Shelter limitations reduce forage frequency by affecting forage motivation in juvenile Pacific abalones (Haliotis discus hannai), but not in adults

Behaviours serve as indicators of the status of, and proxies for the welfare of, cultured specimens. In the abalone farming sector, many factors are known to influence abalone activity, but little is known about how shelter limitations affect behaviour. In this study, we investigated the behavioural responses of juvenile and adult abalones to shelter limitations and the relationship between abalone size and activity level. Cohorts of 139 adult and 272 juvenile Pacific abalones (Haliotis discus hannai) of farm origin were observed under laboratory conditions during a 2-week period with a high level of shelter availability and a 2-week period with limited shelter. We recorded the locomotion under the shelters in the daytime, their positions in aggregations just before the forage time window, and the number of foraging nights. Our data analyses confirmed that in conditions of shelter limitation, juveniles showed increased forage frequency variation, and both active and inactive showed juveniles decreased activity levels. In contrast, adults showed decreased forage frequency variation, and while active adults showed decreased activity levels, inactive adults showed increased activity levels. We found that shelter limitations decreased the forage frequency of juveniles by affecting their forage motivation, not by restricting their access to forage.The results improve our understanding of the differences in Pacific abalone behaviours at the juvenile and adult stages and provide us with valuable information for the future design of abalone culture tanks and cages.