170
5 Spatial and Temporal DistrihuliOI1 Patterns
However, the low recovery rate from the mark-recapture experiments indicates a
large population of C. mamas in the Wadden Sea with a high degree of migration
activity.
The seasonality in the abundance of shore crabs in the sub- and intertidal of the
backbarrier system of Spiekeroog was in agreement with previous records (KleinBreteler 1976b; Beukema 1991). A higher share of male shore crabs in the subtidal, as recorded in the backbarrier system of Spiekeroog, had not been observed
before (Wolf 1997). The frequency distributions of crab sizes indicated that the
average size of crabs was larger in the subtidal than in the intertidal, but this could
be biased by the different sampling techniques which had to be used sub- and
intertidally. A comparison of the population structure in the sub- and intertidal was
also not possible as the two areas were sampled in different years and variations
can occur between years (see Beukema 1991).
Following the ice winter, a severe decline was recorded in the intertidal population of the shore crabs. Low abundances and late appearance of crabs after cold
winters was also reported by Beukema (1991). Up to the end of the field season
for ELA W A T in autumn 1996, neither the abundances nor population structure
had returned to the values of the preceding year. Since newly settled and juvenile
crabs were missing, a prolonged effect of the ice winter on the crab population
was predicted. Giving such a low recovery rate, the stability properties of this
species can only be assessed with long-term surveys.
The part of the shore crab population occurring in the intertidal is not representative for the entire population (Wolf 1997). No crabs < 15 mm had been caught in
the subtidal, confirming that the crabs settle exclusively in the intertidal (KleinBreteler 1976a). The early benthic stages of shore crabs were almost exclusively
recorded in intertidal mussel beds, which has been documented before and is seen
as protection from predation (Klein-Breteler 1976b; Thiel & Dernedde 1994;
Moksnes et al. 1998). The high number of small crabs caught with the traps in the
sandflat was related to their foraging activity, as these traps catch those crabs
active on the tidal flats at high tide (Wolf 1997). Thus the spatial distribution in
the intertidal is related to the activity of the shore crabs and reflects the function of
different intertidal habitats for the crabs of different sizes. The occurrence of the
largest crabs was restricted to the subtidal. Differences in the population structure
of sub- and intertidal shore crabs were also shown by Hunter & Naylor (1993).
Although the intertidal hosts only a small part of the population, it is an important
habitat for C. maenas. The crabs frequenting the tidal flats could be grouped as
follows:
• early crab stages « 15 mm in size) in the mussel beds,
• small juvenile crabs (1.5-2.5 cm in size) in high numbers, derived from the
spatfall of the previous year,
• medium sized or few large crabs, possibly migrating into the tidal flats to avoid
competition with larger crabs in the subtidal.
The size distribution, frequency and seasonality in the occurrence of colour forms
of C. maenas recorded from the backbarrier system of Spiekeroog confirms records from other seas. The higher share of green colour forms in the intertidal
results both from the abundance of juvenile and medium sized crabs in the tidal
5 Spatial and Temporal DistrihuliOI1 Patterns
However, the low recovery rate from the mark-recapture experiments indicates a
large population of C. mamas in the Wadden Sea with a high degree of migration
activity.
The seasonality in the abundance of shore crabs in the sub- and intertidal of the
backbarrier system of Spiekeroog was in agreement with previous records (KleinBreteler 1976b; Beukema 1991). A higher share of male shore crabs in the subtidal, as recorded in the backbarrier system of Spiekeroog, had not been observed
before (Wolf 1997). The frequency distributions of crab sizes indicated that the
average size of crabs was larger in the subtidal than in the intertidal, but this could
be biased by the different sampling techniques which had to be used sub- and
intertidally. A comparison of the population structure in the sub- and intertidal was
also not possible as the two areas were sampled in different years and variations
can occur between years (see Beukema 1991).
Following the ice winter, a severe decline was recorded in the intertidal population of the shore crabs. Low abundances and late appearance of crabs after cold
winters was also reported by Beukema (1991). Up to the end of the field season
for ELA W A T in autumn 1996, neither the abundances nor population structure
had returned to the values of the preceding year. Since newly settled and juvenile
crabs were missing, a prolonged effect of the ice winter on the crab population
was predicted. Giving such a low recovery rate, the stability properties of this
species can only be assessed with long-term surveys.
The part of the shore crab population occurring in the intertidal is not representative for the entire population (Wolf 1997). No crabs < 15 mm had been caught in
the subtidal, confirming that the crabs settle exclusively in the intertidal (KleinBreteler 1976a). The early benthic stages of shore crabs were almost exclusively
recorded in intertidal mussel beds, which has been documented before and is seen
as protection from predation (Klein-Breteler 1976b; Thiel & Dernedde 1994;
Moksnes et al. 1998). The high number of small crabs caught with the traps in the
sandflat was related to their foraging activity, as these traps catch those crabs
active on the tidal flats at high tide (Wolf 1997). Thus the spatial distribution in
the intertidal is related to the activity of the shore crabs and reflects the function of
different intertidal habitats for the crabs of different sizes. The occurrence of the
largest crabs was restricted to the subtidal. Differences in the population structure
of sub- and intertidal shore crabs were also shown by Hunter & Naylor (1993).
Although the intertidal hosts only a small part of the population, it is an important
habitat for C. maenas. The crabs frequenting the tidal flats could be grouped as
follows:
• early crab stages « 15 mm in size) in the mussel beds,
• small juvenile crabs (1.5-2.5 cm in size) in high numbers, derived from the
spatfall of the previous year,
• medium sized or few large crabs, possibly migrating into the tidal flats to avoid
competition with larger crabs in the subtidal.
The size distribution, frequency and seasonality in the occurrence of colour forms
of C. maenas recorded from the backbarrier system of Spiekeroog confirms records from other seas. The higher share of green colour forms in the intertidal
results both from the abundance of juvenile and medium sized crabs in the tidal