Presence of larger flatfish modifies habitat preference by Age-0 northern rock sole Lepidopsetta polyxystra

Abstract

Essential fish habitat (EFH) is poorly defined for the early stages of most commercially important species. Age-0 flatfish making summer use of shallow coastal embayments around Kodiak Alaska distribute themselves according to a variety of bottom features. Both field and laboratory studies indicate that the numerically dominant species, northern rock sole Lepidopsetta polyxystra and Pacific halibut Hippoglossus stenolepis, demonstrate an affinity for seafloor with emergent structure: algae, shell, worm-tubes, etc. We conducted field manipulations, in which bivalve shell was scattered on the seafloor along 100 m transects, with the goal of examining movements of juvenile flatfish in response to these habitat alterations. In the first manipulation, contrary to expectations, Age-0 flatfish emigrated from the shell-enhanced seafloor, producing fish densities that were lower than on adjacent un-enhanced bottom. In comparison, larger flatfish, predominantly Age-1 + rock sole Lepidopsetta spp., increased in abundance on shell-enhanced bottom. Small flatfish may have moved to avoid larger flatfish. The manipulation was repeated at another site with similar physical features, but where large flatfish abundances are generally lower. This time, Age-0 flatfish densities were greater on the shell-enhanced bottom than on adjacent un-enhanced bottom. Again, large flatfish were more abundant on the shell bottom, but they were nearly an order of magnitude less abundant than in the first manipulation. In a subsequent laboratory experiment, Age-1 northern rock sole demonstrated stronger affinity for shell bottom than did Age-0 northern rock sole. In another experiment, Age-0 northern rock sole moved to avoid aggregations of larger conspecifics as well as other species of large flatfish. We conclude that these Age-0 flatfish, predominantly northern rock sole, perceive larger fish as predators and/or competitors, with habitat quality degraded by their presence. These results demonstrate that habitat quality for juvenile fish is influenced by biotic factors as well as by physical characteristics of habitat.

Density-dependent habitat selection in marine flatfish: the dynamic role of ontogeny and temperature

“…Harden-Jones 1968, Livingston 1988. Rock sole also show greater preference for structured habitat in their second year of development (Ryer et al 2007), and generally, flatfish occupy a broader range of substrate types as they grow (Gibson & Robb 1992, Stoner & Ottmar 2003. However, given the fact that biomass explained a large amount of variance when all ontogenetic stages were combined, we suspect that the trends observed in this study were not entirely the result of increased tolerance to coarse grained substrates.…”

“…However, we purposely included higher densities to determine if and when the mechanisms of DDHS begin and plateau. The experimental tanks were maintained at 9 ± 0.8°C, falling well within the range of temperatures (4 to 11°C) experienced by these flatfish species in nursery areas around Kodiak Alaska (Ryer et al 2007). We withheld food from any treatment group for a 24 h period prior to the beginning of any trial to insure that the fish would be sufficiently active to explore the different substrata (see Stoner & Ottmar 2003) We introduced fish to the center of tanks in removable holding rings at 1400.…”

Testing the shallow water refuge hypothesis in flatfish nurseries

“…Juvenile rock sole, halibut and English sole all alter behavior in the presence of predators (Lemke & Ryer 2006, Boersma et al 2008. Further, Ryer et al (2007) demonstrated that juvenile rock sole will move to avoid co-occupancy of habitat with a variety of larger flatfish, both in the field and in laboratory experiments. Hence assume that the juvenile distributions observed in this work were at least partially a consequence of juveniles avoiding predators, as opposed to solely an effect of juvenile removal by predators.…”

Understanding what controls the spatial distribution of fish populations using a multi‐model approach

“…Experimental work on coral‐dwelling gobies show that size‐dependent spatial organization resulting from inter‐cohort competition can be modified by removing old/large individuals (Hobbs and Munday, 2004). Similarly, habitat modification experiments coupled with variations in large fish densities have shown that juvenile flatfishes may change habitat preference and therefore spatial distribution as a result of increased large fish densities (Ryer et al. , 2007).…”