Giménez & Dick: Shore crab settlement and transport mechanisms
163
The period of low colonisation rate was dominated by
strong SW winds; predicted residual currents were towards
the NE, with high speeds. This situation changed
a few days before the increase in colonisation, with
winds relaxing and shifting between SE and NW, and
predicted residual currents tending to move westwards.
For 2004 (Fig. 4b), spectral analysis of colonisation
showed peaks at periods of 10 and 38 d, while
short-term variation (<5 d) was not important.
In 2005, the colonisation rate was mostly moderate,
with several peaks of high density (>15 ind. d' 1 ) and
with only short periods (1 to 3 d) of low density (Fig. 6a);
the only long period of low density was at the end of the
settlement season (after Day 220). Winds showed low
variations in magnitude: they were mostly from the
NW, with short periods (<4 d) from the SE and NE. Predicted
residual currents also showed low variation in
magnitude,- they flowed towards the NW and SW during
most of the settlement season, with short periods in
a NE or SE direction. Spectral analysis of the 2005 data
showed peaks at periods of 5, 10 and 32 d (Fig. 4c).
Relationships among variables
DISCUSSION
Using correlative analysis, this study evaluated the
importance of transport processes on the colonisation
rate of natural substrata by megalopae of the shore
crab Casein us maenas. In the absence of a manipulative
experimental design, causality may be tested by
repeating observations in space and time. For this reason,
a 3 yr data set was used, thus obtaining more
robust conclusions. The data showed variability among
and within years, with patterns observed in one year
not necessarily being repeated in the following year.
For instance, July of 2003 and 2005 was characterised
by high colonisation rates, while in 2004, this month
had lower rates. Within years, 2003 and 2004 were
characterised by periods of 5 to 10 d of very low colonisation,
while in 2005 low colonisation occurred only at
the end of the setdement season.
The observed variability in the colonisation rate of
megalopae may be due to a combination of presetdement
processes (e.g. larval transport affecting
larval supply) and by larval movements, development
and mortality in the intertidal. In Carcinus maenas, lar-In
2003, colonisation rate was significantly
correlated with tidal cycle, suggesting
maximal colonisation 3 to 4 d after spring tide
(Table 1). Correlations were also statistically
significant after northern winds (lag = 0 to 5
d), suggesting that maximum colonisation
followed north winds and minimum colonisation
followed south winds. Significant correlations
between colonisation and predicted
residual currents suggested that minimum
colonisation followed after eastwards currents
(lag = 0lo5d).
In 2004, colonisation rate was not significantly
correlated with tidal cycle (Table 1).
Correlations with wind direction were significant,
suggesting maximum colonisation
after N-NE winds and minimum colonisation
after SW winds. Correlations with predicted
residual currents suggested that
minimum abundance followed after N-E
currents, with a lag of 4 d.
In 2005, relationships between colonisation
rates and tidal cycle were statistically
significant, suggesting maximum values 1
to 3 d after spring tide (Table 1). Correlations
with wind were negative, suggesting
minima after north winds (lag = 5 d); correlations
with residual currents were positive,
suggesting maxima after current relaxation
or slightly eastward currents.
Fig. 5. Time series for year 2004. (a) Colonisation of traps by Carcinus maenas
megalopae; (b) wind direction; (c) predicted residual currents. Further
details as in Fig. 3
