Silver Gulls at Blackmans Bay Beach, Winter 2017
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Water quality and the potential involvement of Silver Gulls at Blackmans Bay Beach, Winter 2017. Report to Kingborough Council, October 2017 Dr Eric J Woehler, BirdLife Tasmania Executive Summary Unusually high numbers of Silver Gulls Chroicocephalus novaehollandiae were present on Blackmans Bay Beach during 2017, with flocks in excess of 2000 birds reported. Local residents expressed concerns to Kingborough Council regarding the potential adverse impacts on water quality and the volume of gull droppings on the beach over a period of several months. Elevated levels of Enterococcus, a potential bacterial source of disease in humans were recorded in water samples from Blackmans Bay. A peak count of Enterococcus occurred following unusual and significant rainfall in December 2016, and another coincided with the presence of Silver Gulls at Blackmans Bay Beach, suggesting that alternative source(s) were likely responsible, rather than the Silver Gulls. The absence of cloacal swabs from Silver Gulls prevents any testing or establishment of a causative link between the birds and high Enterococcus counts. At present, there are no data to support the proposal that the high Enterococcus counts recorded by Council and the DEP at Blackmans Bay Beach originated from the Silver Gulls present during 2017. The cause(s) of the influx of the Silver Gulls to Blackmans Bay during 2017 cannot be determined at this time. Silver Gulls at Blackmans Bay Beach, 11 August 2017. ©Eric J Woehler BirdLife Tasmania Technical Report 2017 - 08 © BirdLife Tasmania 2017. Written E Woehler PhD Reviewed V Ruoppolo DVM Submitted M Steele 24 October 2017 BirdLife Tasmania Technical Report 2017 – 08 Page 2 Context for this Study BirdLife Tasmania was approached by Kingborough Council in August 2017, following numerous enquiries from members of the public who were concerned about the potential human health impacts associated with large (approximately 2000) flocks of Silver Gulls Chroicocephalus novaehollandiae present on the beach. BirdLife Tasmania was commissioned to prepare a report on the potential for human health issues arising from adverse impacts on water quality in the Derwent River and from the quantity of gull droppings on the beach. A meeting was held on site on 11 August 2017, with a subsequent visit on 17 September 2017; in both cases Silver Gulls were present, albeit fewer birds were present on the second visit. This was the first time that many human residents of Blackmans Bay had observed gulls in high numbers present on a daily basis for an extended period (M Steele, Kingborough Council, pers. comm.). The numbers of Silver Gulls present on the beach increased from mid-February 2017 onwards. This contributed to community concerns regarding the potential for human health issues arising from what was perceived to be an “unusual” event. 1. Regional Silver Gull Population The regional population of Silver Gulls (in addition to Kelp and Pacific Gulls) in southeast Tasmania has been monitored using a standardised annual count in June each year since 1983 (BirdLife Tasmania, unpublished data). In that time, the population has varied between fewer than 3000 individuals to more than 16,000 individuals. The regional populations of all three species of gulls are presently decreasing, with the regional Silver Gull population down 34% from its previous maximum in Winter 2014 (Figure 1). Approximately 15 islands in southeast Tasmania are used for breeding by Silver Gulls (Figure 2). At least five islands in the d’Entrecasteaux Channel are used by Silver Gulls: Arch Rock, Green Island, Charity Island, Curlew Island and Little Southport Island. The estimated total breeding population for these islands is approximately 400 breeding pairs of Silver Gulls (BirdLife Tasmania, unpublished data). 600 400 200 % original datum 0 200 100 © BirdLife Tasmania % original datum 0 1980 1985 1990 1995 2000 2005 2010 2015 2020 Winter (mid-point 3Y running mean) Figure 1. Regional (southeast Tasmania) population data (expressed as 3-year running means) for Silver (red), Kelp (green) and Pacific Gulls (blue). No counts were undertaken 1990 to 1994 inclusive. Source: BirdLife Tasmania, unpublished data. BirdLife Tasmania Technical Report 2017 – 08 Page 3 2. Silver Gull Breeding Season In southeast Tasmania, the breeding season for Silver Gulls extends from approximately mid- to late- August until January, depending on the late-Winter and Spring weather conditions. Females lay 3 or 4 eggs over a 6 to 8 day period. Incubation of the eggs is approximately 3 weeks and the chicks fledge (leave the nest at 4 to 5 weeks of age, and fly at 6 – 7 weeks of age. Within a colony, the eggs are laid over an extended period of 4 to 6 weeks, reflecting different arrival times of breeding birds to the colony. The species is highly asynchronous, resulting in the extended breeding season. Individual Silver Gull colonies in southeast Tasmania are deserted during February as adults and chicks leave the colonies and move to adjacent beaches and foreshores where the birds will forage and roost (rest) until the following breeding season. Figure 2. Silver Gull breeding locations (base map from Brothers et al. 2001), with additional sites mapped based on BirdLife Tasmania surveys to 2015/16 (BirdLife Tasmania unpublished data). 