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Population viability analysis of New Zealand sea lions, Auckland Islands, New Zealand's sub-Antarctics: Assessing relative impacts and uncertainty
Abstract and Figures
A common issue faced in wildlife management
is how to assess the uncertainty of potential impacts on the
viability of a species or population. The pup production of
New Zealand (NZ) sea lions (Phocarctos hookeri) has
declined 50% in the last 12 years at their main breeding
area, the Auckland Islands. The two major known atypical
impacts on NZ sea lions are as follows: (1) the direct mortality
as bycatch of trawling and (2) bacterial epizootics,
which can aVect reproduction and mortality. Both of these
impacts include high levels of uncertainty, with Wsheries
data being variable due to percentage observer coverage
and the eVect of sea lion exclusion devises, while the timing
and severity of bacterial epizootics are not predictable.
In this paper, an age-structured model of the NZ sea lion
population at the Auckland Islands was built to examine the
predicted eVects of Wsheries mortality and catastrophes
(bacterial epizootics), both separately and then combined,
on population viability over a 100-year period using the
VORTEX population viability analysis programme. These
models are then compared against 15 years of empirical
Weld data to determine the actual level of impacts being
observed. Model results indicate that although naturally
occurring epizootics reduce the growth rate of the population,
it does not cause a decline in the Auckland Island population.
However, sustained Wsheries bycatch at current
estimated levels, particularly considering its potential
impact on adult female survival, could result in a population
decline and possible functional extinction over the
modelled time period.
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... of time and resources (Chilvers, 2012;Sharpe and Berggren, 2019). Additionally, they often form small and isolated populations due to aquatic habitat fragmentation (Chilvers, 2012), e.g., the Yangtze finless porpoise (Neophocaena asiaeorientalis Huang et al., 2020), Maui's dolphin (Cephalorhynchus hectori maui Baker et al., 2002), and Indo-Pacific humpback dolphin (Sousa chinensis Jefferson and Smith, 2016). ...
... of time and resources (Chilvers, 2012;Sharpe and Berggren, 2019). Additionally, they often form small and isolated populations due to aquatic habitat fragmentation (Chilvers, 2012), e.g., the Yangtze finless porpoise (Neophocaena asiaeorientalis Huang et al., 2020), Maui's dolphin (Cephalorhynchus hectori maui Baker et al., 2002), and Indo-Pacific humpback dolphin (Sousa chinensis Jefferson and Smith, 2016). Unfortunately, small and fragmented populations are more affected by environmental factors (Chilvers, 2012). ...
... Additionally, they often form small and isolated populations due to aquatic habitat fragmentation (Chilvers, 2012), e.g., the Yangtze finless porpoise (Neophocaena asiaeorientalis Huang et al., 2020), Maui's dolphin (Cephalorhynchus hectori maui Baker et al., 2002), and Indo-Pacific humpback dolphin (Sousa chinensis Jefferson and Smith, 2016). Unfortunately, small and fragmented populations are more affected by environmental factors (Chilvers, 2012). ...
Predictions of population viability can provide valid information for guiding the management and conservation of species. However, such studies are rarely conducted on cetaceans due to limited basic data. In this study, a detailed assessment of the population dynamics of Indo-Pacific humpback dolphins (Sousa chinensis) in Xiamen Bay, China was carried out using a Vortex model that was based on model parameters that were derived from monitoring data and published sources. Then, sensitivity analyses were conducted to test the relative importance of the uncertainty of the model parameters. Moreover, potential management models were evaluated to determine their effectiveness in assisting with species recovery. The baseline model projected a decline in the population abundance with a growth rate (r) of -0.031, and the probability of extinction was 58.7% over the next 100 years. The sensitivity analyses revealed that juvenile mortality and fertility were the most pivotal factors for the viability of this population. All the conservation measures, including habitat improvement, catastrophe control, and individual supplementation, contributed to population increases when compared with the baseline model. In particular, decreasing calf and juvenile mortality through habitat improvement, at high levels, was the only way to generate positive growth rates. The findings suggest that these practical management activities can reduce the risk of extinction for this species.
