Laboratory studies of potential competition for food and substrate among early juvenile Missouri River sturgeon and sympatric chub species

Juvenile Pallid Sturgeon Scaphirhynchus albus predominantly consume midges (family Chironomidae) and other macroinvertebrates, while Shovelnose Sturgeon S. platorynchus, Sicklefin Chub Macrhybopsis meeki, and Shoal Chub M. hyostoma feed on those same macroinvertebrates throughout life. The primary objective of this study was to assess the substrate component of habitat selection, specifically selection between sand and mud substrates, as it relates to food availability and quantity relative to presence or absence of a photoperiod (i.e., lack of light). The study was designed to assess the strength of the innate linkage of substrate and availability of food in foraging by early juvenile Pallid Sturgeon, early juvenile Shovelnose Sturgeon, their juvenile hybrids, and adult Sicklefin Chub and Shoal Chub. The ultimate goal of the study was to develop a better understanding of potential competition for resources among these five fish groups. Twenty-four individually isolated aquaria were used to test behavior during a 12 h light : 12 h dark cycle versus continuous darkness. Presence or absence of a photoperiod was combined with feeding levels of maximum ration (C_max) versus half C_max ration and the substrate to which food was delivered. Four-way ANOVA among all five groups and separate ANOVAs for sturgeon and chub groups that included fish group as a factor indicated a selection (i.e., proportion of time over substrate >0.5 given only two substrates equally available) for the substrate to which food was delivered, as expected. However, selection of mud was significantly greater for early juvenile Pallid Sturgeon compared with other fish groups except early juvenile Shovelnose Sturgeon. Neither feeding level nor presence or absence of a photoperiod significantly affected substrate selection. In individual fish group three-way ANOVAs, Sicklefin Chub did not select a substrate significantly more when food was delivered to the substrate. Shoal Chub selected mud more frequently under continuous darkness. All three sturgeon groups showed a significant increase in weight over time. Only Shovelnose Sturgeon and Sicklefin Chub final weight were significantly greater under C_max compared to half C_max. Additionally, early juvenile Pallid Sturgeon, early juvenile Shovelnose Sturgeon, and hybrid sturgeon groups had a greater increase in weight over the study under C_max than half C_max. While caution should always be used when extrapolating results from fine-scaled, controlled laboratory studies to large, complex river systems, the results from this study support the hypothesis of the potential for competition in the lower Missouri River among the studied fish groups.