Interactions between Silver Gulls (Larus novaehollandiae) and Southern Bluefin Tuna (Thunnus maccoyii) aquaculture in the Port Lincoln area

Author: Shelley Jane Harrison

Harrison, Shelley Jane, 2010 Interactions between Silver Gulls (Larus novaehollandiae) and Southern Bluefin Tuna (Thunnus maccoyii) aquaculture in the Port Lincoln area, Flinders University, School of Biological Sciences

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The interaction between seabirds, particularly the Silver Gull (Larus novaehollandiae) and the Southern Bluefin Tuna (SBT, Thunnus maccoyii) aquaculture industry was studied to determine how much tuna feed is scavenged and the impacts of this food source on the local Silver Gull population. Previous research indicated Silver Gulls scavenged approximately 2.3% of the feed used by one tuna farming company and this is likely to be the reason the gull population increased rapidly due to an enhanced reproductive output compared to a reference population without access to tuna feed. The aims of the research described in this thesis were: • To quantify tuna feed loss to seabirds from a broader cross-section of the tuna farming industry. • To determine the reliance of the Port Lincoln Silver Gull population on tuna feed compared to gulls at two reference sites with no access to tuna feed. • To determine whether the reproductive output (and thereby, population growth) of the Port Lincoln Silver Gulls was enhanced by tuna feed compared to the reference site gulls. • To trial practical measures to decrease tuna feed loss to Silver Gulls and to control the Silver Gull population by preventing eggs from hatching and adult birds from re-laying. Sixty thousand tonnes of baitfish were fed out to the SBT per annum across the 13 companies’ pontoons (total ~130 pontoons). Extrapolation of observations of feed loss suggested about 1.3% or 790 tonnes of feed was scavenged by seabirds annually, of which ~570 tonnes (72%) was consumed by Silver Gulls, with around 37,000 Silver Gulls present across the total tuna lease area each day. The proportion of the Port Lincoln area Silver Gull population that is reliant on this tuna feed depends on the method used to estimate dietary intake and varies between 28% (based on pellet/prey collection and stomach flushing) and 72% (tuna farm feed loss estimations). The availability of tuna feed has profound effects on the population of gulls around Port Lincoln. Firstly, the breeding season has been considerably protracted so it now mimics the tuna farming season (January to September/October) with around three peaks in nesting activity per season, compared to reference populations that initiate breeding months later and usually have one, or possibly two, nesting peaks. Secondly, the reproductive output (average number of fledging chicks per nest) of Port Lincoln gulls was 25-50% greater than that of the reference gulls, which coupled with the protracted breeding season further enhances reproductive output. These changes in breeding have, at least partly led to an exponential increase in breeding gulls from 3,300 pairs in 1999 to as high as 27,800 pairs in 2005. However, a proportion of this over abundant population may be caused by gulls migrating to Port Lincoln to exploit the relatively new food source provided by the tuna (and potentially other seafood) industry. This large population causes social and nuisance problems, particularly at the end of the tuna farming season when the gulls migrate into the urban areas of Port Lincoln in search of food, which unfortunately coincides with the busy summer tourist season. The problems associated with this inflated gull population can be significantly reduced by a number of control measures. Feeding the tuna with frozen blocks of baitfish feed in enclosed cages within the pontoon reduced feed losses to 1.08% compared to a 2.38% loss from shovelling chilled or thawed baitfish. Farms that prefer to continue shovel feeding could significantly reduce baitfish losses to gulls by using a scaring device, particularly the float on a rope method, which reduced losses by 87%. Another approach is to control the reproductive output of gulls. Spraying eggs with vegetable oil reduced the hatching rate to 0%. The outstanding success of this trial in 2006 led to the Australian Southern Bluefin Tuna Industry Association (formerly Tuna Boat Owners of South Australia) and the Department for Environment and Heritage implementing an egg oiling program in 2008 to manage the gull population. This project has shown that although the feeding practices of the SBT industry contributed to the exponential increase in the Silver Gull population at Port Lincoln, the industry can (and in some cases has) introduced measures to reduce the feed losses that underpinned the local success of this wonderfully adaptable scavenger species. At an industry-wide level, the Australian Southern Bluefin Tuna Industry Association has played a significant role (with the Department for Environment and Heritage) in the 2006 and 2008 egg oiling programs. This project has been an effective example of scientists working together with industry and regulators to describe, quantify and overcome economic, biological and social issues that threatened to put an important regional industry in conflict with a South Australian native species that is often overlooked.

Keywords: aquaculture and the environment,Silver Gull,Southern Bluefin Tuna,marine ecology,seabird

Subject: Biological sciences thesis

Thesis type: Doctor of Philosophy
Completed: 2010
School: School of Biological Sciences
Supervisor: Jeremy Robertson, John Carragher