A Comparative Study on Two Territorial Fishes: The Influence of Physical Enrichment on Aggressive Behavior

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.

Call Localization of Fat Greenling Hexagrammos otakii Using Two Stereo Underwater Recorders

A recent study revealed that fat greenling Hexagrammos otakii produces call sounds during the spawning season; the number of fish calls increases on the spawning day. However, it is unknown where and how many individual fat greenling produce fish calls. This behavior of fat greenling could be estimated by localizing the fish calls. We propose a sound localization method that uses two stereo recorders, which can then be used to estimate two time differences of arrival for each fish call. First, the calls of fat greenling are recorded by two stereo recorders for 3 weeks during the spawning season. Second, the calls of fat greenling are detected from the recorded data by the automatic detection algorithm, which uses the acoustic features of the fish call, and the positions of fish calls are estimated by the proposed sound localization method. The analysis of the recorded data by using the proposed method localized most of the fish calls near the spawning bed. Furthermore, the number of calling individuals may be estimated by analyzing the relation between the time the fish call is produced and its localized position.