...intraspecific competition among individuals thought to lead to decreased survival of extremely strong year classes (Iles and Beverton, 2000). Juveniles of multiple flatfish species are often observed to coexist in single nursery area (e.g., Burke et al., 1991; Henderson and Holmes, 1991; Reichert and van der Veer, 1991; Norcross et al., 1997), but evidence for interspecific competition is rarely examined. We examined evidence for resource partitioning and interspecific competition among juvenile flatfish species co-occurring in nursery areas.

Flatfish species often have juvenile nursery areas that are segregated from the adult population. In many cases, juveniles use shallow nearshore habitats as nursery areas, and adults are found in deeper offshore habitats (Gibson, 1994; van der Veer et al., 2000). On the Pacific coast of North America, estuaries are typically smaller and occur less frequently than in other areas such as the Atlantic coast of North America. Possibly because of the limited estuarine habitat on the West Coast, fewer species of marine fish and invertebrates use coastal estuaries for nursery areas. In our study, we examined the spatial distribution of co-occurring juvenile flatfish populations within Pacific coast estuaries. The four species examined were English sole (Pleuronectes vetulus), Pacific sanddab (Citharichthys sordidus), sand sole (Psettichthys melanostictus), and starry flounder (Platichthys stellatus). Juveniles of these species are found in Oregon and Washington estuaries during the summer, and adults of each of these species contribute to a lucrative trawl fishery on the coastal shelf.

To date, estuarine residency of juvenile English sole has been well documented (Krygier and Pearcy, 1986; Gunderson et al., 1990; Rooper et al., 2003); however, this is the first comparison of the spatial distribution of the three other species within estuaries, and is the first study to encompass such a wide region of coastal estuaries (ranging over 400 km of coastline). The goal of this study was to compare the summer distribution of juvenile English sole, starry flounder, sand sole, and Pacific sanddab within nursery estuaries over three years, to determine whether the four flatfish species were spatially partitioning the nurseries. We also examined the summer distribution in relation to the abundance of species across years and habitats in order to determine the potential for interspecific competition.

Materials and methods

Study area

This study was carried out at four estuaries on the Oregon and Washington coasts: Grays Harbor, Willapa Bay, Coos Bay, and Yaquina Bay (Fig. 1). Seventeen to 20 trawl sites were chosen within each estuary, and each trawl site was classified into three habitat types: lower main channel, lower side channel and upper estuary. Habitat delineations were previously identified by principle component and discriminant function analysis (Rooper et al., 2003). The lower estuarine sites were on average deeper (depth=5.1 m), colder (13.6[degrees]C), and more saline (28.5 ppt) than upper estuarine sites because of their proximity to the mouth of the estuary. On average, lower side channel sites were slightly warmer (15.6[degrees]C), shallower (depth=3.7 m) than the lower estuarine sites, and had extensive tide flats that surrounded the sites and were exposed during low tides. Upper estuarine sites were typically shallow (depth=4.0 m), very warm (17.0[degrees]C), less...

NOTE: All illustrations and photos have been removed from this article.



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