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horizontal line represents 1,920 strikes and the lower the design enabled systematic exploration of the horizontal black line represent 960 strikes, the curvi- relationship between potential injury to fish and linear contours between represent the RWI values specific sound characteristics such as number of (1-10) derived using the results of testing at 1,920 strikes and sound level, expressed as both single and 960 strikes. It is unknown whether the derived strike (SELss) and cumulative sound exposure lev- relationship plotted would persist if additional els (SELcum). In contrast, all earlier studies on pile levels of strike numbers were tested. However, at driving used caged fishes under conditions in both strike number levels the RWI values increase which the investigators were unable to control any with increase in SELss. The graph of RWI as a func- aspects of sound presentation (e.g., number of tion of number of strikes and SELss is interpreted the strikes, sound intensity), or provide adequate bio- same way as SELcum in Figure 8A. Thus, an RWI of logical control groups (reviewed in Popper and 2 would be achieved when the SELss is about 181 dB Hastings, 2009). for 960 strikes, and an SELss of 179 for 1,920 strikes. Many aspects of pile driving could have been Figure 8C brings both contour layers, SELcum explored during these experiments. These included and RWI, together onto one graph, thereby showing sounds from various types of piles (e.g., steel, con- their relationship to each other as well as their rela- crete), inter-strike intervals, or total number of tionship to SELss and number of strikes. While com- strikes. After discussion with the NCHRP advisory plex, it links a common metric used to manage the group for this project, it was decided to limit param- exposure of fish to impulsive sound generated by eters and the variables to allow for collecting mean- pile driving, SELcum through its constituent parts, ingful and statistically valid data and analyses in a SELss and number of strikes, along with the derived reasonable amount of time. Examination of the lit- RWI in this study. Mathematically, RWI and SELcum erature and talking with scientists, regulators, and are both dependent variables defined by the indepen- industry representatives, found the most important dent variables SELss and number of strikes. The variables were SELss and number of strikes. These composite plot (Figure 8C) identifies the criteria two variables can be used to control driving piles, for acceptable pile driving sound exposure given either through management of the energy applied to a selected response outcome or RWI value. For a pile during each strike or by implementation of example, if a RWI of 2 was selected as the maxi- mitigating actions such as bubble curtains. Since the mum acceptable level of biological response for study focuses on sounds, it is reasonable to conclude a pile driving project, during project planning the that these sound level metrics could be extrapolated likely SELss and number of strikes needed to drive to other impulsive sounds from pile driving, as long each pile could be considered to identify alternative as the sound spectra are reasonably similar to strik- pile driving strategies to avoid exposures that would ing steel piles. risk exceedance of the selected RWI value. These The study's experimental strategy was designed to expectations would be dynamically managed during evaluate the relationship between SELss and SELcum, the project as actual SELss values were observed. number of strikes, and barotrauma damage. To Practically speaking, SELss with the addition of mit- accommodate concerns regarding the variability in igating actions such as bubble curtains are the pri- impulsive sound amplitude distribution, the expo- mary means available to control the exposures fish sure stimulus included eight different impulsive pile experience to pile driving sound. The results of this strike sounds, pseudorandomly presented, all with project suggest that a RWI of 2 or less does not lead the same SELss differing in details of their amplitude to physiological effects that reduce either the imme- (SPLpeak) characteristics. At the same time, a detailed diate or long-term performance and energetics of examination of the variables selected allowed for the Chinook salmon, or probably other species as well. quantitative examination of relationships between sound exposure level and injury. Techniques devel- CHAPTER 4 DISCUSSION oped during and results collected from this study have been incorporated into follow-up studies exam- Overview ining the recovery from Mild and Moderate injuries The work presented here represents the first and responses in Chinook salmon as well as other study to test the effects of pile driving sounds on fish species; these will be published in subsequent fish in a controlled plane-wave acoustic field. Thus, papers (Casper et al., in prep.). 17