How old are whites?
Lobsters are approximately 5 years old (4 years after settling as a puerulus) when they enter their “whites” phase. Catch prediction modelling has demonstrated that catches of white lobster are strongly related to the puerulus settlement that occurred four years earlier (Caputi et al. 1995, de Lestang et al. 2009). Because water temperature is known to impact age at maturity in western rock lobsters, and the whites phase occurs right before sexual maturity, it is likely that water temperature also impacts the age of “whites”. However, as the aforementioned modelling was restricted to the southern and central areas of the fishery, excluding the warmer northern locations, including the Abrolhos Islands, it is unknown how the age of “whites” varies with water temperature.
How big are whites?
The size of white lobsters varies along the coast. Migrating animals are larger towards the southern end of the fishery and smaller in the north, on average 84.1 mm carapace length in Fremantle, compared with 77.1 mm carapace length at the Abrolhos Islands. (de Lestang 2014). This variation is unlikely to be the result of varied growth rates. Research indicates that lobsters grow faster in warmer waters, and as there is a decrease in mean water temperature with increasing latitude, animals of the same age in the south should be smaller than those in the north. This latitudinal variation in the size of whites must therefore be the result of “whites” being younger in the north, and older in the south.
Climate change
Water temperature may also be responsible for the mean size of migrating whites reducing over time (Caputi et al. 2010). Between 1974 and 2006 the mean size of whites decreased at a rate of 0.11 mm/year (Figure 2). Further analysis found that this reduction was significantly correlated with increasing water temperatures over the same period. As warmer water temperatures are associated with increased growth rates, the reduction in the mean size of whites is likely due to warmer water causing lobsters to migrate younger.
Bigger whites in deep water
There is also a change in the mean carapace length of the whites with depth, with those inshore generally being smaller than those offshore (Caputi et al. 2010). This is likely the result of lobsters undergoing two or more migrations. The first small offshore whites migration, from shallow to deeper water would involve smaller lobsters from shallow water. The second whites migration which typically initiates from deeper water (and hence larger lobsters) is usually an extension of the offshore movement undertaken from shallow water coupled with a longer northward migration (de Lestang 2014). Therefore, most of the deeper water whites will be at least one year older, and hence larger, than those migrating from the shallows. Alternatively, as cooler water temperatures have been associated with a larger size at migration (see above), there may be a temperature component to this pattern, as water temperature usually decreases with depth.