EFFECTS OF HYPER- AND HYPOSMOTIC EXPOSURE AND FEEDING REGIME ON POSTPRANDIAL PROCESSING IN SPOTTED SEATROUT Cynoscion nebulosus

Abigail B. Bockus*
 
Louisiana Universities Marine Consortium
8124 Highway 56
Chauvin, LA 70344
abockus@lumcon.edu
 

The success of any aquaculture operation depends on appropriate rearing conditions and feeding protocol. Optimizing these parameters is particularly important during early life-cycle stages, when species are experiencing rapid growth. Hatcheries incur one of the highest operating costs in the industry and ineffective rearing practices can increase production time and materials. Although these factors have a direct affect on post-transition grow out, their influence on digestive processing and physiology is still being determined.

Each feed event leads to a number of physiological disturbances, including acid-base fluctuations and a transient increase in metabolism. In many species, digestion is facilitated by a drop in stomach pH caused by an influx of H+ ions from the blood. Bicarbonate (HCO3-) is simultaneously pumped in the opposite direction, leading to an increase in blood pH referred to as the alkaline tide. Digestion also leads to a spike in postprandial respiration (specific dynamic action), caused by the metabolism of feedstuffs and the costs associated with a return to iono- and osmoregulatory homeostasis. These disturbances are energetically expensive and can last up to 48 hours. Here, we use the spotted seatrout (Cynoscion nebulosus), an estaurine finfish, to examine the effects of hyper- and hyposmotic rearing conditions on postprandial digestive physiology, likely influenced by the passive and active ingestion of water and reverse concentration gradients at the gills. Our research shows how environmental salinity affects the osmoregulatory requirements associated with digestion and defines whether hyper or hyposaline conditions are optimal for growth in this euryhaline species.

Previous studies examining the alkaline tide have investigated this phenomenon after consumption of one large meal. However, many commercial hatcheries implement a continuous feeding protocol. In theory, this alternate feeding regimen should reduce the degree of acid-base and physiological disturbances associated with the ingestion of feed. Our research further defines whether fish fed continuously display temporal differences in postprandial processing when compared to fish fed the same quantity in one discrete meal a day. We hypothesize that individuals fed continuously will maintain an acidic stomach pH for a longer duration, primed in a state of "readiness" for digestion and that blood acid-base disturbances and metabolism will be reduced. This project identifies whether a hyper or hyposaline environment can be used to minimize the cost of digestion and enhance feed efficiency in finfish fingerlings. It also determines how these benefits relate to fishes under timed and continuous feeding protocol. Further, this project elucidates mechanisms of digestive physiology that can be applied across aquaculture industries throughout the United States.