Territorial defense of an excess food supply by an algal grazing fish, ayu

The size of the territory defended by the grazing fish ayu, Plecoglossus altivelis , often increases with decreasing intruder pressure. This territorial extension occurs even after additional growth of the fish reaches a plateau due to a surplus supply of algal food. This raises questions about the ecological advantages of defending an apparently enlarged territory. Enclosures were used to carry out observations on fish maintained as: (i) individuals with algae attached to the whole bottom area (2.4 m 2 ); (ii) individuals with algae covering 0.8 m 2 ; and (iii) with three conspecifics (competitive use) with algae attached to 2.4 m 2 . In the competitive use enclosures, one (occasionally two) fish that exclusively foraged the area appeared in each of the groups. Algal resources were sufficiently renewable that no significant difference in growth rate was detected between the dominants of the competitive groups and the two types of solitary enclosures, indicating that the defended area supplied algal resources that exceeded the dominants’ ability to grow. All the territorial holders realized an increase in relative body weight, suggesting the occurrence of inhibition, which corresponds to Verner’s concept of superterritory. However, as inferred from the pheophytin/chlorophyll  a ratios, algae within a territory showed less senescence, perhaps owing to cropping by the dominant, and this was considered to be an adaptive response by the fish to an unpredictable food supply. In conclusion, the ayu conditionally defends a territory for long‐term benefits, and inhibition is a consequence of facultative territory size and not a cause of it.