Seamount megabenthic assemblages fail to recover from trawling impacts

Abstract

Because the nature, tempo and trajectories of biological changes that follow the cessation of trawling are unknown for seamounts, it is unclear whether closing them to trawling will lead to a recovery of the fauna and, if so, over what time scales. This paper reports on a 'test of recovery' from repeated towed camera surveys on three seamounts off New Zealand in 2001 and 2006 (5 years apart) and three off Australia in 1997 and 2006 (10 years apart). In each region, seamounts where trawling had ceased were compared to adjacent seamounts where trawling was still active, and to seamounts that had never been trawled. If recovery signals existed, the likelihood of detecting them was high because the seamounts were relatively small and topographically simple, and because quantitative survey methods were employed. Multivariate patterns showed no change in the megafaunal assemblage consistent with recovery over a 5-10 year timeframe on seamounts where trawling had ceased. Results based on the number of species and diversity were equivocal, with some cases of increase and decrease on seamounts where trawling had ceased. A few individual taxa were found at significantly higher abundance in the later surveys where trawling had occurred. We suggest this may have resulted from their resistance to the direct impacts of trawling (two chrysogorgid corals and solitary scleractinians), or from protection in natural refuges inaccessible to trawls (unstalked crinoids, two chrysogorgid corals, gorgonians, and urchins). Alternatively, these taxa may represent the earliest stages of seamount recolonisation. They have potential to be dominant for long periods because the pre-trawling composition of benthic assemblages on seamounts includes taxa that grow slowly and ⁄ or have an association with 'thickets' of a single keystone stony coral (Solenosmilia variabilis) that has generated biogenic habitat over millennia. Resilience of seamount ecosystems dominated by corals is low compared to most other marine systems subject to disturbance by bottom trawling because there are no alternative habitats of the same value for supporting associated species, and because trawling typically removes coral habitat from large areas of individual seamounts. Management to conserve seamount ecosystems needs to account for changing oceanographic conditions (ocean acidification), as well as the direct impacts of human activities such as bottom trawling. Networks of spatial closures that include intact habitats over a range of depths, especially <1500 m, and on clusters and isolated seamounts, may be effective by maintaining the resilience of seamount benthic communities.

Recovery of marine animal populations and ecosystems

“…After benthic trawling, slower-growing sponges and corals took up to 8 years to recover compared with <1 years for polychaetes [21]. Yet, a deep seagrass meadow showed only a few signs of recovery 100 years after trawling [75], and a megabenthic seamount assemblage failed to recover 5-10 years after trawling [76]. Population or ecosystem recovery might take even longer if there has been a regime shift, as in the Benguela ecosystem [18].…”

Habitat Differences in Deep-Sea Megafaunal Communities off New Zealand: Implications for Vulnerability to Anthropogenic Disturbance and Management

“…In New Zealand, deep-sea fisheries are economically valuable but can impact habitats with vulnerable benthic communities (e.g., seamounts: Clark and Rowden, 2009;Williams et al, 2010b;canyons: De Leo et al, 2010;seeps: Baco et al, 2010;Bowden et al, 2013), and the fauna of the open slope (e.g., Cryer et al, 2002). Consumers are becoming more aware of the issues surrounding the sustainability of fisheries, thus research that can inform approaches to balance fisheries exploitation with conservation of vulnerable deep-sea communities are of interest to resource and environmental managers.…”

Deep-Sea Mega-Epibenthic Assemblages from the SW Portuguese Margin (NE Atlantic) Subjected to Bottom-Trawling Fisheries

“…Effects on mega-epibenthic fauna include the decline of both standing stocks (abundance and biomass) and species richness, and changes in community composition (Clark et al, 2015 and references therein). In addition, the damage of long lived habitat-forming organisms (i.e., sponges and corals) in seamounts areas, have shown a very low recoverability and marked community shifts of their associated fauna (Koslow et al, 2001;Clark and Rowden, 2009;Williams et al, 2010;Yesson et al, 2016). In more extreme cases, alterations of the megaepibenthic faunal distribution patterns at different spatial scales can also occur (Althaus et al, 2009).…”