spawning provides copious nauplii for cod larvae to eat. Just as important, drift from
this point retains the larvae over the bank until they are ready to swim along the
bottom. Stocks at the many mating sites are distinct genetically, although an endless
debate continues about the fidelity of individuals to mating groups and the precise
statistical evaluation of genetic distinctions.
(^) At least some groups that mix in feeding habitats remain strongly distinct
genetically, each group’s members taking part in migrations to different breeding
sites. For example, Svein-Erik Fevolden and colleagues have studied Atlantic cod in
Norwegian waters, long thought on the basis of blood types and otolith distinctions to
be two stocks. There are “northeast Arctic cod” (NEAC) that mostly feed as juveniles
and adults offshore in the Barents Sea and extending to the Spitsbergen area, and
Norwegian coastal cod (NCC) that mostly feed closer inshore and in fjords. In
December to January, the NEAC migrate along-shore to breeding areas near offshore
banks in the Lofoten vicinity. The hatched larvae are carried in the northward coastal
current back to the Barents, where the juveniles grow to maturity. The adults return
north as well for the summer–fall feeding season. The NCC both spawn and feed in
and near fjords all along the coast, occasionally mixing with NEAC, particularly
adjacent to fjords farthest into the Arctic. Sarvas and Fevolden (2005) examined the
frequencies of the A and B alleles (there are only two) of the nuclear gene for
pantophysin, a membrane protein found in secretory tissues. They developed a PCR
technique for rapid identification of the alleles and found more than 90% B in cod
caught offshore and in the Barents Sea identifiable as NEAC and 50–100% A in cod
captured in fjords and spawning at sites scattered along the Norwegian coast (Fig.
17.2). Selection alone is unlikely to sustain this level of distinction; it derives from
breeding structure, although Case et al. (2006) cite evidence for a degree of selection.
The intermediate gene proportions from some of the areas to the north can be due to
either mixing of NCC and NEAC in the collections, or to actual interbreeding.
Fig. 17.2 Average ±S.E. frequencies of two pantophysin alleles in samples of post-
juvenile (≥1 year) Atlantic cod (Gadhus morhua) from different regions along the
Norwegian coast.
(^) (After Sarvas & Fevolden 2005.)