Welfare Assessment of Laboratory Fish: Beyond Zebrafish

By Dr. Zubair Khalid, DVM, MS, PhD · 2026-08-11

Key Takeaways

• Welfare assessment for laboratory fish beyond zebrafish, specifically medaka (Oryzias latipes) and killifish (Nothobranchius furzeri), necessitates species-specific protocols due to distinct physiology, life history, and behavioral repertoires, rendering direct transfer of zebrafish-based assessments inaccurate.
• Key physiological indicators for stress and welfare compromise in fish include water-borne or whole-body cortisol, plasma glucose, and acute phase proteins, though their kinetics and measurability differ significantly from mammalian models, requiring careful validation for each species.
• Environmental factors such as water quality (ammonia, nitrite, nitrate, pH, dissolved oxygen, temperature) and enrichment strategies are critical welfare determinants, with species-specific needs for structural complexity, social grouping, and feeding predictability that differ from zebrafish requirements.
• Behavioral indicators must be tailored to species-specific repertoires, recognizing that deviations from normal patterns like medaka surface schooling or killifish substrate interaction signal potential welfare compromise, and spontaneous home-tank observations are often more informative than handling-based tests.
• Severity classification and humane endpoints for fish procedures must be clearly defined a priori, considering species-specific vulnerabilities and life history, with examples including loss of feeding response, prolonged abnormal posture, or respiratory distress.
• Common welfare assessment failures include indicator drift, false-negative assessments due to cryptic distress, and instrumentation failure in environmental monitoring, necessitating robust documentation, regular observer calibration, and independent verification of monitoring systems.

This article addresses welfare assessment for laboratory fish species other than zebrafish, with emphasis on medaka (Oryzias latipes) and killifish (Nothobranchius furzeri). It serves veterinary researchers, laboratory animal veterinarians, and animal care and use committees seeking species-specific assessment frameworks. The content answers how welfare indicators, monitoring protocols, and refinement strategies differ when the subject is a small teleost with distinct physiology, life history, and behavioral repertoire from the zebrafish model.

The principles of welfare assessment in laboratory animals rest on structured observation, defined endpoints, and validated indicators. The UK Joint Working Group on Refinement has published practical guidance on establishing welfare assessment protocols, including score sheets and monitoring regimes tailored to individual projects and species. That framework, developed primarily for mammals, requires substantial adaptation for fish. Fish display different pain behaviors, have no facial expressions amenable to grimace scoring, and respond to stressors through physiological axes that differ in timing and measurability from those of rodents. The acute phase response, for example, offers diagnostic potential across species, but its time course and protein components in fish differ from mammalian patterns. Welfare assessment in non-zebrafish laboratory fish therefore demands both adaptation of established principles and development of species-specific tools.

At a Glance

Why Zebrafish Protocols Do Not Transfer Directly

Zebrafish welfare literature has advanced considerably, but the assumptions embedded in those protocols do not automatically apply to other laboratory fish. Medaka and killifish occupy different ecological niches, display different social structures, and have different tolerances for environmental parameters. A protocol designed around zebrafish shoaling behavior, for instance, may misclassify normal medaka behavior as abnormal. The Joint Working Group on Refinement guidance emphasizes that welfare assessment must be tailored to individual projects and species, a principle that becomes critical when moving across fish taxa.

Species-Specific Biology and Welfare Baselines

Medaka

Medaka are small, surface-oriented fish native to rice paddies and slow-moving freshwater in East Asia. They tolerate a wide temperature range but prefer 24 to 28°C. Their social structure involves dominance hierarchies, particularly among males, and aggression can escalate when group composition changes. Medaka are diurnal and display clear behavioral rhythms tied to photoperiod. Welfare assessment should include daily observation of feeding response, position in the water column, and fin condition. Fin nipping from aggressive tankmates is a common and observable welfare compromise that requires intervention before it progresses to skin lesions or secondary infection.

