Tidal mixing fronts: their location, dynamics and biological significance - Introduction to the Physical and Biological Oceanography of Shelf Seas - page 256

Inland Sea, Japan, but higher predation of copepod nauplii in the front was
suggested as preventing this production following through to higher copepod
concentrations (Uye et al., 1992 ). Off northeast Scotland, copepod egg production
and egg concentrations were found to be higher within a front, but successively
older stages of the copepods showed less relation to the front (Kiørboe and
Johansen, 1986 ).

8.6.3 Fronts and larger marine animals

While it would appear to be difficult to demonstrate a simple, clear picture of
zooplankton enhancement at fronts, there are several unambiguous examples of
predators of zooplankton being found to forage preferentially at tidal mixing fronts.
Basking sharks have been found to forage for zooplankton along a tidal front in
the western English Channel (Sims and Quayle, 1998 ). The sharks were observed
to concentrate their swimming close to sea surface slicks, indicators of surface
current convergence, with foraging time being spent mainly in regions with higher
zooplankton concentrations. Convergences associated with smaller scale tidal fronts
around reefs, analogous to the island mixing fronts, have also been shown to
accumulate a range of zooplankton. A variety of reef fish target the convergence
zones to feed (Kingsford et al., 1991 ). Seabirds have long been recognised as
selectively feeding along fronts (Begg and Reid, 1996 ; Jahncke et al., 2005 ). In the
western Irish Sea, observations of seabird distributions have shown a correspond-
ence between frontal structure and foraging behaviour and, in particular, an asso-
ciation of surface feeders with surface convergences (Durazo et al., 1998 ). These
studies indicate that the relatively fine scales of the surface flow convergence, rather
than the broader scale of the phytoplankton distribution, could be providing the
increased prey densities targeted by the sharks or seabirds, and that species can
have different foraging locations depending on their foraging method and the
structure of the front. Thus, tidal fronts are not necessarily important for foraging
animals because they are sites of higher food production. They are also important
because they make food accessible, either by concentrating zooplankton within
convergence zones or by making zooplankton available on the thermocline closer
to the surface.
Tidal mixing fronts have received particular attention in the context of commer-
cially important fisheries. Spawning stocks of Atlantic herring have been shown to be
in close proximity to tidal mixing fronts and the adjacent mixed water, with the
mixed regions acting to prevent wider dispersal of larvae and determining the size of
a stock (Iles and Sinclair, 1982 ) and also maintaining genetic identity of stocks
(Sinclair and Iles, 1989 ). However, this concept of oceanographic retention control-
ling larval recruitment to a fish stock has been seen as contrary to observations of
larval drift from spawning grounds (Cushing, 1986 ), and the idea that recruitment
success depends on spawning being timed to coincide with a flush of suitable food
(e.g. the match-mismatch hypothesis (Cushing, 1990 )). It is possible that all three