1. Introduction
The arapaima (
Arapaima gigas; Schinz, 1822) is endemic to the Amazon and Essequibo basins of South America [
1]. Arapaimas are commonly displayed in public aquariums due to their spectacular size. Adult specimens can grow up to 4.5 m in length and 200 kg in weight [
2]. Their numbers in the wild are unknown and they are threatened by overfishing, habitat degradation, by-catch of juveniles, and recreational angling [
3,
4,
5]. The species is listed in Appendix II of the Convention on International Trade of Endangered Species of Wild Fauna and Flora (CITES), yet continues to be “Data Deficient” in the IUCN Red List [
6]. Therefore, specimens kept in aquarium collections may play an important role in the conservation of the species. A better understanding of the behaviour and the welfare requirements of the species is essential not only for adequate zoological management of the species but also for the future of successful breeding programmes.
Assessing fish welfare has become more critical in the last decades. The welfare aspects of fishes and other aquatic animals are poorly developed compared to terrestrial taxa due to the broad diversity of aquatic species and difficulties in assessing behavioural patterns [
7]. As ethology has evolved from descriptive studies into a meticulous science incorporating sophisticated experimental manipulations and data analyses, well-developed ethograms provide the basis for valid and efficient experimentation [
8]. The establishment of basic ethograms is essential for behavioural analysis. While there is a lack of behavioural studies for arapaimas, there is increasing literature on both freshwater and seawater fishes. For example, Bolhan and her colleagues documented the behavioural repertoire of Arctic charr (
Salvelinus alpinus) under human care by establishing a species-specific ethogram with behaviours grouped into four categories: locomotion, stationary positions, social interaction and displays, and feeding [
9]. In another study, Branconi and her colleagues focused on the differences in live and video coding of Humbug damselfish (
Dascyllus aruanus) behaviour. Their detailed ethogram included 36 different behaviours that were classified into five categories such as aggressive, social, maintenance, reproductive, and submissive [
10]. Ethograms have also been adapted to investigate social plasticity [
11], genotype-environment interaction [
12], play behaviour [
13], or even behavioural responses to interactive robots [
14,
15].
Environmental enrichment plays an important role in the welfare of animals under human care (e.g., [
16,
17,
18]). It provides both mental and physical stimulation, reduces boredom and stereotypy, and ultimately contributes to the health of the animals [
18,
19,
20]. Environmental enrichments for fish represent a developing field and currently, only a handful of studies have been published. Lee and Berejikian investigated steelhead (
Oncorhynchus mykiss) adaptive behaviours by providing them with the choice of two tanks, one of which was barren and the other was enriched with rocks and plastic plants. They documented changes in average behaviour and behavioural variation in response to the differences in the environment [
17]. In other studies, structural enrichment was also documented to positively influence weight gain in juvenile rainbow trout (
Oncorhynchus mykiss) [
21], and improved behavioural flexibility and learning ability were improved in juvenile Atlantic salmon (
Salmo salar) [
22]. Another study found that the affiliative behaviour in zebrafish (
Danio rerio) increased when novel space was present for them to explore [
20]. A meta-analysis of 1171 reviewed studies on the impact of physical enrichment on aquatic animals under human care showed a significant positive welfare effect while under human care and post-release [
23]. In another study, the introduction of black rockfish (
Sebastes schlegelii) increased growth in greenling (
Hexagrammos otakii). Thus, adding the second species is considered an effective social enrichment [
24]. The same team also presented black rockfish (
Sebastes schlegelii) with enriched vs. barren environments. The fish from the barren environment were observed to exhibit increased aggressive behaviour [
25]. Structural environmental enrichment and tryptophan effectively reduced stereotypical and aggressive behaviour in Nile tilapia [
26].
Light provides essential environmental cues for fish [
27] and plays a crucial role in their behavioural ecology (e.g., [
28,
29]). Artificial light was regarded as a behavioural guidance tool for fish, mainly used to increase catch yield in fisheries. Fires on the beach and LED lights on the fishing gear attracted fish at night [
30,
31,
32,
33]. Artificial light was also used to deter the movement of fish, and lower fish mortality during the construction of hydraulic dams by guiding them away from the dangerous place and towards the replacement habitat [
34]. Moreover, artificial light has been commonly used for behavioural ecology research. Behavioural response to light exhibited by different fish species showed a correlation with the species’ diel activity patterns: the diurnal fishes preferred illuminated areas, whereas the nocturnal/crepuscular fishes preferred darkened areas [
28]. In another study, artificial light was used to enhance the passage of juvenile salmonids at Bonneville Dam [
35].
Consequently, the use of light can play a vital role in designing environmental enrichments for fish [
36]. Laser pointers could serve as a tool for light-based environmental enrichment. During a benthic trawl in Lyme Bay, fish were found to be attracted to a laser pointer and exhibited agonistic behaviour [
37]. However, in a recent study, 66 fish species were exposed to a laser pointer. It was found that the majority of the fish showed at least a moderate level of interest in the stimuli [
13].
