Low detection efficiency of candling as a commonly recommended inspection method for nematode larvae in the flesh of pelagic fish.

The third-stage larvae of the parasitic nematode Anisakis simplex commonly occur in most commercially important fish species from the North Atlantic. In this study, the occurrence and site distribution of Anisakis larvae in various size groups of Norwegian spring spawning herring (Clupea harengus), blue whiting (Micromesistius poutassou), and mackerel (Scomber scombrus) from the Northeast Atlantic was investigated. Although most of the larvae were found in the abdominal cavity, the small proportion lodged in the flesh was sufficient to affect food quality and safety. However, the prevalence of Anisakis larvae in fish fillets varied considerably among the size groups of each species: 15 to 60% in herring, 32 to 77% in mackerel, and 89 to 100% in blue whiting. The commonly recommended nematode detection method in the fish processing industry is candling, i.e., a brief visual inspection on a light table. Comparison of successively more accurate detection methods, i.e., candling, enzymatic degradation, and UV illumination, for fillets of each fish species revealed that only 7 to 10% of the nematode larvae present in the fillets were detected by candling. The candling efficiency was apparently independent of fillet thickness. Thus, candling is not sufficient to detect the majority of the nematodes that are actually present in the fillets of pelagic fish from the Northeast Atlantic. These findings emphasize the importance of adequate deep-freezing of any fresh pelagic fish product, especially if it is intended for consumption in a raw or semiraw state.