MacCALL ET AL.: POWER PLANT IMPACT ASSESSMENT 



timated, for example, based on age frequency, the 

 estimate will often be the total mortality rate of 

 adults (Z), where 



Z = M+IF p + ZF,, 



(19) 



The natural mortality rate can be obtained from 



M = Z-TF- IF, 



(20) 





If fish are being removed from the stock by several 

 power plants and fisheries, the impact by a single en- 

 tity cannot be considered in isolation. Our proposed 

 method of long-term impact assessment allows an in- 

 tegrated assessment of impact, but also allows dis- 

 section into individual contributions to the total 

 impact. Equation (15) states that the total rate of 

 removals (F) is the sum of the individual instan- 

 taneous rates of removal. Since Equation (15) is a 

 linear function, the fraction of the total impact which 

 is attributable to any particular entity is the ratio of 

 the sum of its contributions to totalF from all sources 

 as in Equation (15). 



CRITERIA FOR IMPACT 

 EVALUATION 



Removal of fish from a stock, whether by a fishery or 

 by a power plant, will usually lead to lowered equilib- 

 rium abundance. The previous section has presented 

 a method for estimating the approximate reduction 

 in abundance which has taken place. We must now 

 determine whether this impact is "acceptable." In 

 the case of a fishery where tangible values can be 

 assigned to the catch and the stock, optimal catch 

 rates and population sizes can be defined (Roedel 

 1975; Clark 1976). However, losses to a power plant 

 produce no direct consumptive benefit, and, in many 

 cases, the impacted stock is not subject to a fishery 

 and therefore has no conventional value. 



Fortunately, there exists a precedent for evaluating 

 impacts on nonvalued species. The Marine Mammal 

 Protection Act, enacted by the United States in 

 1972, requires that marine mammals be managed for 

 optimum sustainable population size (OSP). Subse- 

 quently, the term was given a working definition: 

 "Optimum sustainable population is a population 

 size which falls within a range from the population 

 level of a given species or stock which is the largest 

 supportable within the ecosystem to the population 

 level that results in maximum net productivity. Max- 

 imum net productivity is the greatest net annual in- 

 crement in population numbers or biomass resulting 

 from additions to the population due to reproduction 



and/or growth less losses due to natural mortality." 

 (Gehringer 1976). 



While power plants clearly do not directly impact 

 marine mammals, the principle of optimum sustain- 

 able population size as defined above may be extend- 

 ed to fish and invertebrates as well. Since OSP is 

 defined to fall within a range, impact is unacceptable 

 when it causes population size to fall below the lower 

 limit of that range, which is the point of maximum net 

 productivity. In the logistic model assumed by our 

 long-term impact assessment, maximum net produc- 

 tivity occurs at one-half the virgin, unimpacted pop- 

 ulation size. This gives a simple criterion for 

 acceptable impact: Abundance should not be driven 

 below one-half of its unimpacted level. In terms of the 

 long-term impact model and its assumptions, this 

 criterion is equivalent to saying that the rate of re- 

 moval should not exceed the adult rate of natural 

 mortality. 



EXAMPLE APPLICATION 



This example is based on data collected during 

 1978 for the purpose of estimating power plant im- 

 pact on topsmelt, Atherinops affinis, inhabiting a 

 California estuary. 4 Topsmelt is a bay-dwelling 

 species with demersal eggs and a short (2-3 mo) 

 spawning period. Topsmelt eggs hatch to produce 6 

 mm larvae, and larvae >15 mm are not entrained. 

 Larvae were categorized by two length stages, 6-10 

 mm and 11-15 mm.'Based on laboratory growth rate 

 experiments for a closely related atherinid, the Cali- 

 fornia grunion, Leuresthes tenuis, duration of these 

 two stages is about 14 d each. 



Standing stock and entrainment data (Table 1) are 

 averages for the 3-mo spawning period based on 

 biweekly sampling. Since larvae are concentrated 

 near the surface during daylight hours, larval stand- 

 ing stock estimates are determined based on daytime 

 surface larval densities sampled by a neuston net. 

 These density values are extrapolated to the area 

 of the estuary and a depth of 1 m. The mean entrain- 

 ment rates are adjusted for variation in cooling water 

 flow during each of the sampling periods. All en- 

 trained larvae are assumed to die. The short-term im- 

 pact of the power plant is estimated to reduce 

 recruitment strengths to 98% of their unimpacted 

 value. Based on seine catches, the standing stock of 

 adult topsmelt in the estuary was 5.4 X 10 5 fish. Since 

 the power plant has long been operational, equi- 



4 Because the power plant impact study has not completed the offi- 

 cial review process, the proprietor wishes not to be identified at this 

 preliminary stage. 



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