Variation in the Trophic Level of Pacific Cod with Changes in Size and Season

The effect of fishing on the marine ecosystem has been often summarized as the trend in the mean trophic level of the catch. This metric has been shown to be declining in many parts of the world, a trend popularly known as "fishing down the food web." The method has been criticized, however, because it typically considers only a single trophic level value for each component of the catch while considerable variation is known to exist in the food habits of marine organisms both seasonally and ontogenically. Examining the seasonal food habits of Pacific cod we determine that significant differences in trophic level exist both with increasing size of the cod as well as by season. However, adjusting the mean trophic level of the entire commercial catch for changing cod trophic levels does little to change the interpretation of. the status of Gulf of Alaska fisheries.