FISHERY BULLETIN: VOL. 80. NO. 2 



position of larval fish populations, since this in- 

 formation can provide estimates of growth, mor- 

 tality, and rates of survival (Gulland 1977). The 

 highest mortality of fishes is during the growth 

 period from larvae to juveniles (Hjort 1914; 

 Tanaka 1972) and consequently, the survival and 

 growth of larval fishes has a pronounced effect 

 upon recruitment (Larkin 1978). It should be 

 possible, by using otoliths for estimation of the 

 age, to determine the growth rates and the age 

 structure of larval fish populations. 



Daily increments have been correlated with 

 natural temperature cycles, light and food for 

 freshwater species by Brothers (1978, 1980 4 ). 

 Taubert and Coble (1977) postulated that daily 

 increments in otoliths of freshwater sunfish re- 

 sulted from a 24-h diurnal light cycle that en- 

 trained an internal clock. 



To utilize daily depositional increments of the 

 otoliths in the analysis of fish population dynam- 

 ics, it is important to understand the physiologi- 

 cal mechanisms involved in the formation and 

 growth of increments and otoliths. Age estima- 

 tion requires knowledge of 1) age when incre- 

 ment formation begins; 2) factors which control 

 the deposition of daily increments in the otoliths; 

 and 3) length of time daily increments are 

 formed without growth interruption. Informa- 

 tion in these areas will make it possible to better 

 understand age and growth in wild populations 

 of fish. 



An important area for research in the field of 

 age and growth is the experimental study of the 

 factors which influence the deposition of incre- 

 ments in otoliths. Brothers et al. (1976) showed 

 that daily increments began to form at different 

 ages in different species. Some species hatch 

 with increments already formed, while others 

 apparently do not form increments until later. 

 Thus, it is necessary to study the formation of in- 

 crements in each species and correlate incre- 

 ment formation with external factors before 

 accurate age determinations can be made. 



The mummichog, Fundulus heteroclitus, is an 

 abundant estuarine fish and an important com- 

 ponent of the estuarine ecosystem (Cain and 

 Dean 1976; Valiela et al. 1977; Kneib and Stiven 

 1978; Merideth and Lotrich 1979). The biology of 

 Fundulus is well-known and its embryology is 

 well-defined (Armstrong and Child 1965). 



The objectives of this study were to 1) delineate 



4 E. B. Brothers, Section of Ecology and Systematics, Cornell 

 University, Ithaca, NY 14850, pers. commun. October 1980. 



the structure and formation of otoliths in the 

 embryological and early larval stages of the 

 mummichog, 2) determine the effect of photo- 

 period on incrementdeposition in embryonic and 

 postlarval mummichog otoliths, 3) measure the 

 effects of temperature on body growth and the 

 deposition of increments in otoliths, and 4) test 

 whether growth and age data can be obtained in 

 wild populations of mummichogs by counting 

 the increments in otoliths. 



METHODS 



Adult F. heteroclitus used as spawning stock 

 were collected from North Inlet Estuary (lat. 

 32°20'N, long. 79°10'W) and North Edisto Estu- 

 ary (lat. 32°26'N, long. 80°12'W), near George- 

 town, S.C. Fertilized eggs were collected as pre- 

 viously described by Middaugh and Dean (1977). 

 Only embryos which developed according to the 

 criteria of Armstrong and Child (1965) were uti- 

 lized in the embryological studies, and only lar- 

 vae which hatched within 6 h of hatch induction 

 were used in the growth studies. The embryo is 

 the stage from fertilization to hatching; from 

 hatching to yolk-sac absorption is the larval 

 stage and the mummichog was considered a 

 juvenile after yolk-sac absorption (Hubbs 1943). 



The terms used to describe growth increments 

 in otoliths are confused, as the increments in lar- 

 vae are variously referred to as lamellae, rings, 

 or layers. The term increment in this study re- 

 fers to a unit formed by an unbroken incremental 

 zone and a discontinuous zone after core forma- 

 tion (Fig. 1), Wild and Foreman (1979). 



Newly hatched larvae were kept at 24°C and 

 30°C±1°C (Radtke and Dean 1979) and were fed 

 brine shrimp, Artemia nauplii, ad libitum and 

 maintained in L12:D12 with a daily change of 

 water (30%„) to determine the effect of the rate of 

 growth on otolith size and increment number. 



A daily sample of 10 larvae was collected for 

 laboratory experiments from each group for the 

 first 10 d, and every 5 d thereafter for 30 d. Stan- 

 dard lengths (SL) were measured on each larva 

 and its otoliths were removed. Photomicro- 

 graphs were made of each otolith for increment 

 counts. 



Juvenile mummichogs were collected from We 

 Creek in North Edisto Estuary on 9 June 1977 

 (28°C, 297..). Each fish was weighed, measured 

 for standard length (SL), and its otoliths extract- 

 ed for increment counts from photographs. Sta- 

 tistical analyses of the data were done with 



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