Age-Related Changes in Survival Behaviour in Parasite-Free Hatchery-Reared Rainbow Trout (Oncorhynchus mykiss).

Millions of hatchery-reared Rainbow trout are currently released in Australian waters to support recreational fisheries objectives, yet many of these fish die soon after release. In addition, little is known whether these fish harbour parasites that can potentially threaten freshwater ecosystems and human health. Here, we tested the behaviour of hatchery-reared trout using six tank-based tests at six different ages to evaluate their chances of survival and then dissected fish to investigate parasite prevalence. At 7 weeks of age fish readily emerged from a hide and showed the greatest number of startle responses to predators. Behaviour around 25-29 weeks of age was relatively "shy", staying in shelter and avoiding open water. At around 37-41 weeks of age though, behaviour changed, with fish emerging from a hide more readily and exploring the environment. Interestingly, at 58 weeks of age fish were slower to initiate exploration, possibly indicating a return to "shyer" behaviour. All fish underwent thorough parasite examination, revealing no infections. We conclude that knowledge of the behaviour of hatchery-reared fish at different ages is useful for decisions around the timing of release that balance the needs of recreational fishers whilst managing the impact on freshwater ecosystem.

Bold-shy personality traits of globally invasive, native and hatchery-reared fish.

Bold behaviour of non-native species is hypothesized to facilitate invasion success, yet extreme boldness in wild and domesticated animals can be maladaptive. The purpose of this study was to compare individual behaviour among Australian native hatchery-reared (n = 33) and wild (n = 38) Murray cod (Maccullochella peelii) with invasive common carp (Cyprinus carpio; n = 30). Three laboratory tests measured individual behaviour: (1) emergence from a shelter, (2) exploration of a novel environment, and (3) approaching a predator. Wild invasive carp and hatchery-reared cod were generally faster and more likely to emerge and explore novel environments when compared with wild Murray cod. The 'bold-type' behaviours of hatchery-reared native cod were more like invasive carp than they were to 'shy-type' wild conspecifics, yet an important difference was that hatchery-reared cod spent substantially more time near a large predator while carp rapidly escaped. We suggest that these results are consistent with a bold-type invasion syndrome in invasive carp and learned boldness of hatchery-reared Murray cod. The propensity of invasive carp to rapidly explore and enter new environments, along with a fast predator escape response may have been important to their invasion success, while extreme risk-taking and predator naivety of hatchery-reared Murray cod may exacerbate post-release mortality rates in fisheries and conservation stocking programmes.

Do parasites influence behavioural traits of wild and hatchery-reared Murray cod, Maccullochella peelii?

This study aimed to investigate the links between parasites and behavioural traits of juvenile Murray cod (Maccullochella peelii). The Murray cod is an endangered Australian freshwater fish for which restocking programs are in place and there is a growing human consumption market. However, little is known about the parasites of these fish and how these parasites influence their behaviour and survival. Fingerlings and yearling fish were sourced from a hatchery and the wild, and after acclimatisation in the laboratory, variation in behavioural traits was examined using emergence, exploration and predator inspection tests. The fish were then euthanised to determine their age and examined for infection with parasites. Wild fish had more camallanid nematodes and lernaeid copepods than hatchery fish. An information theoretic approach using Akaike's Information Criterion (AIC) indicated that infection with protozoan cysts was an important factor for predicting the latency to emerge and explore a new environment, which was interpreted as reduced "boldness". In contrast, the presence of lernaeid copepods was included in two of the four best models predicting predator inspection, indicating that infected fish were less likely to inspect a predator. Source of fish (wild or hatchery) was found to be a strong influence on behavioural responses in all our tests. All parasites found in the present study are known to result in clinical signs of diseases in their fish hosts, raising the possibility that responses in tests of behavioural traits reflect side effects of infection. Additionally, the effect of host adaptation to not show signs of parasite infection, or more simply that the effects on behaviour are subtle and difficult to reveal with small sample sizes, is discussed. Nonetheless, we propose that it is important that infection with parasites is considered in fish behavioural studies both to assess survival behaviour and to avoid misinterpretation of behavioural tests of animal personality.

Aggressive encounters lead to negative affective state in fish.

Animals show various behavioural, neural and physiological changes in response to losing aggressive encounters. Here, we investigated affective state, which are emotion-like processes influenced by positive or negative experiences, in a territorial fish following aggressive encounters and explore links to bold/shy behavioural traits. Eighteen 15-month old Murray cod (Maccullochella peelii) received three tests in order to determine bold/shy behavioural traits then underwent a typical go/no-go judgement bias (JB) test. The JB apparatus had five adjacent chambers with access provided by a sliding door and fish underwent a training procedure to enter a chamber at one end of the apparatus to receive a food reward but were chased using a net if they entered the chamber at the opposite end. Only one third (N = 6) of fish successfully completed the training procedure (trained fish), and the remaining 12 fish failed to reach the learning criterion (untrained fish). Trained fish housed with a larger aggressive Murray cod for 24 h were significantly less likely to enter intermediate chambers during probe tests compared to control fish, demonstrating a pessimistic response. Trained fish showed "bolder" responses in emergence and conspecific inspection tests than untrained fish, suggesting that shyer individuals were less able to apply a learned behaviour in a novel environment. Our limited sample was biased towards bold individuals but supports the hypothesis that losing an aggressive encounter leads to pessimistic decision-making.