S.h SII:C Frequency. Dlstnhutlon and Colour Variation of Coninus //Joel/os
165
total, more than 25 000 crabs were caught during this survey and their size and sex
were recorded.
An epibenthos sledge was used to capture crabs < 15 mm in size. This sledge
had a net opening of 100 x 17 em and a mesh size of 200 11m. It was lowered to the
seatloor around low tide with the net opening facing the current and retrieved after
10 minutes. These samples were taken monthly in 1995.
A mark-recapture experiment was carried out from 1992 until 1994 alongside
the subtidal survey. Crabs caught were colour-coded by gluing a drop of paint on
the dried carapace. Experiments in an aquarium had shown that this colour-code
persisted for about 3 months and caused no harm to the crabs. During the experiment, as many crabs as possible were colour-coded and released in August 1992,
September and October 1993 and June 1996. A different colour was applied at
each date. Their recapture was recorded during the regular monthly surveys in the
subtidal.
In the intertidal, crabs were caught with sediment traps. Buckets of about 500
cm 2 surface area were implanted level with the sediment surface. Five traps were
implanted at each of four transect sites from a mussel bed into a sandtlat on the
Swinnplate (Fig. 3.1.3 and see Chap. 5.2). Monthly samples were carried out in
1995 and 1996. Therefore the traps were first emptied and cleared of sediment and
the crabs collected in the traps within the following 24 h were recorded. In addition, samples were taken with a corer of about 500 cm
2
surface area to 5 em sediment depth. These samples were sieved through a sieve with 2 mm mesh size to
record the smallest shore crabs of < 15 mm in size. Three replicate samples were
taken monthly in 1995 and 1996 on three of the transect sites, the mussel bed, near
the mussel bed and in the sandtlat.
5.6.3
Results
Population size
Mark-recapture experiments had been carried out to assess the population size of
the shore crabs in the subtidal of the Spiekeroog backbarrier system. The rate of
shore crabs retrieved in these experiments was extremely low, which indicates a
large population size. Out of a total of 9368 colour-coded crabs only 68 individuals were retrieved (0.73 %) during the entire experiment. In 1992, a few crabs
were retrieved in the same area four months after their release.
Size frequency and sex distribution of shore crabs in the subtidal
In 1994, about 15 000 shore crabs were caught in the subtidal using the dredge.
These were three times as many crabs as in the previous years, which were not
sampled as frequently as 1994. Over the year, the abundances of male and female
crabs in the subtidal were highest in August/September (Fig. 5.6.1). Ovigerous
females were more abundant early in the year from February to June.
165
total, more than 25 000 crabs were caught during this survey and their size and sex
were recorded.
An epibenthos sledge was used to capture crabs < 15 mm in size. This sledge
had a net opening of 100 x 17 em and a mesh size of 200 11m. It was lowered to the
seatloor around low tide with the net opening facing the current and retrieved after
10 minutes. These samples were taken monthly in 1995.
A mark-recapture experiment was carried out from 1992 until 1994 alongside
the subtidal survey. Crabs caught were colour-coded by gluing a drop of paint on
the dried carapace. Experiments in an aquarium had shown that this colour-code
persisted for about 3 months and caused no harm to the crabs. During the experiment, as many crabs as possible were colour-coded and released in August 1992,
September and October 1993 and June 1996. A different colour was applied at
each date. Their recapture was recorded during the regular monthly surveys in the
subtidal.
In the intertidal, crabs were caught with sediment traps. Buckets of about 500
cm 2 surface area were implanted level with the sediment surface. Five traps were
implanted at each of four transect sites from a mussel bed into a sandtlat on the
Swinnplate (Fig. 3.1.3 and see Chap. 5.2). Monthly samples were carried out in
1995 and 1996. Therefore the traps were first emptied and cleared of sediment and
the crabs collected in the traps within the following 24 h were recorded. In addition, samples were taken with a corer of about 500 cm
2
surface area to 5 em sediment depth. These samples were sieved through a sieve with 2 mm mesh size to
record the smallest shore crabs of < 15 mm in size. Three replicate samples were
taken monthly in 1995 and 1996 on three of the transect sites, the mussel bed, near
the mussel bed and in the sandtlat.
5.6.3
Results
Population size
Mark-recapture experiments had been carried out to assess the population size of
the shore crabs in the subtidal of the Spiekeroog backbarrier system. The rate of
shore crabs retrieved in these experiments was extremely low, which indicates a
large population size. Out of a total of 9368 colour-coded crabs only 68 individuals were retrieved (0.73 %) during the entire experiment. In 1992, a few crabs
were retrieved in the same area four months after their release.
Size frequency and sex distribution of shore crabs in the subtidal
In 1994, about 15 000 shore crabs were caught in the subtidal using the dredge.
These were three times as many crabs as in the previous years, which were not
sampled as frequently as 1994. Over the year, the abundances of male and female
crabs in the subtidal were highest in August/September (Fig. 5.6.1). Ovigerous
females were more abundant early in the year from February to June.