Environmental Engineering Reference
In-Depth Information
modelling tool of choice in marine ecology. This is particularly true in studies of
zooplankton and the early life stages of fish, which often reside in the water column
and have very limited locomotory capabilities. In this context, we generally refer to
biophysical models as
Lagrangian Individual-Centred Models
, although
Eulerian
approaches and those that do not focus on the individual are also found in marine
ecological literature, as well as a number of cases of hybrid coupled
Eulerian-
Lagrangian
models. The definitions of the
Eulerian
and
Lagrangian
approaches
used throughout this chapter follow Turchin (1998).
Marine organisms can live within the sediment, on the seabed (the
benthic
layer)
and above it, the latter ranging from a close association with the bottom (the
demersal
layer) to living on, or just below the sea surface. A considerable number
of organisms, from taxa that span from invertebrates to many species of fish live in
the
pelagic
zone, which is not directly associated with the sea bottom. Many of
these organisms have relatively limited locomotory ability, particularly in relation
to the horizontal flow of ocean currents. However, they are often capable of
significant vertical movement and are referred to by the collective term of
plankton
,
derived from the Greek
planktos
, which means errant or drifter. This also includes
the
ichthyoplankton
, representing the early life stages of many fish. Larval fish have
the ability to swim, and directed horizontal swimming has been demonstrated in a
number of species, although generally among the more developed and behaviou-
rally capable warm-water, rather than temperate, species (Leis 2007).
The physical environment provides planktonic organisms with their means of
transport, their living conditions (e.g. light, temperature, salinity, oxygen) and their
food. The direct observation of biophysical processes at scales relevant to plank-
tonic organisms is difficult. Technological advances such as Video Plankton
Recorders (e.g. Lough and Broughton 2007), Optical Plankton Counters (e.g.
Gallienne et al. 1996), holography (e.g. Sun et al. 2008) and high resolution
acoustic methods (Ross et al. 2008) show promising results. In comparison, tradi-
tional sampling methods based on, for example, nets or pumps are very limited in
their spatial and temporal coverage and resolution, and their sample analysis is time
consuming and dependent on increasingly rare taxonomic skills. Biophysical mod-
els do not suffer from some of the problems of observational methods and can
therefore be used to complement experimental and observational studies in the
marine field.
Cod (
Gadus morhua
) populations in the North Atlantic have been subject to a
high degree of exploitation from the mid-twentieth century, resulting in some
spectacular stock collapses in the western Atlantic (e.g. Myers et al. 1997). Our
study area corresponds to International Council for the Exploration of the Sea
(ICES) Divisions IV (North Sea), IIIa (Skagerrak) and VIId (eastern English
Channel), managed as a single “North Sea stock”, as well as ICES Division VIa
(the “west of Scotland stock”). Following concerns about the effect of an unsus-
tainable fishing pressure on the cod population (Cook et al.1997), ICES recom-
mended a number of stock recovery measures, which culminated in 2002 in the
recommendation of the cessation of fishing in certain areas (ICES 2003).
Although the management of the European cod stocks is implemented at coarse
