Summary
The influence of feeding on swimming performance and exercise recovery in fish is poorly understood. Examining swimming behavior and physiological status following periods of feeding and fasting is important because wild fish often face periods of starvation. In the current study, researchers force fed and fasted groups of largemouth bass (Micropterus salmoides) of similar sizes for a period of 16 days. Following this feeding and fasting period, fish were exercised for 60 s and monitored for swimming performance and physiological recovery. Resting metabolic rates were also determined. Fasted fish lost an average of 16 g (nearly 12%) of body mass, while force fed fish maintained body mass. Force fed fish swam 28% further and required nearly 14 s longer to tire during exercise. However, only some physiological conditions differed between feeding groups. Resting muscle glycogen concentrations was twofold greater in force fed fish, at rest and throughout recovery, although it decreased in both feeding treatments following exercise. Liver mass was nearly three times greater in force fed fish, and fasted fish had an average of 65% more cortisol throughout recovery. Similar recovery rates of most physiological responses were observed despite force fed fish having a metabolic rate 75% greater than fasted fish. Results are discussed as they relate to largemouth bass starvation in wild systems and how these physiological differences might be important in an evolutionary context.
Methodology
Fish husbandry
On 13 April 2008, 127 wild largemouth bass (mass = 134 ± 2 g standard error (SE); starting mean length = 221 ± 1 mm SE) were obtained from the Illinois Natural History Survey. Largemouth bass were of mixed origin collected from various locations across Illinois, and all fish had been held for 2–4 months in outdoor earthen ponds stocked with fathead minnows (Pimephales promelas) as forage prior to experiments. Fish were removed by draining the pond and dip netting fish at the collection box. Collected largemouth bass were taken to the aquatic research facility at the University of Illinois where they were weighed to the nearest g, given an individual fin clip and randomly placed in one of five outdoor 960 L tanks. Tanks were connected to a 0.04 ha earthen research pond using a submersible pump (McMaster-Carr 42945K29, Atlanta, GA, USA), and water supplied to the tanks was returned to the earthen pond for filtration and removal of waste. Tanks were measured daily for temperature (mean = 15.21 ± 0.31°C) and dissolved oxygen (mean = 9.80 ± 0.18 mg/L) with a portable meter (YSI, 550A Yellow Springs Instruments, Irvine, CA, USA), and a commercially available kit (Aquarium Pharmaceuticals Inc, LR8600, Chalfont, PA, USA) confirmed that total ammonia remained <0.25 ppm throughout the study. Each of the five 960 L tanks contained approximately 25 largemouth bass, and the tanks all contained fish from a single experimental treatment: two tanks contained largemouth bass from the ‘force fed’ treatment, two tanks contained largemouth bass from the ‘fasted’ treatment, and the final tank contained fish that were not disturbed during the experiment, hereafter referred to as non-handled controls (NHC).
Feeding and fasting
Starting 14 April 2008, largemouth bass (N = 50) assigned to the ‘force fed’ treatment were collected in dip nets from the 960 L tanks every other day and force fed a standard maintenance ration of food at a rate of 1.6–3.0% average body mass. This feeding treatment lasted for 16 days (until 30 April) and was intended to provide sufficient energy to prevent weight loss (Congleton and Wagner 2006). Food consisted of dense culture fish food (40% protein, Aquatic Ecosystems Inc., Chalfont, PA, USA) mixed with deionized water to form a paste. Food was administered using a 25 mL syringe outfitted with a soft, flexible tube that was inserted orally into the gullet of the fish. Feeding was halted 48 h prior to sampling to allow for the digestion of food prior to experiments.