Killifish

The turquoise killifish (Nothobranchius furzeri) has become a model for aging research because of its short natural lifespan and rapid sexual maturation. This species presents unique welfare considerations. Killifish are annual fish in the wild, inhabiting temporary water bodies that dry seasonally. Their laboratory housing must accommodate substrate preferences for spawning and resting. Adults are short-lived, and age-related pathology develops quickly, requiring vigilant monitoring for neoplasia, spinal curvature, and reduced mobility. The compressed lifespan means that welfare assessment intervals must be shorter than for longer-lived fish, and age-related decline should be distinguished from procedure-related compromise.

Physiological Welfare Indicators

The Hypothalamic-Pituitary-Interrenal Axis

Fish respond to stressors through the hypothalamic-pituitary-interrenal axis, analogous to the mammalian hypothalamic-pituitary-adrenal axis. Cortisol is the primary corticosteroid in teleosts and can be measured from plasma, whole-body homogenates, or water-borne release. Water-borne cortisol measurement offers a non-invasive option but reflects recent stressor exposure and requires careful validation for each species and system. Handling and anesthesia themselves elevate cortisol, so baseline values must be established under conditions that minimize these confounders.

Acute Phase Proteins

The acute phase response provides an additional physiological window into fish health and welfare. As reviewed by Cray and colleagues, acute phase proteins are integral to the innate immune response and have diagnostic applications across animal species. In fish, these proteins respond to infection, inflammation, and tissue damage, though the specific proteins and their response kinetics differ from mammals. Haptoglobin, serum amyloid A, and C-reactive protein analogues have been investigated in various fish species. For routine welfare monitoring, acute phase protein measurement is not yet standardized across laboratory fish species, but it holds promise as an objective indicator when behavioral signs are ambiguous.

Glucose and Metabolic Markers

Plasma glucose rises during acute stress in fish through catecholamine and corticosteroid actions. Glucose measurement requires blood sampling, which is technically challenging in small fish and itself constitutes a stressor. Non-invasive alternatives, such as measuring glucose from mucus or fin clips, remain experimental. Metabolic markers should be interpreted alongside behavioral observations instead of in isolation.

Behavioral Welfare Indicators

Normal Behavioral Repertoire

Welfare assessment requires knowledge of the species-specific normal behavioral repertoire. Medaka display surface skimming, schooling, and distinct courtship behaviors. Killifish spend considerable time near the substrate, bury into sand, and exhibit rapid darting during feeding. Deviation from these baselines, such as lethargy, erratic swimming, or loss of feeding response, signals welfare compromise. The Joint Working Group on Refinement guidance on recognizing and assessing indicators of pain or distress applies here, though the specific indicators must be defined for each fish species.

Behavioral Tests Adapted from Mammalian Models

Rodent welfare assessment has advanced through ethograms, home-cage monitoring, and burrowing or nest-building tests. Fish equivalents are less developed. Novel tank tests and open-field paradigms exist for zebrafish, but their validity for medaka and killifish requires separate confirmation. The review by Turner and colleagues on rodent pain assessment methods notes that spontaneous behavior assessment in the home environment often provides more accurate information than evoked response testing. The same principle applies to fish, where home-tank observation of feeding, social interaction, and locomotion may outperform handling-based tests that themselves induce stress.

Environmental and Husbandry Contributions to Welfare

Water Quality as a Welfare Variable

Water quality is the most consequential environmental factor for laboratory fish welfare. Ammonia, nitrite, nitrate, pH, dissolved oxygen, and temperature must be monitored according to established thresholds. The Guide for the Care and Use of Laboratory Animals provides general expectations for environmental monitoring and veterinary care in laboratory animal facilities, and these principles extend to fish housing. Sudden changes in water chemistry produce measurable physiological stress responses and behavioral changes. Welfare assessment protocols should include scheduled water quality measurement with defined action thresholds for each parameter.