... Another potential influence on population dynamics and trajectory is the dispersal of individual sea lions. Previous population modelling was based on the assumption that dispersal between colonies was negligible as NZ sea lions, particularly adult females, are considered to be philopatric (Chilvers and Wilkinson, 2008;Chilvers, 2012b;Hamilton and Baker, 2016a;Chilvers and Meyer, 2017). However, there is evidence of dispersal from tagged animals (Robertson et al., 2006;Geschke and Chilvers, 2009;MPI, 2018a), the lack of population structure between breeding colonies based on mitochondrial DNA and nuclear loci analyses (Collins et al., 2017), and, unequivocally, the establishment of new breeding colonies along the Otago coast and Stewart Island Chilvers, 2018). ...
... Population Viability Analysis (PVA) supports and advances the PBR approach by providing a tool for assessing bycatch impact, particularly when bycatch sex ratios are uneven, as well as determining key demographic factors driving population trajectory. In the absence of much population-specific demographic data for NZ sea lions at Campbell Island, a preliminary population assessment, particularly the relative efficacy of different management actions, can be appropriately achieved using modelling packages such as VORTEX (Hamilton and Moller, 1995) and, as a proxy, published demographic data from long-term sea lion monitoring at the Auckland Islands (Childerhouse et al., 2009(Childerhouse et al., , 2010Chilvers, 2012b). The modelling packages simulate survival and reproductive events in successive years for each individual in a population by the Monte Carlo method and are stochastic in that they impose variations in annual survival and reproduction by random number generations according to prescribed probability distributions for reproduction and survival rates . ...
... Mortality rates for NZ sea lions have been generated using different approaches and different subsets of data from the wellstudied Sandy Bay (Auckland Islands) colony Roberts et al., 2014;Middleton and Breen, 2016). Mortality values generated by were based on the full tag re-sight database and were adjusted to remove estimated mortality attributed to fisheries bycatch and epizootics (Chilvers, 2012a, b), which enabled modelling of a range of fisheries and disease impact scenarios for the Auckland Islands population (Chilvers, 2012b;Hamilton and Baker, 2016a). Therefore, these sub-adult and adult mortality estimates were also considered appropriate for the Campbell Island models (Table 1; also see Supplementary Material S3.2). ...
The endangered New Zealand sea lion, Phocarctos hookeri is killed as incidental bycatch in a trawl fishery operating near their second largest population on Campbell Island in New Zealand’s sub-Antarctic. Using the Potential Biological Removal (PBR) procedure to assess the sustainability of this bycatch for the sea lion population on Campbell Island indicated that annual bycatch estimates, particularly following the implementation of bycatch mitigation measures, are below the PBR threshold of 25 (derived using a precautionary approach). Preliminary Population Viability Analysis (PVA) modelling supported the finding that current bycatch levels, especially given a strong male bias (98%) in bycatch, are sustainable for this population. Models showed that reducing pup mortality through management actions, such as installing ramps in wallows where large numbers of pups drown, would lead to increased population growth. While obtaining more accurate data on population status and demographic parameters for the Campbell Island population should be a priority, this will take many years of research. The PBR and PVA tools demonstrate that contemporary conservation management should continue to focus on increasing pup survival while maintaining mitigation approaches that have reduced bycatch to low levels, together with high observer coverage to sustain confidence in annual bycatch estimates.
... The extreme diving behaviours and specialisation exhibited by the females at the Auckland Islands suggest that foraging is highly constrained for individuals of this species and has led to slow recruitment and the lowest reproductive rate for any otariid (i.e. Chilvers et al. , 2006Chilvers et al. , 2010Chilvers 2012). Prudent management for this species requires the determination of whether female sea lions at the largest breeding colony, Dundas Island, show the same extreme diving behaviours and foraging ecotype specialisation as those studied on Enderby Island. ...
