Polychaete worm tubes modify juvenile northern rock sole Lepidopsetta polyxystra depth distribution in Kodiak nurseries

• We examined the effect of polychaete tubes on the distribution of juvenile rock sole.
• When tubes were abundant, rock sole co-occurred at intermediate depth.
• When tubes were absent/scarce, rock moved to shallower water.

We have observed inter-annual variability in the depth distribution of juvenile northern rock sole Lepidopsetta polyxystra on their nursery grounds around Kodiak Island, Alaska. This study evaluates whether this variability is a response to inter-annual changes in the availability of habitat created by polychaete tubes; principally Sabellides sibirica. We suspect that worm tubes constitute an alternative refuge and/or feeding habitat for juvenile flatfish. Accordingly, we hypothesized that during years of low worm abundance, fish would concentrate in the shallows (< 10 m depth) where they find refuge from predation, but would move to greater depths (> 15 m, where the worms occur) during years when the worms were abundant. Using data on worm abundance and fish density over 5 yr, we tested this hypothesis at 2 Kodiak nursery embayments. Whether worms were abundant in a given year or embayment had no influence on overall fish abundance, however, worm abundance did influence juvenile flatfish depth distributions. At Holiday Beach, where worms tended to be scarce, fish were typically concentrated in shallow water. However, during the 1 year when worms were abundant, fish were concentrated in deeper water. At Pillar Creek Cove, where worms are more regularly found, fish tended to concentrate in deeper water, the exception being the one year when worms were nearly absent. Regression analysis for both sites and all years indicated that the percent of fish occupying shallow water (< 10 m) decreased with increasing worm abundance. When worms were prevalent, fish were most commonly found on bottom with sparse to moderate worm cover, but avoided bottom where the worms were so dense as to form a ‘turf’. These results demonstrate that the geographic and inter-annual variation in worm tube abundance has significant influence over the distribution of juvenile northern rock sole.