Enrichment for Non-Zebrafish Species

Enrichment strategies developed for zebrafish, such as floating plants and structural inserts, may not suit all species. Medaka benefit from surface vegetation and shallow zones that match their natural habitat. Killifish require substrate for burrowing and spawning, and its absence constitutes a welfare deficit. The NC3Rs provides practical guidance on refinement strategies that can be adapted to fish housing, including environmental enrichment that promotes species-typical behavior. Enrichment should be evaluated for its effect on welfare indicators instead of assumed beneficial.

Social Environment

Social grouping profoundly affects fish welfare. Medaka dominance hierarchies can produce chronic stress in subordinate individuals, detectable through reduced growth, fin damage, and elevated cortisol. Killifish are less social and may require individual housing or carefully managed group composition. The welfare assessment protocol must include social parameters, such as aggression frequency and hierarchy stability, alongside individual health indicators.

Severity Assessment and Humane Endpoints

Severity classification for procedures involving fish follows the same logic as for other laboratory animals, requiring a harm-benefit analysis and defined humane endpoints. The cephalopod guidelines developed through the CephRes, FELASA, and Boyd Group initiative illustrate how species-specific severity assessment can be structured for non-mammalian research animals, and the same approach applies to fish. Humane endpoints for fish include loss of feeding response, prolonged abnormal posture, respiratory distress, and failure to recover from anesthesia. These endpoints must be defined before the study begins and documented in the protocol.

Clinical Welfare Assessment Protocols for Non-Zebrafish Laboratory Species

Building a Species-Specific Assessment Schedule

A welfare assessment protocol for medaka, killifish, or other non-zebrafish species must begin with a defined baseline for that species, strain, and life stage. The Joint Working Group on Refinement guidance emphasizes that effective protocols depend on objective observation and indicators tailored to individual projects instead of generic checklists. For each species, the attending veterinarian should document normal ranges for opercular rate, fin position, coloration, social spacing, feeding response, and response to startle stimuli before any experimental work begins.

The assessment schedule should specify observation frequency, duration, and timing relative to the light cycle. Medaka are diurnal and most active within 2 hours of lights-on, whereas some killifish species show crepuscular peaks. Observations at a single time point will miss diurnal variation in behavior and may misclassify normal inactivity as depression. Schedule observations at consistent times, and record the time of day alongside each score.

A practical scoring framework uses three tiers. Tier one is a daily check of all animals, lasting 30 to 60 seconds per tank, recording mortality, obvious injury, abnormal swimming, and feeding response. Tier two is a weekly detailed assessment of each individual or a representative sample, including body condition scoring, fin integrity, opercular rate, and social behavior. Tier three is a procedure-specific assessment triggered by experimental intervention, anesthesia, or suspected disease, using a score sheet tailored to the expected clinical signs.

Indicator Selection and Interpretation Across Species

The table below compares welfare indicators across the three species groups covered in this article. Selection of indicators should reflect the species' biology, the experimental model, and the sensitivity of each measure to the specific welfare challenge anticipated.

The acute phase response provides a physiological complement to behavioral observation. Cray and colleagues describe acute phase proteins as a core component of the innate immune response activated by trauma, infection, stress, and inflammation, with potential applications to laboratory animal welfare assessment. In fish, acute phase protein assays are species-specific and require validated reagents for each species. Medaka have characterized acute phase responses, but reagents for killifish are limited. Where validated assays are unavailable, the veterinarian should rely on behavioral indicators and glucose or cortisol measurements, acknowledging the reduced sensitivity of this approach.

Decision Points in the Assessment Sequence

The first decision point is distinguishing acute from chronic welfare compromise. Acute compromise presents with rapid changes in opercular rate, coloration, and swimming behavior within minutes to hours. Chronic compromise presents with gradual weight loss, fin erosion, reduced feeding response, and altered social behavior over days to weeks. The monitoring interval should match the expected time course. Acute assessments require repeated observations at 15 to 30 minute intervals, whereas chronic assessments can use daily or weekly scoring.

