Discard Mortality and Welfare in Commercial Fisheries

Discard mortality and animal welfare in commercial fisheries have emerged as critical concerns for sustainable management and policy compliance worldwide. Discard mortality refers to the proportion of non‐retained fish and invertebrates that perish after capture and subsequent release. Factors influencing mortality include gear type, capture stress, handling practices and environmental conditions such as temperature and oxygen levels. Welfare assessments increasingly rely on physiological and behavioural indicators—ranging from blood biomarkers to reflex responsiveness—to estimate the sublethal and delayed effects of capture. Understanding these dynamics underpins mitigation measures such as gear modifications, selective sorting devices and improved slipping techniques in purse seine fisheries. Moreover, integration of welfare science into stock assessment frameworks informs regulatory initiatives like the European Union’s landing obligation by quantifying collateral fishing mortality. This multidisciplinary field spans gear engineering, ecology, physiology and policy, with the dual aim of reducing unintended deaths at sea and preserving catch quality for economic benefit.

Research from Nature Portfolio

Recent investigations into pelagic trawling impacts on Atlantic herring have revealed alarmingly high discard mortality despite minimal external injury. Reflex assessments showed that over 95 % of trawled herring exhibited complete reflex impairment, indicating death, even though only 5 % displayed visible wounding. Blood analyses demonstrated severe osmoregulatory dysfunction, with elevated plasma osmolality and haemolysis linked to scale loss and crowding within the trawl. Extended tow durations and larger catches exacerbated cortisol and lactate responses. These findings underscore the need for rapid release protocols, modifications to pumping systems and reduction of tow times to safeguard fish welfare and improve survival rates in pelagic fisheries.

Discard Mortality and Welfare in Commercial Fisheries publication trend

The graph below shows the total number of articles in discard mortality and welfare in commercial fisheries across all publications each year (not limited to Nature Index journals).

Technical terms

Discard mortality: the proportion of organisms that die following unintended capture and subsequent release.

Reflex Action Mortality Predictor (RAMP): a method using reflex impairment scores to predict delayed mortality in captured animals.

Physiological biomarker: a measurable biological parameter (e.g. cortisol, lactate) indicating stress or health status.

Purse seine slipping: the process of releasing fish through an opening in a purse seine net while still in the water.

Landing obligation: regulatory requirement to retain and land all catches of regulated species, designed to reduce discards and underreporting.

References

1. Investigating the effects of pelagic trawling on the welfare of Atlantic herring (Clupea harengus). Scientific Reports (2024).
2. Survival rates for Nephrops norvegicus discarded from Northern European trawl fisheries. ICES Journal of Marine Science (2020).
3. Pre mortem capturing stress of Atlantic herring (Clupea harengus) in purse seine and subsequent effect on welfare and flesh quality. Fisheries Research (2021).
4. Behavioural and welfare implications of a new slipping methodology for purse seine fisheries in Norwegian waters. PLOS ONE (2019).
5. Towards the integration of ecophysiology with fisheries stock assessment for conservation policy and evaluating the status of the Mediterranean Sea. Conservation Physiology (2022).