Necturus lewisi Study: Distribution & Ecology 21 



detailed statistical analysis for stream depth vs. capture success was 

 done. However, a few generalities were apparent. Capture success in 

 streams less than 5 1 cm deep was very poor (5.3%; 4 of 75 samples). The 

 shallowest stream depth recorded for a positive N. lewisi site was 31 cm. 

 Capture success in streams 51 to 100 cm deep was better (14.6%; 25 of 

 171 samples). Streams over 100 cm deep had the best capture success 

 rate (25%; 126 of 504 samples). 



Water flow rate was recorded for 635 sampling efforts. Attempts to 

 obtain "normal" flow readings were hampered by fluctuating water lev- 

 els that changed flow patterns and rates, so no detailed statistical analy- 

 sis was done. The flow rate data presented in Table 2 suggest reduced 

 capture success in the slowest flow rate category. A lower capture suc- 

 cess rate in slow flowing or lentic water is supported by a lack of any 

 known N. lewisi collections from lakes or ponds. 



Table 2. Comparative Necturus lewisi capture success at different water flow 

 rates. Number of sampling efforts for each flow rate category given in 

 parentheses. Percentages are success rates for each category. 



Capture Flow rates (cm/ sec.) 



success _io ii_20 21-30 31-40 41-50 51-60 61-70 71-80 81-90 91 + 



(0) 



(25) 



(0) 



(10) 





28.6% 



Negative (61) (106) (96) (76) (76) (30) (14) (23) 



Positive (4) (24) (35) (19) (25) (4) (4) (3) 



6.2% 18.5% 26.7% 20.0% 24.8% 11.8% 22.2% 11.5% 



Totals (65) (130) (131) (95) (101) (34) (18) (26) (0) (35) 



Analysis of stream substrate type and capture success rate for 743 

 samples (Table 3) revealed a significantly better capture rate where clay 

 or hard soil was predominant. Capture rates were not significantly differ- 

 ent for the four other substrate types, although the muck and detritus 

 category included leaf beds, said by others to be the preferred habitat 

 (Brimley 1944; Fedak 1971; and Martof et al. 1980). 



Data on maximum and minimum stream temperatures observed 

 during minnow trap sampling revealed that maximum temperatures 

 ranging up to 29° C had little or no bearing on trap success. However, 

 minimum temperatures observed during a trapping period did affect 

 trap success. No N. lewisi were trapped when the minimum temperature 

 exceeded 18° C. Significantly decreased trap success was also observed 

 at a minimum temperature of 0° C, when compared to success at 16° to 

 18° C. 



The potential effects of changes in water temperature on trap suc- 

 cess were investigated by recording no change, general rise, or general 

 fall in temperature during a trapping period. Success rates of 14.8%, 

 20.5%, and 20.2% for no change, rise, and fall, respectively, did not 

 differ significantly. 



