86 Fred C. Rohde and Rudolf G. Arndt 



in age group O at time of capture, 133 (33.0%) in group I, and 8 (2.0%) in 

 presumed group II. Although the data are limited, and assuming that 

 each age group was collected proportional to its actual number in the 

 population, 33.0% of group survived to spawn as group I fish, and 2.0% 

 to spawn a second time. Survival rates have not been calculated for other 

 populations of any species of Cottus. 



Three just-hatched young (measured to nearest 0.1 mm) from one 

 cluster averaged 6.56 mm SL, and 24 from a second cluster averaged 6.05 

 mm. Hann (1927) observed a mean size at hatching of 6.4 mm SL. Newly 

 hatched young in other studies ranged from 5.6 mm SL to 9.8 mm TL 

 (see Nagel 1980). The body of a newly hatched young is slightly opaque, 

 with no evidence of the dark saddles, and with a prominent yolk sac. Five 

 siblings of 98 hours of the second cluster averaged 6.54 mm SL. The yolk 

 sac of these is greatly reduced and only slightly protrudes from the body, 

 and melanophores form weakly-defined saddles. Five siblings of 195 

 hours averaged 6.96 mm SL. Their yolk sac appears fully resorbed, 

 melanophores are conspicuous on the body, and the saddles are better 

 defined. Three additional siblings of 288 hours averaged 6.63 mm SL. 



Two young-of-year taken on 19 May 1981 were 10 mm SL and 12 

 mm SL, and resembled the adult in morphology and coloration. No 

 young were taken until 8 June in 1980 (Table 4, Fig. 5), at which time 

 they averaged 13.5 mm SL (range 1 1-16 mm), an assumed post-hatching 

 increase of 7.5 mm. Subsequent samples taken approximately biweekly 

 revealed a decrease in growth rate, from a 4.9 mm increase over 20 days 

 to 28 June, to but a 2.4 mm increase in the 15 days to 9 August. 



Most growth occurred in the first calendar year of life. Young taken 

 in July 1974 ranged from 16 to 26 mm SL (x= 21.1 mm). From hatching 

 at 6 mm in early to mid-April, growth in the 1974 year class was rapid: to 

 21.1 mm in July, 24.2 mm in September, and 30.0 mm in November 



Table 4. Monthly mean standard lengths, by sex, for 1978, 1979, and 1980 year classes of Cottus bairdi from 

 Butler Mill Branch, Sussex County, Delaware. Number of specimens in parentheses. 



1978 Year Class 1979 Year Class 1980 Year Class 



Date Unsexed Male Female Unsexed Male Female Unsexed 



37.8(10) 



44.0 (8) 23.1 (56) 



59 (1) 

 59.5 (2) 



7 Sept. 1978 



30.9(19) 



- 



16 Mar. 1979 



- 



49.9 (7) 



25 Jul. 1979 



- 



43.3 (3) 



9, 15,22,30, 







Mar. 1980 



- 



59.4 (4) 



4, 20 May 1980 



- 



- 



8 Jun. 1980 



- 



61 (1) 



28 Jun. 1980 



- 



- 



12 Jul. 1980 



- 



- 



25 Jul. 1980 



- 



- 



9 Aug. 1980 



- 



- 



45.0 (24) 



38.2(62) 



- 



47.3 (3) 



41.4(13) 



- 



47.0 (3) 



42.8 (8) 



13.5(23) 



45 (1) 



41.3 (7) 



18.4(51) 



47.0 (4) 



41.5 (4) 



22.2 (69) 



50 (1) 



43 (1) 



24.5 (66) 



51 (1) 



45 (1) 



26.9(53) 



