Hydrologically Regulated Wetlands Act as Breeding Refugia for Mediterranean Gulls in a Flood‐Prone Coastal Lagoon System

ABSTRACT

Long‐term demographic datasets are essential for understanding how colonial waterbirds respond to environmental change. We analysed 30 years (1996–2025) of breeding population data and 10 years (2016–2025) of colony‐level productivity and breeding failure data for the Mediterranean Gull Ichthyaetus melanocephalus in the northern Adriatic coastal lagoons. Breeding numbers in managed fish farms increased steadily from their colonisation in 2003 to 2025 (annual percentage change [APC] = 18.4%, 95% CI: 12.8%–24.3%). During 2016–2025, the regional breeding population showed a modest positive trend (APC = 7.0%, 95% CI: 0.2%–14.3%), whereas the open‐lagoon component showed a negative but nonsignificant point estimate (APC = −20.7%, 95% CI: −39.4% to +3.8%). Open‐lagoon counts fluctuated strongly over the full study period, and breakpoint analyses did not support a clearly identified structural change. Managed fish farms hosted most breeding pairs in most recent years. Mean colony‐year productivity was 0.69 fledged chicks per active nest in managed fish farms and 0.14 in open lagoon habitats. Flooding accounted for a mean of 8.6% of recorded breeding failure in managed fish farms, compared with 37.8% in open lagoon habitats. Habitat differences in productivity and in the proportional contribution of flooding to breeding failure remained strong after accounting for nest elevation and in sensitivity analyses restricted to artificial nesting sites. These findings indicate that hydrologically regulated fish farms provided more reliable breeding conditions than tidally exposed habitats and were consistent with a habitat‐level, flood‐buffered breeding‐refugium function during the study period. Conservation planning in flood‐prone coastal lagoons should therefore integrate protected tidal habitats with compatible management of regulated wetlands, while recognising that their conservation value depends on continued water‐level control and nesting site maintenance.