Combined Use of Otolith Morphometry and Microchemistry to Study the Origin of Spring-Spawning Atlantic Herring in the St. Lawrence Estuary and the Gulf of St. Lawrence

An important goal for the conservation of marine fish metapopulations is to maintain the diversity of components spawning in areas subjected to different environmental pressures. The usefulness of otolith morphometry combined with microchemistry to generate new information on the diversity of spawning components was assessed in declining spring-spawning Atlantic Herring Clupea harengus in the St. Lawrence Estuary (SLE), Restigouche Estuary and adjacent areas (RE), and southern and northern Gulf of St. Lawrence (SGSL and NGSL). Mature Atlantic Herring (6-10 years old) were captured during the spring spawning season from 2013 to 2015 at nine sites with different salinity and temperature conditions along estuarine-marine and southern-northern gradients. Otoliths were measured by image analysis, and various shape indices were calculated. Elemental concentrations (Sr, Ba, and Mg) in the otolith cores were measured using laser ablation inductively coupled plasma mass spectrometry. Natal groups were inferred by hierarchical clustering analysis conducted on shape and microchemical variables separately or combined. Both types of analysis indicated the presence of two main natal groups: one predominant in the SLE and the other predominant in the RE. Proportions of fish having the characteristics of each natal source decreased with increasing distances from the head of the SLE and RE, respectively. Shape analysis provided higher discrimination in the SLE/NGSL, revealing temporal stability of a specific "pygmy" morphotype in the SLE that was previously described in the 1980s and 1990s. Microchemical analysis was more discriminating in the RE/SGSL, revealing a group of fish within the RE that had low Sr in the otolith core. Thus, otolith morphometry and microchemistry provided concordant and complementary information on natal groups, and both markers indicated the potential importance of estuaries as nursery grounds for the recovery of spring-spawning Atlantic Herring stocks in the SGSL and NGSL.