One Stock, Two Stocks, Red Fish, Blue Fish

One Stock, Two Stocks, Red Fish, Blue Fish .... ?
Implications of DNA genomics for fisheries management & conservation

Sequencing of complete mtDNA genomes from Atlantic Cod shows a ‘family tree’ [top left] with two major lineages (red and blue fish), and five sub-families within these lineages (shades of red and blue). What implications does regional variation in the distributions of these lineages across management zones have for stock structure and conservation? Suppose that surveys show an increasing abundance of fish in NAFO 3Ps and continued low density in 3L.

    If the two major lineages occur in isolation in two separate geographic locations, fish found in 3Ps and 3L are "genetically distinct" stocks (top right). Fish in 3Ps are recovering, fish in 3L are not. It may be possible to treat them as separate management units (DFO) or designatable units for conservation (COSEWIC).

     If the two lineages occur at more or less equal frequencies in both geographic locations, fish found in 3Ps and 3L are a single, genetically homogeneous stock (bottom left). Despite differences in relative abundance, what affects one part of the stock also affects the others, and the fish cannot be managed or designated independently. For example, general recovery of the stock may begin in local areas such as 3Ps, which export fish to other areas. Monitoring of the spread of particular sub-families within 3Ps may document this.

     If the two genetic lineages occur at different frequencies in each geographic location, fish found in 3Ps and 3L may be "genetically distinctive" mixed stocks (bottom right) Depending on the degree of mixing and the rate of exchange of particular lineages among geographic zones, they may or may not behave as independent stocks. For example, the major red fish lineage occurs at equal abundance in 3Ps and 3L, but the pink lineage is concentrated in Trinity Bay as a local 'inshore stock'. Blue fish lineages occur as a gradient, with the dark blue lineage occurring in 3Ps, light blue spanning zones, and medium blue occuring only in 3L.

     The mtDNA family tree is resolvable at the level of individual fish (see SM Carr & HD Marshall 2008), and complex patterns of genetic distinctiveness can be measured.