Kootenay Lake kokanee (Oncorhynchus nerka) collapse into a predator pit

Kootenay Lake is a large, oligotrophic waterbody in southern British Columbia renowned for recreational fisheries for piscivorous rainbow trout (Oncorhynchus mykiss) and bull trout (Salvelinus confluentus). Long-term datasets showed a build-up of large-bodied (>2 kg) piscivore abundance followed by a collapse of the kokanee (Oncorhynchus nerka) prey population in 2013 and subsequent decline of large-bodied piscivores. An unprecedented post-collapse state formed in 2015-2018, characterized by low kokanee spawner abundance and biomass and high catch rates for small-bodied (<2 kg), slow growing piscivores. Bioenergetics model estimates of average historic (1961-2008) piscivore consumption was 29.3% of the average historic (1993-2008) kokanee prey supply (biomass and production), but increased to 78.7% in 2011, immediately preceding kokanee collapse. From 2015-2018, kokanee did not recover due to persistently poor juvenile survival; estimated piscivore consumption relative to prey supply remained high (73.0%), suggesting that kokanee were trapped in a predator pit. Although the ultimate and interacting causes of the initial predator build-up remain uncertain, overcoming current depensatory dynamics may be aided by kokanee stocking or increasing harvest on still-abundant, unsatiated piscivores.

Effects of corallivory and coral colony density on coral growth and survival

A suite of processes drive variation in coral populations in space and time, yet our understanding of how variation in coral density affects coral performance is limited. Theory predicts that reductions in density can send coral populations into a predator pit, where concentrated corallivory maintains corals at low densities. In reality, how variation in coral density alters corallivory rates is poorly resolved. Here, we experimentally quantified the effects of corallivory and coral density on growth and survival of small colonies of the staghorn coral Acropora pulchra. Our findings suggest that coral density and corallivory have strong but independent effects on coral performance. In the presence of corallivores, corals suffered high but density-independent mortality. When corallivores were excluded, however, vertical extension rates of colonies increased with increasing densities. While we found no evidence for a predator pit, our results suggest that spatio-temporal variation in corallivore and coral densities can fundamentally alter population dynamics via strong effects on juvenile corals.

THE DISTRIBUTION AND ABUNDANCE OF DECAPOD AND FISH COMMUNITIES IN CLEVELAND BAY, AUSTRALIA

Spatial and temporal variations in the fish and decapod communities were investigated at three stations in Cleveland Bay along with other zooplankton and phytoplankton communities. The linkage between biological assemblages and physical properties of the ocean was explained to develop better understanding of population dynamic of planktonic communities. Biological and physical properties data were gathered in 3 stations by 6 different trips. The results show that there is a significant association between daytime and tidal period to the abundance of planktonic communities (P < 0.05). Spatial distribution of fish and decapod communities are likely explained by “predator pit” and “match/mismatch” concepts to increase the survival probability along with physical properties of the ocean.

Can a low carrying capacity and a highly stochastic environment induce a predator pit in elk populations?

Due to concern over declining elk populations and the unknown effects of wolf predation, Idaho Department of Fish and Game initiated a survival study of elk in north-central Idaho. A total of 82 adult and 80 calves (~6 months old) were captured and fitted with GPS radio-collars from the winter of 2009 – winter 2014. Adult survival rates averaged 86% (range 76% - 97%). Calf survival rates averaged 44% but varied dramatically from year to year. We used a stochastic model of elk-wolf dynamics to investigate the implications of a highly stochastic growth rates indicative of the observed survival rates. In particular, we were interested in the interaction between elk density-dependent growth, carrying capacity and the amount of stochasticity in predation rates. We found that when predation rates were highly stochastic and carrying capacities were low, populations behave as if they were in a predator pit but if carrying capacity was high or stochasticy low, populations grew out of the predator pit. These results suggest that in a stochastic environment, populations may actually be influenced by both bottom-up and top-down forcing simultaneously as opposed to being governed by one or the other.

Can a low carrying capacity and a highly stochastic environment induce a predator pit in elk populations?

