The effect of higher flows on anadromous fish has been difficult to quantify, in part due to the myriad of other factors such as pollution and non-native predators. A 2009 biological opinion from the National Marine Fisheries Service suggested that even higher flows would be required for the fish populations to truly benefit. In 2017, the independent environmental consulting group FISHBIO released a study showing that the number of outmigrating fish may not be as strongly related to artificial pulse flows as previously thought. Data from the 2005–2016 period indicated that fish migration responded the same way to river flows of as they did to the required flows of . Furthermore, the study found that most out-migration occurred during natural rainfall events along the lower river, rather than artificial pulse flows from upstream dams.