A staging system for Atlantic herring (Clupea harengus) larvae based on external morphology and skeletal development

Atlantic herring (Clupea harengus) plays a key role within temperate marine food webs and is targeted by a significant over-regional fishery. Due to its high economic importance, dynamics in herring stock biomass and recruitment are closely monitored, forming the basis for fisheries management advice. As recruitment patterns translate into the adult stock biomass, early life stage ecology has been thoroughly addressed in fisheries research. Larval monitoring programs commonly focus on length measurements and abundance indices, rarely, information on larval developmental stages is given. As length is highly influenced by temperature, salinity and food availability, their size range can significantly vary between cohorts, populations, and ecotypes. Nowadays, a systematic staging system from the 1970s provides the standard guide for herring larval development, although it does not fully resolve important developmental stages. Here, we propose an improved staging system based on external morphology and skeletal development of herring larvae. The staging system has been developed and tested with herring larvae from different populations of the North and Baltic Sea to ensure applicability. The system comprises 15 stages (+substages) in 5 major developmental phases: the yolk sac phase, the dorsal fin development, the caudal fin development, the pelvic fin development, and the juvenile phase. This staging system aims to simplify herring larval staging to gain a more specific picture of early life dynamics. Because of the detailed description of the development, future studies are better equipped to identify stages which, for example, show high mortality rates and better link them to environmental circumstances.