sequence of trophic levels. Smaller fishes in the epipelagic far from land are
dominated by migratory mid-water groups, particularly the Myctophidae. There are
some clupeids: sprat, anchovy, and herrings. Those and other epipelagic species, for
example the flying fish (Exocoetidae), are much more abundant near islands. Larger,
oceanic fish include the so-called dolphin fish or mahi mahi (Coryphaena hippurus),
blue mackerel (Scomber australascius, that reaches 2 kg), sword- and sailfish, several
tuna species, and some oceanic sharks. Most of these larger fish are more abundant in
the vicinity of islands where topographically driven upwelling increases production;
all are subject to active fisheries and most stocks are stressed by those fisheries.
(^) Ommastrephid squids, particularly Ommastrephes bartrami in the Pacific at about
70 cm length, are very deep diel vertical migrators that can be attracted at night to
lights suspended from ships. A complex assemblage of other squid species is
distributed down the water column. Marine mammals are part of the subtropical gyre
fauna, although they are much more important in more productive ecosystems, and
again greater abundance near islands is typical. Seals, for example the endangered
Hawaiian monk seal, pull out to rest on islands. Spinner dolphins (Stenella
longirostris) are abundant close to shore, and whales – particularly humpback whales
– that feed in summer at high latitudes, migrate to subtropical island areas to calve
and mate in winter. There are more examples, e.g. beaked whales that are apparently
more abundant over seamounts and near the Canaries and Azores than elsewhere, and
squid-eating sperm whales that aggregate near northern New Zealand and other
islands. All of these mammals do transit long stretches of open ocean, and a few, such
as the elephant seals (Mirounga angoustirostris) that mate and bear young on the
shores of the American West Coast, make extensive migrations seaward to feed,
during which they do extremely deep dives. The females make subtropical feeding
tours, while the males go toward the Aleutian Islands. Sea turtles, essentially all
tropical–subtropical forms, also migrate seaward to great distances from egg-
deposition sites on sandy beaches into oceanic waters to feed, primarily on jellyfish.
Thus, the system of upper trophic-level links in these waters with the lowest primary-
production rates is substantially complex.
(^) Elaborate efforts are under way (e.g. Spitz et al. 2001) to model subtropical gyre
ecosystem processes. The models do well in representing regional “climatology”
(average seasonality) of nutrients and phytoplankton stock, and progress is apparent in
predicting general responses of those stocks to variations in mixing, irradiance,
temperature, and seasonal timing of events. Models do less well, at least for now, with
other components: primary productivity, dissolved and detrital organic matter, and
zooplankton. Under development are data-assimilating calculations that seek best-
fitting parameters for modestly complex models. The resulting parameters do appear
to be better predictors than those selected on a more ad hoc basis.