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riSHEBY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



interest. There has been, however, a general re- 

 luctance on the part of nianj' engaged in fishery 

 research to accept some of the conclusions put 

 forth. Those investigators who were convinced 

 of a high!}" developed homing ability in anadro- 

 mous fishes found it difficult to reconcile the idea 

 of migrating fishes responding to environmental 

 gradients with that of homing. Many contra- 

 dictory observations made by other workers in 

 the field, because of a general tendency to avoid 

 negative reporting, seldom reached the literature. 

 Furthermore, field workers who were aware of the 

 enormous difficulties in making dependable ob- 

 servations of the movements of fish in large rivers 

 and streams, particularly with only intermittent 

 observations made over a wide area, felt that the 

 evidence upon which the conclusions were based 

 was inadequate. 



In summarizing previous research in this field 

 it might be said that — 



(1) TLe ability of fislies to detect dift'erences in 

 temperature and in amounts of dissolved gases 

 has been established experimentally. 



(2) Laboratory (gradient-tauk) exjierimeuts have 

 also established that differences in temperature 

 and in amounts of dissolved gases can influence 

 the direction of fish movement. 



(3) Differences in water temperature and amounts 

 of dissolved gases have been shown to exist in 

 natural waters at points where they might 

 exert an important influence upon the direction 

 of migrating anadromous fishes. 



(4) It has been suggested that these physical and 

 chemical differences do influence the orientation 

 of the migrating fish. 



(5) Some field observations of migratory-fish be- 

 havior appear to show a relation between the 

 direction of migration and gradients of certain 

 physical and chemical factors. These field ob- 

 servations are relatively few, inconclusive, and 

 contradictory. 



Thus, it is known that some fishes can orient with 

 reference to certain physical and chemical dif- 

 ferences in water, but whether migrating anad- 

 romous fish.es actually do orient with reference 

 to such differences is not known. The following 

 experiments were undertaken to explore this 

 question with one type of anadromous fish. 



EXPERIMENTAL APPROACH 



In studying the influence of gradients of physi- 

 cal and chemical characteristics of water upon the 

 orientation of migrating anadromous fishes a di- 



rect experimental approach was used. Experi- 

 ments were undertaken which attempted to meas- 

 ure the directional responses of migrating fishes of 

 the genus Pomolobus to certain dift'erences in 

 water chamcteristies including temperature, pR, 

 and amounts of dissolved gases (Oo, N2, and CO2). 



The experiments were designed to avoid many 

 complications that have to be considered in labora- 

 tory experiments with live fish. The experiments 

 were made in the stream in which the fish were 

 migrating so that the fish would not be handled or 

 subjected to the shock of being removed from their 

 natural environment. Tests were made on thou- 

 sands of fish, and each fish was tested oidy once so 

 that consitlerations involving learning could be 

 ignored. 



Inasmuch as the special migratory behavior of 

 anadromous fishes is exhibited for only limited 

 periods of time, perhaps only once or twice during 

 the life of the fish, an essential condition of these 

 experiments was that they were conducted while 

 the fish were actually migrating. The impor- 

 tance of this is realized when one considers how 

 greatly the respcuise of a fish to environmental 

 stimuli nuiy differ at various physiological stages 

 of its life. As fiugerlings. the fish are found mi- 

 grating downstream; as maturing adults, they 

 migrate upstream. In those species which survive 

 spawning, the spent fish again migrate down- 

 stream. Frequently, individual fish of the same 

 species may be found in a stream responding in 

 very different ways to identical environmental 

 stimuli. 



The differences in physical and chemical factors 

 tested were artificially produced in these experi- 

 ments but the type and magnitude of the differ- 

 ences were within the range commonly found at 

 stream junctions in nature. 



MATERIALS AND METHODS 



The experiments were conducted in the Herring 

 Kiver at Bournedale, ]Mass.,'in 1949 and 1950 dur- 

 ing the spring herring runs. The herring run at 

 Bournedale actually consists of two overlapping 

 runs of closely related species. The earlier run, 

 made up of alewives, Pomolobus pseudoliarengus 

 (Wilson) , begins about the first of April and con- 

 tinues until the end of jNIay. The second and 

 smaller run of glut herring, Pomolobus aestivalis 