Due to concern over declining elk populations and the unknown effects of wolf predation, Idaho Department of Fish and Game initiated a survival study of elk in north-central Idaho. A total of 82 adult and 80 calves (~6 months old) were captured and fitted with GPS radio-collars from the winter of 2009 – winter 2014. Adult survival rates averaged 86% (range 76% - 97%). Calf survival rates averaged 44% but varied dramatically from year to year. We used a stochastic model of elk-wolf dynamics to investigate the implications of a highly stochastic growth rates indicative of the observed survival rates. In particular, we were interested in the interaction between elk density-dependent growth, carrying capacity and the amount of stochasticity in predation rates. We found that when predation rates were highly stochastic and carrying capacities were low, populations behave as if they were in a predator pit but if carrying capacity was high or stochasticy low, populations grew out of the predator pit. These results suggest that in a stochastic environment, populations may actually be influenced by both bottom-up and top-down forcing simultaneously as opposed to being governed by one or the other.

Additive and compensatory predation mortality by cougar on elk

We studied the impact of predation by cougar ( Puma concolor) on elk ( Cervus elaphus) over a 17 year time span in two watersheds in western Washington. Elk numbers ranged widely in each watershed, from a low of 130 increasing to ≈600 in the Green River and from 600 increasing to ≈1,400 in the White River. We studied adult cow elk survival (n=244) and calf survival (n=548) in both watersheds using radio-marked animals. We manipulated cougar numbers by conducting targeted removals over 7 years and estimated cougar density prior, during, and after the removals. Cougar predation on elk calves was strongly additive (R2=0.80). Cougar predation on cow elk was partially additive in the Green River where other factors had less influence on adult elk survival. Survival of adult cow elk in the White River was less affected by cougar predation than other factors in part due to greater public access. Based on age and femur marrow condition we estimated that 25% of all radio-marked adult elk mortalities due to cougar were compensatory. We have also investigated predation rate by cougar using GPS-marked cougar (n=12). We estimated that 15% of adult elk >= 2 years old that were killed by cougar were likely compensatory and that 75% were likely additive. Removing cougars allowed elk in both watersheds to recover to, or exceed, herd objectives for both watersheds, and restore elk hunting opportunity. Our data strongly indicates that cougar were limiting elk, and that cougar removals allowed elk to escape the predator pit they were in. But simply removing cougar in other elk-cougar systems may not restore elk depending on the relative influence of other factors affecting mortality, amount of additive predation mortality, and suite of interacting predators and prey.

Additive and compensatory predation mortality by cougar on elk

We studied the impact of predation by cougar ( Puma concolor) on elk ( Cervus elaphus) over a 17 year time span in two watersheds in western Washington. Elk numbers ranged widely in each watershed, from a low of 130 increasing to ≈600 in the Green River and from 600 increasing to ≈1,400 in the White River. We studied adult cow elk survival (n=244) and calf survival (n=548) in both watersheds using radio-marked animals. We manipulated cougar numbers by conducting targeted removals over 7 years and estimated cougar density prior, during, and after the removals. Cougar predation on elk calves was strongly additive (R2=0.80). Cougar predation on cow elk was partially additive in the Green River where other factors had less influence on adult elk survival. Survival of adult cow elk in the White River was less affected by cougar predation than other factors in part due to greater public access. Based on age and femur marrow condition we estimated that 25% of all radio-marked adult elk mortalities due to cougar were compensatory. We have also investigated predation rate by cougar using GPS-marked cougar (n=12). We estimated that 15% of adult elk >= 2 years old that were killed by cougar were likely compensatory and that 75% were likely additive. Removing cougars allowed elk in both watersheds to recover to, or exceed, herd objectives for both watersheds, and restore elk hunting opportunity. Our data strongly indicates that cougar were limiting elk, and that cougar removals allowed elk to escape the predator pit they were in. But simply removing cougar in other elk-cougar systems may not restore elk depending on the relative influence of other factors affecting mortality, amount of additive predation mortality, and suite of interacting predators and prey.

Arctic ground squirrel population collapse in the boreal forests of the Southern Yukon

