We have been receiving fish related questions from professionals around the world with increasing frequency. Sometimes it’s a question about study design or research methods, but we even get questions about fish identification. For example, we were recently sent this photo (above) of a juvenile salmon with an inquiry as to whether it was a Chinook or coho. Distinguishing between adult Chinook and coho salmon can usually be mastered with a little practice, and is an important skill for salmon California fishermen. The California Department of Fish and Game recently issued a bulletin to marine salmon fishermen on the importance of properly distinguishing between Chinook and coho salmon, because coho are a protected species and must be released. You can easily differentiate between adults of the two species by looking at the bottom jaw; the base of the bottom teeth on Chinook salmon are all black, whereas coho salmon have a narrow light gray band (See photo guide).
But when it comes to differentiating between juvenile Chinook and coho salmon, you can’t just look at a single morphological characteristic like teeth or gums. Often biologists use the number of fin rays to identify fish to species, but closely related species may have overlapping ray counts, as is the case with Chinook (i.e., 13-19) and coho (i.e., 12-17) anal fin rays. Thus, with juvenile Chinook and coho it is necessary to look at multiple morphological characteristics in order to make the proper identification. Juvenile Chinook, for example, have parr marks (6-12) wider than the spaces in between them, a dorsal fin that usually has a dark leading (anterior) edge, an adipose fin that has some black pigment on the dorsal margin only and an anal fish with the leading edge shorter than the base of the fin. Whereas, juvenile coho may have pale orange fins, parr marks (8-12) that are narrower than the spaces in between them, dorsal and anal fins with the leading edge white followed by black, and a sickle-shaped adipose fin with a longer leading edge than the base of the fin. Most of the time an individual fish will not possess all of these traits, so the best approach is to compare as many features as possible before making a decision. With only a single photo identifying a fish can be even more challenging, so take a look at the photo above and see what you would call it.