Atlantic Coastal Biome
231
to regions north of the Gulf of Cadiz and was first noticed as abundant off Morocco only
in the 1950s. Twenty years later, these sardines had appeared abundantly off Senegal and
were heavily fished there, only to retreat northward again in the early 1980s.
Apparently, the abundance and distribution of European sardines along the African
coastline responds directly to the relative strength and location of upwelling between years,
and hence to changes in the strength of the trade winds. In recent decades, there have been
two periods of high sardine abundance (1976–77, 1989–90), and each of these periods
followed several years of intensified northeast trades, and hence of upwelling strength
(Binet et al., 2001). The actual relationship remains somewhat uncertain, although S.
pilchardus, unlike the zooplanktivore S. aurita, is capable of feeding directly on the
phytoplankton that will have been unusually available during these years. However,
recruitment tends not to succeed when strong offshore advection occurs during the
first 3 months of larval life, so it seems probable that the spawning potential of the
existing stock is enhanced during years of strong upwelling, and that recruitment is
consequently enhanced in subsequent years when offshore transport of larvae is relatively
weak. Off the Iberian segment of the coastline, S. pilchardus also exhibits periodic changes
in population abundance but the cause of these is not yet very well understood. But,
however it is determined, the variance in Iberian sardine year-class strength is very
significant—for a long period, the fishery was sustained solely by the strong 1983 and
1987 year-classes. Subsequently, apart from the moderate 1991 year-class, there was very
little new recruitment to the stock and the fishery once again entered crisis.
How clupeid recruitment responds to variance in environmental forcing in eastern
boundary currents has been a principal focus of fisheries ecology since the 1970’s. I shall
discuss it as a special topic when considering the CALC province, where much of the
critical work has been done. It will be necessary to consider both the consequences of
water column stability in allowing sufficient concentration of food particles for firstfeeding larvae, and subsequently their retention in upwelling regions where sufficient food
for later, more mobile larvae is produced. It is the latter mechanism that has been most
often invoked for the control of recruitment in the CALC province, but I suspect that that
fact merely reflects the interests of individual oceanographers who have worked there.
Regional Benthic and Demersal Ecology
The distribution and ecology of benthic macroorganisms on the continental shelf has
been little studied in this province, most attention having been given to the consumption
of sedimenting organic material from upwelling cells in deep water beyond the shelf. One
survey of benthic macrofaunal biomass on the inner shelf suggested that this decreased
with the seasonal arrival in shoal water of demersal fish.
I can find no general ecological studies of the benthos other than my own brief survey,
performed with quantitative grab samplers at 19 stations over the continental shelf south
of Cape Blanc at 13–14
N in December 1956. Considering the unusually high rate of
primary production in the overlying water column, I was surprised to find that the shelf
was almost entirely covered with clean shell-sand that extended from 10 m off the Gambia
River right out to the break of slope at 180–200 m. The color of the contained organic
material was olive-brown, indicating the dominance of the fecal pellets of macrobenthos,
but this material formed (as settled volume) only around 18% of the deposit samples
over most of the shelf down to 75 m, increasing to 44% at 180–200 m. In very shallow
water in the mouth of the Gambia, organic material was still unusually low for such
situations—only around 35%. These values are much lower than measured with the same
technique on the Guinea shelf in comparable situations (Longhurst, 1958).
The macrobenthic species associations were essentially identical with those on the
Guinean shelf (see GUIN), so that—because of the nature of the deposits—a typical Venus
231
to regions north of the Gulf of Cadiz and was first noticed as abundant off Morocco only
in the 1950s. Twenty years later, these sardines had appeared abundantly off Senegal and
were heavily fished there, only to retreat northward again in the early 1980s.
Apparently, the abundance and distribution of European sardines along the African
coastline responds directly to the relative strength and location of upwelling between years,
and hence to changes in the strength of the trade winds. In recent decades, there have been
two periods of high sardine abundance (1976–77, 1989–90), and each of these periods
followed several years of intensified northeast trades, and hence of upwelling strength
(Binet et al., 2001). The actual relationship remains somewhat uncertain, although S.
pilchardus, unlike the zooplanktivore S. aurita, is capable of feeding directly on the
phytoplankton that will have been unusually available during these years. However,
recruitment tends not to succeed when strong offshore advection occurs during the
first 3 months of larval life, so it seems probable that the spawning potential of the
existing stock is enhanced during years of strong upwelling, and that recruitment is
consequently enhanced in subsequent years when offshore transport of larvae is relatively
weak. Off the Iberian segment of the coastline, S. pilchardus also exhibits periodic changes
in population abundance but the cause of these is not yet very well understood. But,
however it is determined, the variance in Iberian sardine year-class strength is very
significant—for a long period, the fishery was sustained solely by the strong 1983 and
1987 year-classes. Subsequently, apart from the moderate 1991 year-class, there was very
little new recruitment to the stock and the fishery once again entered crisis.
How clupeid recruitment responds to variance in environmental forcing in eastern
boundary currents has been a principal focus of fisheries ecology since the 1970’s. I shall
discuss it as a special topic when considering the CALC province, where much of the
critical work has been done. It will be necessary to consider both the consequences of
water column stability in allowing sufficient concentration of food particles for firstfeeding larvae, and subsequently their retention in upwelling regions where sufficient food
for later, more mobile larvae is produced. It is the latter mechanism that has been most
often invoked for the control of recruitment in the CALC province, but I suspect that that
fact merely reflects the interests of individual oceanographers who have worked there.
Regional Benthic and Demersal Ecology
The distribution and ecology of benthic macroorganisms on the continental shelf has
been little studied in this province, most attention having been given to the consumption
of sedimenting organic material from upwelling cells in deep water beyond the shelf. One
survey of benthic macrofaunal biomass on the inner shelf suggested that this decreased
with the seasonal arrival in shoal water of demersal fish.
I can find no general ecological studies of the benthos other than my own brief survey,
performed with quantitative grab samplers at 19 stations over the continental shelf south
of Cape Blanc at 13–14
N in December 1956. Considering the unusually high rate of
primary production in the overlying water column, I was surprised to find that the shelf
was almost entirely covered with clean shell-sand that extended from 10 m off the Gambia
River right out to the break of slope at 180–200 m. The color of the contained organic
material was olive-brown, indicating the dominance of the fecal pellets of macrobenthos,
but this material formed (as settled volume) only around 18% of the deposit samples
over most of the shelf down to 75 m, increasing to 44% at 180–200 m. In very shallow
water in the mouth of the Gambia, organic material was still unusually low for such
situations—only around 35%. These values are much lower than measured with the same
technique on the Guinea shelf in comparable situations (Longhurst, 1958).
The macrobenthic species associations were essentially identical with those on the
Guinean shelf (see GUIN), so that—because of the nature of the deposits—a typical Venus