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MESA Archives: Effects of Bottom Trawling on Soft-bottom Sea Whip and Other Habitat in the Central Gulf of Alaska

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  picture of sea whip bed Sea whip bed in Chiniak Gully, near Kodiak Island, as seen from the submersible Delta.
(click image to enlarge)

picture of Pacific cod over sea whip bed Pacific cod over a bed of small sea whips in Chiniak Gully.  Sea whips provide vertical relief to this low-relief habitat.  (click image to enlarge)
 

(PLEASE NOTE: These web pages are for archival purposes only and are no longer maintained. For current information please refer to the MESA homepage.)

In April 1987 the North Pacific Fishery Management Council closed two areas around Kodiak Island, Alaska to bottom trawling and scallop dredging (Type 1 Areas).

These areas were designated as important rearing habitat and migratory corridors for juvenile and molting crabs. The closures are intended to assist rebuilding severely depressed Tanner and red king crab stocks. In addition to crab resources, the closed areas and areas immediately adjacent to them, have rich stocks of groundfish including flathead sole, butter sole, Pacific halibut, arrowtooth flounder, Pacific cod, walleye pollock, and several species of rockfish.

These closures provide a rare opportunity to study the effects of an active bottom trawl fishery on soft-bottom, low-relief marine habitat because bottom trawling occurs immediately adjacent to the closed areas. In 1998 and 1999 the NMFS, Auke Bay Laboratory, initiated studies to determine the effects of bottom trawling on these soft-bottom habitats. Direct comparisons were possible between areas that were consistently trawled each year and areas where bottom trawling had been prohibited for 11 to 12 years. The proximity of the closed and open sites allowed for comparison of fine-scale infauna and epifauna diversity and abundance and microhabitat and community structure.

Analyses completed indicate that 1): trawling intensity, although high for the GOA, is relatively low compared to other areas worldwide, and 2) effects on the sedimentary and biogeochemical features of the seafloor and infauna community structure from present levels of bottom trawling were minor and no clear patterns were detectible. Although epifaunal community structure analyses are incomplete, a clear relationship between total epifaunal biomass and sea whip abundance is apparent. This relationship indicates that sea whip habitat may have increased productivity. Recent studies in the Bering Sea have shown a similar functional relationship for sea whip habitat.

  picture of Delta submersible Preparing to launch the two-person submersible Delta.  (click image to enlarge)

picture of submersible tender vessel AJ tracking trawl Submersible tender vessel AJ (background) acoustically tracks the trawl of the FV Peggy Jo
(left foreground).  (click image to enlarge)
 

In 2001, scientists at the Auke Bay Laboratory initiated a study to investigate the immediate effects of intensive bottom trawling on soft-bottom habitat and in particular an area colonized by sea whips. Sea whip biological characteristics and their resistance to two levels of trawling were studied. Sea whips are highly visible and changes in their abundance can be readily quantified. Within the study site, at least two species of sea whips (Halipterus sp., and Protoptilum sp.) are present with densities up to 10 individuals per m2. Sea whip beds provide vertical relief to this otherwise homogeneous, low relief habitat. This habitat may be particularly vulnerable since sea whips can be removed, dislodged, or broken by bottom fishing gear. Furthermore, since sea whips are believed to be long-lived, recolonization rates may be very slow.

The study plan consisted of three phases. In Phase 1, baseline data was collected. The Delta submersible was used to collect in situ videographic documentation of the seafloor along predetermined transects within the study area. Additionally, a bottom sampler was deployed from the submersible tender vessel to collect sediment samples from the seafloor.

During Phase 2, a commercial trawler made a single trawl pass in one corridor of the study area and repetitively trawled (six trawl passes) a second corridor. Phase 3 repeated the videographic and sediment sampling following the trawling phase. A scientist on board the Delta observed the seafloor and vocally identified biota and evidence of trawling including damaged or dislodged biota and marks on the seafloor from the various components of the bottom trawl (e.g., trawl door furrows, and ground gear striations) in synchrony with the external cameras.

Without clear evidence of trawling (e.g., trawl tracks) in the habitat studied in 1998 and 1999 and because of the patchy distribution of the most obvious sessile organisms, sea whips, quantifying immediate changes to the seafloor from trawling has been difficult. The 2001 study will allow quantification of effects resulting from known levels of trawling, and the experimental trawling will allow testing of hypotheses concerning the observed 1998-99 sediment and infauna changes. Additionally, the 2001 study will provide information for evaluating measures to minimize fishing effects such as area closures or gear modifications and if observations can be made in future years, an evaluation of sea whip recolonization and changes in productivity relative to sea whip abundance.


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