Making sense of censuses and dealing with missing data: trends in pup counts of Cape fur seal<i>Arctocephalus pusillus pusillus</i>for the period 1972–2004

Kirkman, SP; Oosthuizen, W. H.; Meÿer, MA; Kotze, P. G. H.; Roux, Jean‐Paul; Underhill, Les G.

doi:10.2989/ajms.2007.29.2.2.185

Cited by 51 publications

(52 citation statements)

References 21 publications

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“…Analysis of trends in pup production as a proxy for total population trend showed that in the SB the fur-seal population grew consistently from the 1970s to the mid-1990s (Fig. 4A) and stabilized thereafter (Kirkman et al 2007c). The numbers of breeding pairs of African Penguins in the SB underwent a substantial decline from about 100,000 in 1956 to 36,000 in 1993, mostly driven by overexploitation of eggs until 1967 (Shelton et al 1984, Shannon andCrawford 1999).…”

Section: Jellyfishes In the Benguelamentioning

confidence: 99%

“…4A). After the late 1980s, large fluctuations in the numbers of pups occurred and were related to poor feeding conditions linked to environmental perturbations (Gammelsrød et al 1998, Hutchings et al 2009a) affecting the prey quantity and distribution, which resulted in mass starvation events and low-pregnancy years (Guinet et al 1998, Roux 1998, Kirkman et al 2007c). In addition the relative distribution of the fur-seal population has changed as new colonies have been established in northern Namibia since the mid-1990s, whereas colonies in southern Namibia have declined.…”

Section: Faunal Trajectories In the Northern Benguelamentioning

confidence: 99%

“…In addition the relative distribution of the fur-seal population has changed as new colonies have been established in northern Namibia since the mid-1990s, whereas colonies in southern Namibia have declined. These changes are attributed to a decline in prey availability in the south (Kirkman et al 2007c). …”

Section: Faunal Trajectories In the Northern Benguelamentioning

confidence: 99%

“…For the SB we have used diet composition estimates from the west coast in the 1950s (Rand 1959) and, since 1980, data reanalyzed and updated by Kirkman et al (2007b). For the NB in the 1980s, we used results from stomach contents collected between 1980 and 1984 (David 1987) and estimates from scat analysis calculated independently for four different subregions (J-P Roux unpubl data) and then weighted those estimates according to estimates derived from aerial surveys of fur-seal populations breeding in each subregion (Kirkman et al 2007c). In the SB, estimates of Cape fur seal diet energy density ranged between 5.22 kJ g −1 in the 1950s and 5.76 kJ g −1 in the more recent period (Fig.…”

Section: Trophic Interactions Between Predators and Their Preymentioning

confidence: 99%

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Jellyfication of Marine Ecosystems as a Likely Consequence of Overfishing Small Pelagic Fishes: Lessons from the Benguela

Roux¹,

Lingen²,

Gibbons³

et al. 2013

BMS

128

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Changes in two contrasting ecosystems of the Benguela upwelling region, one dominated at mid-trophic level by jellyfishes (Namibia, northern Benguela ecosystem, where small pelagic fish abundance has been severely depleted) and one still dominated by small pelagic fishes (South Africa, southern Benguela) were compared in an effort to determine ecosystem trajectories under different exploitation regimes. The role of small pelagic fishes (clupeoids) was highlighted in the context of their importance in maintaining interactions in marine ecosystems. In particular, we examined trophic cascades and possible irreversible changes that promote the proliferation of jellyfishes in marine systems. We found that the presence of large populations of small pelagic fishes has a fundamental role in preserving beneficial trophic interactions in these marine ecosystems. The implications of trophic cascades, such as those observed in the northern Benguela, for ecosystem-based management were apparent. In addition, this comparison provides contrasting case studies to inform the development of management scenarios that avoid ecosystem shifts that affect predators and reduce the value of fisheries production.

