Polychaete worm tubes modify juvenile northern rock sole Lepidopsetta polyxystra depth distribution in Kodiak nurseries Clifford H. Ryer ⁎, Mara L. Spencer, Paul Iseri, Brian A. Knoth, Benjamin J. Laurel, Allen W. Stoner Fisheries Behavioral Ecology Program, Alaska Fisheries Science Center, NOAA Fisheries, Hatfield Marine Science Center, 2030 Marine Science Drive, Newport, OR 97365, USA abstract article info Article history: Received 29 January 2013 Received in revised form 28 May 2013 Accepted 29 May 2013 Available online xxxx Keywords: Biogenic habitat Juvenile flatfish Refuge Sabellides sibirica Video sled 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 (b 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 embay- ments. Whether worms were abundant in a given year or embayment had no influence on overall fish abun- dance, 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 (b 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 bot- tom 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. Published by Elsevier B.V. 1. Introduction The depth range over which a marine species, or life-stage, occurs is perhaps one of the most fundamental descriptors of its habitat. It has been widely observed that small fishes and crustaceans often aggregate in shallow water. The benefits of shallow water occupancy for juveniles are both physiological and ecological. On the physiological side, higher ambient temperature and abundant food in shallow water can acceler- ate growth (Ryer et al., 2012; Yamashita et al., 2001), hastening the point at which juveniles reach size-refuge from predators, potentially allowing greater survival potential both in the shallows and when they migrate to the adult habitat (Blundon and Kennedy, 1982). On the ecological side, aggregation of juvenile fishes and crustaceans in shallow waters is often attributed to lower predation rates; the ‘Shallow Water Refuge Hypothesis’ (sensu Baker and Sheaves, 2007). Lower predatory threat in shallow water may not only increase survival (Linehan et al., 2001; Manderson et al., 2004; Ryer et al., 2010a), but in- crease growth, as juveniles can decrease vigilance and devote more time to feeding (Ryer and Hurst, 2008). Further, in many systems shallow water and structured habitats go hand-in-hand; seagrasses, marshes, mangroves, and littoral vegetation in lakes and rivers (Crowder and Cooper, 1982). In addition to foraging opportunities, the habitat created by physical structure such as vegetation provides refuge from predation by interfering with the detection and pursuit of prey (Bartholomew et al., 2000; Ryer et al., 2004). Yet, many soft bottom communities are devoid of rooted vegeta- tion, and a wide variety of other structures, less conspicuous to the human eye, provide habitat for both juveniles and adults of many fish species (Auster et al., 1996; Thrush et al., 2002). Winter flounder Pseudopleuronectes americanus initially recruit to muddy sediments, but by the time they reach 55 mm total length (TL) they often associ- ate with drift algae, woody debris, and shells (Howell et al., 1999; Sogard and Able, 1991; Stoner et al., 2001). Polychaete worm tubes that project from the sediment surface can also provide juvenile fish habitat. On the northeast Atlantic continental shelf, mats composed of Diopatra cuprea and Asabellides occulata tubes, host twice as many fish as adjacent bare sand bottom (Diaz et al., 2003, 2004). Sim- ilarly, in intertidal nurseries along the Belgian coast, juvenile plaice Pleuronectes platessa are three times more abundant in ‘reefs’ built Journal of Experimental Marine Biology and Ecology 446 (2013) 311–319 ⁎ Corresponding author. Tel.: +1 5418670267; fax: +1 5418670136. E-mail address: cliff.ryer@noaa.gov (C.H. Ryer). 0022-0981/$ – see front matter. Published by Elsevier B.V. http://dx.doi.org/10.1016/j.jembe.2013.05.026 Contents lists available at SciVerse ScienceDirect Journal of Experimental Marine Biology and Ecology journal homepage: www.elsevier.com/locate/jembe