... Small changes in adult female survival probability significantly affect population stability and population trends in large, long-lived mammals (Heppel et al. 2000;Eberhardt 2002;Mills 2012). Female survival has been shown to be critical for the population viability of New Zealand sea lions (Chilvers 2012;Meyer et al. 2015). Population studies of the New Zealand sea lions demonstrate this effect with the Auckland Island sea lion population declining at a rate of 1.9% year −1 , driving a total population decline of 0.6% year −1 over the last 20 years, despite recolonization and population growth on the New Zealand mainland . ...
The diving behaviours of marine predators are thought to be coupled with species demographics. Species that forage at or close to their physiological limits will be limited in their ability to respond to natural or anthropogenic induced changes in their environment. This is the case for the New Zealand sea lion (Phocarctos hookeri). A previous study from Enderby Island, Auckland Islands, the third largest colony for the endangered species, showed lactating females exhibit extreme diving behaviour, and that individuals have either: a benthic diving ecotype or a deeper, more varied mesopelagic diving ecotype which are maintained throughout their adult life. The ecological significance and implications for individual foraging specialisation has major consequences for this species as mesopelagic foragers are more likely to suffer mortality from fisheries than benthic foragers. We investigated the diving behaviour of lactating females from Dundas Island, Auckland Islands, the largest breeding colony for New Zealand sea lions. The diving behaviour of 24 lactating females was examined and confirmed that both diving ecotypes exist and that females at this colony are also operating at the upper limit of their physiological capability. Enderby and Dundas Island are the largest breeding colonies for the New Zealand sea lion and together represent 69% of this species pup production. This population has significantly declined since 1998, predominantly due to direct and indirect fisheries interactions. Measurable conservation and management steps are therefore critical to mitigate the impacts of fisheries on this population.
... These have included direct bycatch in the arrow squid (Nototodarus sloanii) trawl fishery operating around the Auckland Islands, indirect interactions with the fishery (nutritional stress and habitat degradation), climate variation and disease epizootics (Campbell et al., 2006;Roberts & Doonan, 2016;Robertson & Chilvers, 2011). Age-specific differences have been linked to many of these mechanisms; in particular, adult females were identified as being vulnerable to direct mortality in the southern arrow squid fishery and juveniles are considered the most vulnerable to nutritional stress (Chilvers, 2008(Chilvers, , 2012. However, management techniques implemented to mitigate the bycatch of sea lions in the squid trawl fishery have effectively reduced estimated bycatch to low levels (Hamilton & Baker, 2016). ...
Sea lions as a group, display strong site fidelity, and varying degrees of vulnerability to environmental change, disease and fisheries interactions. One of the rarest pinnipeds, the New Zealand sea lion (NZSL, Phocarctos hookeri ) has a very restricted breeding range. At Campbell Island/Motu Ihupuku, one of the two primary breeding sites, at‐sea foraging behaviour is unknown. We hypothesised that NZSL of various sex and age classes would utilise the water column differently due to differing physiological constraints and therefore have different accessibility to prey resources. We tested whether sea lion diving behaviour varied in relation to (i) age and sex class, (ii) time of day and (iii) water depth. We also hypothesised that the proportion of benthic/pelagic diving, and consequently risk of fisheries interaction, would vary in relation to age and sex. Satellite telemetry tags were deployed on 25 NZSL from a range of age/sex classes recording dive depth, duration and location. Adult females and juveniles used inshore, benthic habitats, while sub‐adult males also utilised benthic habitats, they predominantly used pelagic habitat at greater distances from the island. Adult females and juveniles exhibited shorter dives than the same age/sex classes at the Auckland Islands, suggesting a lower dive effort for these age/sex classes at Campbell Island/Motu Ihupuku. Adult females dived more frequently than other age/sex classes, likely operating closer to their physiological limits; however, further data for this age class is needed. Sub‐adult male use of pelagic prey may increase their exposure to mid‐water trawls; however, further research detailing the degree of spatial overlap with fisheries is required. This study highlights the utility of spatially explicit dive data to predict vertical habitat use, niche separation of various age and sex classes of marine predators and attribute potential fisheries interaction risk in relation to predator habitat use.
