Monthly Variation and Influence of Habitat and River Stage on Electrofishing Catch Rates and Population Indices of Flathead Catfish from the Lower Missouri River
Vincent H. Travnichek
Abstract.—Monthly (May–September; 2001–2003) sampling with pulsed DC electrofishing was conducted within five habitat types to determine how sampling month, habitat type, and river stage affect catch rates, observed size structure, and population indices (i.e., proportional size distributions [PSD]) of flathead catfish Pylodictis olivaris from the lower Missouri River. Flathead catfish collected from creek mouths had the largest mean length (320 mm) while trail dike habitats produced the smallest mean length (206 mm). Flathead catfish PSD was lowest from revetted bank (13%) habitat compared with that observed from creek mouth (41%) and wing dike (43%) habitats. Catch rates of flathead catfish were consistently higher from habitats associated with riprap rock (revetted banks, trail dikes, and wing dikes) compared with the creek mouth or mud bank habitats. Mean length of flathead catfish was highest during May collections (267 mm) while the months of July (214 mm) and August (216 mm) had the lowest average length. Similar trends were observed with proportional size distributions with the highest values observed in May and the lowest in July and August. Variations in flathead catfish catch statistics and population demographics were observed among months and habitats from the lower Missouri River. However, river stage was likely as important, if not more, than either month or habitat. There was an inverse relationship (r2 = 0.494; P = 0.001) between river stage and catch rate of flathead catfish, and higher river stage was generally associated with higher average lengths of flathead catfish (r2 = 0.305; P = 0.001). Catch rates of flathead catfish were highest in September from all habitat types, and coefficient of variation in catch rates was the lowest during September. Thus, it is suggested that standardized sampling occur in September to maximize catch rates and minimize variation in catch rates while also collecting enough fish ≥710 mm to have some confidence in assessing this portion of the flathead catfish population. River stage is also generally more stable and at lower levels during this month. Additionally, having a standardized approach to selecting habitats sampled will allow agencies to begin collecting better information on flathead catfish populations.