Introduction

The aggregation of various organisms and the causes of these aggregations have been studied by researchers for decades.  One of the most studied aggregations is the schooling of fish.  Approximately 50% of all fish species exhibit some sort of schooling behavior (1).  Whether a specific species of fish will school is dependant on a number of different factors.  Of the species of fish that do aggregate to form schools, numerous studies have attempted to determine what factors primarily contribute to the fish’s ability to maintain that formation.  Being a member of a school confers several advantages, but the outcome of an individual fish is heavily dependent of the rest of the school.  By swimming in a school, this provides the individual fish with a reduced chance of predator attack compared to being an isolated fish.  In response to a predator, the school will engage in a range of different group avoidance maneuvers (7).  The avoidance maneuvers are heavily dependent of the individual fish assessing the danger correctly and responding to it is an appropriate and decisive manner.  The amount of safety that a school provides depends on several different factors including: the size and shape of the school, the individual’s behavior relative to the group, the group’s behavior, and the number and type of predators present (7).  The schooling behavior of fish is often analyzed using a computerized three dimensional analysis.  One study that was conducted in a laboratory setting, found that by changing the speed and distance between individual neighbors in a school that the behavior of the school as a whole drastically changed (9).  In a study specific to coral reefs, the researchers found that in the shallow coral reef environment, the level of predation fluctuated with the time of day.  The levels of highest predation on schooling fish was during the day, while during the night, the level of predation was minimal (10).  The study also showed that during the afternoon, the level of predation began to slowly decrease as the day progressed.  

            The study conducted in this paper, focused on the schooling patterns of silversides (Hypoatherina harringtonensis and Menidia menidia) that were observed in Whalebone Bay, Bermuda.  In a similar study conducted by Parrish, also in Whalebone Bay, the results indicated that the horizontal stratification pattern of schooling silversides remained constant throughout the summer and winter months (8).  The study also found that smaller fish were found higher in the water column than the larger ones.  Several different species were mixed together within the same school.  In order for the school to effectively function with different species present, the species must have a similar morphology so they don’t stick out to predators against the background (8).  The study in this paper was conducted to determine the impact of that the level of predation has on the behavior of the school.