Eye Color Pattern During Isolation Indicates Stress-Coping Style in Nile Tilapia Oreochromis niloticus L.

Abstract

Abstract. It has previously been shown that changes in eye color pattern (ECP) in Oreochromis niloticus is a consequence of a fish's relative position in the social hierarchy. However, it is still an open question if ECP during isolation is correlated to stress coping styles in fish such as ventilation rate (VR). To evaluate this, 94 all-male juvenile O. niloticus of similar size were introduced in a novel environment and were isolated in aquaria for 7 days. The ECP and VR of each fish were recorded daily during the isolation period. Eye color change was observed to be fractional changes of the color of the iris and sclera around the pupil which were transformed into scores ranging from 0 (no darkening) to 8 (total darkening). Results of the study showed that ECP was significantly correlated with VR, both individual daily values (r = 0.48, n = 658, P<0.001) and mean values (r = 0.68, n = 94, P<0.001). Thus, ECP indicates stress coping style in O. niloticus. Based on the trend of ECP values during isolation, fish with ECP values of less than 4 after the isolation period of 7 days may be classified as proactive individuals while those with ECP values of more than 4, as reactive individuals. The ECP is a good, easy and inexpensive tool for assessing both alertness and stress level in the fish.

Density-dependent changes in neophobia and stress-coping styles in the world's oldest farmed fish

“…Melanocyte-stimulating hormone (α-MSH) and melanin-concentrating hormone (MCH) are the two main hormones involved in the modulation of chromatophore dispersal [68–71], and it has been shown that social stress can increase α-MSH in plasma and result in body darkening in salmonids [72,73]. Eye and body darkening are associated with subordinance [41,74] and a reactive coping style in Nile tilapia [42], as found in our study. Thus, our results are consistent with the idea that crowding during aquaculture intensification makes Nile tilapia chronically stressed, and as seen in other species, this results in body and eye darkening, neophobia and, more generally, in a shift from a proactive to a reactive stress coping style.…”

“…Tilapias rank among the oldest and most widely farmed fish worldwide [19,36], but are also included in the ‘100 world's worst invasive alien species' [40], so knowledge about how these species respond to novelty when they escape from fish farms and become feral might be important for reducing impacts. To address these questions, we reared juvenile Nile tilapia at high and low densities, measured the extent of eye and skin darkening (two metrics of chronic stress, [41,42]), and screened them for neophobia, as well as for activity, aggression and boldness, as these are measures of stress coping style [43,44]. We used fry because this allowed us to address the effect of rearing density at a critical stage of development, when the young leave the care of their parents and variation in exploratory behaviour is first manifested [45].…”

Morphological and Physiological Changes in Red Tilapia (Oreochromis spp.) Subjected to High Temperature and Confinement Stress

“…For ECP, it was quantified as a darkened area of both the iris and sclera. The circular area of the eye was divided into 8 equal parts using an imaginary diameter line and the ECP value that was observed ranged from zero (no darkening) to eight (total darkening) (based on Volpato et al, 2003;Vera Cruz and Tauli, 2015).…”

“…This happened when the fish sensed a change in their environment. The eye color may also indicate the level of stress; with more stress as the color became darker (Vera Cruz and Tauli, 2015).…”

“…This indicates that when the VR increases, the eyes and skin color of red tilapia darkens and intensifies, respectively. According to the study of Vera Cruz and Tauli (2015), the ECP of O. niloticus that was exposed to confinement stress was positively correlated to its VR. This generally implies that VR, ECP, and skin color can be an easy, inexpensive, and non-invasive way to measure conditions of fish stress.…”