The second decision point is determining whether observed changes reflect welfare compromise or normal biology. Female medaka develop coelomic distension as they mature and may carry eggs for several days. Annual killifish naturally lose condition as they age and approach the end of their short lifespan. Seasonal color changes in killifish can mimic stress responses. The veterinarian must maintain species-specific baseline records and consult the literature or colony history before classifying a finding as abnormal.

The third decision point is selecting the intervention threshold. The score sheet should define a trigger score at which the animal is removed from the study, treated, or humanely euthanised. This threshold must be set before the study begins and agreed with the research team. The Joint Working Group guidance recommends that score sheets include clear criteria for when to intervene and that all staff are trained to recognize these criteria. For medaka, a sustained loss of feeding response for 48 hours combined with opercular rate above 150 percent of baseline warrants intervention. For killifish, the threshold may be lower because of their shorter lifespan and more rapid decline once compromised.

Documentation and Communication of Findings

Welfare records should include the assessment date and time, observer identity, housing conditions, water quality parameters, and individual or tank scores. Digital records with timestamped entries reduce transcription errors and allow trend analysis. Photographic documentation of fin lesions, color changes, and body condition provides an objective record that can be reviewed by the veterinary team and the animal care and use committee.

The attending veterinarian should review welfare records at least weekly and produce a summary for the research team. Trends that are not apparent in individual observations often emerge in longitudinal data. For example, a gradual decline in feeding response across a tank may indicate a water quality problem before individual fish meet the intervention threshold. The National Research Council guide emphasizes that the veterinarian has the authority to intervene when animal health or welfare is compromised, and this authority should be exercised based on documented evidence.

Communication with the research team should include a clear explanation of the welfare findings and the scientific implications. A welfare compromise that requires intervention may confound experimental data, and the veterinarian should document this possibility. The decision to remove an animal from a study should be recorded with the reason, the score at removal, and the outcome. This documentation supports both welfare assurance and scientific integrity.

Recognized Complications and Failure Modes

Welfare assessment protocols fail in predictable ways. The most common failure is indicator drift, where observers unconsciously recalibrate their baseline after weeks of exposure to suboptimal animals. This is particularly insidious in killifish colonies, where gradual deterioration in fin condition or opercular rate may go unnoticed until a concurrent stressor precipitates overt disease. Guard against drift by maintaining written baseline descriptions with photographs or video, and by rotating observers between colonies.

A second failure mode is the false-negative assessment in species with cryptic distress. Medaka, like many small fish, suppress behavioral signs of compromise until physiological reserves are exhausted. Reliance on behavior alone will miss early decompensation. The guidance on welfare assessment protocols from the UK Joint Working Group on Refinement emphasizes that effective monitoring requires integration of multiple indicator classes, with predetermined thresholds that trigger intervention regardless of whether the observer believes the animal appears distressed.

Water quality instrumentation failure constitutes a third category. Probes drift, calibration solutions expire, and automated monitoring systems can report plausible but incorrect values. A welfare assessment that depends on a single oxygen probe without periodic manual verification is vulnerable to silent failure. The National Research Council guide for laboratory animal care identifies environmental monitoring as a core veterinary responsibility, which implies independent verification of automated systems.

Common Errors and Corrective Action

Less experienced assessors frequently mistake activity for welfare. A fish that is vigorously swimming may be expressing escape behavior or stereotypic circling instead of positive welfare. Conversely, resting fish are often misclassified as lethargic. The solution is systematic observation at consistent times relative to feeding and light cycle, with attention to the distribution of behaviors across the tank, also their presence or absence.

A second error is treating all species as interchangeable in indicator selection. Killifish tolerate handling poorly and may show prolonged cortisol elevation after net capture, whereas medaka habituate more readily. Applying a medaka-derived handling schedule to killifish will produce artefactual stress readings. The review of acute phase responses in animals notes that inflammatory and stress biomarkers vary substantially between species, and this variation extends to the magnitude and time course of responses to identical stimuli.

