MURPHY and CLUTTER: PLANKTON PURSE SEINE 



are required are maximum sustainable speeds 

 over distances ranging from to 100 cm or so. 

 Houde (1969) found that yellow perch larvae 

 larger than 9.5 mm could sustain speeds up to 

 4 body lengths per sec for 1 hr. Larimore and 

 Duever (1968) observed swimming speeds over 

 10 body lengths per sec for over 3 min for small- 

 mouth bass 20-25 mm in length. Hunter and 

 Zweifel (1971), Figure 4, present data for 

 sustained swimming of jack mackerel 4.5-27.0 cm 

 for short periods. These data fitted to the ex- 

 pression y = ax^ yielded an a of 18.06, h of 0.829, 

 and r of 0.997, y being speed in centimeters per 

 second, and x body length (L) in centimeters. 

 This gives an extrapolated speed of 10 cm per 

 sec for a 5-mm nehu larva. More recently. 

 Hunter (1972) observed burst speeds of very 

 short duration as high as 28 lengths per sec 

 for a 4.2-mm larva and 25 lengths per sec for a 

 12.1-mm larva. Whether such speeds can be 

 sustained long enough to explain plankton net 

 avoidance is not known. In the computations to 

 follow, we assume that they can be sustained as 

 follows. One trial {u/) assumes that the back- 

 ward extrapolation of the data by Hunter and 

 Zweifel (1971) holds (cm/sec = 18.06 Lcm"-"-') , 

 and the second trial {u/') assumes that cm/sec 

 equals body length in centimeters times 10. 



The catch data were processed as follows. The 

 raw data (second and third columns in Table 2) 

 were fitted to exponential expressions (fourth 

 and fifth columns) . The purse seine data were 

 then multiplied by 46.883/29.685 to adjust the 

 data to the point of assumed 100% meter net 



efl^iciency (see Figure 2). P', the fraction re- 

 tained, was then calculated as 1 — P. Ue and 

 He" were then calculated from the expressions 

 in the preceding paragraph. Minimum alarm 

 distance, :ro, was then calculated from: 



Xo = 



R{V' — tie')y^ [1 — {P')y^] 



lie 



(5) 



which is a straightforward rearrangement of 

 equation (4). The resulting minimum alarm 

 distances (last two columns in Table 2) do not 

 seem unreasonable. For example, it seems rea- 

 sonable that a 7. 5-mm larva could detect a meter 

 net 200-400 cm away and begin to take meaning- 

 ful evasive action. The greater effectiveness of 

 towed nets at night might be caused by a re- 

 duction in detection distance as well as reduced 

 ability to take early, well-directed evasive action. 



OTHER TOWED NETS 



Two additional sets of data will be considered 

 here. The first is a comparison between a 10-ft 

 Isaacs-Kidd trawl and a standard meter net. The 

 ratio of mesh area-to-mouth opening was the 

 same for both nets in order to ensure compara- 

 bility of hydrodynamic and clogging character- 

 istics. The trawl was meshed throughout with 

 Nitex having an opening of 0.505 mm. This is 

 nearly the same as that used in the standard 

 CalCOFI (California Cooperative Oceanic Fish- 

 eries Investigations) meter net which, according 

 to Smith, Counts, and Clutter (1968), had silk 

 gauze with a mesh width of 0.55 mm as its main 



Standard 

 length 

 (mm) 



Table 2. — Calculation of escape parameters for meter net and purse seine data. 



Purse 



seina 



observed 



Meter 

 net 



observed 



Purse 

 seina 



calculated^ 



Meter 

 net 



calculated^ 



Purse 



seina 



adjusted* 



P' 



X 't 

 ' 



* "t 

 ' 



795 



