these fibres was picked up by electrodes whose tips 

 lay in the thin lateral red muscle strip. Relative 

 proportions of red and white muscle fibres were 

 determined by dissection, and their innervation 

 patterns were examined in Formalin-fixed mate- 

 rial. Cryostat sections stained for lipid and for 

 succinic dehydrogenase by routine methods were 

 used to distinguish different fibre types. 



Similar studies were carried out on carp, Cy- 

 prinus carpio Linnaeus, 25-30 cm FL, caught by 

 seining in the Fraser Valley, B.C., and rainbow 

 trout, Salmo gairdneri Richardson, 17-30 cm FL 

 obtained from a commercial supplier. Before trial 

 in the respirometer both species were held in cir- 

 cular tanks around which water was pumped to 

 give a constant flow of 30-40 cm/s around the cir- 

 cumference where the fish normally swam. For 

 these freshwater fish, much improved sig- 



nal : noise ratios were obtained by adding small 

 amounts of sea water to the freshwater in the res- 

 pirometer; this did not affect the behavior of the 

 fish, which were allowed to acclimate for 18-20 h in 

 the apparatus before testing. 



Results 



Pacific Herring 



The records of muscle activity shown in Figure 

 lA-C are from electrodes with their tips lying in 

 the lateral strip of red muscle. These records show 

 that bursts of irregular potentials around 200-300 

 /uV peak to peak are recorded from the red fibres 

 during slow sustained swiming, becoming more 

 synchronous and shorter as swimming speed in- 

 creases and tail beat frequency rises. At sustained 



■••^M^^^^^ 



B ^Mf^^y^'^r^'^r^^ 



^^«/^|4.^^^ 



D 



4/ftW^H4i 



ii^f^fJ^aXt^W|^ 



Figure l, — Records from lateral red muscle of a juvenile Pacific herring ( 16.4 cm fork length) swimming at different speeds. A: 1.8 

 body lengths per second; B: 2.8 body lengths per second; C: 3.1 body lengths per second; and D: 4.3 body lengths per second. Note 

 gradually increasing synchrony of potentials as swimming speed and tail beat frequency rise (A-C), and the appearance of large 

 potentials in D < picked up from underlying white fibres) when the fish struggled to maintain position at high flow speed. Vertical bar: 

 500 ^V; time marker: seconds. 



692 