Students commonly over-interpret single observations. A single day of reduced feeding in a killifish may reflect the natural feeding rhythm of the species instead of compromise. The pain assessment literature in laboratory rodents makes the parallel point that repeated observation with consistent methods improves diagnostic accuracy, a principle that transfers directly to fish. Corrective action is to record trends instead of snapshots, and to define a priori what change over what time period constitutes a welfare concern.

Limitations of Current Evidence

The evidence base for welfare assessment in non-zebrafish laboratory species is thin. Most published work derives from zebrafish, salmonids, or ornamental species, and extrapolation to medaka and killifish carries uncertainty. Species-specific validation of behavioral indicators, physiological biomarkers, and enrichment efficacy is largely absent. Expert opinion differs on whether killifish require substrate for burrowing, whether medaka benefit from surface vegetation, and whether group housing is preferable to pair housing for either species.

There is also genuine disagreement about the utility of physiological measures in routine welfare monitoring. Some authorities advocate regular cortisol sampling, while others argue that the handling required for sampling introduces more stress than the measurement prevents. The discussion of food and fluid restriction in laboratory animals illustrates the broader principle that monitoring procedures themselves can compromise welfare, and this trade-off must be weighed explicitly in protocol design.

Referral, Consultation, and Reporting

Veterinarians should seek specialist consultation when welfare concerns persist despite correction of husbandry variables, when disease is suspected but not confirmed, or when mortality exceeds expected baselines for the species and life stage. Laboratory animal specialists, fish pathologists, and aquatic veterinary consultants can provide diagnostic support that is not available in most facilities.

Regulatory reporting obligations vary by jurisdiction. The World Organization for Animal Health terrestrial animal health standards address disease notification and surveillance obligations that may apply to laboratory fish populations. Institutional animal care and use committees must be notified of unexpected welfare events, protocol deviations, or humane endpoint breaches as defined by local oversight requirements. When in doubt about reporting obligations, consult the institutional veterinary staff and the AVMA professional practice resources for guidance on professional responsibilities.

Frequently Asked Questions

How Can We Implement a Welfare Assessment Protocol When Budget Constraints Limit Equipment and Staff Time?

Prioritize observation-based indicators that require no specialised hardware. The UK Joint Working Group on Refinement guidance emphasizes that structured observation schedules, score sheets, and staff training in behavioral recognition form the core of any assessment system, and these are inexpensive to establish. Allocate the most frequent checks to water quality parameters that carry the highest mortality risk for the species in question, such as ammonia and temperature for medaka. Reserve biochemical sampling, including acute phase protein assays, for animals already undergoing procedures or at defined experimental endpoints instead of as routine screening. A two-tier system, daily rapid assessment and weekly deep assessment, concentrates resources where they detect the most welfare compromise.

What Should We Do When Species-Appropriate Enrichment Items Are Not Commercially Available?

Fabricate enrichment from materials already approved for use in the facility. Structured refuges can be constructed from inert plastic pipe sections, provided edges are smooth and materials are certified safe for aquatic use. Spawning substrates for medaka, such as bundled nylon fibers, are simple to produce and serve dual enrichment and husbandry functions. Killifish, particularly annual species, benefit from peat moss or coconut fiber substrates that mimic natural oviposition sites. Validate any novel item by observing interaction rates and checking for injury or water quality deterioration over a two-week trial period before facility-wide adoption. Document the fabrication method and approval in the animal care and use protocol.

How Does Welfare Assessment Differ Between Laboratory Fish and Laboratory Rodents in Practical Terms?

Rodent welfare assessment benefits from validated grimace scales, burrowing tests, and nest-building scoring, methods that rely on terrestrial behaviors and facial anatomy. Fish lack these validated tools, so assessment depends more heavily on water quality parameters, opercular rate, fin posture, and species-specific behaviors such as substrate interaction or shoaling cohesion. The aquatic environment itself is a welfare variable, since water chemistry directly affects physiological state. The Joint Working Group on Refinement notes that monitoring regimes must be tailored to the species and project, and for fish this means integrating environmental monitoring with animal observation into a single assessment schedule. Pain assessment in rodents increasingly uses spontaneous behavior in the home cage, an approach that translates well to fish if the observer knows the normal behavioral repertoire.

