CAMPANA: DAILY GROWTH INCREMENTS IN OTOLITHS OF PLAINFIN MIDSHIPMAN 



increments in all environments. However, foremost 

 among the age-associated effects (Table 2) was the 

 prominence of daily increments in juveniles relative 

 to larvae. Distinction between daily and subdaily 

 increments was seldom difficult in juveniles (outside 

 of the acclimation period) unlike the situation in lar- 

 val otoliths. If this age-related difference in daily 

 increment formation is universal, daily increment 

 counts in larvae may be unreliable relative to slightly 

 older fish. This suggestion has serious implications 

 for the application of growth increments in aging lar- 

 val fish. Similarly, the absence of definitive criteria 

 for differentiating daily and subdaily increments 

 could cause problems in aging field-collected fish. 

 Subdaily increments can be numerous and confusing 

 in some species (Campana, unpubl. data). 



The demonstration of and age-related rhythm and 

 the existence of an acclimation period may have 

 resolved some of the conflicting results in the litera- 

 ture concerning the zeitgeber effect of light. In a pre- 

 vious study, a constant light regime did not influence 

 the production of daily increments in juvenile starry 

 flounders (Campana and Neilson 1982). The floun- 

 ders were about 8 mo-old, suggesting that the 

 necessary acclimation period would be short. In addi- 

 tion. the fish were exposed to the experimental 

 environment for 2 wk prior to tetracycline injection 

 (marking the start of the experiment); it is probable 

 that acclimation occurred during this period, resulting 

 in clear daily increment production by the time the 

 experiment began. An analogous explanation may 

 explain the results of another study, where chinook 

 salmon eggs, reared in darkness, produced daily 

 increments after hatch (Neilson and Geen 1982). The 

 embryos were held in total darkness for 50 d before 

 hatch, suggesting that their endogenous circadian 

 rhythm had time to acclimate before hatch. 



A fluctuating temperature regime did not entrain 

 increment production under constant light con- 

 ditions. Fish reared in this environment produced 

 more increments than would be expected of daily 

 production, similar to those of 24L/CT fish. The 

 variance of larval increment counts was similar to 

 that produced under a constant environment, both of 

 which were significantly larger than the 1 4L: 1 OD/CT 

 variance (Bartlett's test,P< 0.01). Once acclimation 

 occurred, daily increments were produced through 

 an apparently endogenous periodicity, and not 

 through temperature entrainment of an internal 

 clock. However, the formation of a broad, optically 

 dense, sharply delineated opaque zone in postac- 

 climation daily increments indicates that tempera- 

 ture fluctuation did affect increment production. The 

 opaque portion of a daily increment consists of 



calcium carbonate and a proteinaceous matrix, with 

 the latter component predominating (Brothers 

 1981;Mugiyaetal. 1981). Falling temperatures, such 

 as would occur at night, may have increased the pro- 

 portion of protein deposited in the opaque region, 

 resulting in an increment that had increased visual 

 contrast. Accentuation of contrast renders in- 

 crements visually prominent, and could easily be 

 interpreted as an entraining mechanism. Diel tem- 

 perature fluctuations noticeably accentuated incre- 

 ment contrast in young chinook salmon otoliths ( J. D. 

 Neilson 2 ). A correlation of increasing protein deposi- 

 tion with decreasing temperature suggests that the 

 broad opaque zone formed during the low tempera- 

 ture, 1 0-h, experimental "night", overlaid the opaque 

 zone formed under circadian control through a 3-h 

 period (Mugiya et al. 1981). If temperature does 

 exert a "masking" effect (Enright 1981), a low 

 temperature-induced opaque zone would appear 

 independently of any endogenous circadian rhythm 

 of deposition. Therefore, multiple daily oscillations 

 in temperature could conceivably produce a distinct 

 increment after each cycle, in addition to the daily 

 increment formed under endogenous control. In 

 some situations, the masking effect of temperature 

 fluctuations may be substantial, obscuring most of 

 the increments formed through the action of an 

 endogenous rhythm of deposition (E. B. Brothers 3 ). 

 This hypothesis is consistent with studies that 

 demonstrated that temperature cycles do not entrain 

 daily increment production (Campana and Neilson 

 1982; Neilson and Geen 1982), but can influence 

 increment formation (Brothers 1981). 



My results suggest that a diel light cycle entrains an 

 endogenous circadian rhythm of increment deposi- 

 tion. Increasing age mitigated the zeitgeber effect of 

 photoperiod, while temperature fluctuation influ- 

 enced increment appearance, rather than perio- 

 dicity. In other studies, the incidence of subdaily 

 increments was correlated with feeding periodicity 

 (Neilson and Geen 1982; Campana 1983). The fact 

 that so many variables may affect increment deposi- 

 tion suggests that the environment does not 

 influence the rhythm of otolith deposition directly, 

 but acts through some penultimate process. 

 Metabolic rate is susceptible to environmental 

 influence, as well as being subject to an endogenous 

 circadian rhythm (Matty 1978) that changes with age 

 (Davis 1981). However, metabolic rate is in turn 



-J. D. Neilson, Marine Fish Division. Biological Station, St. 

 Andrews, New Brunswick, Canada EOG 2X0, pers. comraun. Jan- 

 uary 1983. 



'K. H. Brothers, Division of Biological Sciences, Cornell Univer- 

 sity, Ithaca, XV 1 1850, pers. comraun. May 198 3 



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