... Unfortunately, similar to many pinniped populations, it is not their on-land habitat that is causing their decline but their at-sea foraging range overlap and interactions with fisheries that is causing the significant decline in the Auckland Islands region (Robert son , Meyer et al. 2015. The Auckland Islands population continues to decline at a rate that modelling projected in 2010, which would lead to this population being functionally extinct by 2035 if better management of fisheries interactions is not undertaken (Robertson & Chilvers 2011, Chilvers 2012, Meyer et al. 2015. Given the Stewart Island sea lion population is smaller and more restricted in distribution than the Auckland Islands population, it makes them extremely vulnerable to localised impacts such as is impacting the Auckland Islands. ...
New Zealand sea lions Phocarctos hookeri have experienced a 46% decline in pup production in the last 25 yr, driven by female deaths in trawl fisheries around the Auckland Islands. The only colony for this species recorded away from the subantarctics and this fishing impact is on Stewart Island. Despite the Stewart Island colony only being 1% of the species, it is significant, as it is currently the only stable population and is located in an area away from most on-land human impacts, although not away from potential fisheries pressures. To ensure this colony’s persistence, more information is needed on these sea lions’ diet, foraging behaviour, and local fisheries interactions for management that promotes their protection. The present research investigates the long-term foraging behaviour, and infers the diet, of female New Zealand sea lions on Stewart Island using their known foraging behaviour and linking this to their proximal and full-length whisker stable isotope values. Previous foraging studies found that these animals have small foraging areas with consistent mesopelagic diving behaviours. These behaviours, linked with their whisker stable isotope values, show consistency in foraging behaviour and inferred diet across their entire adult life. Currently, there are no known commercial fisheries bycatch issues for sea lions around Stewart Island. However, precautionary measures ensuring fisheries pressure does not increase and management to safeguard that deepwater trawling and finfish aquaculture remain absent would be proactive steps that should be undertaken for the protection of this new colony and for this Endangered species as a whole.
... Population viability analysis (PVA) has become a favoured tool for conservation dealing with long-lived species, for which generation time dictates slow recoveries and measurable responses to management intervention. PVA has been applied in relation to marine and freshwater turtles (Mazaris, Fiksen & Matsinos, 2005;Famelli et al., 2012), sea lions and fur seals (Chilvers, 2012;Hernandez-Camacho & Trites, 2018), whales (Lacy et al., 2017;Verborgh et al., 2021), sharks (Bradshaw et al., 2013;White, Nagy & Gruber, 2014), anadromous fishes (Jager et al., 2000;Gross et al., 2002;Thomas et al., 2013), and freshwater fishes (Vélez-Espino & Koops, 2009;Roberts, Angermeier & Anderson, 2016), which have become imperilled as a result of a variety of threats acting on survival, growth, fecundity, or recruitment. In Canada, the Species at Risk Act (SARA) listing process was initiated for the lake sturgeon (Acipenser fulvescens) following the release of the 2006 Committee on the Status of Endangered Wildlife in Canada (COSEWIC) status report (COSEWIC, 2006). ...
• Regulatory agencies and fisheries managers tasked with implementing recovery plans for endangered species must frequently make decisions based on limited data, while also considering general uncertainty associated with prediction. Population viability analysis (PVA) is a modelling tool that is particularly useful for long-lived species and can be applied even in the absence of robust estimates of life history parameters. As empirical data are accumulated over time, PVA inputs, model structure, and parameter estimates can be refined to increase model realism and accuracy.
• Since the first iterations of lake sturgeon (Acipenser fulvescens) PVA were run, approximately 15 years ago, based on conservative inputs for Canadian population units, important empirical data have accumulated from a variety of river systems that the species inhabits. Erratic recruitment patterns have been revealed, juvenile survival has been determined to be higher than initially believed, and growth rates have been found to vary by habitat (river) type.