Context The arctic ground squirrel (Urocitellus parryii) comprised 17% of the biomass of herbivores in the Yukon boreal forest during the summer months from 1987 to 1996 and was responsible for 23% of the energy flow at the herbivore level. By 2000, ground squirrel populations in this region collapsed to nearly zero and have remained there. Aims We summarise the population monitoring (since 1975) and recent experimental work that has been done on this key herbivore in the Kluane area of the southern Yukon to test one mechanistic hypothesis as the possible explanation for this population collapse and subsequent lack of recovery: predation. Methods Ground squirrels are the preferred summer prey of bird and mammal predators when snowshoe hare (Lepus americanus) populations are declining. We used translocations into formerly occupied habitat and radiotelemetry to determine movements and causes of death from 2009 to 2014. We surveyed 158 sites between 2008 and 2013 to measure the disappearance of colonies in alpine and forest habitats over 25 000 km2. Key results Ground squirrels from 2000 to 2013 comprised a small fraction of the herbivore biomass in the boreal forest zone, down from 17% earlier. Most forest populations (~95%) are currently extinct, whereas just over half (65%) of low-elevation meadow populations are locally extinct. One hypothesis is that ground squirrels in the forest have been driven into a predator pit from which they cannot recover. They remain abundant in alpine tundra (93% occupancy rate) and around airport runways and human habitations (97% occupancy), but there is no apparent dispersal from alpine areas down into the boreal forest. Conclusion The predator pit hypothesis is a likely explanation for the initial collapse and sustained decline in population size from 2000 to 2013. Recent attenuation of the hare cycle and milder winter climate have allowed shrubs to expand throughout the forest, thereby reducing visibility and increasing predation risk. This conclusion will be tested in further research using reintroductions to formerly occupied sites. Implication If the loss of this herbivore from the boreal forest is not reversed, predator pressure on the other major herbivores of the montane forest zone is likely to change significantly.

Modelling prey consumption and switching by UK grey seals

Smout, S., Rindorf, A., Hammond, P. S., Harwood, J., and Matthiopoulos, J. Modelling prey consumption and switching by UK grey seals. – ICES Journal of Marine Science, 71: . Grey seals (Halichoerus grypus) are adaptable generalist predators whose diet includes commercial fish species such as cod. Consumption by the seals may reduce the size of some fish stocks or have an adverse effect on stock recovery programmes, especially because predation may trap sparse prey populations in a “predator pit”. To assess the likely impact of such effects, it is important to know how consumption and consequent predation mortality respond to the changing availability of prey. We present a model of grey seal consumption as a function of the availability of multiple prey types [a Multi-Species Functional Response (MSFR)]. We fit this MSFR to data on seal diet and prey availability (based on the overlap between the distributions of predators and prey). Bayesian methodology was employed to account for uncertainties in both dependent and independent variables, improve estimation convergence by the use of informative priors, and allow the estimation of missing data on prey availability. Both hyperbolic (Type 2) and sigmoidal (Type 3) functional response models were fitted to the data and the Type 3 model was clearly favoured during model selection, supporting the conclusion that seal–prey encounter rates change with prey abundance (sometimes referred to as “switching”). This suggests that some prey species may be vulnerable to predator pit effects. The fitted model reproduced contrasts in diet observed between different regions/years and, importantly, added information to the prior distributions of prey abundance in areas where the availability of some prey species (such as sandeels) was not known. This suggests that the diet of predators such as seals could provide information about the abundance and distribution of prey in areas that are not covered by fisheries and research surveys.

Natural and human-induced predation on Cape Cormorants at Dyer Island

SummaryTo develop conservation strategies for vulnerable seabird species that need attention, it is important to know which factors influence their breeding productivity. Predation of eggs and chicks can have large influences on seabird reproduction, especially when human disturbance facilitates predation. On Dyer Island, Kelp GullsLarus dominicanusprey on Cape CormorantPhalacrocorax capensiseggs and chicks, whereas Cape fur sealsArctocephalus pusillus pusillusprey on Cape Cormorant fledglings in the waters surrounding the island. Kelp Gulls were estimated to predate 3.8% of the total number of Cape Cormorant eggs and 2.0% of the chicks on the island. These percentages can be expressed as a loss of 4.8% of Cape Cormorant fledglings, which is low compared to the estimated 24.3% mortality of Cape Cormorant fledglings by Cape fur seal predation. Human disturbance facilitated Kelp Gull egg and chick predation and increased the mobbing of cormorant fledglings by Kelp Gulls. Cormorant egg predation by gulls was more frequently reported in the late afternoon. Seal predation was more abundant at the northern side of the island compared to the southern side, was recorded more frequently in the morning, and increased through the breeding season. The altered abundance and distribution of prey, the availability of suitable breeding habitat and mortality from avian cholera, have also influenced the Cape Cormorant’s population size. Hence, the possibility that Cape Cormorants may be locked in a predator-pit, where seals and gulls prevent the population from increasing in size, needs further attention.