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An analysis of aggressive behavior, growth, and competition for food and space in medaka (Oryzias latipes) : pisces, cyprinodontidae Magnuson, John Joseph

Abstract

The role and consequences of aggressive behavior in competition, for food and space were studied among laboratory populations of juvenile medaka. Growth rate and relative condition were used to measure the success of an individual fish in different competitive situations. Both were measured relative to sibs of the same age and size raised in isolation under the same conditions. Temperature, day length, and light intensity were held constant, and fresh water was circulated. All fish were raised in nylon baskets (30 meshes/cm) suspended into a common water bath. Length or weight, or both, of each fish was measured every 6 days for at least 24 days, between 0 and 66 days after hatching. Quantitative records of aggression, activity, and location preference were taken throughout the day. Paramecium, Artemia salina nauplii and pellets (diameter = 0.25 mm to 0.5 mm) were used as food. A. salina were assumed to be "in excess" if active nauplii were present at all times. Limited food was 10 pellets per fish per day (0.68 mg per fish per day). Growth was followed for 648 fish in populations of 1, 2, 4, 8, or 16 fish in 1, 4, or 8 liter baskets. No difference in average growth was observed at different densities, and growth depensation was no greater than would be expected from genetic differences in growth potential, as long, as food was supplied "in excess" and the accumulation of waste products was prevented. Aggressiveness was at a low level, and both large and small fish were equally, aggressive. Spatially localizing excess food did not alter the relationship. When food supply was limited a social hierarchy developed in which large fish were socially dominant, chased small fish away from food, and grew faster than small fish. Aggressive actions increased in frequency just after limited food was presented. If food was localized spatially the social hierarchical society changed into a territorial society in which the dominant defended the food area, and the dominant1s competitive advantage increased. Aggressive behavior was initiated by internal state of "hunger" and the presence of food stimuli and smaller medaka. Visual isolation between competitors increased the dominant's advantage if food was contagiously distributed, but decreased it if food was evenly distributed. When food was evenly distributed and the environment had a semi-isolated subsection for each fish both dominant and subordinate grew equally well. If population size was large the dominant could not chase all subordinates from the food area, and consequently the growth advantage of social dominance was in part lost. In addition frequency of aggressive actions by the dominant decreased. Aggressive behavior only dispersed medaka through the habitat if food was evenly distributed. Small fish could not eat pellets as fast as large fish and if all fish had equal access to the limited food supply the rate at which they ate was important in determining their growth rate. Action of aggressive behavior as a competitive mechanism for space or Lebensraum and the influence of environment on both the expression of aggressive, behavior and the extent to which it reserves the food supply is discussed. Applicability of these findings to field situations and other species of fish is also considered.

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