In light of the above, we investigated the use of a green laser pointer as a potential visual enrichment for our three arapaimas at Ocean Park Hong Kong. We hypothesised that the successful use of the laser pointer would attract the fish, engage them in increased locomotive activity and motivate them to utilise more areas of the habitat.
4. Discussion
This pilot study examined whether a laser pointer could be applied as an environmental enrichment for arapaimas under human care. The results demonstrated that during the use of the laser pointer, the fish increased their presence, activity, and habitat use, supporting the use of the novel enrichment.
With the laser pointer, the arapaima used the exhibit tank more frequently than the back-of-house tank. During the baseline phase, two of the three arapaimas (Aaron and Beth) often stayed inside the back-of-house tank. At least one was partially absent in all 18 baseline sessions and entirely absent in 11. The increased presence of the arapaimas in the exhibit tank was also apparent in the increased use of the upper layer of the water body during the test, while the time spent in the lower layer showed no difference between baseline and testing conditions. Thus, the increased time spent in the upper layer reflects the overall increased presence in the exhibit tank. Their tendency to stay off-exhibit (thus out of sight) could reflect on potential territorial behaviour [
42,
43,
44]. It is also possible that the fish simply stayed at the back-of-house pool due to individual preference, or both. Nonetheless, when the laser pointer was in use, Aaron and Beth entered the exhibit tank more frequently and for a longer duration. There was no change in Casmir’s presence as it remained nearly equal between the baseline and the test session. It is important to mention that during the simultaneous presence of the three arapaimas in the exhibit tank, no inter- or intra-species aggression was documented. The laser pointer provided an attractive stimulus for the fish without creating social conflict. Similar to other studies, the introduction of novel enrichments increased and enhanced the animals’ use of their habitat [
45,
46,
47]. Thus, we conclude that the positive change in the time spent present in the exhibit tank reflects the arapaimas’ interest towards the enrichment item and is considered a positive outcome for the fish and the visitors.
The laser pointer enrichments also enhanced activities, as indicated by increased travelling, layer changes, and physical contact during test sessions. Adequate environmental enrichments have been documented to increase exploratory behaviour in steelhead (
Oncorhynchus mykiss, [
17]), prompted attentiveness/tracking, chasing, and catching in fish [
13], and increased activity is generally accepted as a positive welfare indicator [
48]. Thus, the documented increased travelling further supports the positive effect of the laser pointer.
As the laser pointer was used equally between the upper and the lower layers of the waterbody, the fish were recorded not only travelling more but also changing between the layers more often than during baseline. Hence, the use of the laser pointer resulted in greater habitat use. Successful utilisation of novel environmental enrichments has been previously recorded to increase positive utilisation of the habitat in other species [
7,
20]. Therefore, the documented increase in layer change provides further support for the use of the laser pointer as environmental enrichment for arapaimas.
The observed difference in activity level between baseline and test conditions was the most prominent in Beth’s behaviour while it was least prominent in Casmir’s behaviour. We cannot exclude the possibility that individual differences may be responsible for the difference between the three arapaimas’ behaviour. Individuals may respond differently toward the same stimulus or environment even though they are the same species, the behavioural components of their response can be regarded as part of the animal’s personality [
49]. However, as mentioned before, the differences could also reflect on the territorial behaviour [
42,
43,
44] of the three fish. Thus, we modelled the variation of the same fish in repeated measurements as a random effect. As the laser pointer was used in the exhibition tank which was the preferred habitat of Casmir, the effect of the laser pointer was more prevalent in Aaron’s and Beth’s behaviour.
The three arapaimas spent more time at the exhibit tank and moved around in their habitat during test sessions which resulted in a higher chance of physical contact between the three fish and the other inhabitants of the tank. Only one laser pointer was used, and consequently, it is possible that the simultaneous interest of multiple fish might have led to the increase in physical contact. It was difficult to characterise finer movement patterns, such as target following, synchronous swimming, or leader-follow swimming due to the relatively slow movements of the fish and the design of the habitat. However, we cannot exclude the possibility of stimulus enhancement, attraction to the stimulus could be generated by the vicinity or by the interest of a conspecific [
50]. Future studies in larger habitats, where fish have more room for movement and manoeuvrings could provide more in-depth data on the arapaima’s response.
While there has been an increase in the number of studies on environmental enrichment use with charismatic and flagship species under human care (e.g., [
46,
47]), there is certainly a need for investigations with lesser-known species such as the
A. gigas. To our knowledge, this study is the first to explore the use of any environmental enrichment with arapaimas. The current literature on this species is limited to publications regarding its biology and farming. In line with the recent findings of other fish species’ response to the use of a laser pointer [
13], our arapaimas’ response could be categorised as moderate or even high.
In conclusion, while our study only included three individuals in one habitat, we believe our data provide a valuable baseline for further investigations. We hope that our initial success will inspire other care facilities to replicate our study and build on it. Our results show positive changes in the behaviour of the three fish supporting the value of a laser pointer as environmental enrichment for arapaimas.