Studies on recruitment in the Western Australian rock lobster Panulirus longipes cygnus George: density and natural mortality of juveniles

Abstract

Density of juveniles on shallow coastal reefs was measured by a single census mark-recapture method, samples being taken by baited traps. Verification that marked and unmarked individuals were equally vulnerable to trapping was obtained by double marking and direct underwater checking. Very little dispersal or migration of the juveniles occurred from January to October, but in November and December the larger specimens moved to deeper water while smaller juveniles (age group 2+ years) appeared on the reefs by January. In a test area away from the centre of the coastal range of this species, density of juveniles at the beginning of each of the past five years fluctuated greatly from one year to another (ranging from 5385 per ha in January 1967, to 1212 per ha in January 1969). Natural mortality of juveniles varied with density (M = 0.499D₀ × 10^-3), shelter possibly limiting the carrying capacity of a reef. In aquaria, cannibalism of newly moulted rock lobsters has been shown to increase with crowding, despite surplus feeding.

Density-independent factors operating upon planktonic larvae and densitydependent factors limiting survival of juveniles play complementary roles in determining the level of recruitment to the adult stock. In the centre of the population range, the second set of factors is of major importance; towards the periphery of the range, the first set of factors is the more important.

Catchability declined with water temperature in autumn and increased in spring. In January of five successive years (temperature 22.2-23.2°C) catch per unit effort was directly proportional to population density (c/g = 5.05D × 10^-3. However, catchability rose from 2.5 &3215; 10^-3 in January 1966 to 15.8 × 10^-3 in January 1969, possibly due to a decline in the availability of natural food.