Abstract
The size at which sexual maturity is reached is a key population parameter used to guide the setting of minimum legal size limits in fisheries. Understanding spatial and temporal variations in size at maturity is fundamental to management because the relationship between size at maturity and minimum legal size limits affects the fraction of the mature population biomass that is harvested, and resulting egg production, larval settlement and recruitment. This study measured the size at maturity of female Southern Rock Lobster (Jasus edwardsii) across South Australia between 1991 and 2015 in relation to known oceanographic characteristics, surface and subsurface temperature data, and relative changes in lobster abundance. There was pronounced north to south spatial variation in estimates of size at maturity. Larger average size at maturity was recorded in warmer north-western areas of the fishery relative to the cooler waters of the south-east. Estimates of size at maturity also differed over 25 years across the fishery. However, the nature of temporal responses varied spatially, and were more consistent with variations in surface and subsurface water temperature at local-scales than changes in lobster density. In the well-mixed waters of the north-western, western and south-eastern parts of the fishery, relatively high rates of increase in sea-surface temperature and size at maturity were recorded since 1991, indicating that size at maturity may be responding to ocean warming associated with global climate change. In more central parts of the fishery, contrasting temporal signals in sea-surface temperature (positive) and bottom temperature (negative) indicated increases in upwelling strength over the study period, and formation of a bottom cold pool below a warm surface layer, with corresponding decreases in size at maturity recorded. The spatio-temporal changes in size at maturity measured in this study highlight the need for oceanographic information to be integrated into future stock assessment models to enhance harvest strategy development, allow timely adaptive management decisions and increase the resilience of fisheries to the impacts of climate change.
Highlights
Fisheries management requires stock assessments to account for variations in the life history traits of fish populations that alter the way they respond to fishing pressure [1][2]
While minimum legal size limits are often set, understanding spatial and temporal variations in size at maturity across a fished stock is crucial to fisheries management because the relationship between minimum legal size limits and size at maturity is intrinsically linked to the fraction of mature biomass that is harvested, and resulting egg production, larval settlement and recruitment [7][8]
The study area encompasses waters targeted for fishing of Southern Rock Lobster by the South Australian Rock Lobster Fishery
Summary
Fisheries management requires stock assessments to account for variations in the life history traits of fish populations that alter the way they respond to fishing pressure [1][2]. The division into two zones reflects ecological and biological characteristics that differ in both regions in relation to habitat [12], growth [13] and recruitment [14]. Fishing in both zones is managed using a suite of controls including limited entry, vessel-size and power restrictions, protection of ovigerous females, and seasonal and spatial closures. Individual minimum legal size limits that reflect spatial differences in growth [13] and size at maturity [17][18] have been set in each zone to protect brood stock and maintain egg production. In the Northern Zone, minimum legal size limits have changed over time, increasing from 98.5 to 102 mm CL in 1995, and further to 105 mm CL in 2000 [15]
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