Distribution and Abundance of the Bioeroding Sponge Cliona sp. in Oyster Populations of Ostrea chilensis Across New Zealand

Bioeroding sponges are recognised pests in shellfish fisheries, as their boring activity can reduce market value, contribute to oyster mortality and cause economic losses. In New Zealand, bioeroding sponges in the shell‐burrowing alpha form are frequently observed in shells of the flat oyster Ostrea chilensis , a historically and culturally important species supporting several fisheries, including those in Foveaux Strait. Although sponge infestation has been noted for decades and linked to reduced meat quality, quantitative data on its distribution and abundance in oyster populations remain limited. Here, we investigated the distribution and abundance of Cliona sp. in O. chilensis populations across New Zealand, with a particular focus on Foveaux Strait. Cliona sp. was widespread in all oyster populations, but the proportion of infested individuals varied strongly among sites. The proportion of infested oysters increased along an east–west gradient within Foveaux Strait, and large oysters were more frequently infested than smaller size classes. Infestation intensity differed between large and small oysters, but not among sites, suggesting that spatial differences in sponge occurrence may be linked more strongly to recruitment or spread than to within‐oyster sponge growth. Our findings provide the first quantitative assessment of boring sponge infestation in New Zealand flat oyster fisheries and establish a baseline for future monitoring.