The whites phase of the western rock lobster life cycle is associated with a large-scale, mass migration, in which these prepubescent lobsters migrate from their inshore juvenile habitats to the offshore reefs where the majority of the adult population reside. Not all white lobsters will undergo a migration. In fact, studies have shown that between 30 and 95% of tagged white lobsters on inshore reefs failed to migrate offshore (MacArthur et al. 2008; Melville-Smith and Beale 2009). Because lobsters are capable of more than one migration, it is possible that these lobsters will undergo a second whites phase the following year, at which time they will migrate.
Two Migrations
An extensive tag study by de Lestang (2014) have shown that lobsters may undergo at least two migrations in their lifetime; the first dominated by a shorter offshore movement, and the second associated with an extension of the offshore movement, together with a strong northern movement. During the first inshore-to-offshore migration, the study found that 10 – 50% of lobsters of migrating size, migrate around 25 km offshore in a westerly direction, and 2 km northward. The following year, it was found that up to 15% of lobsters of migrating size undergo a second migration, moving approximately 50 km further west, and, on average, another 20 km to the north.
Orientation of migration
Migrating lobsters have been found to all move in the same direction, irrespective of their starting latitude (de Lestang 2014) (Figure 1). Initially, lobsters travel in a west north-westerly direction, at a bearing of approximately 280 degrees, towards the shelf-break. Once they reach depths of 100-200 m they change direction and head north, remaining within this depth range. The fact that all migrating lobsters follow the same bearing indicates the use of orientation cues to guide their movements. There are numerous potential cues used by spiny lobsters to orientate themselves while migrating, including environmental factors, such as bathymetry, current flow, and wave surge, as well as by detecting variations in the magnetic field.
In the initial part of the migration, when lobsters are moving west-northwest across the continental shelf (<100 m), bathymetry and hydrodynamics vary greatly, both spatially and temporally, making it highly unlikely that lobsters use them as cues. It is therefore assumed that lobsters use the magnetic field to orientate themselves during this part of the migration, by following regions of equal magnetic intensity from their inshore habitats (de Lestang 2014). The ability to orientate and move using magnetic reckoning has been shown for other similar spiny lobsters (Lohmann 1985).
The Leeuwin Current
For the second part of the migration, the deepwater northward migration, the south-flowing Leeuwin current presents a consistent hydrodynamic feature that lobsters are likely able to use as an orientation cue. This current is always present off the coast of Western Australia and has a consistent directionality counter to the lobsters’ migration. Lobsters can detect current flow direction via chemosensory (smell) and/or mechanosensitivity (feel). The setae, tiny sensory hairs, covering many parts of the lobster body, sense minute changes in odour plumes and water chemistry, as well as water motion. The whites may orientate their migration by sensing a chemical signal unique to the northern breeding areas, and/or orientating themselves against the south-flowing water movement of the Leeuwin (de Lestang and Caputi 2015). It is therefore interesting that, some lobsters tagged south of Rottnest, where the Leeuwin Current is much weaker, have been returned after migrating southwards (not northwards which is the norm). If currents are used to orientate in deeper waters, it is possible that the northward flowing Capes Current, which can be quite strong south of Rottnest in late Summer, may confuse the lobsters into thinking this is the direction back to their main breeding grounds.
At the completion of their deep-water migration, the whites appear to turn inshore again where they settle on reefs in depths of around 35 – 75 m. The latitude at which lobsters finish their migration has been correlated with the strength of the Leeuwin Current. In years when the Leeuwin current is stronger, the lobsters finish their migration further south (de Lestang and Caputi 2015). In some years, this has meant migration has ceased 100s of km short of their “normal” distribution (de Lestang and Caputi 2015). This is thought to be the result of hydrodynamic drag caused by high-velocity currents slowing the lobsters’ movements (like riding a bike into the wind), rather than impacting the duration of migration.
Why North?
