Intraspecific aggression is detrimental to body/fin damage, physiological stress, and other problems in aquaculture. Environmental enrichment has been proposed to have positive effects on fish aggressive behavior, physiological stress, and fish welfare, but there are mixed results. Here, we examine the impact of physical enrichment levels (i.e., the intensity of physical enrichment) on aggression in black rockfish (Sebastes schlegelii) and fat greenling (Hexagrammos otakii). Generally, with the increase in the enrichment level, the frequency of the aggressive behavior of black rockfish gradually decreased. In contrast, a non-monotonous effect of the enrichment level on aggression was observed for fat greenling, with low and intermediate levels leading to no or more aggression, while a high enrichment level reduced aggression. After three days, the high-level enrichment groups in both rockfish and greenling reached social stability (i.e., a relatively stable social structure indicated by lower aggression), while aggression in the other groups continued increased. These results show the significant regulatory effect of enrichment levels on the aggressive behavior in both black rockfish and fat greenling. This study may promote the development of environmental enrichment measures, and it provides useful information for reducing fish aggression and improving fish welfare in aquaculture.

Fish often undergo food shortages in nature, especially for juveniles that are relatively vulnerable in most aspects. Therefore, the effects of food deprivation on fish behavior and physiology are worth exploring. Here, we investigated the behavioral and metabolic adaptations of the juvenile black rockfish Sebastes schlegelii to different durations of food deprivation. In this study, three treatments were set: control group, short-term food deprivation (STFD) group, and long-term food deprivation (LTFD) group. The rearing lasted for six weeks. During this period, videos were recorded three times per week to assess the locomotor activity and aggression. After this, the fishes’ boldness, neophobia, and aggressiveness were assessed using different behavioral assessment devices, while the standard metabolic rate (SMR) was measured by flow-type respirometry. In general, the values of the four indicators (swimming time, the number of turns, movement number, and attack number) for the STFD group were significantly higher than those for the control and LTFD group during the same period of rearing. In a subsequent personality assessment, the STFD group was observed to be significantly bolder in an assessment measuring boldness than the control and LTFD group (indicated by time in the circle, swimming time, number of times through the circle, and frequency in the circle). The LTFD group was observed to be more exploratory in the assessment of neophobia (indicated by duration in the exploratory area, distance from novelty item, and frequency of stay in the exploratory area). Indeed, the LTFD group was observed to be significantly less aggressive in the assessment of aggressiveness than the control and STFD group (indicated by attack number, attack range frequency, first attack, and winner). SMR was noted to be significantly higher in the STFD group than in the control and LTFD group. In conclusion, this study firstly reported the effects of food deprivation duration on the personalities of black rockfish, as well as the behavioral and physiological mechanisms. Thus, we hope to provide insights into the work of stock enhancement.