Experimental Analysis of Intra- and Interspecific Competitive Interactions between Cutthroat Trout and Sculpins in Small Streams
Ramirez, B.S.

In the Pacific Northwest ecoregion of North America, sculpins represent a major constituent of freshwater assemblages in coastal rivers. Little is known of their interactions with co-occurring species, such as widely studied salmon and trout (salmonines). In this study, I evaluated inter- and intraspecific interactions involving cottids (Cottus sp.) and coastal cutthroat trout (Oncorhynchus clarkii clarkii). I used a response surface experimental design to independently evaluate effects of cutthroat trout and sculpin biomass on growth and behavior. There was evidence of both intra- and interspecific interactions between cutthroat trout and sculpins, but the interactions were asymmetrical with biomass of cutthroat trout driving both intra- and interspecific interactions, whereas sculpins had little influence overall. Cutthroat trout biomass was positively related to conspecific aggressive interactions and negatively related to growth. Sculpin exhibited increased use of cover during the day in response to greater biomass of cutthroat trout, but not sculpin biomass. Nocturnal use of cover by sculpins was unaffected by biomass of either species.  This experiment provides insights into the species interactions and the mechanisms that may allow sculpins and salmonines to coexist in nature. As cutthroat trout appear to be superior competitors, coexistence between sculpins and cutthroat trout may depend on some form of refuge.

DISCIPLINE: Fisheries    STUDY: Alsea, Hinkle Creek, Trask    TYPE: Theses    TAGS: sculpins, intraspecific interactions, cottus, Oncorhynchus, coastal cutthroat trout, biomass, Oregon, competition
Fish Population Response to Harvesting with Contemporary Forest Practice Regulations: The Alsea Watershed Study Revisited
Bateman D., R. Gresswell, D. Hockman-Wert, D. Leer, and J. Light

Coastal cutthroat trout Oncorhynchus clarkii clarkii are the most widely distributed native salmonid in the forested watersheds of western Oregon. The initial Alsea Watershed Study demonstrated negative impacts on the abundance of cutthroat trout due to logging practices of the day. Here we report on abundance, size, growth, and condition of coastal cutthroat trout before and after logging under the current forest management practice regulations using a before, after, control, impact (BACI) study design with Flynn Creek and Needle Branch as the control and impact streams respectively. Relative abundance estimates are from a census of pool habitats using single-pass electrofishing and relative growth is from the recapture of individuals implanted with passive integrated transponder tags. A significant increase in age 1+ cutthroat trout biomass and abundance was observed post-harvest in Needle Branch relative to Flynn Creek (p=0.04 and 0.01 respectively). There was also a significant shift in the spatial distribution of cutthroat biomass in Needle Branch (p=0.04) in an upstream direction post-treatment suggesting that increases in cutthroat trout were spatially linked to the location of the harvest unit. There was no evidence for a treatment effect on mean fork length or the 90th percentile of fork length for age 1+ cutthroat trout (p=0.32 and 0.24 respectively). This result was supported by an absence of evidence for a treatment effect on relative growth rate.

DISCIPLINE: Fisheries    STUDY: Alsea    TYPE: Presentations    TAGS: Cutthroat Trout, single-pass electrofishing, biomass, habitat
Subscribe to biomass