There is a clear benefit to the overall migration being in a northward direction. During the 9-11 month larval phase of the western rock lobster’s life, there is a general redistribution of larvae downstream by the same Leeuwin Current that the lobsters migrate into. Additionally, modelling shows that larvae released in the northern parts of the species’ distribution have greater settlement success rates. Therefore, a mass migration towards the north will redistribute adults along the coast and, allow the release of eggs from more northerly parts of the lobster’s distribution (de Lestang 2014).
This particular migration path along the continental shelf edge also places the whites in an area on the ocean side of the west coast’s westernmost reef systems. Anecdotal evidence from fishers indicates that this area is dominated by sand and silt habitat, with very few of the lobster’s natural predators, such as octopus and demersal fish, making it ideal for rapid migration with relatively low levels of mortality (de Lestang 2014).
High mortality rate
It is thought that migrating white lobsters, due to the fact they are more exposed to predators as they walk over bare sandy areas, have a higher rate of natural mortality than do similar-sized red lobsters who remain on and around reef systems.
Catchability
While migrating, the whites’ behaviour is markedly different to the residential reds’. Anecdotal evidence from fishers indicates that these animals are far more catchable during this life stage. This is often attributed to their recently moulted state and, as a consequence, their greater food requirement. Additionally, because these lobsters are migrating over large distances, their general increased activity and net movement will also likely increase their catchability as they are, 1) more likely to come across a pot (or a bait plume), and, 2) they will have higher energetic demands, and therefore increased food requirements. Research on lobster movement has indicated that whites move at significantly greater speeds than residential animals. When foraging, juvenile lobsters have been found to move on average 1 m per minute, compared to similarly-sized migrating lobsters which have been tracked moving at speeds between 3.3 and 11.33 m per minute, with an average speed of 7.4 m per minute (Melville-Smith and Beale 2009). Moving at this increased pace will have a significant energetic toll, which may result in increased attraction to bait and hence catchability. Additionally, migrating lobsters cover barren habitats with very little reef or structure. Therefore, lobster pots may have greater catchability in these environments, as they provide an attractive refuge for lobsters in which to shelter (see Video 2 below).
Filming the behaviour of reds and whites
Interestingly, a study that sought to investigate differences in reds’ and whites’ behaviour, found no difference in the catchability of the two life stages (Konzewitsch 2009). The study found that the same proportions of reds and whites that approached a pot, subsequently entered it; i.e. after approaching a pot the whites were no more likely to enter the pot than the reds (Konzewitsch 2009). However, this study also found that whites were less cautious compared to the reds.
The study used pots with lights and cameras to film lobster pot behaviour. To test whether the lights impacted catch rates, control pots were also fished without cameras and lights. The pots with and without lights caught comparable numbers of lobsters in the whites. However, during the reds, pots with lights caught significantly fewer lobsters than pots without lights (Figure 2). This suggests that white lobsters are not deterred by lights as they are when red. This is also in line with anecdotal evidence that whites are less affected by the lunar cycle than reds, who are significantly less catchable during the full moon. This may be reflective of their increased energetic demands and resulting desire to feed. Therefore, although reds are just as likely to enter a pot as whites, the whites may still be more catchable due to their being less reluctant to approach a pot.
“Backpacks” to track the whites
Between 2004 and 2007, a study was conducted to investigate the movement patterns of the whites (Melville-Smith and Beale 2009). A total of 135 white lobsters had data storage tags attached to their backs, that were capable of recording temperature and pressure (Figure 3). These “backpacks” were designed so that they didn’t affect the lobster movement, but would float to the surface when the lobster moulted and were therefore able to be collected on beaches by the public so the data could be retrieved. The results of this study indicated that the whites’ movements, as with the reds, are nocturnal, with migration only occurring at night. This was also supported by the above-mentioned behaviour study, which observed no lobsters outside the pots between sunrise and sunset (Konzewitsch 2009). This is in contrast to many other migratory spiny lobster species, which continue their migration during the day. However, there is agreement among commercial fishers and scientists that on occasions the whites migrate during the day, with pots continuing to catch during the daylight hours (additionally see Video 1 above). This research also found that migration is constrained by the moon, with moonrise ending the nightly migration. However, the periods of activity became longer as they migrated deeper where moonlight was less influential.
References