Both the regression analysis and graphical
analysis of the change in temperature and the level of predation’s effect on
the average distance between individual fish in a school was carefully
analyzed. The results of the regression
analysis indicate that there is a significant correlation between the predation
level and the average distance apart between individuals. When two points, which contained trumpet fish
as the major predator, were removed from the analysis, the correlation became
stronger. This change indicates two
different things. First, it suggests
that trumpet fish, because of their relatively small size (12-15 inches in
length), do not provide as big a threat to the school as other larger
predators, such as the tarpon and the barracuda. Even when present in a large school roughly
30 individuals, the trumpet fish did not provide a large predation threat to
the school. The other thing that the
results indicate is that the level of predation has a negative correlation with
the average distance between individual fish in the school. As the number of predators increases, the
distance between the fish in the school decreases. The distance decreases because the school is
attempting to become more compact to successfully avoid predators. In a study by Parrish of the University of Washington, the different types of predator
avoidance were discussed (1). The most common
formations that the school formed to avoid predators included: the formation of
a ball, a vacuole, and a herd. The ball is
a formation that is spherical and solid, and the school will remain as compact as
possible. A vacuole formation occurs
when the school forms a circle around a potential predator and every member of
the school keeps a certain distance from the predator. The heard formation was less commonly seen
than the other two and it involves a predator effectively herding and directing
the movement of the school. The study
also showed that school formation provides many benefits and disadvantages to
the individual fish. In regards to
predation, the chance that one individual will be consumed by a predator is
significantly reduced in a school. But
at the same time, the large school will inevitably attract predators and some
members will be consumed. Also, by being
in a school, each individual will have a longer foraging time, the amount of
food available to consume is decreased because of competition. Throughout the study in Whalebone Bay,
the number of predators was quite high compared to other locations. The tight school formations were advantageous
to the silversides because it offered each fish the best chance of survival. In the absence of predators, the fish still
remained schooled, indicating that there was also an advantage to foraging as a
school. The regression and graphical analysis of temperature
compared with the distance between fish in the school did not reveal any
significant findings. The P value
obtained from the regression analysis was higher than 0.05, indicating that
there is not correlation between the two variables. The temperature of the water throughout Whalebone Bay constantly fluctuated from one area
to the next. The school did not seem to
prefer any specific temperature during times of both low and high predation.
 | From the
behavioral observations that were made of the schooling behavior several
conclusions can be reached. First of
all, the behavior and structure of the school completely changed from day to
night. During the two night snorkel
trips to Whalebone, there were no predators present and the structure of the
school had completely changed. The
silversides had formed small schools of only 20-30 individuals that were either
foraging or resting. The majority of the
school had left the bay to venture into deeper water to feed. One reason for the fish to leave the bay and
to venture into deeper water is that deeper water may offer more food than the
bay and there is less competition. The
schools that remained in the bay during the night were very small and dispersed
throughout the bay. By sunrise the next
day, all of the individuals returned to the bay and the dense school had
reformed. Clearly, the bay provides
greater protection from predators than the shallow waters along the coast that
are exposed to the open ocean.
As the week in Bermuda
progressed, the size and dynamic of the school changed significantly. During the first day of the study, the size
of the school was at its largest, which was also the time when the largest predators were |
present. Several tarpon
and barracuda were present during the first two days of the study, but then
disappeared. The second to last day, the
school was at it smallest size and this was also the point when there were no
predators present. The last day of the
study, the large barracuda returned to Whalebone Bay
and the size of the school increased significantly. From these observations, it appears that the
absence of large predators allows individuals within the school to leave the
bay for alternative foraging and resting areas.
These observations also indicate that the bay provides more protection
from predators than other shallow water locations along the coast.
It is
important to note that during the middle of the study period, a number of dwarf
herring (Jenkinsia lamprotaenia) were
present within the school of silversides.
Several hundred dwarf herring were present in Whalebone Bay
for three days and then disappeared. In
a study conducted by Coblentz in the Virgin Islands,
they found that the reproduction of dwarf herring is heavily dependant on the
lunar cycle (6). The study found that
this is main time that they reproduce and they can found in large numbers in
shallow water during the time of the full moon.
The three day period that the dwarf herring were present was also during
the time of the full moon. The study
also found that during the mating period of the herring, they leave the shallow
waters during the night to feed. This
also explains why during the night snorkels, the herring were no longer present
in the bay.
Towards the end of the study, large, dense mats of seaweed
washed up from the ocean in Whalebone
Bay. Upon further analysis it became clear that the
seaweed had both positive and negative effects of the school. It offered the school additional protection
from predators. Small groups of the
school were often found foraging and resting beneath the mats. This allowed the groups to increase both foraging
resting time. On the other hand, several
individuals got caught in the dense seaweed and died. Roughly 10 different species of fish were
found washed up dead in the seaweed. Another
alternative explanation is that as the seaweed was decomposing, it released some
sort of toxin that was harmful to the fish.
The
behavior experiment that was described in the results section, revealed that
the there was some sort of threat level associated with predation. When the predation level was high, the school
was not affected by my swimming and splashing through the school. However, when the level of predation was low,
the school avoided me and a kept a constant distance away. Because they only responded to my actions
during times of low predation, it implies that the school must assess the level
of predation and respond primarily to the largest threat (their natural
predators).
Along
with the data and observations that were collected and analyzed, an aerial
photograph was taken of school during each trip to Whalebone Bay. The photographs were used primarily to ensure
the presence and location of the school within the bay. It was also used to roughly estimate the size
of the school and density. A rough idea
of the density could be determined based on the light or dark color of the
school. Finally, the photos were used to
look for specific patterns of movement on a day to day basis. No significant movement patterns were found
because the location, size, and behavior of the school fluctuated from one day
to the next. When the dense seaweed
washed up in the bay, it completely changed the location of the school. |  |