• Exploiting an improved biological understanding of the species, somatic growth models derived from three well-studied lake sturgeon populations were used to examine the effects of recruitment variability and incremental levels of adult harvest (mortality) on the probability of population recovery and risk of population decline. A total of 110 PVA scenarios (with 1,000 replicates per scenario) were run for each of the three somatic growth models (slow, medium, and fast).
• The PVA results suggest that the recovery potential for lake sturgeon populations may be higher than previous models, based on conservative inputs, have indicated. A few scenarios resulted in recovery after 100 years, and nearly half of the scenarios resulted in recovery after 250 years. Similarly, the probability of decline for small populations was most sensitive to adult harvest, with little change in the number of scenarios resulting in decline at the time points of 100, 250, and 500 years. A worked example highlights how the targeted monitoring of both juvenile and adult life stages can be used to evaluate the recovery potential of more than one population and prioritize management initiatives, such as harvest reduction and stocking.
... We carried out population viability analyses (PVA) to evidence the effect of present culling rate on the size and survival probability of the wild boar population and to test the effectiveness of additional culling to reduce the population size in the Park (Galimberti et al. 2001;Chilvers 2012;Carroll et al. 2014;Meriggi et al. 2016). For PVA, we used the control data provided by the Park administration from 2006 to 2016, assuming that control was not selective on sexes and age classes (Meriggi et al. 2016). ...
Wild boar foraging impacts the crops, pastures, and meadows causing remarkable losses to agricultural income. Protected areas located in plains, such as the Ticino Valley Natural Park, are characterized by the coexistence of important natural habitats and intensive agricultural areas. In the Park, from 2010 to 2017, 49% of the complaints report an event of damage to maize and 43% to meadows. The total expense for reimbursements of the maize amounted to € 439,341.52, with damages concentrated in May, after sowing period and between August and September, during the milky stage of maize. For meadows reimbursements amounted to € 324,768.66, with damage events concentrated in February and March. To reduce damage to crops, the Park administration carried out lethal control of the wild boar population. From 2006 to 2017, the most used control method was culling from hunting hides. In our analysis, we did not find significant relationships between the number of shot boars and the damage amount. The factors that determine the decrease of damage probability to crops are mainly related to human disturbance and the characteristics of the fields. The predictive model of damage risk built comparing damaged and undamaged fields showed a good predictive ability. The population viability analyses showed that it is impossible to obtain a drastic reduction of population with the current harvest rate. By tripling it and focusing on the females and sub-adult a numerical reduction of 50% of the population would be achievable in 7 years and the probability of population survival would be halved in 3 years.
... Pero las estimaciones de PMV pueden ser inciertas y difíciles de generalizar, costosas y con requerimientos de datos abstractos y modelos, especialmente para especies en peligro de extinción o de distribución restringida (Brook et al., 2006). Por esto, está la alternativa de usar el Análisis de Población Viable (APV) (Chilvers, 2012), que también calcula el riesgo de extinción, pero puede elaborarse a partir de información observada directamente en terreno. Este análisis también puede ser usado para estimar probabilidades de sobrevivencia de una población bajo diferentes condiciones ambientales y de manejo, usando información sobre su historia natural y de la influencia de factores ambientales y antrópicos (Miller y Lacy, 2005). ...
Los criterios más relevantes e inclusivos para una gobernanza coherente y efectiva de la biodiversidad son: (a) valorar la diversidad biológica; (b) promover un acceso justo a los beneficios de ella; (c) incluir todo el territorio nacional en las estrategias de conservación; (d) proteger el 10% de los ecosistemas de cada ecorregión, incluyendo los ecosistemas protectores, los productores, los difusos y los urbanos; (e) considerar como motor el actuar sinérgico de todos los actores involucrados en esta gobernanza, como los organismos gubernamentales, los organismos no gubernamentales, la sociedad civil, el sector empresarial, los agricultores, campesinos y pescadores artesanales, y las universidades y centros de investigación independientes. Se analizan estos criterios y se relevan los actores involucrados, proponiéndose algunas iniciativas de acción de efecto múltiple como el manejo y restauración de ecosistemas en la ordenación territorial, la restauración ecológica, corredores biológicos en las redes hídricas y viales y ecología urbana.
