Using fine‐scale catch predictions to examine spatial variation in growth and catchability ofPanulirus cygnusalong the west coast of Australia

Abstract

Puerulus settlement has been monitored throughout the western rock lobster Panulirus cygnus fishery for nearly 40 years. These data, in combination with indices of effort and water temperature, were used to produce recruitment-catch relationships for each 1° transect of latitude in the coastal part of this fishery from Kalbarri to Cape Leeuwin, as well as at the offshore Abrolhos Islands (total of eight transects). The fine spatial scales of these models provided estimates of certain life history traits that are known to affect lobster catches between adjacent fishing ports. This catch modelling showed that the proportions of 3-and 4-year-old post-settlement lobsters contributing to the catches varied markedly from the southern to northern transects, suggesting that juvenile lobsters grow substantially faster in the warmer northern and offshore waters of this fishery. These proportions provide accurate estimates of juvenile growth rates, which are vital in the construction of location-specific growth algorithms required by the age-structured models used in the management of this fishery. Model estimates of density-dependent mortality were greater in the more densely populated centre of the fishery and markedly lower at the northern and southern limits of this species distribution. Annual increases in fishing efficiency were also found to be lowest at the northern and southern extremes of the fishery and greatest in the centre of the fishery, where technology advances and increased fleet mobility have enabled the fleet to increase efficiency by 1-3% each year. Catchability (q) was found to be most influenced by water temperatures in the cooler southern transects, whereas at the Abrolhos Islands, changes in water temperature produced almost no discernable change in q. The catch modelling was also used to quantify the impact of management changes introduced in the 1993/94 fishing season. Increased protection of female lobsters and an 18% pot reduction resulted in a 3-4% permanent reduction in the catch rates of lobsters throughout most of the coastal fishery, whereas at the offshore Abrolhos Islands, catch rates increased by c. 20%, presumably owing to a reduction in the level of pot saturation.

Climate variability affecting the contranatant migration of Panulirus cygnus, the western rock lobster

“…Time lags (t) of 3 and 4 (or their average) years were applied to the puerulus data since the majority of white migrating lobsters caught between December and January are a combination of lobsters of those ages (Caputi et al 1995;de Lestang et al 2009). The environmental data tested in the above model were restricted to those observations that were derived for areas that corresponded to those occupied by P. cygnus, i.e.…”

“…between November and February the following year (de Lestang 2014). At the time of the migration, the pale-colour migrating lobsters (commonly called whites), on average, are 5 years old (4 years since puerulus settlement) and range in carapace length (CL) from 60 to 90 mm (Melville-Smith et al 2003;de Lestang et al 2009;.…”

Interannual Variations of Wind Regimes off the Subtropical Western Australia Coast during Austral Winter and Spring

“…An annual ''puerulus settlement index'' is derived from a monthly sampling program that looks at the abundance of late larval-stage lobsters (puerulus) settling on inshore reefs along the WA coast, peaking between August and January each year (Caputi et al 2001). This index has always shown a strong correlation with commercial catches of lobsters three to four years later de Lestang et al 2009). Evident from the correlation of SST with the settlement along the WA coast (Fig.…”

Maximum economic yield of the western rock lobster fishery of Western Australia after moving from effort to quota control

“…A feature of the stock assessment and management of the western rock lobster fishery for over 30 years has been catch predictions based on the level of puerulus settlement (post-larval settling stage) recorded 3-4 years earlier [2,3,4]. Over the seven years (2006/07 to 2012/13), puerulus settlement has been below average, with the settlements in 2008/09 and 2009/10 being the lowest recorded since monitoring began over 40 years ago.…”