for each treatment during its week of longest day length exposure demonstrated that the phase adjusted group had both the largest and fastest growing fish (Table IV). Smoltification

With the exception of week 22, the phase adjusted treatment maintained the lowest average plasma sodium levels following seawater challenge (Table V). A declining trend in sodium values was observed in this treatment, reaching a low point of 174 meq/l at week 17. Plasma sodium values increased above 190 meq/l by week 21. The value of the adjusted treatment at week 17 was determined to be significantly different from the others. The only other significant difference in plasma sodium values was observed at week 21, when fish from the sustained maximum group exhibited notably higher values indicative of extremely poor saltwater adaptability.

110 TABLE V Average post seawater challenge plasma sodium values (meq/l) during a 26-week exposure to natural, rate extended, sustained maximum and phase adjusted photoperiods Week No.

Natural

Rate extended

Sustained max.

Phase adjusted

11 12 13 14 15 16 17 18 19 20 21 22 23 24

197 200 201 203 207 192 198 194 194 198 202 210 203 204

197 198 205 195 203 198 202 197 205 206 203 204 205 199

202 195 200 208 206 189 202 184 216 202 225(s) 192 212 215

192 189 190 187 187 179 174(s) 180 185 189 200 195 197 196

(s) Denotes a significant treatment difference.

DISCUSSION

The phase adjusted exposures resulted in the fastest, most efficient and greatest growth, and the highest level of saltwater adaptation. The development of the observed osmoregulatory response was focused around and peaked at the week preceding the longest daylength exposure. This would indicate the existence of some type of relationship between the peak of smoltification and the peak of photoperiod in the phase adjusted test group. No similar relationships were found among the other experimental treatments just prior to their longest daylength exposure. An explanation of this phenomenon has been developed involving the growth results. For example, during week 13 when the natural and sustained maximum groups were at their maximum daylength exposure, not only were their growth rates comparatively low, but the fish were smaller than the minimum smolt size postulated by Brannon et al. (1975). In contrast, presumably adequate mean weights were achieved in the phase adjusted and rate extended groups, due primarily to the extended growth period gained by delaying the longest daylength exposure for 5 weeks. However, only in the case of the phase adjusted group, with the greatest growth rate, was an elevated level of hypoosmoregulatory ability observed. This correlation between growth rate, smoltification and an increasing photoperiod is consistent with previously reported results (Komoudjian et al., 1976; Clarke et al., 1981). In conclusion, the results of this study indicate the following:

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(1) The timing of coho salmon smoltification can be manipulated by adjusting the phase of the natural spring photoperiod. (2) For smoltification to occur, certain size and growth rate limitations must be satisfied.

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