Discerning population connectivity and natal origins of Pacific herring (Clupea pallasi): inferences on population structure from otolith chemistry

Pacific herring, Clupea pallasi, undertake annual migrations between feeding and spawning grounds that link life stages, habitats, populations, communities, and ecosystems. However, movement patterns of these highly mobile fish are poorly understood. Declines in Pacific herring abundance and slow population recoveries in the absence of fishing pressure have elevated concerns over the status of this ecologically, economically, and culturally important species. Pacific herring spawn on substrate in nearshore habitats where eggs and larvae develop for approximately two weeks before hatching. Early development within discrete spawning grounds could facilitate the incorporation of distinctive chemical signatures within otoliths that could be used as intrinsic markers to trace movements and mixing among groups or regions. Identifying the direction and strength of connectivity among groups can reveal source populations and promote the development of population- and ecosystem-based management strategies that reflect ecologically relevant spatial scales. We applied otolith microchemistry data to: 1) test the utility of elemental signatures to distinguish the natal origins of larval herring; 2) evaluate inter-annual variation in natal signatures within spawning sites; and 3) assess the similarity of edge and natal signatures of adult herring within and among spawning sites. In 2015 and 2016, we sampled actively spawning adult herring and their offspring in the northern Salish Sea and across British Columbia, Canada. Otoliths were extracted, aged, and their elemental composition analyzed using laser ablation inductively coupled plasma mass spectrometry. Cohort-specific analyses were applied to assess consistency among elemental signatures and broader, age-specific movement patterns. Our analyses show that otolith elemental signatures of Pacific herring can provide insight into complex population structure at scales of 10s – 1000 kms to inform and enhance spatially-explicit approaches to conservation and management