PREDATOR FUNCTIONAL RESPONSES IN SEVERAL LARVAL SUNFISHES (CENTRARCHIDAE)
Abstract
The farm pond is an important aquatic ecosystem in the midwestern United States. Characteristic fish population imbalances in ponds are often caused by wide variations in the size of predator or prey year classes. These variations are controlled primarily by fluctuations in larval mortality. This study examines the importance of food availability and competition for food as primary determinants of larval mortality and growth by quantifying the predator functional responses of larval largemouth bass (Micropterus salmoides), bluegill (Lepomis macrochirus) and green sunfish (L. cynaellus). Analysis of data on survival, length, weight, and prey items eaten per hour at various prey density revealed a differential ability of bluegill, green sunfish, and largemouth bass protolarvae to handle the same size prey. Green sunfish survival was superior to either bluegill or bass survival at all experimental prey densities. Bluegill survival improved as prey density was increased, but was consistently poor even at very dense prey concentrations. Bass protolarvae also had fairly low survival at most prey levels. Slight differences in the body characteristics of bluegill and green sunfish larvae may cause the considerable differences observed in the predator functional responses of the two species. Relative body width and depth were greater in green sunfish than in bluegill of the same age. Green sunfish had a slightly larger muscle cross-sectional area and were stouter in appearance. Muscle cross-sectional area may correlate with power that can be used in the pursuit and capture components of foraging activity. Even slight increases in available thrust might reasonably increase the number of prey captures possible and provide the superior survival and growth observed in green sunfish larvae. Competition for food between larval bluegill and green sunfish was investigated as a factor that might influence mortality and ultimately pond balance. The presence of larval green sunfish in experimental containers significantly depressed bluegill survival and growth. Bluegill ate fewer prey per hour, grew less in weight and length, and had poorer survival when in competition with the congener. Green sunfish responses were not depressed by the presence of bluegill. The influence of competition between bluegill and green sunfish on either species' natural diet was investigated in a series of partitioned areas in a local pond. Species composition was manipulated in each section to obtain data on larval bluegill and green sunfish feeding characteristics with and without interspecific competition. The presence of interspecific competition did not appear to alter the diet of either species. Competition with older individuals of either species produced major dietary shifts. Daphnids were consumed by smaller larvae when larger individuals were absent, but a shift to rotifers occurred when late mesolarvae or juveniles were present. Developmental responses (the relationships between larval stage and survival and growth functions) were measured for three stages of largemouth bass. Survival rates and relative growth rates were generally greater in older than younger larvae. However, at low prey densities mesolarval bass used the experimental prey (Artemia nauplii) more efficiently than did younger and older larvae. The relationship between increased capture success and increased predator metabolic cost as larvae develop may be important in explaining bass development responses.
Degree
Ph.D.
Subject Area
Ecology
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