3. Blackmans Bay Silver Gull Counts Data from the annual Winter Gull Counts conducted by BirdLife Tasmania for Blackmans Bay Beach in toto are shown in Table 1. These counts are undertaken on the Sunday of the Queen’s Birthday long weekend in early- to mid-June annually, and counts of all species of gulls are obtained. The Blackmans Bay Beach counts cover the entire length of the beach. For the period 2009 to 2016 inclusive (ie excluding 2017), the mean number of Silver Gulls recorded on BirdLife Tasmania Technical Report 2017 – 08 Page 4 Blackmans Bay Beach in total was 43 ± 36 birds. Including the 2017 count (1000 birds), the mean is 150 ± 321 birds. For the period 2009 to 2016 inclusive, the numbers of Silver Gulls present on Blackmans Bay Beach represented fewer than 1% of the annual estimated regional population of Silver Gulls in southeast Tasmania (mean 0.6 ± 0.5%). Including the 2017 data, the mean for 2009 to 2017 increased to 1.5 ± 2.9% of the annual estimated regional population of Silver Gulls in southeast Tasmania. The 2017 count at Blackmans Bay Beach comprised approximately 9% of the annual estimated regional population of Silver Gulls in southeast Tasmania in Winter 2017 (BirdLife Tasmania unpublished data). Winter Silver Gull Proportion of SE Count Tasmania population % 2009 79 1.1 2010 36 0.4 2011 115 1.2 2012 15 1.2 2013 17 0.1 2014 20 0.1 2015 41 0.3 2016 24 0.2 2017 1000 9.2 Table 1. Numbers of Silver Gulls present on Blackmans Bay Beach from BirdLife Tasmania’s annual Winter Gull Counts and the proportion of annual estimated regional populations, expressed as a percentage (BirdLife Tasmania unpublished data). 4. Water Quality Data, Blackmans Bay South The recreational water sampling in undertaken by Kingborough Council in conjunction with the DEP. The Derwent Estuary Program (DEP) provided data on Enterococcus counts from water samples collected from Blackmans Bay South (Table 2) for the period December 2011 to June 2017. The Kingborough Council provided data on Enterococcus counts from water samples collected from Blackmans Bay South (Table 3) for the period May 2017 to October 2017. Further details of the sampling regime (typically monthly from December to March each summer) and the analytical procedures can be found at https://www.derwentestuary.org.au/beach-watch/. The monthly data for Enterococcus counts (expressed as CFU/100ml) from Blackmans Bay Beach South and annual Winter Gull Counts from Blackmans Bay Beach for the period December 2011 to June 2017 are shown in Figure 3. Overall, the Enterococcus count at Blackmans Bay Beach has increased in the period December 2011 to March 2016, and significantly so during the 2016/17 season (Table 2). A high Enterococcus count in December 2016 was attributed to a significant rainfall event that resulted in poor water quality at most beaches in the Derwent River. In addition, the variances in the seasonal data have also increased, reflecting greater variability in the Enterococcus count data. The reasons for this increased variability are presently unknown, and it would be useful to compare the Blackmans Bay data with other data sets for the Derwent Estuary to see if similar increases in Enterococcus counts and the variances have been obtained elsewhere over the same time period. BirdLife Tasmania Technical Report 2017 – 08 Page 5 Season Enterococci Blackmans Bay Beach South 2012/13 26 ± 22.2 2013/14 38 ± 47.8 2014/15 44 ± 69.8 2015/16 31 ± 37.4 2016/17 221 ± 493.3 Table 2. Enterococcus counts from Blackmans Bay South water samples. Source Derwent Estuary Program (DEP), unpublished data. The counts are expressed as seasonal means of monthly samples collected from December to March each summer. The monthly counts are expressed as CFU/100ml. Date Enterococci Blackmans Bay Beach South 16-05-2017 20.0 22-05-2017 121.0 07-06-2017 262.0 20-06-2017 496.0 27-06-2017 135.0 04-07-2017 107.0 18-07-2017 3609.0 26-07-2017 10.0 01-08-2017 450.0 09-08-2017 183.0 16-08-2017 10 22-08-2017 10.0 29-08-2017 122.0 05-09-2017 64.0 09-09-2017 183.0 12-09-2017 10.0 20-09-2017 63.0 26-09-2017 10.0 03-10-2017 10.0 Table 3. Enterococcus counts from Blackmans Bay South water samples 2017. Source Kingborough Council unpublished data. The counts are expressed as CFU/100ml.