During the juvenile life stage, western rock lobsters are considered residential, with no migration or large scale movements occurring (Chittleborough 1970, Chittleborough & Phillips 1975). Western rock lobsters are nocturnally active, with the majority of activity, including foraging, occurring at night (Cobb 1981, Jernakoff 1987, Jernakoff 1990). Studies have demonstrated that this foraging begins in response to decreasing light associated with dusk and not changes in water temperature or currents (Jernakoff 1987). Tracking in the field and observations in aquaria indicate that juvenile western rock lobster foraging activity is nocturnal; beginning immediately after the onset of darkness, and continuing throughout the night until sunrise (Morgan 1978, Jernakoff 1987). However, in a post puerulus grow-out trial (study examining whether post-puerulus could be grown to a markable size), activity was noted to be crepuscular in nature, with activity peaking around dusk and dawn (Moyle et al. 2009). There are no field observations to corroborate these findings, most likely due to the difficulty of finding/tracking post pueruli owing to their small size and cryptic nature, however, Jernakoff et al. (1993) while collecting post-puerulus in the field, observed that “post-pueruli usually begin to forage within one hour after sunset”.
Foraging patterns
Post-puerulus forage close to their day-time shelters, on the reef face and reef tops, and therefore have a relatively restricted foraging distribution (Jernakoff 1990, Jernakoff & Phillips 1993). Larger juveniles, on the other hand, undertake nightly movements away from the reef, to forage in nearby seagrass and algal habitats. Most foraging appears to occur within a radius of between 15 m (Chittleborough 1974) and 150 m (Jernakoff et al. 1987) of their “home reef”. Within this area the total distance travelled by a lobster per night can vary greatly, from ~100 to ~600 metres (Jernakoff et al. 1987, MacArthur et al. 2008).
Surveys indicate that 90% of lobster foraging activity occurs within 60 m of the nearest high relief limestone reef (MacArthur et al. 2008). Therefore, even if a lobster travels extensive distances from its “home reef”, it will usually still be within 60 m of the nearest reef. This foraging behaviour means that individuals will often live in one reef, but travel to forage on seagrass beds in front of another, often visiting other lobster dens while there (Jernakoff 1987).
There is some evidence that suggests individual juvenile western rock lobsters follow consistent foraging patterns, following the same path to and from their dens in the reef, and visiting the same seagrass or algal beds each night (Cobb 1981, Jernakoff 1987), however, how long this consistent behaviour persists is unknown. During foraging, juvenile lobsters have been observed to move at a rate of ~ 1 m per minute, and up to 18 m per minute when walking over bare sand (Jernakoff 1987).
Site fidelity and home reefs?
Surveys indicate that foraging continues throughout the night, before the lobsters return to the reef when dawn light increases (Jernakoff 1987). There is some evidence that juveniles are relatively residential, remaining in the same general geographic region for the duration of their juvenile years. During underwater surveys, Cobb (1981) found that lobsters returned to the same area of the reef after foraging, and in a tag-recapture study, Chittleborough (1974) found there was little dispersion of lobsters over the reef, with a high proportion of tagged lobsters remaining at the site of original capture up to 12 months later.
Conversely, however, a tracking study found that over a 3 week period 50% of the juveniles in one reef moved to another, indicating much lower reef fidelity (Jernakoff 1987). The authors hypothesised that site fidelity may be influenced by other factors such as population density and food availability. During the aforementioned tag-recapture survey the site had unusually low densities of lobsters (compared with previous years), while the site of the tracking survey had much higher densities. It may be that site fidelity is high when density, and therefore competition for food and space, is low, and low when competition is high. Mortality rates have been correlated with density, with higher mortality rates observed at sites with higher densities (Ford et al. 1988). As noted by the authors, the interpreted reduced mortality rates at low densities may actually reflect reduced emigration at these sites, i.e. fewer animals moving to new reefs, thus supporting the concept of density-dependent site fidelity. One study demonstrated that, upon being translocated, juvenile lobsters will generally disperse rapidly from their new location, have higher mortality rates, and will sometimes display homing behaviours, moving towards their original location of capture (Chittleborough 1974). This homing behaviour indicates the use of a “home reef” and supports the theory of high site fidelity.
Factors affecting movement
A number of studies have reported a range of variables that influence the movement rates of juvenile lobsters in shallow areas. Lobsters in the laboratory have been observed to increase their movement with increasing water temperatures between 17 and 25°C, and then reduce it again above this temperature (Morgan 1978). This finding has been corroborated by studies on lobster catch rates, which have shown a general trend of increasing catches with increasing temperatures (Morgan 1974, de Lestang et al. 2009). Similar studies have also found that catch rates of juvenile western rock lobster are correlated with the lunar cycle, with the lowest catch rates observed during the full moon (Morgan 1974, Srisurichan et al. 2005). Additionally, catch rates may be affected by swell, with lower juvenile catch rates during times of high swell in some locations (Srisurichan et al. 2005). It is highly likely that these lower catch rates are a result of reduced foraging and movement during these periods. Lobster activity has been found to decrease substantially while in a premoult condition, both in the laboratory (Morgan 1978), and in the field (Morgan 1974, Joll & Phillips 1984, Jernakoff & Phillips 1993).
There is some evidence that salinity may also affect lobster movement patterns, with one study finding a correlation between catch rates and salinity (Morgan 1974). However, the authors caution against interpreting this relationship as causal, stating that “high and low salinity waters off the Western Australian coast have different origins, and so it may be that the rock lobsters are reacting to some other component of the water system which was not measured but which is associated with the salinity and hence origin of the water”. It is therefore unclear if and how salinity affects lobster movement.
It is also unclear whether juvenile lobster activity is impacted by sex or carapace length (beyond the ontogenetic shift mentioned above). Two studies concluded that sex and carapace length do not impact movement (Morgan 1978, MacArthur et al. 2008), while another study found that males were more active foragers than females (Jernakoff 1987). It is highly likely that bait plumes influence lobster foraging behaviour and this effect appears to be influenced by currents. Chittleborough (1974) estimated the radius effect of a pot to be relatively small, approximately 20 m. However, when positioned upstream, lobsters have been observed to travel up to 120 m to enter a baited pot (Jernakoff & Phillips 1988). On the other hand, if the pot is located downstream, lobsters have been observed to pass within 20 m of the pot without entering it.
References
Chittleborough RG (1970) Studies on recruitment in the Western Australian rock lobster Panulirus longipes cygnus George: density and natural mortality of juveniles. Mar Freshwater Res 21:131–148.