Spatial variation in body size and reproductive condition of subtidal mussels: Considerations for sustainable management

Population characteristics such as body size and reproductive condition are widely used by industry and resource managers as criteria for harvesting commercial species. Given the broad-scale approaches commonly adopted by managers to evaluate stocks, any spatial heterogeneity in the structure and functioning of those stocks may result in inaccurate assessments, interpretation and inappropriate management. Spatial heterogeneity in body size and reproductive condition has been shown in intertidal populations of Mytilus spp., but no assessment of subtidal structure has been made despite the importance of these populations as a mariculture resource. A spatially stratified sampling programme was used to test hypotheses of differences in bed structure depending on position within the bed. Commercial gears were used to harvest seed mussels and two condition indices were tested to identify a rapid accurate approach to evaluate reproductive condition for resource managers. Differences in the size and condition of mussels were dependent on spatial position within the population, with mussels exhibiting strong seasonal growth patterns. Edge-zone mussels showed both greater reproductive condition and body size than bed centre mussels. Reproductive condition also covaried with body size. Differences in spatial structure and function have the potential to confound stock assessments if appropriate sampling programmes are not utilised. Appropriate survey and reproductive condition assessment methods should be used to support subtidal stock persistence and optimal exploitation practices.