Caspian Terns as a Sentinel Species—September KRMB Meeting Report

A good-sized audience of members and visitors gathered for our September meeting: Caspian Terns as a Sentinel Species for the Southern Metropolitan Coastal Waters presented by Dr Nic Dunlop. Caspian Terns (Hydroprogne caspia) inhabit coastal and inland waters in sub-tropical and temperate regions around the world. In south-western Australia, Caspian Terns nest in small colonies along the coast. One of the biggest colonies, consisting of around 60 pairs, nests on Penguin Island. Initially they nested on Cormorant Island until it became too overgrown, then on Seal Island, then the colony moved to Penguin Island about four years ago. Nic and his team have been studying this colony and use walk-in nest traps to capture birds for study and banding. Data from their study showed that outside of the breeding season the colony moved to the Peel Harvey Estuary (99 per cent of sight/digital photo recoveries have been from the Peel Harvey Estuary). They possibly also hunt in this area during the breeding season.

Estuary systems such as the Swan River Estuary and the Peel-Harvey Estuary are highly eutrophic as a result of high nitrogen and phosphorus inputs from industry, land development and agriculture. As a consequence of this, the Peel-Harvey system suffers from algal blooms, high organic carbon levels, anoxic conditions and toxic mono-sulphidic black ooze which can result in fish kills. Caspian Terns are a fish-eating predator and catch prey species by plunge diving in shallow waters and catch their prey in their beaks. As such, the Penguin Island/Peel-Harvey Estuary population of Caspian Terns appeared to provide a good opportunity to evaluate the effectiveness of using predator species as ecological indicators to monitor the health of coastal and estuarine waters in the Perth region.

The diet of the Caspian Terns was investigated using three methods: identification of fish regurgitated by trapped adults; identification from the hard parts (scales, fins or otoliths) in regurgitated pellets; and identification from telephoto shots of birds with fish in their beaks. The data showed a predominance of shallow-water, demersal fish such as Hardyhead, Herring, Striped Trumpeter, Yellow-finned Whiting, King George Whiting and Sea Mullet (50 per cent of diet). The study used carbon and nitrogen stable isotope ratios to determine the characteristics of the foraging habitat. The fifth tail feathers (R5) of captured birds were sampled from a total of 39 adults during banding operations in 2012 and 2016 and analysed for stable isotope ratios and heavy metals. The results for stable isotope ratios showed that prey species are mainly consuming plant species and that adult Caspian Terns foraged in seagrass/benthic algal mat food chains in estuaries and coastal waters. The results for chicks showed that they were provisioned more from seagrass closer to the breeding colony. The average nitrogen value of 11.75 for Caspian Tern adults indicated a low trophic level for a fish-eating predator; by comparison the mean for Crested Terns from Penguin Island ranged from 12.97 to 14.77.

When the R5 feathers were analysed for heavy metals, the results for mercury in adult feathers show a high frequency of mercury concentrations above 0.9mg/kg. Expected levels for uncontaminated waters are 0.7mg/kg in our region. Fitness impacts in sea birds have been reported above 5mg/kg and two birds from Penguin Island were found to have levels above 5mg/kg. One of these birds was observed dying in Peel-Harvey three months after sampling. The mercury concentrations were not normally distributed in adult Caspian Terns—indicating different foraging patterns. Mercury levels were strongly correlated with the nitrogen stable isotope ratio, indicating a relationship between mercury exposure and mineralised nitrogen, i.e. eutrophic waters. The more anoxic the environment the higher the mercury levels in the feathers due to methyl mercury formation. Selenium can replace mercury in vertebrate animals, reducing the rate of uptake or the assimilation of mercury. Selenium is relatively abundant in seawater and marine prey species, but less so in estuarine waters. For the Caspian Terns there was a strong correlation between selenium and mercury concentrations in feathers. The high selenium availability from marine prey species is likely to provide the terns some protection from mercury.

The study concluded that the Caspian Tern colony nesting at Penguin Island and foraging in the Peel-Harvey Estuary at other times showed significant potential as indicators of the health of coastal and estuarine waters of the Perth region. Ongoing banding and feather analysis could be used in a monitoring programme for the Peel-Harvey Estuary.

It was a very interesting presentation and the audience showed their interest in the topic by exploring the subject further with several pertinent questions for Nic. For those readers who are interested in this topic, a full scientific paper containing a lot more detail was published in Marine Ornithology 45: 115–120 (2017). ⇐link

Colin Prickett