Exploitation drives an ontogenetic-like deepening in marine fish

Significance

The occupation of progressively deeper waters with increasing size and age of fish is common among commercially exploited species, a behavior attributed to evolved ontogenies. Recently deepening has been attributed to ocean warming. We evaluated the possibility, ignored in previous analyses, that these patterns result from selective exploitation of larger individuals. We found that size-selective exploitation accounted for >70% of the deepening of cod on the Scotian Shelf (Northwest Atlantic). This deepening declined dramatically when exploitation was banned. Ontogeny contributed to the remaining variance. The claim that deepening can be used as an index of ocean warming should be exercised with caution; the overarching effect of exploitation should be specifically addressed in all such analyses.

Abstract

Virtually all studies reporting deepening with increasing size or age by fishes involve commercially harvested species. Studies of North Sea plaice in the early 1900s first documented this phenomenon (named Heincke’s law); it occurred at a time of intensive harvesting and rapid technological changes in fishing methods. The possibility that this deepening might be the result of harvesting has never been evaluated. Instead, age- or size-related deepening have been credited to interactions between density-dependent food resources and density-independent environmental factors. Recently, time-dependent depth variations have been ascribed to ocean warming. We use a model, initialized from observations of Atlantic cod ( Gadus morhua) on the eastern Scotian Shelf, where an age-dependent deepening of ∼60 m was observed, to assess the effect of size- and depth-selective exploitation on fish distribution. Exploitation restricted to the upper 80 m can account for ∼72% of the observed deepening; by extending exploitation to 120 m, all of the deepening can be accounted for. In the absence of fishing, the model indicated no age-related deepening. Observations of depth distributions of older cod during a moratorium on fishing supported this prediction; however, younger cod exhibited low-amplitude deepening (10–15 m) suggestive of an ontogenetic response. The implications of these findings are manifold, particularly as they relate to hypotheses advanced to explain the ecological and evolutionary basis for ontogenetic deepening and to recent calls for the adoption of evidence of species deepening as a biotic indicator or “footprint” of warming seas.