Vegetation preference and the puzzle of abandonment: high-quality spawning habitat remains unused by Pacific herring (Clupea pallasii)

Understanding factors that drive the spawning behaviour of migratory fish populations is essential for effective and sustainable fisheries management. Pacific herring (Clupea pallasii) in the Strait of Georgia, British Columbia, Canada, have undergone a significant contraction in spawning range over the last century, abandoning numerous historical spawning sites in favour of a single dominant region. The mechanisms underpinning the reoccupation of these abandoned regions remain unclear. This study evaluated if habitat type limits reoccupation by quantifying spawning substrate availability at both active and abandoned sites and assessing spawning vegetation preferences. We developed a predictive model of egg deposition based on 36 years of dive surveys (1988-2024) and applied it to vegetation data collected via remotely operated vehicle surveys at eight sites. We found that herring prefer structurally complex vegetation, particularly the invasive macroalga (Sargassum muticum), which supports significantly higher predicted egg densities than native flat kelps or seagrasses. However, contrary to the habitat limitation hypothesis, abandoned sites frequently possess vegetation assemblages equal to or exceeding the quality and density of those at active spawning grounds. Substrate availability is thus not a primary bottleneck for spawning recovery at abandoned sites. Other factors may play a role, such as learned migratory behavior and the loss of diverse spawning contingents. Consequently, while vegetation restoration can benefit ecosystem functioning, the rapid recolonization of historical spawning grounds is unlikely without management interventions targeting population-level constraints rather than habitat enhancement alone.