ORIENTATION- OF MIGRATING AXADROMOUS FISHES 



379 



(Mitrhiin, usually starts alxnit the last week of 

 May and lasts until the middle of June. 



There are visible external differences by which 

 these two species can be identified. The glut her- 

 ring is generally smaller than the alewife, has a 

 -mailer eye in relation to the head, and when ex- 

 amined out of water has darker dorsal pigmenta- 

 tion. The individual variations in size within the 

 two groups overlap considerably, and in the water 

 each species has the ability to modify its pigment 

 to blend with the background in the matter of a 

 few seconds. Consequently, it is very difficult to 

 distinguish between the two species with any de- 

 gree of certainty without first removing the fish 

 from the water. Attempts to separate the two 

 species while the experiments were in progress 

 jM-oved to be impractical. When the studies were 

 completed, a careful comparison of the responses 

 shown by the fish in experiments conducted at the 

 beginning of the early run wlien only alewives 

 weie present with those in experiments made near 

 the end of the later run when only glut herring 

 were present, failed to reveal any differences be- 

 tween the responses of the two species. 



These anadromous members of the herring 

 family (Clupeidae) proved to be ideally suited to 

 experimental purposes. They are small and 

 migrate in enormous numbers through easily ac- 

 cessible brooks and streams (fig. 1). The con- 

 struction of the experimental apparatus and the 

 conditioning of water for experiments with these 

 fishes can be done on a smaller, less expensive scale 



than would be necessary for fishes such as the 

 shad or the salmon. At Bournedale each year 

 during the herring run, more than half a million 

 of these fishes enter the small Herring Kiver 

 (completely fresh water; average flow less than 

 20 c. f. s.) from tlie sea water of the Cape Cod 

 Canal and migrate upstream for approximately 

 a mile to their spawning grounds in Great Herring 

 Pond. The experiments were conducted in the 

 stream a short distance below its entrance into 

 Great Herring Pond. 



EXPERIMENTAL METHOD 



As the fish migrated upsti-eam, they were di- 

 lected by wire screens into a shallow experimental 

 trough. The upstream end of the trough was 

 divided into two channels of equal size (fig. 2). 

 As a fisli progressed upstream through the experi- 

 mental trough, it was presented with a choice be- 

 tween the two channels. Differences in water 



* 

 o 



UJ 



a: 



z 

 o 



r- 



•recording point" 



FOR TEMPERATURE TESTS 



•point OF DECISION* 



ZONE OF MIXING 



Figure 1. — The bcri-iiis run at limirneilalc, .Mass. This 

 pool is immediately below the experimental station. 



SCREEN DIRECTING FISH 

 INTO TROUGH 



FIGURE 2.— Dia;;ram of experimental troii^'h. Dimensions : 

 18 ft. ions, 21 in. wide, and 10 in. deep. 



