Ices cm 2017/F: 414 Contingent structure of Northwest Atlantic mackerel evaluated using otolith stable isotopes

Atlantic mackerel (Scomber scombrus) is an abundant migratory species that supports important commercial fisheries and is major prey for North Atlantic piscivores. Northwest Atlantic mackerel stock assessments currently assume a single stock, comprised of northern and southern contingents, with natal regions centered in the southern Gulf of St. Lawrence and southern New England. We hypothesized that otolith δ¹⁸O and δ¹³C values could discriminate between fish from these natal regions based upon hydrographic differences and thereby illuminate seasonal migrations and contingent structure. Archived otoliths from both the United States and Canada, as well as otoliths from Iceland and Norway were carefully milled to extract carbonates corresponding to the first year of life. Sub-regional differences were not found within the US-origin southern contingent, but comparisons among source countries showed significant variation within the northwest Atlantic and across the Atlantic basin. Additionally, interannual differences in δ¹⁸O values were observed within the northwest Atlantic that appeared to be related to regional ocean warming trends and the position of the Gulf Stream. Analysis of natal material in older Atlantic mackerel revealed substantial mixing at older ages between the northern and the southern contingents. These results indicate a complex pattern of how contingents contribute to US and Canadian fisheries, and have implications for assumptions of spatial structure in assessment and management.

Chesapeake Biological Laboratory