Ontogenetic deepening of Northeast Atlantic fish stocks is not driven by fishing exploitation

For many marine fish species, the average size of individuals increases with depth. This phenomenon, first described a century ago, is known as ontogenetic deepening (1, 2). Several mechanisms have been proposed to explain it: optimal foraging; predation avoidance; and different optimal growth temperature for larger individuals, causing them to seek deeper and cooler waters to optimize growth and reproduction (3). In their recent paper in PNAS, Frank et al. (4) suggest an alternative explanation. They examined age-structured data from Atlantic cod (Gadus morhua) on the eastern Scotian Shelf, a stock that has experienced successive periods of intense, and absence of, fishing. In their study, fishing explained 72% of the variation in the observed age-related deepening, with the remaining variability attributed to ontogenetic deepening. They conclude that higher abundances of large fish in deeper waters is an artifact of greater fishing intensity at shallower depths and question whether ontogenetic deepening is a real ecological phenomenon.