Observation of the eye-attacking behaviour of Koi Cyprinus carpio var. koi in a koi–goldfish polyculture system

ABSTRACT

The eye-attacking behaviour of Koi fishes was recorded in a koi–goldfish polyculture system. The attacked fishes suffered from eye ablation, abnormal swimming and suppressed feeding behaviour. There was a significant difference in mean sizes of victims and attackers but the exact motivating stimuli for the observed action was not clear. For future research, the effects of different stocking ratios on aggressive behaviour of koi should also be examined.

KEYWORDS:

• Eye
• ablation
• fish
• aggression

Polyculture is economically more beneficial than monoculture, providing more efficient utilization of resources and higher production rates (Shang and Costa-Pierce 1983; Papoutsoglou et al. 1992; Wahab et al. 1995; Sharma et al. 1999). Cyprinid polyculture has become extensively practised in recent years (da Silva et al. 2006). It is well demonstrated that cyprinid polyculture has several advantages over the culture of single species, including optimized utilization of the available food, high biomass production and less impact on the environment (Kestemont 1995; Wahab et al. 1995; Milstein et al. 2002). Interactions between each species and the environment and/or other species are considered to be key factors impacting the success of polyculture systems. Synergism and antagonism are among the main species–species interactions. Synergistic interactions usually occur when the stock sizes and densities are balanced (Milstein 1992). These interactions are accompanied by an improvement in the condition of the environment and an increase in food resources (Milstein 1992). In contrast, unbalanced conditions can lead to the dominance of one (or more) species over others, resulting in suppressed growth and reduced activity of subordinate fish, or even physical injuries (Gibbard et al. 1979; Milstein 1992; Mork 1995). This note reports the occurrence of antagonistic interactions between koi (Cyprinus carpio var. koi) and goldfish (Carassius auratus) in a polyculture system.

The observation occurred for 10 days during August 2014 at a local carp farm (Tabriz, Iran). The farm's grow-out indoor system consisted of nine 2000-L circular concrete tanks stocked at 0.5 fish/L with goldfish (three tanks), koi (three tanks) and koi–goldfish (1:1 mixed culture, three tanks) fingerlings. The stocked fingerlings were the offspring of Asagi and Bekko koi types and Ranchu and Calico Ryukin goldfish varieties. Rearing conditions were maintained at 23–25.5°C; 7.0–7.3 pH, 7–7.3 mg/L dissolved oxygen, and 12/12 (light/dark) illumination. Fishes were fed twice daily at a level of 4% body weight.

According to our observations, a total of 45 Ranchu goldfishs were injured in the attack. One hundred per cent of the specimens attacked in the polyculture tanks, suffered from ablation of both eyes, with no distinct sign of haemorrhage in the orbit (Figure 1). No remains of the removed eyes were found in the tanks. The attacked fish (6.32 ± 0.50; mean standard length ± SD) were significantly (t = 4.33; p = .00) smaller than the attackers (14.80 ± 0.21; mean standard length ± SD). The victims exhibited head down swimming and suppressed feeding activity. Although the aggressive behaviour of koi carp against goldfish has been noted before (Jha et al. 2005, 2006), to our knowledge this is the first evidence of an antagonistic eye-attacking behaviour in koi fish. Eye-attacking is a common phenomenon among carnivorous fishes (Foxx 1972; Strand et al. 1995), which is usually exhibited by starved fish.

Figure 1. Photograph of the attacked goldfish suffered from ablation of both eyes.

We did not dissect koi specimens to find remains of the removed eyes in the gut, but it seemed as if the removed eyes were consumed by the attacker(s) since no remains were found in the tank. Jha et al. (2005) showed that presence of food can motivate aggression in koi carp but we did not observe any cases of eye-attacking behaviour at feeding times. Although the exact stimuli for the incidence of eye-attacking behaviour was not investigated in our study, we suspect that the same feeding strategy of these species to be the one of the foremost reasons of these attacks. Both common carp and goldfish are omnivorous bottom-feeders and may compete for food. Reducing stocking density of koi or sorting job may moderate occurrence of aggressive behaviours. For future research, the effects of different stocking ratios on aggressive behaviour of koi should also be examined

Disclosure statement

No potential conflict of interest was reported by the authors.