Caught in the middle: bottom-up and top-down processes impacting recruitment in a small pelagic fish

Abstract

Understanding the drivers behind fluctuations in fish populations remains a key objective in fishery science. Our predictive capacity to explain these fluctuations is still relatively low, due to the amalgam of interacting bottom-up and top-down factors, which vary across time and space among and within populations. Gaining a mechanistic understanding of these recruitment drivers requires a holistic approach, combining field, experimental and modelling efforts. Here, we use the Western Baltic Spring-Spawning (WBSS) herring (Clupea harengus) to exemplify the power of this holistic approach and the high complexity of the recruitment drivers (and their interactions). Since the early 2000s, low recruitment levels have promoted intense research on this stock. Our literature synthesis suggests that the major drivers are habitat compression of the spawning beds (due to eutrophication and coastal modification mainly) and warming, which indirectly leads to changes in spawning phenology, prey abundance and predation pressure. Other factors include increased intensity of extreme climate events and new predators in the system. Four main knowledge gaps were identified related to life-cycle migration and habitat use, population structure and demographics, life-stage specific impact of multi-stressors, and predator–prey interactions. Specific research topics within these areas are proposed, as well as the priority to support a sustainable management of the stock. Given that the Baltic Sea is severely impacted by warming, eutrophication and altered precipitation, WBSS herring could be a harbinger of potential effects of changing environmental drivers to the recruitment of small pelagic fishes in other coastal areas in the world. Graphical abstract

Combining modeling with novel field observations yields new insights into wintertime food limitation of larval fish

“…It remains an open question whether the productivity of the North Sea will increase in the future, or Atlantic herring will adapt its spawning strategy, including phenology and/or distributional shift, to promote larval survival as it is known for various marine fish species (e.g., Bakun 2006; Ottmann et al 2021). Long‐term climate variability have caused past shifts in the spawning season of herring in different ecosystems (ICES 2005; Moyano et al 2023) and the ongoing global warming could potentially drive the shifts in phenology (or prey requirements) beyond tolerable thresholds for the persistence of some populations in the future. Therefore, feeding processes and adaptation strategies are of great interest in species with considerable plasticity such as Atlantic herring.…”

Local reflects global: Life stage‐dependent changes in the phenology of coastal habitat use by North Sea herring

“…As these factors all change similarly within the year, their effects can be difficult to disentangle in statistical models, and establishing cause and effect is problematic. This is, in general, a limitation of any study relying on systematically collected field data, and highlights the importance of using not only long‐term monitoring surveys but also complementary laboratory experiments and modeling examinations to advance process knowledge as was recently highlighted in a review of efforts to understand the population dynamics of an Atlantic herring stock in the southwestern Baltic Sea (Moyano et al., 2023).…”

Persistence of fish populations to longer, more intense, and more frequent mass mortality events

“…HABs can cause mass mortality of fish (Karlson et al., 2021) including high mortality at the early life stages (Gjøsæter et al., 2000). For many marine organisms, and particularly for fish, survival through early life stages plays a key role in population dynamics (Gaillard et al., 2008; Moyano et al., 2023; Subbey et al., 2014). Early life stages of fish are known to have a relatively limited thermal tolerance (Dahlke et al., 2020), highlighting the link between MHWs and elevated mortality at these stages.…”