Tailfin clipping, a painful procedure: Studies on Nile tilapia and common carp
Research Highlights
►Nerve bundles are present in the tail of teleosts suggesting a possibility of nociception. ►Classical stress parameters don't allow discrimination between clipped and handled groups. ►Fin clipping induces changes in behavior regarding spatial preference. ►Fin clipping induces an adrenergic response characterized by branchial mucus release.
Introduction
In humans, awareness of pain, fear and stress depends on functions controlled and executed by the highly developed hippocampus, amygdala, and cerebral frontal lobes and neocortex [1]. In fish, the telencephalon, which will evolve to these cerebral structures in higher vertebrates, is far less complex and anatomically and fundamentally different, which has led many to conclude that fish cannot experience pain, fear or stress [2], [3]. One of the endeavours in research on fish welfare is the assessment of consciousness which is at the basis of pain and fear experience. There is ample evidence to conclude that fish experience stress and successfully mount behavioural and neuroendocrinological responses to cope with stress [4].
Reviews by Braithwaite and Huntingford [5] and Chandroo et al., [6] present convincing evidence that fish, despite their less developed telencephalon, have learning abilities at a level that implies cognitive abilities. For some species (rainbow trout Oncorhynchus mykiss, Atlantic cod Gadus morhua, common goldfish Carassius auratus, and Atlantic salmon Salmo salar), the first evidence has been advanced that fish may have the capacity to perceive painful stimuli and have a nervous substrate to experience fear and to suffer [7], [8], [9]. However, it has to be emphasized that it is unlikely that fish, as well as other animals, except maybe higher primates, have the capacity to experience suffering as human do [5]. Nociception, the detection of potentially harmful stimuli, is at the very basis of experiencing pain, i.e., interpreting the nociceptive stimulus. Pain perception thus involves both the nociceptive sensory machinery and the actual translation of harmful stimuli to the feeling of pain. Fish should possess then both a nociceptive system and some cognitive capacities to experience pain in a human sense. Indeed, a limited, yet firm, literature supports that fish detect harmful stimuli, respond to nociceptive stimuli and may conceptualize pain [5], [6], [7], [10], [11], [12].
Next to the feeling of pain, fear and stress are motivational affective states that are relevant to fish welfare. In their seminal reviews Braithwaite and Huntingford [5], and Chandroo et al., [6] conclude that these affective states may well be attributed to fish. Recently, Nilsson et al., [8] demonstrated explicit memory in Atlantic cod and, therefore, it is reasonable to hypothesize that fish indeed have capacities to have some form of consciousness and be aware of pain.
Studies that deal with the welfare of fish are limited to only a few out of an estimated total of 35,000 species; indeed, the knowledge on fish can only be called fragmentary. Beyond natural variation, human influences on fish, e.g., through prolonged farming and domestication, may impinge on welfare-related aspects such as aquaculture-related stress physiology [13]. Clearly, big gaps in the knowledge on fish welfare exist. Nevertheless, the current literature suggests that fish deserve a better moral consideration than they have received so far [14].
The international association for the study of pain (IASP) defined pain as an unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage [15]. Although pain has a subjective component that is difficult to convey without words, a non-verbal individual can still experience pain and benefit from pain-relieving treatment. In humans, methods to assess and quantify pain focus on cognitive abilities and subjective feelings. In studies on other mammals, emphasis is put on physiological parameters and behavioural activity, with little interest in the cognitive abilities and subjective feelings as is done for humans. However, few of these methods have been applied to demonstrate or quantify painful stimuli in fishes. A complicating factor in pain research is that the application of painful stimuli goes with an inherent stress response, for instance to handling (e.g., when blood is sampled) that interferes with the response to the fin clip. It is difficult to distinguish between stress responses and mild pain responses as these responses share a larger part of the stress physiology.
In this study, behavioural and stress-endocrine responses of the Nile tilapia (Oreochromis niloticus) to a presumed pain stimulus (tailfin clip) were investigated. In common carp (Cyprinus carpio), the clipped tissue was investigated at the ultrastructural level to identify nerve fibres classified in mammals and rainbow trout as pain fibres. Swimming activity was monitored and the fish's preference to reside in the lightened or darkened section of a compartmented aquarium. The stress parameters plasma cortisol, glucose and lactate, were measured. Parameters for osmoregulatory performance including Na+/K+-ATPase enzymic activity and chloride cell abundance and position in gills and plasma concentrations of Na+, K+ and Ca2+ were determined. In addition, mucus content of mucus cells in the gills was quantified.