What Minimum Record-Keeping Structure Supports Defensible Welfare Decisions in a Fish Facility?

Maintain three linked records: a daily environmental log, an individual or cohort observation sheet, and a procedure-specific score sheet. The environmental log captures water quality parameters, photoperiod, and any system failures. The observation sheet records behavioral and physical indicators with predefined descriptors and severity grades, following the score sheet model described in the Joint Working Group on Refinement guidance. The procedure sheet documents expected welfare impacts, actual observations, and interventions taken. Each record must include date, time, observer identity, and the unique animal or tank identifier. Review all three together at weekly intervals to detect trends that single observations would miss, such as declining condition preceding overt clinical signs.

How Should We Explain Welfare Findings to a Principal Investigator Who Prioritizes Experimental Throughput?

Frame the discussion around data quality instead of moral obligation. Stress that physiological stress responses, including activation of the hypothalamic-pituitary-interrenal axis and acute phase protein changes, can confound experimental endpoints and increase data variability. The acute phase response is a recognized source of nonspecific variation in research animals, and controlling it improves statistical power. Present the welfare protocol as a refinement that reduces background physiological noise. Offer a pilot comparison showing endpoint variability before and after implementing the assessment schedule. Reference the broader scientific expectation that refinement is integral to acceptable animal research, as articulated in the Guide for the Care and Use of Laboratory Animals, and position the welfare protocol as compliance with that standard.

When Should We Seek External Consultation for a Fish Welfare Problem instead of Resolving It In-House?

Seek external input when a welfare concern persists despite corrective action, when mortality exceeds facility baseline by more than 50 percent for two consecutive weeks, or when clinical signs are ambiguous and no pathognomonic pattern emerges. Consult a veterinarian with aquatic animal experience through professional networks or referral services, and consider the MSD Veterinary Manual for species-specific disease guidance. External consultation is also appropriate when designing a novel enrichment or housing system, since published evidence for non-zebrafish species is limited and independent review can identify overlooked risks. Document the consultation request, the advice given, and the outcome, as this record supports both animal care and use committee review and future protocol refinement.

Related Clinical & Scientific Guides

• Refining IACUC Protocols to Minimize Animal Pain and Distress
• Health Monitoring Programs for Laboratory Animal Facilities
• Anesthetic Risk Assessment in Laboratory Animals: Preoperative Evaluation

References and Further Reading

• Guidelines for the Care and Welfare of Cephalopods in Research -A consensus based on an initiative by CephRes, FELASA and the Boyd Group.. 2015.
• Food or fluid restriction in common laboratory animals: balancing welfare considerations with scientific inquiry.. 2007.
• Acute phase response in animals: a review.. 2009.
• A guide to defining and implementing protocols for the welfare assessment of laboratory animals: eleventh report of the BVAAWF/FRAME/RSPCA/UFAW Joint Working Group on Refinement.. 2011.
• A Review of Pain Assessment Methods in Laboratory Rodents.. 2019.
• Animal welfare implications of neonatal mortality and morbidity in farm animals.. 2004.
• Guide for the Care and Use of Laboratory Animals, 8th Edition. National Academies Press, 2011.
• NC3Rs Resources on Replacement, Reduction and Refinement. NC3Rs.
• MSD Veterinary Manual, Professional Edition. MSD Veterinary Manual.

Related Articles

• Welfare Assessment Tools for Laboratory Rodents
• Husbandry and Welfare of Laboratory Guinea Pigs
• Zebrafish Welfare in Research: Housing and Environmental Needs
• Anesthetic Risk Assessment in Laboratory Animals: Preoperative Evaluation
• Pain Assessment in Laboratory Animals: Behavioral and Physiological Indicators

This article is educational professional reference material for veterinary audiences. It is not a substitute for veterinary diagnosis, individual clinical judgment, current product labeling, or applicable regulatory requirements.