Seasonal variations in fish species diversity in Nelson Bay
RLS surveys have been conducted at the Halifax Divesite in Nelson Bay for several years. However more intensive surveying has been conducted in the last 18 months due to concern over sand movement causing damage to the site. Since October 2010, surveys have been conducted at the site every 2 months, and these surveys provide an insight into seasonal variations in fish diversity in the region.
Surveys have been conducted at 2 sites 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 below shows the number of fish species observed on each survey over the last 18 months. The survey data shows a clear trend, with a rapid increase in species occurring over summer, high species diversity throughout autumn, a rapid decline in species numbers over winter, and low diversity in spring. The data also shows that there is good correlation between the species numbers at the 2 sites, with the Western site consistently having a lower numbers of species.
Figure 2 shows the trend, over the last 18 months, in the average number of species across the 2 sites, plotted on a graph, along with the average water temperatures over the same interval. The graph shows that, as would be expected, changes in species diversity at the site follow the same trend as changes in water temperature. As the temperature increases the number of species increases and as temperature falls the species count falls. This of course is all due to the East Australia Current which sweeps a host of tropical species into Nelson Bay over the summer.
Surveys have been conducted at 2 sites 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 below shows the number of fish species observed on each survey over the last 18 months. The survey data shows a clear trend, with a rapid increase in species occurring over summer, high species diversity throughout autumn, a rapid decline in species numbers over winter, and low diversity in spring. The data also shows that there is good correlation between the species numbers at the 2 sites, with the Western site consistently having a lower numbers of species.
Figure 2 shows the trend, over the last 18 months, in the average number of species across the 2 sites, plotted on a graph, along with the average water temperatures over the same interval. The graph shows that, as would be expected, changes in species diversity at the site follow the same trend as changes in water temperature. As the temperature increases the number of species increases and as temperature falls the species count falls. This of course is all due to the East Australia Current which sweeps a host of tropical species into Nelson Bay over the summer.
To date more than 250 species of fish have been identified on RLS surveys in the Port Stephens area. Of these species some are present all year round, and some come and go over the summer months. Table 1 shows the 10 most common ‘temperate species’ seen in the bay, and the 10 most common ‘tropical species’, although some of the tropical species can be encountered throughout the year.
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!