Early signs of recovery suggested by changes in the structure and function of deep-sea megabenthic communities on a seamount 19 years after fishing

Abstract

Bottom trawling is the most widespread and severe human disturbance affecting deep-seafloor environments. Seafloor communities inhabiting seamounts (undersea hills or mountains) are especially vulnerable to trawling impacts as they are often dominated by long-lived, sessile epifauna (e.g., cold-water corals) that are easily damaged and/or removed by fishing gear. Trawling on seamounts is spatially concentrated, compounding effects on the benthos. However, the extent to which communities on seamounts that are closed to fishing can recover from earlier trawling impacts is uncertain. Previous studies have hypothesised that any post-trawling recovery of benthic communities on seamounts will probably be initially patchy, recolonised by surviving remnant populations, and may take several decades or more to occur. A fine-scale approach is thus needed to understand and determine the spatio-temporal dynamics of recovery. To address this need, we studied a New Zealand seamount located on Chatham Rise, Morgue, that was heavily trawled in the 1990s before being closed to fishing in 2001. We analysed seafloor imagery collected with a towed camera during repeat surveys in 2001, 2005, 2009, 2015, and 2020 to examine potential changes in benthic community structure and function after trawling ceased. A temporal shift in community structure driven by changes in the abundances of several taxa (mainly bryozoans, stylasterid hydrocorals, and comatulid crinoids) was observed over the study timescale, indicating the communities may have been in the early stages of post-trawling recovery. However, structural variation between different seamount sides and the summit (i.e., spatial variation) was still more pronounced than temporal variation and reflected the trawl fishing footprint. Community function showed little sign of ongoing recovery overall, though some change (driven by colonisation by stylasterid hydrocorals and bryozoans) indicative of early recovery was observed for communities on the seamount summit. Juvenile colonies of the reef-forming stony coral Solenosmilia variabilis were also observed between 8 and 19 years after the closure, demonstrating that the fisheries closure may have facilitated the early recovery process of this species and its associates. Overall, this work has important implications for the spatial management of seamounts vulnerable to bottom-contact fishing operations.