... Previous studies of seal quasi-extinction risks have mostly focused on single stressors (Elmgren 1989, Harding et al. 2005, Olsen et al. 2014). However, natural populations are typically affected by multiple stressors (Munns et al. 2007, Chilvers 2012) that have to be taken into account to successfully design management efforts (Nacci et al. 2005, Munns 2006). Our multistressor analysis revealed that even if a seal population is healthy and close to carrying capacity, the combined effects of the several mild stressors can have severe consequences. ...
Conservation efforts have mainly been focused on depleted species or populations, but many formerly reduced marine mammal populations have recovered to historical abundances. This calls for new management strategies and new models for ecological risk assessment that incorporate local density dependence and multiple environmental stressors. The harbor seal metapopulation in Swedish and Danish waters has increased from about 2500 to 25,000 over the past 40 yr. Trend analysis based on aerial survey data and somatic growth curves indicates that the population is close to carrying capacity. We performed a population viability analysis based on realistic life history parameters and investigated a range of potential scenarios caused by future stressors. If the population is able to resume its high intrinsic rate of increase at about 11% annually, when pushed down below carrying capacity, it can also sustain additional mortality such as modest hunting and infrequent epizootics. However, if xenobiotics will cause even a slight reduction in average fecundity, the population becomes significantly more vulnerable. In the absence of epizootics, and given full reproductive capacity, hunting of a few hundred animals annually is not harmful to the long‐term persistence of the population. Nevertheless, a slight decrease in growth potential, for example, caused by exposure to endocrine disruptors, makes even limited hunting risky. Our study shows how an apparently stable and abundant marine mammal population can be close to a point of rapid population decline. Thus, careful monitoring of population size, growth rate, health, and exposure to xenobiotics as well as recording of the age and sex structure of the hunt is required to avoid repeating the history of overexploitation and another population collapse.
Nine weeks field work was completed during two trips in January/February and March/ April 2003 to investigate the distribution and abundance of New Zealand sea lion Phocarctos hookeri pups at Campbell Island. A total of 161 pups were tagged and a further 138 dead pups were found. A closed mark‐recapture model was used to estimate the total number of live pups (e.g., tagged plus untagged pups) at Campbell Island in April as 247 (SE = 28, 95% CI 198–308). Pup production at Campbell Island is estimated at 385, which comprises 13% of the total pup production for the species in the 2003 season. This is the first robust estimate of pup production for New Zealand sea lions at Campbell Island. The figure of 385 pups is considerably higher than any of the previous estimates reported from Campbell Island. The high level of pup mortality (36%) at Campbell Island for approximately the first 2 months after birth is higher than the 17% reported for the Auckland Islands for approximately the same period in 2003, but is similar to unusually high levels of mortality (20–30%) reported at the Auckland Islands in recent years. It was not possible to determine the cause of death of the 138 dead pups owing to scavenging and decomposition. Pups were found over the whole Island, with the exception of its northern end. Male pups were significantly heavier and faster growing than female pups over the same period.
Threatened species lists are widely consulted as sources of information on the conservation status of species. However, their application to planning for conservation is limited because they have not been developed systematically, and because the criteria used to judge extinction risk are subjective. Recently, new proposals have been made to increase the broader usefulness of these lists, and some elements of these proposals are reviewed here. Apart from indicating the geographical and taxonomic groups containing most threatened species, these lists can provide other kinds of information. Some data from threatened species classifications using quantitative criteria provide a new method for estimating extinction rates in a variety of vertebrate taxa. This analysis suggests that over the next 100 years, the extinction rate could be as high as 15-20% in these groups. These values are comparable to those based upon extrapolations from species-area curves. However, allocating threatened species categories is only a first step towards developing rational systems for setting conservation priorities. These systems will need to consider a quite different set of variables, including those for incorporating species conservation priorities in area-based planning.
