Roger Kirkwood

Day roost selection by female little forest bats (Vespadelus vulturnus) within remnant woodland on Phillip Island, Victoria

The day roosting behaviour of the little forest bat (Vespadelus vulturnus), Australia’s smallest bat, was investigated in the context of the planned removal of dead timber within managed woodlands on Phillip Island, Victoria. Between August 1999 and March 2000, 14 female little forest bats were fitted with VHF microtransmitters and tracked to a total of 16 roost trees. All roosts were located in dead timber, 11 in severely decayed remains of eucalypt trees, and five in dead sections of live trees. Roost trees were compared with randomly chosen trees from within the available habitat, for a range of tree characteristics. Female little forest bats selected roosts in trees with dead timber offering many hollows and reduced canopy cover. Furthermore, roost trees were located in areas (0.1-ha plots) with higher densities of these types of trees than in the available habitat. However, there was no difference in the height or diameter of roost trees or roost plots compared with available habitat. Emergence time from roosts was strongly associated with civil twilight (when the centre of the sun is 6° below an ideal horizon), and the number of bats exiting a single roost tree ranged from 1 to 120 (median = 20). Dead trees provide critical roosting habitat and we recommend retention of dead standing trees for conserving little forest bat roosts in managed woodlands.

Drivers and annual estimates of marine wildlife entanglement rates: A long-term case study with Australian fur seals

Methods of calculating wildlife entanglement rates are not standardised between studies and often ignore the influence of observer effort, confounding comparisons. From 1997-2013 we identified 359 entangled Australian fur seals at Seal Rocks, south-eastern Australia. Most entanglement materials originated from commercial fisheries; most frequently entangling pups and juveniles. Using Generalized Additive Mixed Models, which incorporated observer effort and survey frequency, we identified that entanglements were observed more frequently amongst pups from July to October as they approached weaning. Neither the decline in regional fishing intensity nor changing seal population size influenced the incidence of entanglements. Using the models, we estimated that 302 (95% CI=182-510) entangled seals were at Seal Rocks each year, equivalent to 1.0% (CI=0.6-1.7%) of the site population. This study highlights the influence of observer effort and the value of long-term datasets for determining the drivers of marine debris entanglements.

Adaptation and assessment of M-44 ejectors in a fox-control program on Phillip Island, Victoria

This study aimed to adapt M-44 ejectors for use in sandy soils and to assess the feasibility of incorporating the modified M-44s into a long-term fox-control program on Phillip Island, Victoria. M-44s were adapted by burying a plastic cylinder around them, which prevented sandy soil from collapsing and inhibiting the trigger mechanism, and at the same time orientated the foxs mouth vertically over the M-44 to maximise the dose of poison delivered. The fast-acting poison sodium cyanide was used to ensure the collection of fox bodies and any non-target animals. A fox was killed on 78.6% of occasions that an M-44 was triggered. No non-target species triggered M-44s, although rodents and birds occasionally ate the unpoisoned baits. The modified M-44 ejector technique accounted for 19% of foxes killed by all techniques during one year on Phillip Island. To assess whether M-44s were a worthwhile technique to include in the fox-control program on Phillip Island, we compared their catch per unit effort (number of foxes killed per 1000 person-hours) with other control techniques (spotlight shooting, treadle snaring and hunting with fox hounds). Deployment of M-44s with cyanide was labour intensive, due to safety considerations, and cyanide can be used only as a research tool. Future management use of the M-44s would be with sodium monofluoroacetate (1080), so an estimate was made of the catch per unit effort of M-44s with this poison. Results suggest that deployment of M-44s with 1080 is likely to be more time-effective than the other techniques.

Lessons from long-term predator control: a case study with the red fox

Abstract Context. Predator-control aims to reduce an impact on prey species, but efficacy of long-term control is rarely assessed and the reductions achieved are rarely quantified. Aims. We evaluated the changing efficacy of a 58-year-long campaign against red foxes (Vulpes vulpes) on Phillip Island, a 100-km2 inhabited island connected to the Australian mainland via a bridge. The campaign aimed to eliminate the impact of foxes on ground-nesting birds, particularly little penguins (Eudyptula minor). Methods. We monitored the success rate of each fox-control technique employed, the level of effort invested if available, demographics of killed foxes, the numbers of penguins killed by foxes and penguin population size. Key results. The campaign began as a bounty system that ran for 30 years and was ineffective. It transitioned into a coordinated, although localised, control program from 1980 to 2005 that invested considerable effort, but relied on subjective assessments of success. Early during the control period, baiting was abandoned for less effective methods that were thought to pose fewer risks, were more enjoyable and produced carcasses, a tangible result. Control was aided by a high level of public awareness, by restricted fox immigration, and by a clear, achievable and measurable target, namely, to prevent little penguin predation by foxes. Carcasses did prove valuable for research, revealing the genetic structure and shifts in fox demographics. The failure of the program was evident after scientific evaluation of fox population size and ongoing fox impacts. In 2006, the campaign evolved into an eradication attempt, adopting regular island-wide baiting, and since then, has achieved effective knock-down of foxes and negligible predation on penguins. Conclusions. Effective predator control was achieved only after employing a dedicated team and implementing broad-scale baiting. Abandoning widespread baiting potentially delayed effective control for 25 years. Furthermore, both predator and prey populations should be monitored concurrently because the relationship between predator abundance and impact on prey species is not necessarily density dependent. Implications. Critical to adopting the best management strategy is evaluating the efficacy of different methods independently of personal and public biases and having personnel dedicated solely to the task.

