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

A recently published article by Frank et al. titled “Exploitation drives an ontogenetic-like
deepening in marine fish” claim that the deepening of large individuals
commonly observed in exploited marine fish species is driven by fishing pressure. These conclusions
fundamentally challenge our current understanding of ontogenetic deepening in marine fishes,
including a range of hypotheses that have been put forward to explain it (optimal foraging, optimal
temperature, avoidance of predation mortality) and have significant implications for the use of
species’ deepening as an indicator of warming seas. However, Frank et al.’s findings are based on a
single exploited stock, and in a region where sea temperatures have remained within the species
thermal preference range.
If Frank et al.’s findings are widely applicable, then the depth at which large fish are observed should
correlate positively with fishing intensity in other stock as well. Here we have performed a brief
statistical analysis on several Northeast Atlantic fish stocks which experienced varying degrees of
fishing exploitation. Our results showed no evidence that ontogenetic deepening became less
evident with declining fishing intensity. If anything the depth was negatively correlated with fishing,
meaning that as fishing mortality dropped the ontogenetic deepening was more evident. This
questions the universality of Frank et al.‘s findings and challenges their conclusion that the
deepening of marine species may not be an adequate indicator of warming seas.