feeding. Atlantic and Pacific salmon species provide extreme examples of the
phenomenon, with each stream area defining a unit stock. The freshwater provides
their larvae (“alevins”) hydrographic containment in a sense, and probably more
importantly it serves as a refuge from marine predators. Genetic studies, similar to
those on Atlantic cod, have been done for many species, generally with similar
results. The unit stock and hydrographic containment concepts are useful idealizations
for a wide range of marine animals.
(^) While mating sites are many and widespread, there are also large ocean areas and
stretches of coastal ocean, even very productive ones, with little spawning by fish
with pelagic larvae. That is because young left there would be swept away to
unsuitable habitats. Breeding adults do not know this; natural selection installs
breeding times and locations as part of a reproductive program that produces
survivors, a program by which the young arrive in appropriate places at appropriate
times. Fish leaving young where they will be carried out of a suitable habitat do not
have grandchildren. A prime example of a spawning gap (Parrish et al. 1981) is the
California coastal zone from Cape Mendocino to Point Conception, where eddying
streamers carry newly upwelled water far out to sea (represented by the fat arrow in
Fig. 17.3), mixing it eventually into the oligotrophic central gyre. Largely separate
stocks of northern anchovy (Engraulis mordax) spawn to the north of Cape Blanco
(southern Oregon) and to the south of Point Conception (at Santa Barbara), but not
much between. Adult anchovy, and sardines also, wander through central Californian
water, but they belong to spawning stocks to the north or south. The Pacific hake
(Merluccius productus) actually migrates back and forth across this long stretch of
coast in large schools, moving north to feed off Oregon, Washington, and British
Columbia from March into summer, then moving south again starting about October
to spawn seaward of northern Baja California in winter (Bailey et al. 1982; Fig. 17.4).
The exact location of hake spawning is not well known because it is offshore, deep,
thought to be broken into patches, and seems to be in different places different years.
The eggs, which float upward, and early larvae are carried north and inshore by the
seasonal surface flow. The extent to which larvae are retained inshore after spawning
determines subsequent recruitment to the fishery. Thus, an index of “year class
strength” (see below) is negatively correlated with the amount of offshore flow
estimated from an index of the along-shore wind stress in January (Bailey et al. 1982).
Fig. 17.3 Generalized summer circulation off the west coast of North America. Broad
arrows indicate the relative level of offshore Ekman transport, which mostly occurs in
jets at topographic projections. Simple arrows show the direction of mean geostrophic
current, including the inshore gyre in the Southern California Bight. Contours
represent a wind-mixing index (m^3 s−3). Pelagic spawning is minimal from Cape
Mendocino to Point Conception.
(^) (After Bakun 1993.)