Stability of cabergoline in fox baits in laboratory and field conditions

Cabergoline is a potent inhibitor of prolactin release and a potential fertility control agent for foxes. To understand how cabergoline could behave in baits deployed for fox control, we conducted laboratory and field trials to investigate the stability of cabergoline when (1) in solution, (2) injected into a bait (deep-fried liver and Foxoff®) and (3) exposed to a range of environmental conditions, including burial. Cabergoline, dissolved in a 1% acetic acid solution, and its carboxylic acid hydrolysis product can be assayed using high-performance liquid chromatography. When stored at 4°C and at room temperature, cabergoline in solution was stable for up to 36 days. When stored under cool (≤15°C), dry conditions, cabergoline (800 µg) in commercial Foxoff® and deep-fried ox-liver baits was stable for 28 and 7 days, respectively; stability was reduced by increases in temperature (tested up to 40°C) and humidity. Recovery of cabergoline from buried baits exposed to a range of field conditions decreased rapidly in the first week, but after 56 days remained detectable at levels of 6–22% of the injected amounts. This study has important implications for baiting campaigns that use cabergoline for fox control.

Echoes from the past: Regional variations in recovery within a harbour seal population

Terrestrial and marine wildlife populations have been severely reduced by hunting, fishing and habitat destruction, especially in the last centuries. Although management regulations have led to the recovery of some populations, the underlying processes are not always well understood. This study uses a 40-year time series of counts of harbour seals (Phoca vitulina) in the Wadden Sea to study these processes, and demonstrates the influence of historical regional differences in management regimes on the recovery of this population. While the Wadden Sea is considered one ecologically coupled zone, with a distinct harbour seal population, the area is divided into four geo-political regions i.e. the Netherlands, Lower Saxony including Hamburg, Schleswig-Holstein and Denmark. Gradually, seal hunting was banned between 1962 and 1977 in the different regions. Counts of moulting harbour seals and pup counts, obtained during aerial surveys between 1974 and 2014, show a population growth from approximately 4500 to 39,000 individuals. Population growth models were developed to assess if population growth differed between regions, taking into account two Phocine Distemper Virus (PDV) epizootics, in 1988 and 2002 which seriously affected the population. After a slow start prior to the first epizootic, the overall population grew exponentially at rates close to assumed maximum rates of increase in a harbour seal population. Recently, growth slowed down, potentially indicative of approaching carrying capacity. Regional differences in growth rates were demonstrated, with the highest recovery in Netherlands after the first PDV epizootic (i.e. 17.9%), suggesting that growth was fuelled by migration from the other regions, where growth remained at or below the intrinsic growth rate (13%). The seals’ distribution changed, and although the proportion of seals counted in the German regions declined, they remained by far the most important pupping region, with approximately 70% of all pups being born there. It is hypothesised that differences in hunting regime, preceding the protection in the 1960’s and 1970’s, created unbalance in the distribution of breeding females throughout the Wadden Sea, which prevailed for decades. Breeding site fidelity promoted the growth in pup numbers at less affected breeding sites, while recolonisation of new breeding areas would be suppressed by the philopatry displayed by the animals born there. This study shows that for long-lived species, variable management regimes in this case hunting regulations, across a species’ range can drive population dynamics for several generations.

Harbour seals are regaining top-down control in a coastal ecosystem

Historic hunting has led to severe reductions of many marine mammal species across the globe. After hunting ceased, some populations have recovered to pre-exploitation levels, and may again act as a top-down regulatory force on marine ecosystems. Also the harbour seal population in the international Wadden Sea grew at an exponential rate following a ban on seal hunting in 1960’s, and the current number ∼38,000 is close to the historic population size. Here we estimate the impact of the harbour seal predation on the fish community in the Wadden Sea and nearby coastal waters. Fish remains in faecal samples and published estimates on the seal’s daily energy requirement were used to estimate prey selection and the magnitude of seal consumption. Estimates on prey abundance were derived from demersal fish surveys, and fish growth was estimated using a Dynamic Energy Budget model. GPS tracking provided information on where seals most likely caught their prey. Harbour seals from the Dutch Wadden Sea fed predominantly on demersal fish, e.g. flatfish species (flounder, sole, plaice, dab), but also sandeel, cod and whiting. Total fish biomass in the Wadden Sea was insufficient to sustain the estimated prey consumption of the entire seal population year-round. This probably explains why seals also acquire prey further offshore in the adjacent North Sea, only spending 13% of their diving time in the Wadden Sea. Still, seal predation was estimated to cause an average annual mortality of 43% and 60% on fish in the Wadden Sea and adjacent coastal zone, respectively. There were however large sources of uncertainty in the estimate, including the migration of fish between the North Sea and Wadden Sea, and catchability estimates of the fish survey sampling gear, particularly for sandeel and other pelagic fish species. Our estimate suggested a considerable top-down control by harbour seals on demersal fish. However predation by seals may also alleviate density-dependent competition between the remaining fish, increasing fish growth, and partly compensating for the reduction in fish numbers. This study shows that recovering coastal marine mammal populations could potentially become an important component in the functioning of shallow coastal systems.