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Central Coast Trawl Impact and Recovery Study: 2009-2012 Final Report

Lindholm, J., M. Gleason, D. Kline, L. Clary, S. Rienecke, M. Bell, and B. Kitaguchi (January 2013)

Report to the California Ocean Protection Council, 49pp.


This report summarizes the results of a multi-year study (June 2009 to December 2012) to assess the impacts of bottom trawling on seafloor habitats and associated biological communities. The Central Coast Trawl Impact and Recovery (CCTIR) study was 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 project used small foot-rope trawl gear in an experimental study conducted in unconsolidated, soft-sediment habitat on the continental shelf off of Morro Bay, California. It was a collaborative research effort that involved numerous federal, academic, NGO, and fishing partners in the design and execution of the research.

Research Objectives

The objectives of this research project were to compare any changes to the structural and biological attributes of the seafloor before and after bottom trawling at known levels of trawling effort, as well as to monitor the recovery of the seafloor post-trawling. Specifically, we assessed changes to the small-scale topographic complexity and abundance of mobile and sessile (attached) invertebrates that are important attributes of fish habitat that could be attributed to trawling impacts. The research questions that were addressed by this study included:

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

Research Design

The experimental design for this project underwent extensive peer-review by the Ocean Science Trust and external reviewers, including representatives of the fishing industry. 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. Four of the plots were selected to be trawled at specific levels of intensity (based on historical effort data), while the remaining four plots served as non-trawled control plots against which changes in the trawled plots were evaluated over time. The study employed a thirty-three foot small-footrope otter trawl for the directed trawling; this is the gear required to be used in this shelf habitat shoreward of the trawl Rockfish Conservation Area. A remotely-operated vehicle (ROV) was used to collect photographic and video data and a modified Van Veen bottom grab sampler was used to collect sediment samples in the study plots before and after trawling.

Research Timeline:

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 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 was conducted in May 2012, approximately one and one-half years post-high-intensity trawling.

Results and Conclusions

Our results showed that the heavy trawl doors that hold the trawl net open leave persistent scour marks in the seafloor, but there were few other measurable effects that could be attributed to the experimental trawling in this habitat. Our results on the effects of bottom trawling activity on the structural attributes of fish habitat in soft sediments of the outer continental shelf include:

  • Trawl door scour marks can persist for up to a year in unconsolidated sediments.

We found that tracks left in the seafloor by the heavy trawl doors scouring the seafloor, measuring as wide as 20 cm and as deep as 10 cm, persisted for at least a year following low-intensity trawling.

  • Minimal impact to micro-topographic structure on the seafloor following both low- and high-intensity bottom trawling.

Some smoothing out of the seafloor, as measured by reductions in bioturbated sediments, were observed in the trawled plots compared to controls; however, these minimal differences with respect to micro-topographic structure on the seafloor were statistically significant only during one sampling period.

  • No measurable impacts of bottom trawling on macro-faunal invertebrate densities

There were no observed differences in densities of either sessile invertebrates or mobile invertebrates between the trawled or control plots that could be attributed to trawling.

  • High spatio-temporal variation in macro-faunal invertebrate densities.

We found that densities of both sessile and mobile invertebrates were both low in the study area, and that densities varied considerably across the study plots and between study periods. This suggests that any effects of trawling on epifaunal invertebrate communities must be considered in the context of significant background environmental variation.

  • No difference in the composition of infaunal invertebrates between trawled and control plots.

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.

In conclusion, this study showed that bottom trawling with small footrope trawl gear impacted the unconsolidated sediments of the study area via the trawl door scour marks that persisted for up to a year following trawling. However, the small reductions in micro-topographic complexity observed in trawled plots relative to control plots were generally not significant, and there were no measurable effects of trawling on densities of invertebrates, including sessile and mobile epifauana and infauna. The study area was characterized by a high degree of patchiness in space and time in the invertebrate assemblage, particularly for opportunistic species such as polychaete worms and brittlestars.

Management Implications

Trawling is still the primary way to catch flatfish and remains an important component of California's fisheries. It is important to reconcile our need for local seafood, fish landings, and fishermen's livelihoods with environmental impacts of trawling in different habitats, as well as bycatch and discard rates associated with the gear. The types of management measures this study could inform include trawl gear regulations, trawl effort controls, and spatial management of trawling to limit the footprint to avoid more sensitive habitats.

One of the great benefits of this project was the collaborative partnership that 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.

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