Taxonomic Discrimination Between Larval Herring (Alosa pseudoharengus and A. aestivalis): Is It Feasible?

Anadromous populations of Alewife (Alosa pseudoharengus) and blueback herring (A. aestivalis) in the Northeast are commonly referred to and managed collectively as “river herring” since their distribution is overlapping and their morphology, ecological role, and environmental requirements are similar. Nevertheless, significant differences in certain physical, physiological and biological characteristics exist between the two species.  There is compelling recent evidence that river herring populations in the Northeast are declining. For example, annual passage within the Connecticut River at various fish passage facilities has recently declined in orders of magnitude over the last 20 years.  In response to these declines, emergency closures and moratoriums have been instituted by management agencies in several New England States. Facilities located within estuaries with large intake water demands are commonly required to assess species-specific impacts.  Species-specific impacts are typically assessed using Equivalent Adult Models (EAM), in which stage-specific mortality rates are a model input. Sampling programs designed to generate these data often require large sample sizes and result in the collection of relatively high densities of herring eggs and larvae; making ichthyoplankton sorting and identification very labor intensive.  Due to the perceived difficulty of taxonomically distinguishing among species at the egg and larval life stages, laboratories do not typically identify to the species level, consequently mortality estimates are only derived to the genus level (Alosa).  Since the northern populations of river herring are in decline, it is important to understand the individual population dynamics of these species separately. Several taxonomic methodologies for distinguishing larval life stages reference techniques including the analysis of preanal myomeres and preanal length for discrimination.  Myomere analysis is extremely labor intensive and often thought to give inconsistent taxonomic information.   This paper compares and contrasts the utility of various taxonomic methods used to discriminate between larval alewife and blueback herring with the goal of developing a procedure that will yield a high degree of efficiency and accuracy.  Ichthyoplankton samples will be collected in the spring of 2011 within the Taunton River, Massachusetts and river herring will be identified to species utilizing at least two of the most promising methodologies.  Taxonomic identification will be verified by developing a genetic marker for herring in the Taunton River.