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Section: Jellyfishes In the Benguelamentioning

confidence: 99%

Section: Faunal Trajectories In the Northern Benguelamentioning

confidence: 99%

Section: Faunal Trajectories In the Northern Benguelamentioning

confidence: 99%

Section: Trophic Interactions Between Predators and Their Preymentioning

confidence: 99%

See 2 more Smart Citations

Jellyfication of Marine Ecosystems as a Likely Consequence of Overfishing Small Pelagic Fishes: Lessons from the Benguela

Roux¹,

Lingen²,

Gibbons³

et al. 2013

BMS

128

View full text Add to dashboard Cite

show abstract

“…A search on Google Scholar revealed 118 citations for Underhill and Prys-Jones (1994), including those of Cayford and Waters (1996), Goss-Custard et al (1998), Perez-Arteaga (2004), Atkinson et al (2006), Rendón et al (2008) and Dalby et al (2013) who apply the Underhill index on bird data and Kirkman et al 2007 andWright et al 2013) who apply it to data on mammals. Dennis et al (2013) apply a similar technique but with a different model on insect data.…”

mentioning

confidence: 99%

Working with population totals in the presence of missing data comparing imputation methods in terms of bias and precision

2016

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Missing observations in water bird censuses are commonly handled using the Underhill index or the birdSTATs tool which enables the use of TRIM under the hood. Multiple imputation is a standard technique for handling missing data that is rarely used in the field of ecology, but is a well known statistical technique in the fields of medical and social sciences. The purpose of this paper is to compare these three methods in terms of bias and variance. The bias in the Underhill method depends on the algorithm and starting values. birdSTATs and multiple imputation are unbiased in the case of missing values that are missing completely at random; more missing values implies less information, and so wider confidence intervals are expected as the missingness increases. The Underhill method and birdSTATs tool underestimate the variance; omitting data from a complete dataset and applying the Underhill index or birdSTATs tool results in smaller confidence intervals. Multiple imputation with an adequate imputation model provides wider confidence intervals. Biased parameter estimates with underestimated variance can potentially lead to incorrect management and policy conclusions. Hence, we dissuade the use of Underhill indices or the birdSTATs tool to handle missing data, rather we suggest that multiple imputation is a more robust alternative, even in suboptimal conditions.

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Geographical, temporal, and individual‐based differences in the trophic ecology of female Cape fur seals

Botha

Trueman

Kirkman

et al. 2023

Ecology and Evolution

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Information on resource use and trophic dynamics of marine predators is important for understanding their role in ecosystem functioning and predicting population‐level responses to environmental change. Where separate populations experience different local environmental conditions, geographic variability in their foraging ecology is often expected. Within populations, individuals also vary in morphology, physiology, and experience, resulting in specialization in resource use. In this context, isotopic compositions of incrementally grown tissues such as keratinous hairs offer a valuable opportunity to study long‐term variation in resource and habitat use. We investigated the trophic ecology of female Cape fur seals ( Arctocephalus pusillus pusillus ) using carbon and nitrogen isotopic compositions of serially sampled whiskers collected at four breeding sites along the coast of South Africa. Drawing on over 900 isotopic measurements, we assessed geographic variability in isotopic niche width between colonies and the degree of individual specialization. We found slight, but clear geographic differences in isotopic ratios and isotopic niche widths, seemingly related to ecological setting, with niche widths being proportional to the area of available shelf and shelf‐slope habitat surrounding the colony. We further identified periodic oscillations in isotopic ratios, which likely reflect temporal patterns in foraging distribution and prey type, linked to shifts in the availability of prey resources and their interaction with constraints on individual females throughout their breeding cycle. Finally, individual specialization indices revealed that each of the study populations contain specialist individuals that utilize only a small subset of the total population niche width. The degree of individual specialization was, however, not consistent across colonies and may reflect an interactive influence between density‐dependent effects and habitat heterogeneity. Overall, this study provides important information on the trophic ecology of Cape fur seals breeding in South Africa and highlights the need to consider geographic and individual variability when assessing the foraging ecology of marine predators.

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Making sense of censuses and dealing with missing data: trends in pup counts of Cape fur sealArctocephalus pusillus pusillusfor the period 1972–2004

Cited by 51 publications

References 21 publications

Jellyfication of Marine Ecosystems as a Likely Consequence of Overfishing Small Pelagic Fishes: Lessons from the Benguela

Jellyfication of Marine Ecosystems as a Likely Consequence of Overfishing Small Pelagic Fishes: Lessons from the Benguela

Working with population totals in the presence of missing data comparing imputation methods in terms of bias and precision

Geographical, temporal, and individual‐based differences in the trophic ecology of female Cape fur seals

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