Authors:
J. de Marignac1, J. Hyland2, J. Lindholm3, A. DeVogelaere1, W.L. Balthis2, and D. Kline3
1NOAA/NOS Office of National Marine Sanctuaries (ONMS), Monterey Bay National Marine Sanctuary, 299 Foam Street, Monterey, CA 93940
2NOAA/NOS National Centers for Coastal Ocean Science (NCCOS), Center for Coastal Environmental Health and Biomolecular Research (CCEHBR), 219 Fort Johnson Road, Charleston, SC 29412-9110
3Institute for Applied Marine Ecology, California State University Monterey Bay, 100 Campus Center, Building 53, Seaside, CA 93955
A number of studies have shown that mobile, bottom-contact fishing gear (such as otter trawls)
can alter seafloor habitats and associated biota. Considerably less is known about the recovery
of these resources following such disturbances, though this information is critical for successful
management. In part, this paucity of information can be attributed to the lack of access to
adequate control sites areas of the seafloor that are closed to fishing activity. Recent closures
along the coast of central California provide an excellent opportunity to track the recovery of
historically trawled areas and to compare recovery rates to adjacent areas that continue to be
trawled. In June 2006 we initiated a multi-year study of the recovery of seafloor microhabitats
and associated benthic fauna inside and outside two new Essential Fish Habitat (EFH) closures
within the Cordell Bank and Gulf of the Farallones National Marine Sanctuaries. Study sites
inside the EFH closure at Cordell Bank were located in historically active areas of fishing effort,
which had not been trawled since 2003. Sites outside the EFH closure in the Gulf of Farallones
were located in an area that continues to be actively trawled. All sites were located in
unconsolidated sands at equivalent water depths. Video and still photographic data collected via
a remotely operated vehicle (ROV) were used to quantify the abundance, richness, and diversity
of microhabitats and epifaunal macro-invertebrates at recovering and actively trawled sites,
while bottom grabs and conductivity/temperature/depth (CTD) casts were used to quantify
infaunal diversity and to characterize local environmental conditions.
Analysis of still photos found differences in common seafloor microhabitats between the
recovering and actively trawled areas, while analysis of videographic data indicated that biogenic
mound and biogenic depression microhabitats were significantly less abundant at trawled sites.
Each of these features provides structure with which demersal fishes, across a wide range of size
classes, have been observed to associate. Epifaunal macro-invertebrates were sparsely
distributed and occurred in low numbers in both treatments. However, their total abundance was
significantly different between treatments, which was attributable to lower densities at trawled
sites. In addition, the dominant taxa were different between the two sites. Patchily-distributed
buried brittle stars dominated the recovering site, and sea whips (Halipteris cf. willemoesi) were
most numerous at the trawled site though they occurred in only five of ten transects. Numerical
classification (cluster analysis) of the infaunal samples also revealed a clear difference between
benthic assemblages in the recovering vs. trawled areas due to differences in the relative
abundances of component species. There were no major differences in infaunal species richness,
H 2 diversity, or J 2 evenness between recovering vs. trawled site groups. However, total infaunal
abundance showed a significant difference attributable to much lower densities at trawled sites.
This pattern was driven largely by the small oweniid polychaete Myriochele gracilis, which was
the most abundant species in the overall study region though significantly less abundant at
trawled sites. Other taxa that were significantly less abundant at trawled sites included the
polychaete M. olgae and the polychaete family Terebellidae. In contrast, the thyasirid bivalve Axinopsida serricata and the polychaetes Spiophanes spp. (mostly S. duplex), Prionospio spp.,
and Scoloplos armiger all had significantly to near significantly higher abundances at trawled
sites. As a result of such contrasting species patterns, there also was a significant difference in
the overall dominance structure of infaunal assemblages between the two treatments.
It is suggested that the observed biological patterns were the result of trawling impacts and
varying levels of recovery due to the difference in trawling status between the two areas. The
EFH closure was established in June 2006, within a month of when sampling was conducted for
the present study, however, the stations within this closure area are at sites that actually have
experienced little trawling since 2003, based on National Marine Fishery Service trawl records.
Thus, the three-year period would be sufficient time for some post-trawling changes to have
occurred. Other results from this study (e.g., similarly moderate numbers of infaunal species in
both areas that are lower than values recorded elsewhere in comparable habitats along the
California continental shelf) also indicate that recovery within the closure area is not yet
complete. Additional sampling is needed to evaluate subsequent recovery trends and persistence
of effects. Furthermore, to date, the study has been limited to unconsolidated substrates.
Ultimately, the goal of this project is to characterize the recovery trajectories of a wide spectrum
of seafloor habitats and communities and to link that recovery to the dynamics of exploited
marine fishes.
Key Words:
Fishing gear impacts, bottom trawling disturbances, benthic fauna, seafloor microhabitats, habitat recovery, central California continental shelf, National Marine Sanctuaries.