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Central Coast Trawl Impact and Recovery Study: 2009-2011 Summary Report

Lindholm, J., M. Gleason, D. Kline, J. Adelaars, L. Clary, H. Kelley, S. Rienecke, M. Bell, and B. Kitaguchi (January 2012)

A Report to the California Ocean Protection Council, 39pp.

EXECUTIVE SUMMARY

This report summarizes accomplishments and results for the period from June 1, 2009 to December 31, 2011, and covers Years 1 and 2 of a multi-year study to assess the impacts of bottom trawling on seafloor habitats and associated biological communities. The Central Coast Trawl Impact and Recovery (CCTIR) study is funded by the California Ocean Protection Council (OPC) through a State Coastal Conservancy grant (#10-058) to The Nature Conservancy (TNC) and by private funders. This is a collaborative research project conducted in unconsolidated, soft-sediment habitat on the continental shelf off of Morro Bay, California that has involved numerous federal, academic, NGO and fishing partners in the design and execution of the research.

The aim of this research project is to compare any changes in microtopographic complexity of the seafloor and associated species that is attributable to bottom trawling across a gradient of trawling effort on the continental shelf and to monitor the changes in seafloor communities' recovery post-trawling. The research questions that are being addressed by this study include:

  • How does microtopographic complexity of the seafloor, invertebrate density and fish density differ between trawled plots and control plots over time in a depositional soft-sediment environment?
  • How do spatial and temporal patterns in seafloor community structure vary under different levels of trawling intensity?
  • What is the catch of flatfish and bycatch of associated species using trawl gear in this soft-bottom habitat?

These questions are being addressed using a remotely operated vehicle (ROV) to survey fishes, epifaunal macroinvertebrates, and seafloor microhabitats; a modified Van Veen bottom grab sampler to sample infaunal invertebrates; and a 33 ft. small-footrope otter trawl to disturb the seafloor and to collect additional information on trawl-caught fishes. The experimental design for this project underwent extensive peer-review by the OPC science advisory team and external reviewers.

The study area was apportioned into eight treatment plots, each measuring 1000 m x 300 m at a water depth of approximately 170 m, over soft-bottom habitat on the continental shelf off Morro Bay, California. Four of the plots were selected to be trawled at specific levels of intensity (based on historical effort data), while the remaining four plots serve as non-trawled control plots against which changes in the trawled plots can be evaluated over time.

Pre-trawling baseline surveys were conducted in the fall of 2009. The first directed trawling treatment occurred in October 2009, with 'low-intensity' trawling equivalent to 5 two trawl passes over the entirety of each of the four trawl treatment study plots. In October 2010 the 'high-intensity' trawling treatment was conducted, with five trawl passes over the entirety of each trawl treatment plot. Post-trawling surveys to assess impacts and recovery occurred at two-weeks, six months, and one year after each of the directed trawling efforts. A final survey to complete Year 3 of the study is planned for May 2012, approximately one and one-half years post-high-intensity trawling.

Analyses of project data are on-going pending the completion of the final research cruise. In this report we present the results of our analyses to-date, which offer preliminary insights into the ecological effects of bottom trawling activity on the structural attributes of habitat in unconsolidated sediments of the outer continental shelf. These are initial results and no conclusions should be drawn at this point in the study, however the primary results thus far include,

  • Both low- and high-intensity bottom trawling reduced microtopographic complexity of the seafloor.

We quantified a small, but persistent, difference between control and trawled study plots with respect to the percentage of the seafloor that was 'bioturbated,' the primary source of microtopographic complexity in unconsolidated sediments.

  • Significant spatiotemporal variation in macro-faunal invertebrate densities.

We found that densities of both sessile and mobile invertebrates varied considerably across the study plots and between study periods. This suggests that any effect on epifaunal invertebrate communities that is attributable to low- or high-intensity bottom trawling must be considered in the context of significant background environmental variation. Potential effects of the trawling treatments on invertebrates groups are still being analyzed.

  • Significant spatiotemporal variation in demersal fishes

We found that while community composition remained fairly constant over the entire study period, there was considerable seasonal and inter-annual variability in the demersal fish community with respect to abundance and spatial distribution across the study plots and between study periods.

  • No difference in the composition of infaunal invertebrates

We found that species diversity in the infaunal community was low relative to other locations along the continental shelf at similar depths, and that diversity did not vary significantly between trawled and control plots immediately following low-intensity trawling.

  • An aperiodic 'event' may confound results for recovery following high-intensity trawling

We observed that the trawl door scour marks visible in November 2010, immediately after the high-intensity trawling in October 2010, were no longer visible in May 2011 and there was a noted decline in abundance of mobile 6 invertebrates. This was in stark contrast to the persistence over a full year of the low-intensity trawl scour marks in year 1 of the study. We have hypothesized that some kind of large scale (across the whole study site or larger) event may have smoothed out the seafloor sediments.

  • Unique research partnership advancing discourse on fisheries management

One of the great benefits of this project has been the collaborative partnership that has evolved among diverse stakeholders interested in moving toward a more quantitative evaluation of the impacts of bottom trawling on seafloor communities and a greater understanding of ecosystem dynamics and resilience to inform fishery management.

Analysis of project results will be on-going through the end of 2012.

URL: http://montereybay.noaa.gov/research/techreports/trlindholm2012.html    Reviewed: March 04, 2014
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