Can biologists look at scales to discriminate between sea run alewife and blueback herring from the Sheepscot River, Maine?

Sea run alewife (Alosa pseudoharengus) and blueback herring (Alosa aestivalis) are managed collectively as ‘river herring’ because the two species are difficult to distinguish visually.  Most reliable methods for differentiating the species are lethal (i.e., examining the peritoneum) or require extensive laboratory work (e.g., meristic counts and measures, DNA sampling). We evaluated non-lethal methods by performing ‘blind’ identification tests using collections of river herring scales.  We collected river herring in spring 2009, and identified species by examining the peritoneum. PowerPoint slides were then created of scale images from 30 alewives and 29 blueback herring. Twelve scale readers, whose experience with river herring scales varied from novice to expert, identified the species represented by the scale images.  Differences in classification accuracy among readers were not detected.  Readers were generally more accurate when examining multiple scale images than a single scale image, but mean classification accuracy was less than 76% indicating that scale pattern approaches are not very accurate in visually identifying the two species.  Slight, significant differences in the snout length to eye diameter ratio of the two species may enable scientists to quantitatively separate the two species, but this approach usually requires excessive handling of the fish.