What is the black stringy thing in cod fish? An Expert's Guide
                                
                                
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                                    5 min read                                
                            
                                According to seafood experts, parasitic worms are a natural occurrence in wild-caught fish, including cod, and do not present a health risk when the fish is properly prepared. However, a dark, string-like substance in a cod fillet is often just the fish's natural bloodline. Understanding the difference is key to safely preparing and enjoying your meal.