This study was designed to discriminate the acute stress response inherent to the application of a fin clip as presumed pain stimulus from the fin clip proper through inclusion of the appropriate controls, and to select key parameters for future studies into this field of research.
Peripheral nerve fibres are categorized according to their diameter, conduction velocity and degree of myelinisation as A-α, A-β, A-γ, A-δ B- and C-fibres [16]. The A-fibres are myelinated for fast conduction of action potentials. The A-δ fibres are involved in the transmission of well-localized acute pain, while C-fibres lack a myelin sheet (are very simply isolated by glia) and therefore slowly conduct action potentials and involved in poorly localized unpleasant slow dull pain [7], [13], [17]. Fibres conducting in the velocity range of A-δ and C-fibres were identified in the trigeminal nerve of the rainbow trout and characterized as nociceptive fibres by Sneddon [7]. A-δ fibres (25%) were predominant over C-fibres (4%), displaying a different pattern compared with other vertebrates, where C-fibres can comprise from 50% (cat, human) up to 65% (frog) of the total fibre type [18].This difference in fibres composition is attributed to the water-to-land transition in vertebrate evolution [7].
A tailfin clip was chosen as pain stimulus; all the handling around the clipping procedure, but omitting the clip, served as control procedure to quantify the handling stress. Fins are vulnerable body parts that are easily damaged as a result of aggressive behavior between fishes or of aquaculture practices, such as sorting and transport.
Section snippets
Nerve bundles
Tailfin clips of common carp were immersed in glutaraldehyde (2.5% v/v), K2Cr2O7 (1% w/v) and OsO4 (1% w/v) in 0.15 M cacodylic acid (pH 7.5) and embedded in Spurr's resin. Ultrathin sections (70–90 nm) were cut with an ultratome and mounted on square mesh nickel grids. On-grid sections were post-stained for 2 min with uranyl acetate and then lead citrate for 2 min and rinsed thrice with doubly distilled water. Nerve fibre types in cross sections were categorized based on diameter and the presence
Ultrastructural analysis of common carp (C. carpio) tailfin
In carp tailfin clips, nerve bundles were found, both within the lepidotrichia segment and in the soft tissue (hypodermis) between the finrays. The nerves were symmetrically distributed (Figs. 1 and 2). Morphometric analyses revealed four categories of neurites, three types of myelinated A-fibres and one type of unmyelinated C-fibres (Fig. 2). Neurites in five nerves were analysed for diameter to score them as C and A-δ, A-β and A-α type (Table 1). The neurite type distributions in the nerves
Ultrastructural analysis of common carp (C. carpio) tailfin
This study investigated acute physiological and behavioural responses of Nile tilapia to a presumed painful stimulus and the stress response inherent to the application of the painful stimulus (i.e., the handling to clip the tailfin). In carp, the nerve in fin clips fulfilled all requirements to be designated as nerves that can carry noxious stimuli. Nervous tissues were observed in similar region of tail of the false mouth-breeder tilapia, Tilapia melanopleura[25].
Nerves bundles were found
General conclusions
This experiment aimed to confirm involvement of pre-selected parameters in the response to a presumed pain stimulus in the form of a fin clip and to select key parameters for future studies into this field of research. In addition, the study aimed to confirm differential responses to the fin clip compared to the accompanied stress response. A wealth of new insights was obtained with great promise for the near future of our welfare research in fishes.
The response that was found for several
Acknowledgements
We thank Tom Spanings (Radboud University Nijmegen) for the fish husbandry and invaluable assistance during the experiments.
This research project has been carried out within the Knowledge Base Research / Policy Supporting Research / Statutory Research Task for the Dutch Ministry of Agriculture, Nature and Food Quality. Project BAS-project KB08 009001ASG.
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2023, Veterinary Clinics of North America - Exotic Animal PracticeCitation Excerpt :Since then numerous empirical studies have demonstrated the capacity for pain in teleost fish using neuroanatomical, electrophysiological, molecular, physiological, imaging, and behavioral techniques.2–4 Besides rainbow trout, several other species of fish have been investigated, including Atlantic salmon (Salmo salar),5,6, goldfish (Carassius auratus)6–8, common carp (Cyprinus carpio),7 Nile and Mozambique tilapia (Oreochromis niloticus, O mossambicus),9,10 piauçu (Leporinus macrocephalus),11 and zebrafish (Danio rerio).7,12–14 Given that there are well over 30,000 species of fish and studies so far have shown profound differences in response to pain between species, more research is needed to explore species-specific differences.
- 1
Contributed equally to this study.