EYE COLOR AS A PREDICTOR OF SOCIAL DOMINANCE IN NILE TILAPIA Oreochromis niloticus Emmanuel M. Vera Cruz, Reggie May L. Bero, Remedios B. Bolivar College of Fisheries Freshwater Aquaculture Center Central Luzon State University Russell J. Borski Department of Biology North Carolina State University

Introduction Heterogeneous fish growth is a common phenomenon in cultured fish populations Variation in individual growth rates attributed in part to: social interactions formation of feeding hierarchies Fish status in a stable dominance hierarchy classified as: dominant subordinate, or intermediate In Nile tilapia the eye color pattern is a consequence of fish’s relative position in the social hierarchy (Volpato et al., 2003, Vera Cruz & Brown, 2007) Can eye color pattern prior to the interaction predicts the outcome of contest for social dominance?

Experimental fish Seventy five 3-wk old all-male Nile tilapia (mean wt = 0.87 g) was obtained from the Phil-Fishgen, CLSU, Science City of Muñoz Nueva Ecija Maintained in a rectangular tank (2 x 1 x 1 m) receiving continuous flow of water for around 3 months Fed four times a day (2 % BW)

Isolation Weight & length of each fish were obtained Isolation of 40 fish for 10 days

Monitoring of eye color pattern (ECP) ECP was monitored for six days ECP was measured by quantifying the darkened area between the iris & sclera; from zero (no darkening) to eight (total darkening) The eye was divided into 8 equal parts using 4 imaginary diameter lines 2/8 or 2 3/8 or 3

Social interaction ECP was recorded prior to the interaction After 10 days of isolation, fish were size- matched for the later pairing with maximum size difference of 17.74% (mean = 2.32%±2.09) Fish were individually marked by cutting the lower (with darker ECP) or upper (with paler ECP) part of the caudal fin To prevent the effect of place familiarity, the fish in a pair were introduced at the same time in a new environment (30x15x 30 cm aquarium)

Data gathered The period from time of the introduction up to the time of first agonistic attack Number of pairs that interacted Duration of social interaction The change in ECP Social status of each fish after the interaction

RESULTS

Duration of time before observance of first attack This time duration corresponds to the duration of familiarization to the new environment of at least one individual in a pair The mean was 4.97 min (±0.69) & only one pair exceeded 10 min Fish can adapt to the new environment as early as 2.17 minutes.

Prior to interaction Circling at each other, provoking the other fish to start the interaction Fins of both fish were erected indicating that they are ready to fight

During the interaction Biting directly at each others mouth Biting at body part of the opponent Biting at the caudal part of the opponent

After the interaction Subordinate retreating during attacks of dominant

Eye color patterns before and during the interaction and the resulting social ranks Fish pair Eye color pattern Before the encounter During the encounter After the encounter Dominant individual Early part Middle part Late part 1 & 131, 22, 88, 28, & 112, 28, 8 8, 21, 82 3 & 53, 78, 82, 88, & 371, 80, 02, 62, 28, & 153, 7NI 7 & 223, 88, 82, 88, & 362, 40, 86, 82, 88, & 211, 48, 28, & 162, 32, 28, 88, 08, & 102, 72, 68, 08, & 86, 88, 82, 87, 88, & 20*6, 88, 28, 8 2, & 297, 7.56, 84, 80, 88, & 396, 8NI 26 & 38*3, 74, 82, & 312, 66, 68, 80, 28, & 334, 88, 82, 22, 88, & 30*0, 48, 88, 20, & 124, 88, 8 8, & 282, 78, 82, 88, 2 28 NI – no interaction

Social interaction 18 of the 20 pairs interacted In this 18 pairs, 14 (77.78%) fish with darker ECP prior to pairing became dominant Frequency difference was highly significant (Binomial test, P = 0.009) It is important to note that during the start of the social conflict in all the 18 pairs, the opponents with paler ECP were the ones that initiated the encounter & they were the ones that first bit & chased the opponent

Duration of interaction GroupNumber of pairs (n) Mean duration of interaction (min) ECP difference Short Interaction (< 5 min) Long Interaction (> 5 min) ANOVA P = P = 0.008

Change in ECP of competing individuals during and after the encounter as compared to their ECP prior to the encounter Part of the social encounter Pairs with the following change in ECP (%) Pale- Paler: Dark- Darker Pale- Darker: Dark- Darker Pale- Darker: Dark-Paler Pale- Paler: Dark-Paler Early part Middle part Late part After the encounter In dominant fish, any change in ECP after the interaction was either towards paler or darker than the original ECP while for subordinate fish it was only for darker

Conclusions Tilapia with darker ECP before the interaction has a greater possibility to win the fight Subordination increased the ECP patterns of the fish after pairing In dominant fish, any change in ECP after the interaction was either towards paler or darker than the original ECP Shorter duration of social encounter is needed for the establishment of dominance hierarchy in pairs having greater difference in ECP prior to social encounter.

Recommendations Further investigations on the ECP of fish is needed to answer the ff. questions: 1. Is ECP of the fish consequences of prior social (dominance or subordination) experience or is it a coping style used as a flexible behavioral strategy? 2. Is the degree of change in ECP after the interaction compared to the original ECP prior to interaction caused by the level of stress (i.e aggressive encounter) received during the interaction? 3. What are the factors influencing the change in ECP in both opponents during the early stage of the conflict?

Funding for this research was provided by the C OLLABORATIVE R ESEARCH S UPPORT P ROGRAM The AquaFish CRSP is funded in part by United States Agency for International Development (USAID) Cooperative Agreement No. EPP-A and by US and Host Country partners. The contents of this presentation do not necessarily represent an official position or policy of the United States Agency for International Development (USAID). Mention of trade names or commercial products in this presentation does not constitute endorsement or recommendation for use on the part of USAID or the AquaFish Collaborative Research Support Program. The accuracy, reliability, and originality of the work presented are the responsibility of the individual authors.

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