The roles of food supply and shelter in the relationship between fishes, in particular Cnidoglanis macrocephalus (Valenciennes), and detached macrophytes in the surf zone of sandy beaches

Abstract

Regular night-time sampling was conducted over a 1-yr period in the surf zones of two adjacent sandy beaches, located in the Marmion region near Perth, Western Australia. The catches of most of the 37 fish species were dominated by juveniles. An earlier series of daytime and night-time nettings at the same sandy beach sites established that the abundance of these surf-zone fishes was positively correlated with the volume of drift macrophytes. This relationship reflected the provision by the drift weed of a rich invertebrate, primarily amphipod, food supply and a refuge from diurnal predation by the cormorant Phalacrocorax varius (Gmelin). The present study demonstrated a significant positive relationship between the total number of fishes and the volume of drift macrophytes. This relationship was based on sampling performed throughout the year; demonstrating a very persistent association between those two variables. Seasonally also contributed to the observed variation in the number of fish caught. However, the most important result of the present study demonstrated that at night, when the risk of predation and thus the need to seek shelter was low, the abundance of one of the dominant nocturnal and commercially important fish, 0 + -yr-old Cnidoglanis macrocephalus (Valenciennes), caught per netting was also significantly positively correlated both with the volume of drift macrophytes and with the volume of fine red algae and dead seagrass components of the drift macrophytes. However, these two components of the drift have previously been shown to provide an essential habitat and food supply for amphipods, primarily Allochestes compressa Dana which is the major food item for Cnidoglanis macrocephalus. These results provide conclusive evidence that the night-time drift weed-juvenile C. macrocephalus association is related primarily to the food requirements of this fish. There appears to have been no other instances reported of an important commercial species being so dependent on a particular assemblage of drift macrophyte species for its survival.