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Rejection of the Equal Energy Hypothesis injuries have a clear impact on performance because for Pile Driving of the consequences, likely mortality and/or damage to organs. Severe injuries would require consider- The equal energy hypothesis was suggested to able opportunity for recovery, which under most cir- be irrelevant for fishes exposed to pile driving cumstances would not unlikely be available to the (discussed in Carlson et al., 2007). The significant dif- fish (e.g., predator-free refuge, ideal flow rates, eas- ference (p = 0.015) between the 1,920- and 960-strike ily accessed nutrition-rich foraging). In contrast, the regression lines (Figure 4) strongly supports this sug- Mild physiological injuries seemingly would not gestion, and similar results have been shown for affect the performance of fish because life functions mammals exposed to impulsive sounds (Hamernik would not be compromised and recovery needs et al., 2003). As a result, the use of a single metric, would be minimal. typically SELcum, is not sufficient to determine regu- The biological response metric, RWI, derived in latory criteria but remains an important variable to this study, permits identification of impulsive sound observe during pile driving activity. Other metrics exposure thresholds, expressed in terms of common related to SELcum, such as SELss and number of strikes sound measures, which protect fish from levels of need to be taken into consideration. physiological injury that would likely affect their performance and ability to survive. None of the Mild Relationship Between Pile Driving Exposure injuries singularly or in combination would be likely and Biological Response to reduce individual performance or affect ecologi- cal endpoints. This is not necessarily the case for This study has shown that the severity of baro- Moderate injuries and certainly not the case for trauma is a function of the energy in each strike Mortal injuries. At the lower end of RWI values, an (SELss) summed over the total number of strikes RWI level of 1 or 2 can only be realized by 1 or 2 needed to drive a pile, SELcum. For each strike that Mild injuries respectively. An RWI of 3 can occur occurs during pile driving activity, the energy deliv- with 3 Mild injuries or 1 Moderate injury. While 3 ered to the pile is used to overcome resistance Mild injuries would not likely reduce performance of encountered from substrate changes and other factors. fish, the same may not be true of 1 Moderate injury. Furthermore, for each strike, the energy delivered to With this in mind, an option would be that an the pile is managed by manipulating the stroke (the RWI level of 2 be used as an acceptable level of distance the hammer travels). For a particular pile, physiological injury for juvenile Chinook salmon generally the greater the energy in the strike on a pile, exposed to pile driving sound. This level of injury is the higher the energy level in the impulsive sound not overly cautious, but still very protective of fish generated (Carlson and Weiland, 2007). This means exposed to pile driving sound. This recommendation that the elemental unit of exposure for fish to pile is specifically for juvenile Chinook salmon in the driving sound is the energy in individual strikes, range of 93115 mm (standard length) and an aver- SELss. This complex relationship between exposure age wet weight of 11.8 g. An RWI of 2 could be and response is summarized for juvenile Chinook carefully extrapolated to include other fish within salmon in Figure 8. the salmonid family of similar size. It may also be Data from this study demonstrate that as energy possible to extrapolate to other species, but that levels of pile driving exposures increase there is a would be beyond the scope of this report. In addi- statistically significant increase in the severity of tion, based on predictions made by Carlson et al. barotrauma injuries. The highest energy levels pre- (2007), it is likely that larger fish would show less sented in this study caused injuries that resulted in effects at the same exposure levels as those used substantial physiological costs to the fish. The less here. Thus, at higher exposure levels, it is possible severe exposures caused fewer barotrauma injuries, that larger fish would potentially have an RWI of 2 and these tended to be Mild injuries, which imposed or below. minimal or no physiological effects on the fish. Figure 8 can be used to identify fish responses The survival of fish exposed to pile driving (RWI) to impulsive pile driving sound, in terms that sound that experience injury is dependent on the map directly to the pile driving activity. This mapping cumulative effect of those injuries on the perfor- will protect fish from having a biological response mance and energetics of the fish. The most severe that is greater than an RWI of 2, for example. If an 18