Seasonal variations in fish species richness in Nelson Bay

Informal report on RLS surveys by RLS survey diver Tom Davis

RLS surveys have been conducted at the Halifax Divesite in Nelson Bay for several years. More intensive surveying has been conducted in the last 18 months, however, due to concern over sand movement causing damage to the site. Surveys have been conducted at the site every 2 months since October 2010 to assess recovery from this disturbance. As an added bonus, these repeated surveys also provide an insight into seasonal variations in fish diversity in the region, which is heavily influenced by the East Australian current.

Surveys have been conducted on 2 transects at Halifax, with each survey starting at a known landmark on the reef. One transect is at a depth of 8m on the Eastern side, and the second transect is at a depth of 11m on the Western side.

Figure 1 shows the trend over the last 18 months in the average, across the 2 transects, of the total number of fish species encountered on each transect, along with average water temperatures over the same interval. Species richness at the site has followed a very clear seasonal trend, tracking changes in water temperature. The East Australia Current is known to sweep a host of tropical species into Nelson Bay over the summer, and the marked seasonality in species richness is primarily due to patterns of recruitment by tropical species, which apparently either emigrate or die-off during the cooler months.

Figure 2: Comparison of average water temperature and average number of species at Halifax

To date more than 250 species of fish have been identified on RLS surveys in the Port Stephens area. Table 1 shows the most common species with temperate and tropical or sub-tropical affinities recorded on transects in Nelson Bay. Note that while many of the tropical/sub-tropical species are only found during summer, some of these are also regularly encountered during winter; for example Parupeneus spilurus and Stethojulis interrupta.

Common temperate species		Common tropical/sub-tropical species
Latin Name	Common Name	Latin Name	Common Name
Hypoplectrodes maccullochi	Half-banded seaperch	Parupeneus spilurus	Black spot goatfish
Notolabrus gymnogenis	Crimson-banded wrasse	Stethojulis interrupta	Cut-ribbon wrasse
Ophthalmolepis lineolata	Maori wrasse	Thalassoma lunare	Moon wrasse
Parma microlepis	White-ear	Chaetodon guentheri	Gunthers butterflyfish
Scorpaena jacksoniensis (formerly S. cardinalis)	Red scorpion cod	Labroides dimidiatus	Cleaner wrasse
Cheilodactylus fuscus	Red morwong	Cheilinus bimaculatus	Two spot maori wrasse
Atypichthys strigatus	Mado sweep	Cirrhilabrus punctatus	Small spotted wrasse
Achoerodus viridis	Blue groper	Pomacentrus coelestis	Neon damsel
Mecaenichthys immaculatus	Immaculate damsel	Halichoeres nebulosus	Clouded wrasse
		Chromis nitida	Yellow-back puller

Table 1: Common Temperate and Tropical Species in Nelson Bay

To date there is little evidence that sand movement at Halifax has had a significant impact on fish species at the site, however there is no question that sand inundation has devastated much of the site’s sponge gardens. Monitoring is ongoing and hopefully in the future the sponge gardens will recover. In the meantime, Halifax is still a fantastic place to see a wide variety of fish species, especially in Autumn!

For more information on diving in Nelson Bay see divingnelsonbay.weebly.com

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