Daily rings formed on otoliths of known-age, laboratory-raised pumpkinseed (Lepomis gibbosus), green sunflsh (L. cyanellus), bluegill (L. macrochirus), and mozambique mouthbrooder (Tilapia mossambica) for at least 176, 170, 125, and 60 days, respectively. Subdaily rings found in young laboratory and wild fish were easily distinguished from daily rings. Width of daily rings on otoliths of green sunfish was linearly related to daily increase in length of fish, but the number of rings was a product of age of fish only, not length of fish or otolith radius. Growth and daily ring formation on otoliths in wild bluegill and largemouth bass (Micropterus salmoides) appeared to be similar to those in laboratory-raised fish. Otoliths of green sunfish held under simulated winter conditions ceased to produce daily rings, but did form an annulus. Two kinds of otolith tissue were present in most of the larger laboratory fish and wild bluegill but were not observed in wild largemouth bass. The first type, present in all areas of the otolith except the extreme posterior end, was translucent and had well-defined daily rings. The second type, present only in the posterior end, was opaque and had poorly etched daily rings that were difficult to discern. Both tissues were calcium carbonate in the aragonite form. Daily rings were found on otoliths offish held at constant temperature. Results of experiments with young mouthbrooders held under various light–dark and feeding cycles suggested that a 24-h light–dark cycle that entrained an internal, diurnal clock was required for daily ring production.
A field study of spawning gravel conditions affecting the survival of steelhead trout (Salmo gairdneri Richardson) embryos was conducted in two small streams in the Alsea River Basin in Lincoln County, Oregon, from February to June 1959. Holes 10 inches deep, approximating natural redds. were dug in arbitrarily selected spawning locations. Plastic mesh sacks containing gravel and 100 fertilized trout eggs were placed in the upstream end of each hole. A standpipe was placed in the lower end of each excavation about 10 inches away from the eggs, and the hole was filled with gravel to the streambed level. Periodically, determinations were made of gravel permeability and of the apparent velocity and dissolved-oxygen content of the intragravel water. A month after calculated hatching times, the bags were removed from the streambed, and the fry contained in them were counted and preserved. The permeability of the spawning gravel fluctuated while embryos were in the gravel. During this period mean gravel permeabilities ranged from 80 to 400 meters per hour; apparent velocities from 5 to almost 110 centimeters per hour; and dissolved-oxygen concentrations from 2.6 to 9.25 milligrams per liter. Embryonic survival percentages ranged from 16 to 62. There was positive correlation between the apparent velocity of ground water and embryonic survivals, and between the dissolvedoxygen levels of the gravel water and survivals. Apparent velocities and dissolved-oxygen concentrations were closely related in the intra-gravel water, and effects of these factors could not be separated.
To determine if first winter mortality were greater for small than for large freshwater fish, we conducted eight experiments in lakes and hatchery ponds during which fish were measured in late fall and early spring. Overwinter increase in mean length of the bluegill, Lepomis macrochirus, in one lake and largemouth bass, Micropterus salmoides, in another could have been caused by higher mortality of smaller fish. Size‐related mortality was not detected in six other populations–two of bluegill and one each of largemouth bass, green sunfish, Lepomis cyanellus, fathead minnow, Pimephales promelas, and white crappie, Pomoxis annularis. Available evidence indicates that size‐related mortality is probably not a common phenomenon for bluegill and largemouth bass, but it may be important for walleye, Stizostedion vitreum, and sander, Stizostedion lucioperca.
Angling would affect a fish population if it increased the total mortality rate, reduced numbers or biomass, or reduced the average size of fish in a population by removing the larger ones. Data from the literature indicate that angling commonly affects populations of bluegills Lepomis macrochirus. Substantial exploitation rates (mean, 27%) are not unusual; exploitation is directly related to fishing effort, and angling reduces average size and increases total mortality. Many exploited bluegill populations comprise mainly small fish, less than 150 mm long. Consistent high recruitment of young of the year, size‐selective exploitation, and reduced growth and increased natural mortality from competition would tend to produce and maintain such a population size structure. Because angling affects populations, management steps to reduce angling mortality of larger fish, as well as to reduce density of small fish, seem desirable.
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