Environmental Engineering Reference
In-Depth Information
Table 13.2
Long-term changes for the period 1869-1981 of 100 macrobenthic species common at one time or
another in the Wadden Sea area close to Sylt, Schleswig-Holstein, Germany. This table gives the numbers of
species showing a long-term decrease, no change or an increase. Modified from Reise (1982).
Taxonomic grouping
No. of species
Decrease
No change
Increase
Bivalves (Bivalvia)
8
3
1
Other molluscs (Gastropoda and Placophora)
1
7
3
Crustaceans (Crustacea)
6
12
8
Other invertebrates (Porifera, Coelenterata and Echinodermata)
8
7
0
Polychaete worms (Polychaeta)
5
13
18
species to the loss of the many microhabitats provided
by the complex physical structures such as oyster beds,
tubeworm (
Sabellaria
) reefs, and seagrass (
Zostera
marina
) beds. He noticed that of the 30 species
showing a long-term increase, half are particularly
abundant in mussel beds, a habitat that had only
disappeared in the period 1970-80. When these com-
plex intertidal structures (see Table 13.1) disappear,
so does the fauna that associates with them. Reise's
(1982) assessment can be regarded as a summary of
the changes to intertidal flat communities brought about
by a century of human use.
Human fishery activities are no doubt very old, but
the scale and the intensity of the fisheries has shown
dramatic increases over the past century, and especially
over the last 30 years. For example, so-called mussel
farming has been around in the western Dutch
Wadden Sea since the specialists from the province
of Zeeland moved in during the early 1960s. This indus-
try not only involves the filling up and dredging out
of the artificial subtidal mussel beds, it also entails
the bringing together of mussel spat from much
larger areas, including mussels from the nearby inter-
tidal zone and from stocks outside the Netherlands
(Kamermans & Smaal 2002). Such replacement may
take place several times in the beds before mussels
are finally transported to market, and each replace-
ment involves bottom dredging.
In response to the development of markets for bait
used in sport fishing (angling), techniques to mech-
anically dredge for lugworms
Arenicola marina
were
developed in the Netherlands in the early 1980s.
Given the considerable depth at which lugworms live
(30-40 cm), dredging for lugworms is very invasive
(leaving gullies 40 cm deep) with considerable con-
sequences for the intertidal biota (Beukema 1995).
Over a 4-year dredging period, lugworm densities
over 1 km
2
declined by half. Simultaneously, total
biomass of benthic organisms declined even more with
an almost complete local disappearance of the large
sandgaper (
Mya arenaria
) that initially comprised
half of the biomass. The small, short-lived species were
quite resistant to the dredging, although the worm
Heteromastus filiformis
showed a clear reduction
over the 4 years. Recovery took several years.
Common cockles (
Cerastoderma edule
) have not
been a popular food in the Netherlands, but the
demands by foreign markets nevertheless made this
fishery profitable (albeit on a limited scale) from the
early 1900s onwards. With the discovery of new
markets, notably in countries in the Mediterranean
region, and the development of mechanical harvest-
ing techniques this fishery has, over the past decades,
seen a large expansion. From the late 1970s onwards,
dredging for cockles became a veritable industry.
Ecological studies have shown long-term, near
decadal, effects on rates of recruitment of cockles and
Balthic tellins (
Macoma balthica
; Fig. 13.2; Piersma
et al.
2001). Also in the shorter term there appear to
be strong negative ecological effects of mechanical
cockle-dredging practices. In 1998-2001 the cockle
dredgers concentrated their efforts on the most bio-
diverse intertidal areas in the western Dutch Wadden
Sea, with the declines a year after fishing being
