Giménez & Dick; Shore crab settlement and transport mechanisms 
165 
Table 1. Carcimjs maenas. Results of cross-correlation analy 
sis evaluating potential relationships between colonisation 
rate by megalopae and physical variables. Only lags (X in 
days) between -5 and 0 d and correlations (r) larger than the 
expected 95% confidence interval for white noise are shown. 
Negative lags indicate that the colonisation at Time t is 
explained by the physical variable at Time i - X. Positive cor 
relations indicate increasing abundance at spring tide, with 
winds from N or E, and with eastward or northward currents, 
ns: non-significant correlation 
Forcing 
2003 
2004 
2005 
factor 
X 
r 
X 
r 
X 
r 
Tide 
-3 
+ 0.32 
-1 
+ 0.28 
-4 
+ 0.34 
ns 
-2 
+ 0.31 
-5 
+ 0.31 
-3 
+ 0.27 
Eastern 
wind 
ns 
-1 
+ 0.18 
ns 
Northern 
0 
+ 0.35 
-4 
+ 0.45 
-5 
-0.31 
wind 
-1 
+ 0.51 
-5 
+ 0.55 
-2 
+ 0.49 
-3 
+0.47 
-4 
+0.38 
-5 
+0.39 
Eastward 
0 
-0.40 
-4 
-0.41 
-4 
+0.32 
current 
-1 
-0.41 
-5 
-0.38 
-5 
+0.39 
-2 
-0.47 
-3 
-0.48 
-4 
-0.52 
-5 
-0.51 
Northward 
ns 
-4 
-0.44 
ns 
current 
-5 
-0.41 
In 2003, colonisation rate was positively correlated 
with spring-neap tidal cycles, peaking at or after 
spring tide, except at times of strong SW winds. In 
2004, the correlation was not statistically significant: 
during one of the 2 spring tides sampled, strong SW 
winds and eastward currents may have overridden the 
effect of the tides. Thus, the data from these years sug 
gest that (1) strong and persistent SW winds and east 
ward currents preclude colonisation of the intertidal at 
Helgoland (under other wind regimes the colonisation 
rate was moderate to high); (2) there may be a tidal 
effect if SW winds and eastward currents are not 
strong and persistent. In 2005, colonisation rates ten 
ded to be moderate to high; concurrently, SW winds 
and predicted eastward currents were not strong and 
never persisted longer than 3 to 4 d. In this year there 
were colonisation peaks around spring tide. Although 
spectral analysis suggested that peaks of colonisation 
occurred every 10 d, correlations of colonisation with 
the tidal cycle support the idea that when SW winds 
are not persistent, a tidal effect is present. In 2005, the 
correlations between colonisation rates and wind 
direction or predicted residual currents were opposite 
to those in the 2 previous years. However, there were a 
Fig. 7. Proximate residual circulation patterns around Helgo 
land for (a) a southwesterly wind and (b) winds from other 
directions 
number of differences between 2005 and the previous 
years: (1) In 2005, periods of low larval abundance 
(<5 individuals) were very short, with the expected 
exception of the end of the settlement season. This was 
not the case in 2003 and 2004, when long periods 
(>5 d) of low abundance occurred. (2) The ranges in 
wind force (northerly component) and eastward cur 
rent speed were narrower in 2005 than in 2003 and 
2004, This was especially clear for the eastward cur 
rent component, which never rose beyond +10 cm s" 1 
and was greater than +5 cm s” 1 on only 5 occasions, 
during the whole settlement period. In contrast, in 
2003 and 2004 the eastward component was >5 cm s“ 1 
for periods longer than 5 d, reaching >10 cm s' 1 within 
these periods, which were periods with low colonisa 
tion rates. (3) With regard to wind direction, southerly 
winds in 2005 were from the SH (e.g. on Days 180 to 
185, Fig. 6), not from the SW as in 2003 and 2004. In