The goal of this project was to examine a series of locations in the far Northwestern Hawaiian Islands (NWHI) and the Emperor Seamount Chain (ESC) to address the hypothesis, based on predictions of low resilience and decadal recovery times for seamounts, that deep-sea corals have not recovered despite the end of trawling 30+ years ago. We also hypothesized that the initial colonization of a seamount may take decades, and that the initial colonizers would be the key source of propagules for subsequent recruitment to a given site. To test these hypotheses, we surveyed and sampled a series of replicate seamounts using an AUV and submersibles in 2014-2017.  We focused on three specific aspects: community structure, age structure of precious corals, and population connectivity of precious corals. 

The seamounts of the far NWHI and ESC provided excellent test locations because they have had some of the heaviest trawl impacts in the world, from both fish and precious coral fisheries.  Additionally, because of the coral fishery, we expected that the pre-trawling benthic community included precious corals and that they would be an important part of the recovering communities. Another advantage of this location is that the history of trawling allows for a three ?treatment? design that includes sites that have never been trawled, recovering sites that were protected from trawling with the establishment of the US EEZ in 1977, and sites that are still trawled outside the EEZ.

The results of this study show that despite visible evidence of substantial historic fishing impacts, a subset of the seamounts that have been protected showed multiple measurable signs of recovery including corals re-growing from fragments, and higher abundances of benthic megafauna and deep-sea corals than still trawled sites. This indicates that protection of heavily trawled seamounts does allow for measurable recovery on time scales of 30-40 years.  These findings are contrary to the expectations and previous observations of little or no recovery of seamount communities following disturbance. Further work is needed to determine if the recovering sites are returning to the same communities that were present prior to fishing.

A second important outcome was the discovery of deep-sea scleractinian reefs.  In the Central and Northeast Pacific, the shallow aragonite saturation horizon (ASH) and high carbonate dissolution rates fueled the hypothesis that reef formation was improbable. Despite this, live scleractinian reefs were discovered on six seamounts at depths of 535?732 m and aragonite saturation state (Ωarag) values of 0.71?1.33. Although the ASH becomes deeper moving northwest along the chain, the depth distribution of the reefs becomes shallower, suggesting the ASH is having little influence on their distribution. Other environmental variables like currents and chlorophyll also cannot explain the depth distribution, requiring further study. While exciting, the discovery of these scleractinian reefs in this region is also of concern because the majority of the sites occur on seamounts with active trawl fisheries.

Further outcomes include an increased understanding of the distribution and spatial variability of benthic megafauna on seamounts. This work included the most extensive seamount surveys that have ever been undertaken, and revealed that high levels of variability in benthic communities can occur over small spatial scales for both fishes and invertebrates.  The most pronounced difference in all diversity, abundance and assemblage structure metrics was with depth, but there was also variation among sides of a seamount when controlling for depth. 

These findings are important to management of the region.  Imaging surveys conducted on the four actively fished seamounts showed vulnerable marine ecosystem (VME) taxa (octocorals, scleractinian reefs, and sponges) were present in sufficient abundances and densities to constitute reproductively viable populations and to be acting as habitat for invertebrates and fishes (and therefore can be considered VMEs). Evidence of significant adverse impacts (SAIs) from bottom-contact fisheries was also observed including large areas with scars from bottom contact gear, the presence of lost gear, including many observations of coral rubble in or around nets, lines or floats, and the fact that coralliid octocorals, once sufficiently abundant on the seamounts to support the world?s largest precious coral fishery, were extremely rare. Despite these SAIs, some images suggested remnant and/or recovering VME populations on all four currently fished seamounts.

The ecological implications of these results are that recovery of deep-sea corals on seamounts may be possible, aided by pockets of remnant populations, if protections are put into place. In a management context, the results of this project indicate the need for closure of currently untrawled and fished areas in all NHR and ESC seamounts to bottom contact fisheries until fishing gear can be proven to not cause SAIs.

Additional broader impacts include training of three PhD and one M.S. student who derived the bulk of their research from this project along with numerous undergraduates who gained both laboratory and field research experience.

Last Modified: 05/28/2020
Modified by: Brendan Roark