308 
Fishery Bulletin 116(3-4) 
is frequently worse than in fish occurring in the wild. 
For example Graham and Orth (1987) reported that 
the clarity of otolith increments in laboratory-reared 
smallmouth bass was less than that viewed in wild in¬ 
dividuals. It could also have been the case that in the 
present study, the clarity of otoliths of northern pike 
larvae was not as good as it possibly could have been 
if the otoliths were collected from wild fish. As with 
the present results, lapilli are found to be useful for 
aging in many species (e.g., Hoff et ah, 1997; Bestgen 
and Bundy, 1998). Ichimaru and Katsunori (1995) pre¬ 
ferred lapilli for collecting age data for two species of 
flyingfish larvae (Cheilopogon doederleini and Cypselu- 
rus hiraii ) because the increments in the lapilli were as 
clear as those in the sagittae and did not require any 
preparation. Bestgen and Bundy (1998) reported that 
increments deposited on the sagittae of Colorado pike- 
minnow (Ptychocheilus lucius) were difficult to distin¬ 
guish after the fish were 30 days old. Therefore, those 
authors used lapilli to age older fish. 
On some of the otoliths (mostly sagittae) a stress 
check was deposited on day 7 (and less frequently on 
days 14 and 22), which corresponded with the time of 
sample collection from the aquaria. Increment counts 
between the otolith edge and the check formed on day 
7 provided additional confirmation of daily periodicity 
of increment deposition. Stress checks related to han¬ 
dling are frequently observed on larval fish otoliths, 
and some authors use them, as we did in this study, 
as markers for increment validation (Volk et al., 1984; 
Boehlert and Yoklavich, 1985). Moreover, lower SD for 
the counts from the stress check to the otolith edge, 
compared with SD for counts from the otolith center 
to the edge, suggests that the source of error in aging 
northern pike is related largely to distinguishing incre¬ 
ments in the central otolith area (approximately first 
7 days). A more irregular increment pattern in the oto¬ 
lith center was also reported in northern pike by Wang 
and Eckmann (1992). 
Acknowledgments 
This article is a contribution to statutory project 
Doll 8/P IKE conducted at the National Fisheries Re¬ 
search Institute and financed by the Ministry of Sci¬ 
ence and Higher Education, Poland. Part of this work 
was co-fmaneed by statutory project no. 18.610.001-300 
conducted at the Department of Fish Biology and Pisci¬ 
culture, University of Warmia and Mazury in Olsztyn. 
The authors would like to thank M. Teodorowicz from 
Komorowo Fish Farm for technical support during in¬ 
cubation of fish eggs and H. Wrobiewska for help with 
otolith extraction. 
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