MEDVED and MARSHALL: FEEDING AND BIOLOGY OF YOUNG SANDBAR SHARK 



RESULTS 



A total of 73 h from 17 fishing stations con- 

 ducted in May 1977 and 1978 resulted in no 

 catches. The first catch occurred on 6 June 1977 

 and on 1 June 1978 at water temperatures of 

 20.5° C and 21.0° C, respectively. After the initial 

 catch, sandbar sharks were caught consistently 

 and CPUE did not differ significantly among 

 June, July, and August during either year (Ta- 

 ble 1). 



The number of catches and hours of fishing con- 

 ducted after the arrival of sandbar sharks are 

 summarized in Table 2. A total of 318 sharks 

 were caught in 478 h of fishing. Mean CPUE 

 for the 62 fishing stations was 1.02 sandbar 

 sharks /h. CPUE varied among the 3 study yr 

 (Table 3). CPUE in 1979 was 2.06 sandbar 

 sharks /h, but was based on only one 24-h fishing 

 station. In 1977 and 1978, CPUE was based on 

 numerous fishing stations and was significantly 

 higher in 1977. 



Sandbar sharks were caught during all hours 

 of the day and night. CPUE was calculated at 1-h 



Table l.— Summary of fishing data for the 3 study mo of 1977 

 and 1978. The value given for "Mean rank" is the mean of the 

 ranks assigned to the observations on CPUE in a Kruskal-Wallis 

 test for differences in CPUE among months within the indicated 

 year. 



Year Month 



No. of fishing 

 stations 



Mean CPUE 

 (sharks/h) 



Mean rank P-value 



neous stations, and all stations combined. 



Stations 



Hours of 

 fishing 



No. of sharks 

 caught 



No. of 

 stations 



Mean CPUE 

 (sharks/h) 



24 h 



Miscellaneous 



Overall 



232 

 246 

 478 



160 

 158 

 318 



10 

 52 

 62 



0.67 

 1.07 

 1.02 



Table 3. — Summary of fishing data for each study year The 

 value given for "Mean rank" is the mean of the ranks assigned to 

 the observations on CPUE in a Mann-Whitney [/-test. 



Year 



Hours of 

 fishing 



No. of 

 sharks 

 caught 



No. of 

 fishing 

 stations 



Mean CPUE 

 (sharks/h) 



Mean 

 rank 



P-value 

 (2-tail test) 



1977 128 



1978 325 



1979 25 



119 



148 



51 



23 



38 



1 



1.39 



.76 



2.06 



39.2 

 26.0 



0.005 



intervals for each 24-h fishing station, but be- 

 cause of the relatively small sample size, numer- 

 ous observations of zero CPUE, and great station- 

 to-station variation in CPUE, these data were 

 not sufficient for a statistical analysis of diel 

 rhythmicity. Visual inspection did suggest a day- 

 night difference in CPUE. On all but one 24-h 

 station, CPUE was higher during the night than 

 during the day. Grouping the data on this more 

 general level and treating day and night observa- 

 tions (on CPUE) as samples paired on fishing 

 station indicates CPUE was significantly higher 

 during the night than during the day (Table 4). 



CPUE did not differ significantly between flood 

 and ebb tidal current periods (Table 4). During 

 both current periods CPUE was higher during 

 the night than during the day, but, because of 

 multiple testing on the same data, the probability 

 values given for the tests should be considered as 

 only rough approximations (Table 4). 



Catch data were grouped into four tidal current 

 speed categories. Sandbar sharks were caught 

 over the entire range of current speeds recorded 

 during 24-h stations and CPUE was not signifi- 

 cantly different among the four current speed 

 categories (Table 5). 



CPUE was calculated for the three fishing 

 depths used during 24-h fishing stations. Al- 

 though sandbar sharks were caught at all three 

 depths, CPUE differed significantly among them 

 (Friedman test, P = 0.02) and was higher at the 

 bottom than at the surface or middepth. CPUE 

 did not differ significantly between the surface 

 and middepth (Table 6). The difference in CPUE 

 among the depths was not the same for both day 



Table 4. — Summary of fishing data for various periods during 

 10 24-h fishing stations. The values given for "Rank sums" are 

 the sums of the positive and negative differences of paired obser- 

 vations on CPUE in a Wilcoxon signed rank test. Differences for 

 day-night and for day-night by current stage categories were for 

 CPUE during the night minus CPUE during the day Differ- 

 ences for the flood-ebb category were for CPUE during flood 

 minus CPUE during ebb. Dusk and dawn periods were split 

 equally between day and night categories. 



Period 



No. of 

 Hours of sharks 

 fishing caught 



Mean CPUE 

 (shark/h) 



Rank sums 



(+) (-) 



P-value 

 (2-tail 

 test) 



' Probabilities should be considered as rough approximations because of 

 multiple testing on the same data. 



443 



