Factors affecting colour change in ‘white’ western rock lobsters, Panulirus cygnus

Abstract

At 4–5 years old, western rock lobsters migrate offshore to the fishery, soon after their colouring has changed from red to a pale pink colour. Although a large number of these animals (known as ‘whites’) are landed by the commercial fishery during November to January each year, they are less popular with consumers in the Japanese market, and so have a lower market value than the ‘red’ nonmigrating recruits. ‘White’ rock lobsters darken in colour without moulting, so that by February, virtually all are classified as ‘reds.’ This study examined methods of increasing the value of the catch by speeding up their change back to red while held live in tanks after being caught. The first experiment tested the effect of background colour (three treatments) and diet (six treatments); the second tested the effect of colour of incident light (three treatments), period of light exposure (two treatments), and light intensity (two treatments). In both experiments, tanks kept in total darkness were used as controls. Changes in lobster colouration over the experiment were recorded in Munsell Colour Notation, using a colorimeter. The classification tree method was used to define the colour represented by different colour components, making up production categories of ‘white,’ ‘coral,’ and ‘red’ lobsters. Treatments were gauged in terms of their ability to speed up the natural colour change from white to red. Few of the treatment combinations were successful in producing ‘red’ lobsters and, except for those held on a white background, the lobsters darkened in colour from a white to a coral colour. The rate of change within a moult was no different from the rate in the field, although field lobsters generally attain the more sought-after red rather than coral colour. Of the treatments tested, a dark background with high light intensity levels produced the most optimally (red) coloured lobsters. Most of the experimental treatments (14 of 16) recorded a further progression towards a red colouration after moulting. While the intermoult durations of the lobsters held under most treatments were similar, those held under high light intensity (5–30 mmol quantum m −1 s −1) had significantly shorter intermoult periods ( P<0.01). However, none of the treatments produced red lobsters more quickly than in the wild, and the research did not therefore produce commercial benefits.