FISHERY BULLETIN: VOL. 82, NO. 2 



of their functional response where smolts could 

 potentially be more vulnerable. Contrary to re- 

 sults of Ricker (1941) and Cameron (1958), con- 

 sumption rates by char were proportional to smolt 

 abundance and smolts did not find refuge at low 

 migration numbers. On occasion (primarily in 

 1980) smolt abundance was great and consump- 

 tion rates of char were disproportionately low, a 

 response observed by Neave (1953). Thus, char at 

 Little Togiak River exhibited a Type II functional 

 response where vulnerability of smolts to preda- 

 tion may be greater at lower migration densities. 

 This increased vulnerability ultimately depends 

 on the numerical response of char to smolt abun- 

 dance. 



The inclusion of smolt weight and char length in 

 the functional response multiple regression model 

 further described important variables that influ- 

 ence char predation, as well as reducing within- 

 and between-year variability. The exponential 

 increase in consumption rates by char during 

 migrations of smaller smolts is probably due to 

 more smolts needed to decrease feeding activity 

 and a greater ease in capturing small smolts. Be- 

 cause juvenile salmon growth is density depen- 

 dent (i.e., smaller smolts at greater densities; 

 Rogers 1968), increased consumption during mi- 

 grations of smaller smolts may act to cancel the 

 proportionately lower consumption rates of char 

 at greater smolt abundances. Thus, it is important 

 to test concurrently the effect of smolt weight and 

 smolt density when describing the functional re- 

 sponse of char. 



The relationship of char length to consumption 

 of smolts by char was best described by the al- 

 lometric conversion (Moriarty 1977) of char length 

 to char weight. According to regression analysis, 

 the significance of char weight (as converted from 

 char length) is questionable; however, char length 

 was included in the model because it seems rea- 

 sonable that a larger predator would require more 

 food and may be able to capture mobile prey easier 

 than a smaller predator. Rogers et al. (1972) re- 

 ported larger char consumed more smolts than 

 smaller char. 



The average number of smolts consumed per 

 char, as predicted by the model, was 0.8 smolts/24 

 h, and the maximum was 5.6 smolts/24 h. These 

 values were corrected for smolt weight and char 

 length. The low average of consumed smolts re- 

 flects the low number of smolts that generally 

 migrate. The predicted maximum of 5.6 smolts/ 

 char corresponds quite well with the observed 

 maximum of 6.0 smolts/char per 24 h. These esti- 



408 



mates are lower than the average and maximum 

 consumption rates by char at the Agulowak River 

 (3.4, 8.4 smolts/char per 24 h, respectively) calcu- 

 lated from weekly estimates (Meacham and Clark 

 1979). This difference between the two rivers may 

 be explained, in part, by the larger char size (Mor- 

 iarty 1977) and the probable extension of the daily 

 migration period at the Agulowak River, which is 

 a large river that intercepts smolts from several 

 lakes in the Wood River system. 



Percent Smolt Mortality 



The shape of a percent-mortality curve can pro- 

 vide valuable information on the stability charac- 

 teristics of a salmon population (Peterman 1977) 

 and provide information to a hatchery manager 

 planning to release smolts. For example, percent 

 smolt mortality could vary as in a Type III or 

 modified Type II curve where smolt mortality in- 

 creases up to a certain threshold density of smolts 

 before decreasing. In this example, a hatchery 

 manager should release smolts at densities 

 greater than the threshold density. 



Results from this investigation indicate the char 

 numerical response (number feeding) may influ- 

 ence the type of percent-mortality curve. If the 

 char numerical response is constant, then percent 

 mortality will decrease as more smolts migrate. 

 However, if the numerical response of char varies 

 with smolt abundance, as we suspect, then percent 

 mortality may increase with more smolts up to a 

 threshold density. Beyond this threshold density, 

 percent mortality decreases. The importance of 

 the percent-mortality curve is to indicate the 

 smolt density at which mortality is minimized. 

 Smolts at Little Togiak River experience less risk 

 of predation at daily migration abundances of 

 about 20,000 smolts or greater. However, migra- 

 tion densities of this magnitude were rare. 



Char Consumption of Smolts by Length 



Char with less full stomachs contained smolts 

 that were, on average, significantly larger than 

 those in full char and those in the migration. A 

 plausible explanation for the greater average 

 smolt length in less full char than full char in- 

 volves the effect of hunger on feeding behavior. 

 Char containing only a few smolts may be hungry 

 and aggressive (Ware 1972), which may induce a 

 high success rate when feeding on the larger, more 

 mobile smolts. When char approached stomach 

 fullness, their hunger and aggressiveness may 



