mental tagged carcasses and carcasses of natural 

 river-killed fish have similar floating charac- 

 teristics; (2) that floating carcasses pass rapidly 

 through the recovery area and are available for 

 recovery on only 1 day. 



Floating Qualities of Fresh and Experimental Carcasses 



Salmon sink after death, but decomposition 

 gases cause them to float after a period of time. 

 In the mark-and-recovery method that we used 

 to estimate the population of dead fish, it was 

 essential to ascertain whether tagged experimental 

 carcasses had floating qualities similar to those 

 of other salmon that die in the Columbia River. 

 Diff'erences between the two kinds of carcasses 

 could influence results. We made a series of ex- 

 periments at various water temperatures with 

 fresh and frozen experimental chinook salmon car- 

 casses to determine whether there are differences 

 in elapsed time between death and rise of the 

 carcass to the surface. 



Some information was already available from 

 experiments by Hanson et al. (1950). They de- 

 termined the elapsed time between death and 

 floating for 21 fresh salmonids at water tempera- 

 tures of 13.9° to 20.6° C. We performed similar 

 experiments over a wider range of temperatures 

 with 24 frozen and 14 fresh chinook salmon. Cold- 

 water experiments were done in a spring-fed pond 

 at the Oregon Fish Commission Laboratory, 

 Clackamas, Oreg.; warm-water experiments were 

 done in the Columbia River near Bonneville Dam. 

 Frozen fish were thawed in air for 24 hours before 

 submersion for testing; they were from the same 

 lots as those that were later tagged and released 

 to estimate the population of dead salmon. Fresh 

 fish were purchased from commercial fishermen 

 and were submerged within a few hours after 

 death. The time required to float was calculated 

 from the time the fish was placed in the water 

 until some part was visible at the water surface. 



In the cold-water temperature range (7.2°- 

 11.1° C.) frozen salmon took 2.3 days longer to 

 float than fresh salmon (appendix table 1), 

 whereas in the warm range (12.8°-17.2° C.) the 

 difference was reduced to 1.7 days (appendix 

 table 2). We tested the significance of these dif- 

 ferences by using Wilcoxon rank sum tests for 

 comparison of group means. This nonparametric 

 test does not require that any assumption be 



made concerning the distributions of floating 

 times in the populations from which our samples 

 are drawn. When fresh and frozen carcasses were 

 compared under cold-water conditions (appendix 

 table 1), the null hypothesis that there is no dif- 

 ference in floating times between the two types 

 of carcasses was accepted at the 90-percent level of 

 significance. A similar result was obtained for 

 warm-water conditions (appendix table 2). Ap- 

 pendix figure 1 shows the similarity of floating 

 characteristics of fresh and frozen carcasses, 

 especially at relatively warm water temperatures 

 of over 10° C. The temperature of the Columbia 

 River was 14.4° C. during the experiment to esti- 

 mate the population of dead salmon. 



Next we compared floating times of carcasses 

 in warm and cold water, again using the Wilcoxon 

 rank sum test. Because no diff'erences had been 

 found between fresh and frozen carcasses, both 

 types of carcasses were included in the comparison 

 between water temperatures. We rejected (at the 

 99-percent significance level) the null hypothesis 



Appendix Table 1. — Days required for fresh and frozen 

 chinook salmon (a float in cold water (7.2°-ll.l° C.) 



Days 

 Water required 



temperature to float 



Fresh chinook salmon 



Average 



days 

 required 

 to float 



Water 

 temperature 



Frozen chinook salmon 



Average 

 Days days 



required required 

 to float to float 



7.2-10.0. 

 7.2-10.0. 

 8.3-10.6. 

 8.3-10.6. 



Number Number 

 11 

 12 

 12 

 12 



Number Number 



14.1 



Appendix Table 2.^Days required for fresh and frozen 

 chinook salmon to float in warm water {12.8°-17.2° C.) 



Fresh chinook salmon 



Water 

 temperature 



Frozen chinook salmon 



Average 

 Days days 



required required 

 to float to float 



° C. Number 



13.3-13.9 7 



13.3-13.9 6 



13.3-14.4 ' <6 



Number 



13.3-14.4. 

 15.6-16.1. 

 15.6-16.1. 

 15.6-16.7.. 

 16.7-17.2. 

 16.7-17.2. 

 16.7-17.2. 



' <6 

 3 

 4 

 5 

 4 

 4 

 4 



°C. 

 12.8-13.3- 

 12.8-13.3.. 

 12.8-13.3. 

 13.3-14.4.. 

 13.3-14.4.. 

 16.6-16.7.. 

 15.6-16.7.. 

 15.6-16.7.. 

 15.6-16.7.. 

 16.7-17.2.. 

 16.7-17.2.. 

 16.7-17.2.. 



Number 

 11 

 11 



8 

 ' <6 



7 



4 



5 



5 



5 



5 



4 



4 



Number 



' These observations omitted from calculations of means and statistical 

 comparisons of floating times. 



490 



U.S. FISH AND WILDLIFE SERVICE 



