Mugiya and Oka: Biochemical relationship between otolith and somatic growth in Oncorhynchus mykiss 



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Discussion 



RNA-DNA ratios in muscle have been widely accepted 

 as an index of current somatic growth rates in various 

 species of marine and freshwater fish (Bulow 1987). 

 These ratios are affected by various factors such as 

 season (Bulow et al. 1981), ration size (Bulow 1970, 

 Buckley 1979, Wilder and Stanley 1983, Jiirss et al. 

 1986), temperature, salinity (Jiirss et al. 1987), and 

 lunar cycles (Farbridge and Leatherland 1987). Various 

 toxicants also reduce the ratios (Barron and Adelman 

 1984). The present study presents an additional find- 

 ing with regard to variations of RNA-DNA ratios: the 

 ratios had distinct diel variations, showing higher 

 values during daytime than nighttime. It is desirable 

 to confirm such a profile of variations by further ex- 

 aminations at shorter and longer time-intervals. 



Endocrinologically, RNA-DNA ratios are under the 

 control of growth-regulating hormones. Hypophysec- 

 tomy reduced the ratios, and replacement therapy with 

 beef growth hormone restored the ratios to a normal 

 level in bullheads Ictalurus melas (Kayes 1979). Orca- 

 dian periodicities in the surge of growth-regulating 

 hormones are well documented in higher vertebrates 

 (Kato et al. 1982). Although few comparable references 

 are available in fish, cyclic variations in growth hor- 

 mone have been reported in plasma and pituitary levels 

 in salmonids (Leatherland et al. 1974, Leatherland and 

 Nuti 1982, Bates et al. 1989). Therefore, the cyclic 

 surge of this hormone in combination with other hor- 

 mones is probably a cause for diel variations in RNA- 

 DNA ratios in the present fish. 



Effects of starvation or restricted food on RNA-DNA 

 ratios have been repeatedly reported. A 4-week star- 

 vation induced an approximately 40% decrease in the 

 ratios in rainbow trout Oncorhynchus mykiss (Jiirss et 

 al. 1986). Brook trout Salvelinus fontinalis also had the 

 ratio reduced by 44% after 22 days of restricted feed- 

 ing (Wilder and Stanley 1983). Bulow (1970) showed 

 that RNA-DNA ratios directly reflected different soma- 

 tic growth rates induced by nutritional manipulation. 

 However, it is not clear how fast food deprivation 

 affects this ratio. Although the time course of the ef- 

 fect will depend on various factors such as tempera- 

 ture, fish sizes, sexual status, etc., the present study 

 revealed that the first effect of starvation on RNA- 

 DNA ratios in muscle appeared on day 2 (67 hours from 

 last feeding) in adult rainbow trout Oncorhynchus 

 mykiss. This inhibitory effect was exaggerated with 

 time, but the ratios never decreased below half the con- 

 trol level even on day 15 after starvation (data are not 

 presented here). This level (55% of the control) is 

 similar to trout values in the literature mentioned 

 above, and therefore it appears to be a basal level in 

 RNA-DNA ratios in white muscle of rainbow trout. 



Starvation-induced decrease in RNA-DNA ratios 

 recovered with the resumption of daily feeding at a 

 level of 1.5% of body weight. The recovery processes 

 were rather steady, taking 4 days in the present study. 

 Bulow (1970) showed a more rapid recovery in golden 

 shiners Notemigonus crysoleucas: 2 days feeding at 6% 

 body weight seemed to be enough for the recovery of 

 a starvation-induced decrease in ratios to the normal 

 level. This disparity may be explained by the amount 

 of food used and/or species difference. Lied et al. (1983) 

 reported that a starvation-induced decrease in RNA- 

 DNA ratios recovered to the normal level within 8 

 hours after refeeding ad libitum in cod Gadus morhua. 

 However, it is uncertain whether this recovery was due 

 to the result of food assimilation or to diel variations 

 in the ratios, because control values were not available 

 at each determination time in the study. 



Recently Miglavs and Jobling (1989) reported that 

 a growth spurt (compensatory growth) transiently oc- 

 curred following realimentation after a period of food 

 restriction in juvenile Arctic char Salvelinus alpinus. 

 Nevertheless, no corresponding increase occurred in 

 tissue RNA-DNA ratios. This led them to present a 

 limit to using the ratios as a growth index. However, 

 since their somatic growth rate was calculated from 

 a cumulated increase in body weight for 28 days, it is 

 uncertain whether or not the rate remained high on the 

 last day, when the RNA-DNA ratios were determined. 



When somatic and otolith growth rates were bio- 

 chemically compared at 6-hour intervals over a 24-hour 

 period, or after food deprivation, they were coupled at 

 a qualitative level. In spite of these positive relation- 

 ships, evidence of an allometric relationship between 

 otolith and somatic growth has been accumulated, 

 especially under suboptimal conditions. For example, 

 Mosegaard et al. (1988) examined the effect of tem- 

 perature, fish size, and somatic growth rate on otolith 

 growth rate in Arctic char Salvelinus alpinus and found 

 an uncoupling between somatic and otolith growth rates 

 at hyperoptimal temperatures. Based on these results, 

 they suggested that metabolic activity, not necessarily 

 somatic growth rate, governs otolith growth rate. 

 Somatic growth rate results mainly from the balance 

 between protein synthesis and degradation, and hyper- 

 optimal temperatures would accelerate both compo- 

 nents, especially degradation, resulting in no somatic 

 growth (Houlihan et al. 1988). However, since somatic 

 growth is comparable with components from metabolic 

 rates within the range of optimal temperatures (Webb 

 1978), the muscle RNA-DNA ratio, an index for pro- 

 tein synthesis, will be a reflection of metabolic com- 

 ponents at an appropriate temperature. Therefore, it 

 appears reasonable to use this ratio for examining the 

 relationship between somatic and otolith growth rates, 

 even if otolith growth is a function of metabolic rate. 



