NOTE Lmdquist and Shaw: Effects of current speed and turbidity on catches of larval and juvenile fishes 439 



tion and an off-platform light trap collection in random 

 order. During sampling, turbidity (Nephelometric tur- 

 bidity unit: NTU) was measured every 5 sec by using 

 a Hydrolab DataSonde3 suspended in surface waters 

 within the platform structure. Current speed and direc- 

 tion were measured every 10 min with an InterOcean 

 S4 Current Meter suspended 1-2 m below the surface 

 on the up-current side of the platform. Because the 

 platform structure undoubtedly reduced current speeds 

 (Forristall, 1996), current data taken from this location 

 should be considered as relative estimates for the light 

 trap collections. 



Samples were preserved in 10% buffered formalin and 

 transferred to ethanol within 12 hours. Fish were enu- 

 merated and identified to the lowest possible taxonomic 

 level. Preflexion larvae were measured to notochord 

 length, and postflexion and juvenile fish were measured 

 to standard length. Data from light trap catches were 

 standardized to a catch per unit of effort (CPUE) of 

 number of fish per 10 minutes. 



Data analyses 



We assumed that there were no inter-location differences 

 in the relationship between light trap CPUE and current 

 speed or turbidity; therefore, data from all platforms 

 for the months May to September were combined. The 

 relationship between total light trap CPUE and current 

 speed or turbidity was analyzed by using regression 

 analysis. Current speed and turbidity were analyzed 

 separately, rather than in a multiple regression analysis, 

 because there was a limited number of sampling sets 

 where we had data for light trap CPUE, current speed 

 and turbidity together (n = 60, or 31% and 37% of the 

 available turbidity and current data, respectively). There 

 were no significant differences in the regression coef- 



ficients of CPUE vs. current speed or turbidity between 

 within- and off-platform light traps (P>0.15); therefore, 

 the CPUEs from both light traps were averaged for each 

 sampling set. Mean total CPUEs were log-transformed 

 (log 10 (.y+l)) and analyzed with the mean current speed 

 or turbidity from each respective sampling set. Mean 

 CPUEs were also calculated for the dominant families 

 collected; however, regression analyses could not be 

 performed because variances remained heterogeneous 

 after transformation. 



To investigate how fish size (i.e., locomotive ability) 

 influenced light trap catches with increasing current 

 speed, length-frequency distributions of all fishes col- 

 lected at different current speed intervals (0-9, 10-19, 

 20-29, 30-39, 40-49 and >49 cm/sec) were compared by 

 using Kolmogorov-Smirnov tests (a=0.05). The length- 

 frequency figures were subdivided by three ecological 

 groupings: clupeiforms (Clupeidae and Engraulidae); 

 demersal taxa (predominantly Synodontidae and Blen- 

 niidae); and scombrids and carangids, to further assess 

 whether any changes in the size of fish collected over 

 the current intervals were due to a particular group. 

 All statistics were performed with SAS version 6.12 

 (SAS Institute, Cary, NO. 



Results 



Current speed 



Mean total CPUEs generally decreased with increasing 

 current speed (Fig. 1). At current speeds s30 cm/sec, 

 light trap catches were highly variable (CPUEs ranged 

 from to 138 fish per 10 min); however, CPUEs >20 

 fish per 10 min occurred only at these lower speeds. 

 Although there were fewer samples at speeds >30 cm/sec, 



