LANDER and HENRY: 1965-66 BROOD COHO SALMON 



Fredin O. nerka Mi(lower) > M2(upper) = 0.042/ 

 (1964) mo after the first few months or 



first year at sea. Value is from 

 Model 1 (constant natural mortality 

 during all of ocean life), averaged 

 for smolts migrating in their third 

 third and fourth years. 



Parker O. gorbu- Mi ~ 0.78 1/mo during first 40 days 

 (1968) sella in coastal waters and M2(upper) = 



0.169/mo during last 410 days of 

 ocean life (hence M1IM2 ^ 4.0). 

 Reported daily rates are averaged 

 here for the 1961-63 broods and 

 multiplied by 30. 



In contrast to the situation for 1965-66 brood 

 coho salmon, offshore fishing had a negligible 

 effect on data from which the foregoing values 

 were derived. Thus Ricker (1962) and Fredin 

 (1964) demonstrated sharp decreases in the total 

 and monthly average rates of natural mortality 

 during all of ocean life as the size of smolts in- 

 creased; they correctly emphasized that the 

 actual risk of death is much greater during the 

 first part of ocean life when fish are smallest. 

 Their values for M2(upper) — 0.038/mo and 

 0.042/mo — are compatible with Parker's (1962) 

 value of M2 ^ 0.026/mo which excludes mor- 

 tality on small, juvenile fish before they leave 

 coastal waters. Although Lander (1973) con- 

 trasted the assumptions of Ricker's (1962) 

 mortality computation and Model 1 of Fredin 

 (1964), the distinction is mainly academic: 

 natural mortality is the overwhelming agent of 

 removal for salmon smolts and values calcu- 

 lated from a given set of real data differ little 

 between the two models. 



Parker's (1968) results appear to be the only 

 ones which determine with reasonable success 

 the real time distribution of natural mortality in 

 salmon from direct marking/recovery at sea. 

 Thus Parker's value of Mi =^ 0.781/mo greatly 

 exceeds the highest possible values in Figure 3: 

 Mi(upper) is about 0.50/mo for the 1965 brood 

 and 0.48/mo for the 1966 brood. This is not 

 surprising since relatively higher natural mor- 

 tality is associated with smaller sockeye salmon 

 smolts as just noted (Ricker, 1962; Fredin, 

 1964). The pink salmon in Parker's analysis 

 were tiny when marked during their first 40 

 days at sea — roughly 0.3-8.0 g judging from 

 LeBrasseur and Parker (1964) — compared with 



average release weights of about 14.6-33.1 g (at 

 different hatcheries) for 1965-66 brood coho 

 salmon. Beyond this initial size difference, 

 Parker's value of Mi ^ 0.781/mo is for only 

 the first 40 days at sea, but values of Mi(upper) 

 for the coho data of about 0.50 (1965 brood) 

 and 0.48 (1966 brood) are for the first 6.0 mo 

 after release. With respect to M2(upper) = 

 0.169/mo, Parker (1968) emphasized that his 

 reported escapement was considerably low. 



The foregoing evidence, although indirect so 

 far as 1965-66 brood coho salmon are concerned, 

 does suggest that Mi > M2, and further that 

 M2 is closer to the lower limit (M2 = 0) than to 

 M2 = Ml (Figure 3). Assuming M2 < Mi, one 

 can choose any value of M2 in this range to get 

 a reasonable idea of the relative magnitudes of 

 survival during the first 6.0 mo (.si) and last 

 12.0 mo (.S2) after smolts were released. Based 

 on the observation that M2(upper) = 0.038/mo 

 (Ricker, 1962) or 0.042/mo (Fredin, 1964), we 

 chose M2 = 0.03/mo to illustrate relative mag- 

 nitudes of .si and ,S'2. Values for Mi and F were 

 approximated from Figure 3, and values for Si 

 and .S2 were calculated from the relations given 

 below. 



Relation 



1965 brood 1966 brood 



-6.OM1 



si — e 



■i2.OM2-4.5f 



S2 = e 



0.06 



.14 



0.08 



.17 



To present unique point estimates with 

 unknown bias for all parameters, we applied 

 the limit-mean model; its basic assumption is 

 that si (upper) = S2 (lower) = s (Lander, 1973). 

 Its possible effect on estimates is discussed in 

 the Appendix, where the scheme for calculating 

 is summarized also. 



The resulting estimates for all parameters 

 are in Table 7. The exploitation rate (p) in off- 

 shore fisheries evidently was slightly higher for 

 the 1965 brood (70% ) than the 1966 brood (68% ). 

 The proportion of smolts recruited to oceanic 

 fishing was slightly lower for the 1965 brood 

 (5% ) than the 1966 brood (6% ). as noted earlier. 

 The estimates indicate also that slightly more 

 of the surviving smolts matured as jacks in the 



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