Demersal fisheries generate large quantities of unwanted by-catch. Nephrops norvegicus is the most important shellfish resource in UK waters and although the fishing effort has increased considerably over past decades the ecological effects of this fishery have yet to be evaluated. This study provides a baseline assessment of the composition and fate of invertebrates regularly discarded in the Clyde Nephrops fishery. Monthly sampling on local fishing boats revealed that only 18% of the catch is landed, invertebrates accounting for up to 90% of the discards. Crustaceans (Liocarcinus depurator, Munida rugosa) and echinoderms (Asterias rubens, Ophiura ophiura) were the most important groups discarded. Monthly damage assessments of invertebrates collected from commercial trawlers showed the severity and frequency of damage was mainly correlated with species-specific morphological and behavioural characteristics: almost all O. ophiura and some 40%-60% of the crustaceans incurred injury. By contrast, damage was low in more flexible invertebrates such as A. rubens (30%) or hard-shelled animals such as Pagurus bernhardus (14%) and Aequipecten opercularis (2%). Biochemical analyses of haemolymph samples taken from trawled and emersed crustaceans revealed significant increases in ammonia, glucose and lactate concentrations along with a decrease in pH. Recovery started after 4 h. Longer-term survival experiments showed that post-trawling mortality was 100% for O. ophiura 14 d after trawling; by contrast, almost all hermit crabs and whelks survived. Experimental ablation of appendages and autotomy increased L. depurator post-trawling mortality significantly (78% and 26%) cf. creel-caught controls (8%). Asterias rubens with multiple arm loss showed >90% mortality 21 d after trawling possibly due to an increased risk of infection with Vibrio metschnikovii. While mortality figures for most species doubled over the last 14 d of the experiment, most previous studies covered monitoring periods of only <5 d and may have underestimated discard mortality. Traps baited with crustacean and echinoderm discards deployed at sea attracted a number of species. While whelks and A. rubens were found in significantly higher numbers in traps baited with crustaceans, P. bernhardus was more attracted to A. rubens bait. The two amphipods Orchomene nanus and Scopelocheirus hopei occurred in high numbers (n=2745) and preferred traps baited with crustaceans. These findings are discussed with respect to their ecological significance and relevance to fisheries management.