554 



Fishery Bulletin 88(3), 1990 



Discussion 



The salient result of these studies is that both rainbow 

 trout and coho salmon exposed as embryos to a sub- 

 lethal dose of B[a]P exhibit decreased ability to swim 

 upstream after emergence from artificial redds. 



Emergence and orientation are activities that ter- 

 minate a sequence of early-life behaviors critical for 

 salmonid survival. Following hatching, developing 

 salmonid eleutheroembryos remain in gravel redds for 

 2-4 months until yolksac absorption and fin develop- 

 ment are nearly complete (Fast 1987). As yolksac 

 reserves are exhausted the eleutheroembryos migrate, 

 primarily at night, to the upper layers of the redd. 

 Emergence is complete when eleutheroembryos, now 

 called fry, leave the redd and swim to the water sur- 

 face to inflate the air bladder and begin feeding. The 

 direction in which emergent fry migrate is species- 

 specific. Coho salmon and rainbow trout fry typically 

 orient and swim upstream to counteract downstream 

 transport and to colonize natal and nearby streams 

 unavailable to, or not used by, spawners (Mason 1976). 

 Soon after emergence, fry establish and defend ter- 

 ritories as feeding begins (Chapman 1962, Mason and 

 Chapman 1965). 



Our previous work (Ostrander et al. 1989) demon- 

 strated that the lipophilic nature of B[a]P coupled with 

 the expansive yolksac reserves of the embryo resulted 

 in significant uptake of the compound and, further- 

 more, the B[a]P was only gradually lost from the egg 

 throughout development. We found that over 60% of 

 the B[a]P is retained in the eleutheroembryo at hat- 

 ching when this exposure regimen is utilized. Conse- 

 quently, rather than exposing our B[a]P-treated fish 

 to an acute 24-hour pulse of B[a]P, they faced con- 

 tinuous exposure to B[a]P and its harmful" metabolites 

 (Gelboin 1980) as they exhausted their yolk reserves 

 throughout early development. 



There were no significant differences in numbers of 

 fish emerging among the various control and exposure 

 groups. Yet, fewer rainbow trout exposed to B[a]P as 

 embryos exhibited t^^jical upstream swimming behav- 

 ior following emergence. These results are similar to 

 previous findings for coho salmon (Ostrander et al. 

 1988). In that study, however, significantly fewer coho 

 salmon, successfully emerged (Table 1). Perhaps the 

 larger size of the coho alevin coupled with compromised 

 abilities due to B[a]P exposure resuked in reduced suc- 

 cess in negotiating the intersticial spaces of the redd 

 during emergence. Fish in the downstream compart- 

 ment did generally appear less active than fish in the 

 upstream compartment. In neither study, however, 

 were significant differences in weight or fin develop- 

 ment among control and treatment groups observed. 



nor were any morphometric differences detected 

 between fish going upstream or downstream in the 

 experiments. 



In these experiments, fish emerged from the gravel 

 and oriented into the water flow. After about 1 minute, 

 the majority of fish moved into the upstream compart- 

 ment of the test apparatus. A smaller number of fish, 

 generally less active, either swam or drifted into the 

 downstream compartment. Successful upstream move- 

 ment requires that the time to travel the distance 

 across the redds is less than the time to become 

 fatigued, assuming that fish in the downstream com- 

 partment became fatigued before they reached the 

 upstream compartment. This is expressed, Tf>T,, 

 where T, is the travel time to the upstream compart- 

 ment and r, is the fatigue time of the fish. Travel time 

 to the upstream compartment depends on the average 

 velocity of the current across the redds, U. the average 

 fish swimming velocity, V, and the distance, L, it 

 travels across the redd, and is expressed: 



T, = LI(V-U). 



Fatigue time also depends on swimming velocity. 

 Laboratory experiments indicate that among adult 

 rainbow trout, the time to fatigue decreases exponen- 

 tially with swimming velocity for velocities above 3 

 body lengths per second (bl/s). However, at 2 bl/s adult 

 rainbow trout can swim indefinitely (Beamish 1978). 

 Alevins probably have a similar relationship, although 

 (due to smaller size) they can maintain considerably 

 larger swimming velocities on a body length basis, ex- 

 ceeding 30 bl/s (100 cm/s) (Ostrander et al. 1989). The 

 model assumes time to fatigue is infinite when swim- 

 ming velocity is zero and decreases as swimming 

 velocity increases. A simple power function has these 

 properties: 



T, = k V-" 



where A' and « are parameters that depend on condi- 

 tion of the environment and the fish. Equating the two 

 time scales, a critical stream velocity is defined: 



U* = r - L/kV 



where successful upstream migration requires U<U*. 

 The intersection of U and U* defines a swimming 

 velocity range required for successful movement into 

 the upstream compartment (Fig. 2). For velocities out- 

 side the allowable range fish become fatigued before 

 they reach the upstream compartment. 



The experiments indicated that fish exposed to B[a]P 

 had a greater chance of ending up in the downstream 

 compartment. This result could be achieved if fish 



