Implications of stock structure in understanding juvenile Atlantic cod (Gadus morhua) habitat suitability in the Gulf of Maine

The misalignment between the spatial structure of biological stocks and management units can be a potentially significant obstacle to the recovery of collapsed stocks. This study examines how ignoring biological stock structure in habitat modeling can prevent the detection of spatially explicit habitat relationships, using juvenile Atlantic cod (Gadus morhua) as an example. A recent reevaluation of cod stock structure found that there are likely two separate biological units in the eastern Gulf of Maine (EGOM) and western Gulf of Maine (WGOM) within the current Gulf of Maine (GOM) management unit. These two areas differ in their ranges of depth, bottom temperature, and sediment type, variables that influence habitat use by juvenile Atlantic cod (20–50 cm). When applying a Habitat Suitability Index (HSI) model within these distinct spatial areas, juvenile cod in the EGOM biological unit were found to occupy wider ranges of habitat, including deeper and colder waters, with larger seasonal differences in overall habitat quality than in the WGOM biological unit. When modeling HSIs over the entire GOM management unit and ignoring biological stock structure, results aligned closely with WGOM trends, but did not identify the same seasonal habitat suitability shifts, and the unique habitat associations in the EGOM were not identified. Juvenile survival may be more important than adult survival and fecundity in rebuilding cod stocks in the GOM, and can increase through the protection of quality habitat. Habitat conservation measures, such as area closures, may be implemented over suboptimal areas if spatially explicit habitat relationships are ignored, failing to slow population declines and further hindering recovery of collapsed stocks such as Atlantic cod.