Interactions between Australian fur seals (Arctocephalus pusillus doriferus) and the Southern Squid Jig Fishery (SSJF) were investigated in April–May 2002. Details on the number of seals present, distance from the vessel, age and gender, and their behaviour were recorded using scan sampling techniques over 26 nights from eight vessels operating out of Portland and Port Fairy on the southwest coast of Victoria. Seals were observed on all nights but none were recorded in 30% of all (777) scan observations. Of the seals attending vessels at any one time (1.89 ± 0.24), 67% were involved in activities unrelated to jigging operations with the most common behaviour category being resting/grooming. Only 3.6% of observations involved seals targeting squid caught on jig lures whereas a further 29% were of foraging on squid within 40 m of the vessel. Damage to fishing gear attributable to seals was recorded on only three occasions. There was no evidence of negative impacts on seals from vessel operations. The majority of seals foraging on squid around vessels were adult females (71%) with the remainder being almost exclusively juvenile males. The current level of interactions between Australian fur seals and vessels in the SSJF appears minor.
A common ‘rule of thumb' in conservation biology is that populations of more than 100 individuals are not threatened by demographic stochasticity alone. However, the effect of demographic stochasticity on population survival may be strengthened if the population has a non-homogeneous demography. A recently developed measure, the demographic effective population size ND, scales population size by taking the differential contribution of individuals into account. We exemplify the estimation of ND with the current population of the Saimaa ringed seal, comprising about 200 individuals. The estimate of ND is shown to be dependent on how independently individuals can be assumed to reproduce. Completely independent reproduction leads to ND∼ 250, suggesting a safe future for the population. However, when acknowledging the potential of breeding failure due to habitat limitation and difficulties of dispersal among the different subareas of Lake Saimaa, ND lies in an alarming range from six to 30 individuals. The effective means to increase the ND value are limited. Although subdivision itself decreases the value of ND, improving dispersal of individuals is shown to be of limited aid. Instead, the most effective way to enhance future prospects of the Saimaa seal population is to improve pup survival to maturity.
Cited By (since 1996):88, Export Date: 26 November 2013, Source: Scopus
The probability of extinction is sensitive to the presence and character of density dependence controlling the dynamics of a population. This means that our capacity to estimate a population's risks of extinction under varying environmental conditions or competing management regimes is linked to our ability to reconstruct from data the density-dependence relationships governing the natural dynamics, especially when data do not reveal a trend of population growth or decline. In an example using Gadus morhua, we show that even 10- or 20-year data sets are too short to make precise estimates of these risks. We also observe, however, that under moderate or weak density dependence, the computed risks are lower than when density dependence is not included in the model. We propose, therefore, that when available data sets are insufficient for reconstructing reliable measurements of density dependence, conservative estimates of extinction probabilities can be made from models that simply omit density dependence.
In New Zealand, a fishery for squid (Nototodarus sloanii) incidentally catches a threatened sea lion, Phocarctos hookeri. Bycatch is managed with an annual limit designed to ensure rebuilding of the sea lion population. We explore the conservation and cost effects of the current limit and two simple alternative rules, comparing them with no fishing and unrestricted fishing. We fitted an age-structured Bayesian model to sea lion pup estimates to obtain samples of the joint posterior distribution of parameters; from these we made 100-year simulations with five harvest control rules under six different sets of environmental conditions. The base-case fit suggests that the current sea lion population may be near its carrying capacity, although this may be sensitive to modelling choices. The fishery bycatch constitutes little risk to the sea lion population in the absence of catastrophes and generates small marginal risks when catastrophes are simulated. The current management rule does not minimise the marginal risk of extinction, is much more costly to the fishery than simple alternative rules, and incurs greatest cost when risk is smallest. The model appears to be a good tool for evaluating alternative management strategies against predefined objectives.