February 15-18, 2009 Washington State Convention Center Seattle ...

EYE COLOR AS A PREDICTOR OF SOCIAL DOMINANCE IN NILE TILAPIA Oreochromis
niloticus
Emmanuel M. Vera Cruz*, Reggie May L. Bero, Remedios B. Bolivar and Russell J. Borski
Freshwater Aquaculture Center – College of Fisheries
Central Luzon State University
3120 Science City of Muñoz
Nueva Ecija, Philippines
bongbee15@hotmail.com
Heterogeneous fish growth is a common phenomenon in cultured fish populations. Variation in individual growth rates may
be attributed in part to social interactions and the formation of feeding hierarchies. Breeding is also largely driven by social behavior
and an understanding or ability to predict dominance may contribute to breeding programs. Fish in a stable dominance
hierarchy may be classified as dominant, subordinate, or intermediate in status.
It has previously been shown that eye color pattern in Nile tilapia is a consequence of a fish’s relative position in the social
hierarchy. However it is still an open question if this physiological difference can also be a cause of social status. To evaluate
this, 40 all-male juvenile Oreochromis niloticus of similar size and with no apparent differences in social history were isolated
for 10 days and were used in a social pair study. Eye color pattern of each fish was observed and recorded several times during
the isolation period, during an aggressive encounter and after the social interaction settled. Eye color change was marked by
fractional changes of the color of the iris and sclera around the pupil which was transformed into scores ranging from 0/8 (no
darkening) to 8/8 (total darkening).
Results of the study demonstrated that the ability to win a fight for social dominance can be predicted using the fish’s eye color.
Fish with darker eye color pattern are most likely to win the contest for social dominance. In addition, during an aggressive
encounter, shifts in the eye color pattern were observed in both the subsequent dominant and subordinate fish. This change in
eye color patterns in both fish served as a signal to the opponent to indicate preparedness to fight. When the social interactions
had settled, dominant and subordinate fish shifted their eye color patterns to a paler and darker configuration, respectively. The
eye darkening in the subordinates served as a social signal for social submission.
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