FISHERY BULLETIN: VOL. 81, NO. 3 



TABLE 1. — Example calculation of short-term impact by entrainment of top- 

 smelt larvae, n.e. indicates not estimated. 



Equation (5): R c = exp (-2.0 X 10" 2 ) = 98. 



Equation (11): -4 e = 5.4 X 10 s (0.98 - ' -1)= 1 .1 X 1 4 fish. 



valent adult losses are estimated by Equation (11) 

 and are 1.1 X 10 4 fish. 



Estimation of long-term impact requires informa- 

 tion on all sources of mortality. The annual estimate 

 of adult impingement by the power plant, based on 

 360 d of sampling, is 5,147 fish. According to Califor- 

 nia Department of Fish and Game records, there 

 were no commercial landings of topsmelt in 1978, 

 and recreational fishermen landed < 1,000 topsmelt 

 in this area (we use 1 ,000 fish for this example). Total 

 mortality rate was estimated by regressing log abun- 

 dance against age (Ricker 1975), giving an instan- 

 taneous total mortality rate (Z) of 1.8. Nonnatural 

 mortality rates of adults were negligible (Table 2), 

 giving a natural mortality rate of M= 1.8. Application 

 of the long-term impact approximation (Equation 

 (14)) indicates that topsmelt may be near 99% of 

 their unimpacted abundance despite impacts by 

 both power plants and fisheries. The majority of im- 

 pact on the local resource is probably due to power 

 plant operation, with entrainment impact being 

 about twice as large as impingement impact. In any 

 case, this estimated small reduction in long-term 

 abundance indicates that there is no cause for con- 

 cern with regard to power plant impact on topsmelt 

 abundance in this estuary. 



DISCUSSION 



This approximation of long-term impact is not in- 

 tended to be a substitute for proper studies of pop- 

 ulation dynamics. Rather, just as in the case of fishery 

 management, it is intended to be a working approx- 

 imation which should be discarded as more definitive 

 information and analyses become available. In the 

 case of impact assessment there will always be a suite 

 of organisms, particularly invertebrates, which un- 

 doubtedly are impacted, but lack sufficient "status" 

 to justify the expense of close monitoring and study. 

 For these organisms, approximation is the most that 

 reasonably can be asked. In this respect the potential 

 yield approximation is well established in fishery 



618 



Table 2. — Example calculation of long-term impact from all 

 sources of topsmelt mortality. 



Equation (20): M = 1 .8 - 1 .0 X 1 0" 2 -0.2 X 1 0" 2 = 

 Equation (14): R = 1 - (3.2 X 1 0" 2 /3.6) = 0.99. 



1.8. 



management and should therefore be an equally 

 applicable approximation for power plant impact 

 assessment. 



It is likely that the bases of the approximation can 

 be improved in two ways. First, organisms may be 

 classifiable into types with various productivity 

 curves, of which the logistic is a special case. For 

 example, Fowler (1981) observed that species with 

 high reproductive rates and short life-spans show 

 most density-dependent compensation at low pop- 

 ulation levels, whereas species with low reproductive 

 rates and long life-spans show most of their density- 

 dependent compensation near carrying capacity. 

 Thus, it may be possible to specify the shape of the 

 curves in Figure 1 as a function of observable or 

 measurable traits of specific organisms. The second 

 improvement consists of developing better scaling 

 criteria for the production curve, once its shape has 

 been established. A survey of population growth 

 rates of many species could form the basis of an em- 

 pirical estimator of compensatory capacity; the use 

 of natural mortality rate may not be appropriate in 

 many cases. Clearly, an improved approximation 

 method would be of value both to fishery manage- 

 ment and to power plant impact assessment. 



LITERATURE CITED 



Alverson, D. L., and W. T. Pereyra. 



1969. Demersal fish explorations in the northeastern Pacific 

 Ocean — an evaluation of exploratory fishing methods and 





