Juvenile abundance both mirrors and masks a settlement pulse of the rock lobster Jasus edwardsii

Abstract

Interannual changes in juvenile abundance can validate interannual changes in levels of settlement on collectors and address such postsettlement processes as density-dependent mortality. A strong pulse of postlarval Jasus edwardsii recruitment took place near Wellington in 1991–92, followed by much lower settlement. To determine whether juvenile abundances by age would reflect this pulse and so validate the collector catches, divers estimated quarterly the abundance and size of juveniles at six sites, from 1993 to 2000. Juveniles younger than 2 years were uncommon. At four sites, 2+ and 3+ lobsters were present in large proportion until March 1995. From June 1995, most were at least 3+. These data were consistent with progress of the 1991–92 modal group through the age-frequency distributions, after which juvenile abundances declined, thereby reflecting the settlement data. The other two sites differed in that 2+ and 3+ lobsters became more abundant with time. The reasons for the disparity are unknown, but clearly sufficient site replication is needed if juvenile year-class strengths are to reflect settlement patterns accurately. Where pueruli are numerous enough to register on collectors, crevice collectors appear to be more accurate and cost-effective in following recruitment highs and lows than dive surveys.