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Fisheries Behavioral Ecology Program - Newport Laboratory

Emergent Biotic Structure Controls Juvenile Flatfish Distribution in Nursery Areas Off Kodiak Island, Alaska

polychaete Sabellides sibirica
Figure 14.  During summer 2008, the polychaete Sabellides sibirica formed a dense turf between depths of 21 and 30 m at Pillar Creek Cove.  Photo courtesy of Stephen Jewett, University of Alaska Fairbanks.

Acrylic microcosm inserted into substrate
Figure 15.  Acrylic microcosm inserted into substrate by divers allowed video monitoring of juvenile flatfish behavior on bare sand and worm turf seafloor.  Fish were introduced to the microcosm through a port on the top of the cylinder and monitored for approximately 1 hour.

Field studies around Kodiak, Alaska, reveal that age-0 juvenile Pacific halibut and northern rock sole (Lepidopsetta polyxystra) aggregate in shallow coastal waters (<50m) for their first summer, where they are closely associated with seafloor characterized by sparse to moderate coverage by ampharetid polychaete worm tubes.

During the summer of 2008 the ampharetid polychaete worm Sabellides sibirica was more common at the Pillar Creek Cove study site than during any of the 8 years this site has been monitored (Fig. 14). This allowed for the testing of specific hypotheses regarding how juvenile flatfish interact with this worm tube habitat.

We had hypothesized that while attracted to areas of bottom covered by sparse or patch worm tubes, juvenile flatfish avoid dense worm ‘turf’, because it interferes with their first line of defense against predation; namely burial. To test this hypothesis, divers placed juvenile northern rock sole into microcosms over bare sand seafloor or seafloor covered by dense worm turf (Fig. 15).

In sand treatments fish immediately buried into the bottom. In contrast, on worm bottom fish moved around, making repeated, but unsuccessful burial attempts (Fig. 16). After 5 minutes, fish on sand periodically emerged and moved about, but continued to bury at will. On worm turf bottom, fish continued to move about making unsuccessful burial attempts.

Fish distribution at Pillar Creek Cove was tightly associated with worm tube cover. The worm tubes formed a continuous turf from depths of 21–30 m ( Fig. 17 top). Video from our towed camera sled revealed that age-0 flatfish density increased with depth, with the highest density at the edge of the worm turf, where the worm tubes were patchy ( Fig. 17 middle). However, age-0 flatfish were nearly absent from bottom where worm tubes formed a dense turf. Importantly, fish were also infrequent along the outer (deeper) patch edge of the worm turf.

figure 16, see caption
Figure 16.  Burial attempts by juvenile northern rock sole on sand and worm turf seafloor.  Along the x-axis, time is divided into 2.5 min intervals.

figure 17, click image to enlarge
Click image to enlarge.

We had hypothesized that aggregation of fish in areas of sparse/patchy worm tube cover was attributable to 1) lower predation intensity, and/or 2) more or better quality forage base in this habitat. We tested this first hypothesis by tethering juvenile flatfish over a range of depths from 4 to 47 m.

Predation mortality of tethered fish increased with depth from 4 to 16 m ( Fig. 17 bottom), but then declined significantly at 21 m; the area of sparse worm tubes along the turf edge where juvenile flatfish density was highest. This suggests that reduced predation risk in the vicinity of sparse/patchy worm tube cover may partially explain juvenile flatfish preference for this habitat. We are also exploring the nonmutually exclusive hypothesis that juveniles are attracted to this habitat by its associated forage base.

During July through September, we took both benthic grab samples and fish for stomach analysis across a range of depths to determine habitat specific prey availability and diets of juvenile flatfish. These samples are currently being analyzed at the University of Alaska Fairbanks (benthic samples) by Stephen Jewett and at the Kodiak Fisheries Research Center (stomachs) by Brian Knoth.

It is particularly noteworthy that the prevalence of this worm tube habitat varies greatly from year to year, and in some years, is completely absent from our study sites. Additional studies are ongoing to elucidate the role this conspicuous habitat feature plays in controlling the quality of shallow water nursery embayment for juvenile flatfish utilizing the nearshore waters of the Gulf of Alaska, as well as examining how natural anthropogenic disturbance influences this habitat.

By Clifford H. Ryer


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