Geoscience Reference
In-Depth Information
This circulation is believed to act as a retention mechanism for the Nephrops eggs and
larvae, aiding the settlement of young Nephrops back onto the required muddy
substrate (Hill et al., 1997 ). It is probable that the 'leakiness' of this gyre, the inter-
annual variability in the timing of spring stratification and the strength of the
stratification, and the fishing pressure on the Nephrops population will combine to
play important roles in the survival of Nephrops.
(ii)
Fish stocks over Georges Bank
Georges Bank, off the northeastern United States, has long been recognised as an
important feeding area for fish, particularly Atlantic cod and haddock, and has been
commercially fished since at least the early nineteenth century, possibly much longer.
Fish stocks generally need to maintain sufficient integrity for enough of the adults to
meet up and spawn each year despite living within a dispersive marine environment.
In 'choosing' a region for spawning, adult cod and haddock have to solve the
problem of how their initially non-motile eggs and weakly motile larvae can reach
the feeding area over the shoals of Georges Bank rather than being advected or
dispersed elsewhere. Considerable research effort has been focused on understanding
the links between the physical regime of Georges Bank and the life stages of the fish,
primarily in response to the importance of the commercial fishing and the consequent
pressures that the fish stocks have been experiencing.
The residual circulation around Georges Bank has already been shown ( Fig. 8.7 ).
Rectification of the barotropic tidal currents (see Sections 3.7 and 8.3.2 ) provides
a clockwise circulation around the bank all year, but the development of the
tidal mixing front separating mixed bank water from the surrounding stratification
augments the flow considerably (e.g. from
20 cm s 1 along
the southern flank of the bank (Loder and Wright, 1985 )). During the summer
stratified period far weaker, but as we shall see vitally important, cross-frontal
residual flows have been observed and modelled (Lough and Manning, 2001 ; Chen
et al., 2003 ).
These two components of the flow, along-bank transport followed by weak cross-
bank transfers, provide the fish eggs and larvae with their conveyor back to the
shoals of Georges Bank. The pathway is shown in Fig. 8.21 . Both cod and haddock
spawn prior to and over the onset period of spring thermal stratification on the
northeast end of Georges Bank. The residual along-bank flow then moves the eggs
and subsequent larvae along bank isobaths towards the southwest, shown in
Fig. 8.21a . This transport has been inferred from tracking the ages of eggs and larvae
against the known background flows (Butman and Beardsley, 1987 ; Lough and
Bolz, 1989 ). Observing the weaker cross-bank flows proved to be more difficult.
A combination of residual flow set up by the tidal mixing front ( Fig. 8.21b ) and
the vertical position of the larvae (either transported downward by the frontal
downwelling, or actively swimming downward in the case of older larvae) is thought
to lead to transport of larvae into the shoals over Georges Bank. The tidal mixing
front thus plays a key role, partially by augmenting the along-isobath transport of
larvae and then by providing a mechanism for cross-isobath transport.
10 cm s 1
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