(^) A complex literature considers the level of genetic distinctions among Atlantic cod
in detail, without fully explaining within-population variation in allele proportions
(usually compared between years), but establishing with certainty that fish stocks
diverge genetically over wide geographical ranges (e.g. Pogson et al. 2001). That
shows that gene flow is slow. In part, that may simply be distance, but it also can be
attributed (particularly for intra-regional cases like the Norwegian-Arctic cod and
Norwegian fjord cod) to separation into stocks with spawning-site fidelity, As a
further example, Kovach et al. (2010) have used the pantophysin alleles to show that
the group of cod spawning at the northeast peak of Georges Bank (Fig. 17.1b) are
distinct from spring-spawning cod along the southwest coast of the Gulf of Maine,
and that both are distinct from those spawning over shallow bottoms around Cape
Cod. While catches from all of those stocks would be landed at, say, Plymouth,
Massachusetts, USA, their distinct habits might warrant trying to manage them
separately (Hauser & Carvalho 2008), although it could not be done easily.
(^) More examples of spawning migrations can be found among the hake, herring
(Sinclair 1988), tuna, elasmobranchs, squid, and a wide range of demersal fishes:
plaice, flounder, halibut. Even whales migrate, often thousands of miles, switching
from subpolar feeding grounds to subtropical areas where they visit special local sites
for calving and mating. In their case, it cannot be for hydrographic containment, but
the common feature applies that some sites are best for reproduction, others for
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