Importance of adjacent habitats in structuring fish communities within Posidonia australis seagrass beds in S.E. Australia
Within coastal seagrass ecosystems, bed canopy height, shoot density, heterogeneity and blade morphology are commonly attributed to the structuring of resident and transient fish communities, however proximity to adjacent ecosystems, potentially harboring predators or food likely influence fish community structure within the seagrass. Here we describe how distance from a seagrass/reef boundary alters the fish community within two Posidonia australis seagrass beds in Jervis Bay, NSW. Four 60-m long transects, each 15-m wide were sampled at each site into the seagrass beds away from the rocky algal reef. Observations were made at four 15-m intervals i.e. 0-15m (seagrass immediately adjacent to reef), 15-30m, 30-45m and 45-60m along each transect using both a seine net to observe cryptic slower moving fish and GoPro video cameras for faster moving, larger pelagic individuals. Data collection was done over two seasons - Austral winter (August / September 2013) and Austral summer (January / February 2014). Video footage was also analysed for habitat use and behaviour.
Seine net catches revealed lower abundances at 0-15m and an increase in species diversity away from the reef. While some fish species showed no consistent trend with distance others namely Atherinosoma microstomata, Atherinomorus vaigiensis, Siphamia cephalotes, Hyporhamphus regularis, Haletta semifasciata and Neodax balteatus increased steadily with distance. Spotted pipefish (Stigmatopora argus) avoided the immediate edge but were found in greatest abundance at 15-30m and declined thereafter. GoPro video footage showed a greater abundance of schooling juvenile Atherinidae closer to the reef (0-15m and 15-30m) with the larger luderick (Girella tricuspidata), trevally (Pseudocaranx wrighti), seapike (Sphyraena obtusata) and mullets (Mugilidae) more abundant away from the reef (>30 m). Sparidae primarily tarwhine (Rhabdosargus sarba) were sited more frequently closer to the reef. Video analysis documented greater accounts of feeding events, slow among-blade meandering and slower swimming movements > 30 m from the reef. The seagrass immediately adjacent to the reef tended not to be utilized greatly by larger fish, potentially due to wave action disturbing individuals, and washing food material away from the area including reef-derived algae. The reef/seagrass boundary does appear however to offer desirable protection for schooling individuals small enough to take cover within the blades. These dynamics highlight the importance of distance from an adjacent habitat in structuring fish communities and determining fish behaviour. We therefore propose that distance from adjacent ecosystems is an important abiotic component of seagrass ecosystems, necessitating inclusion as a seagrass bed descriptor.