Microhabitat use and its effect on growth of age-0 smallmouth bass in the North Anna River, Virginia

• Main Author: Sabo, Matthew J.
• Other Authors: Fisheries and Wildlife Sciences
• Format: Others
• Language: en
• Published: Virginia Tech 2014
• Subjects: LD5655.V856 1993.S236; Black bass > Virginia > North Anna River; Smallmouth bass > Growth
• Online Access: http://hdl.handle.net/10919/38640; http://scholar.lib.vt.edu/theses/available/etd-06192006-125734/

This study examined the relationship between microhabitat use and growth of age-0 smallmouth bass in the North Anna River. The study objectives were to describe microhabitat use during summer, assess the profitability of microhabitats, and determine if and how microhabitat use could determine which individuals gained a growth advantage. Age-0 smallmouth bass changed their microhabitat use as they passed through stages of ontogenetic development and became familiar with their environment. More than 30% of the area available to brood larvae contained mean water column velocities greater than 4 cm/s and no cover. More than 80% of larvae observed after dispersing from the nest site occupied areas approximately one m deep with velocities near 0 cm/s, or large cover objects that created low velocity shelters throughout the water column. By six weeks after dispersal (when all juveniles were > 40 mm), more than 50% of juveniles occupied depths less than 60 cm and focal point velocities > 3 cm/s, and microhabitat use by large and small juveniles did not differ. In these shallow microhabitats with moderate to fast current velocities, juveniles foraged at a higher rate (5.1 bites/min on average) than in deeper and slower velocity areas (1.3 bites/min). When these foraging rates were translated into estimates of energetic profit, juveniles in the shallow-fast microhabitats gained approximately 5 j/min more than juveniles foraging in other microhabitats. Microhabitat use was the only aspect of behavior that affected the foraging rate or energetic profit gained by juvenile smallmouth bass. I examined daily rings on otoliths to track the growth of individual smallmouth bass through time. Temperature affected growth rates below 22-23°C, so that individuals spawned later grew in warmer thermal regimes and grew relatively fast during early life stages. However, individuals that spawned early and grew slowly in cooler temperatures did not suffer higher mortality and compensated for their slow growth by accumulating growth over a longer period of time. Above 22-23°C, relative growth rates were not consistent through time; a fast growing individual during one life stage could grow relatively slow during the next. If growth above the temperature threshold depended on foraging success and foraging success depended on microhabitat use, then few individuals were consistently occupying the most profitable microhabitats. In experiments in an artificial stream, juvenile smallmouth bass assessed the potential profitability of an area by the foraging rate they achieved there. They usually did not remain in an area unless they foraged well at the time they searched it, and did not appear to associate habitat characteristics with profitability. In the river, most of the microhabitats available were relatively unprofitable, so an individual that abandoned a profitable area might (depending on its foraging behavior) spend Significant time in unprofitable microhabitats and consequently reduce its growth rate. Increasing the availability or density of food resources may improve growth of juvenile smallmouth bass in the North Anna River by increasing the rate at which juveniles encounter profitable microhabitats and elevating their intake rates in profitable areas. === Ph. D.