After the ‘whites migration”, adult western rock lobsters are highly residential, with no migration or large scale movements occurring (Chubb et al. 1999). Some tagged females have been caught on the same part of a reef multiple times up to 13 years later. Not all lobsters stay in exactly the same spot however, as nightly foraging behaviour can result in a progressive slow movement in the magnitude of kilometres over the course of years. In a tag recapture study, 91% of “red” lobsters, i.e. non-migratory lobsters, were recaptured within 10 km of their release location, after one or more years at liberty (Chubb et al. 1999). This 91% is likely an underestimate, due to some lobsters being tagged and recaptured as “red”, but passing through a “white” phase while at large.
Foraging patterns
Significantly less is known about foraging and movement patterns in adults compared with juveniles and whites. Aquaria observations of large juveniles and adults collected from shallow water habitats, indicate that foraging is nocturnal, with activity beginning immediately after the onset of darkness, and continuing throughout the night until sunrise (Morgan 1978). However, as adults predominantly reside in deep water offshore reefs, that are markedly different from the shallow water reefs inhabited by juveniles, is it possible that their foraging behaviour is also different, potentially less constrained by daylight hours.
Additionally, while nightly foraging distances and speeds have been tracked in juveniles, no such tracking/movement studies have been published on deep water adult populations. Similarly, whether adults return to a “home reef” as it appears some juveniles do, is also unknown.
Movement & Catchability
Research on adult western rock lobster activity levels and catch rates has demonstrated that movement can be impacted by various factors. 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 (Morgan 1978). This finding has been corroborated by studies on lobster catch rates, which have shown a general trend of increasing catch rates with increasing temperatures (Morgan 1974, de Lestang et al. 2009). Similar studies have also found that catch rates of adult western rock lobster are higher in times of high swell (while the opposite may be true for juveniles) (Srisurichan et al. 2005), and correlated with the lunar cycle, with the lowest catch rates observed during the full moon (Morgan 1974, Srisurichan et al. 2005). It is highly likely that these lower catch rates are a result of reduced foraging and movement during full moon periods, likely due to higher light levels at night and hence a greater chance of predation. Brooker et al. (2020) found that as swell levels increased many fish species disappeared from reef systems (up further into the water column), which presumably resulted in safer foraging conditions for lobster and may explain why lobster catch rates increase with swell.
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 lobster activity is impacted by sex or carapace length. Two studies on juveniles and early adults concluded that sex and carapace length do not impact movement (Morgan 1978, MacArthur et al. 2008), while another study, which looked at juveniles only, found that males were more active foragers than females (Jernakoff 1987).
In adult females, reproduction impacts food consumption rates and foraging activity, with females carrying late-stage eggs having lower food consumption rates and moving less than un-berried females, females with early-stage eggs, and males (Waddington et al. 2005). Moult stage is also known to significantly impact lobster activity. Activity has been found to decrease substantially while in a pre-moult condition, both in the laboratory (Morgan 1978) and in the field (Morgan 1974, Joll & Phillips 1984, Jernakoff & Phillips 1993). 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. While the aforementioned studies were conducted on juveniles, it is likely that bait plumes have a similar impact on adult activity.