The effects of future consumption by the Cape fur seal on catches and catch rates of the Cape hakes. 3. Modelling the dynamics of the Cape fur seal<i>Arctocephalus pusillus pusillus</i>

Butterworth, Doug S; Punt, André E.; Oosthuizen, W. H.; Wickens, P. A.

doi:10.2989/025776195784156511

Cited by 53 publications

(40 citation statements)

References 6 publications

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“…Instead, within-sample variability was used by determining the variability associated with estimates of fish mass derived from measurements of subsamples of otoliths within each month's sample (Mecenero et al, 2006a). No estimates of variability were available for numbers of lactating females (N ym ), because these counts were based on once-off counts of pups from aerial photographs, and for the age distribution because those values were derived from a separate study (Butterworth et al, 1995) that did not provide measures of error. Each run of the model produced a consumption estimate for the lactation period (MR).…”

Section: Uncertainty Estimatesmentioning

confidence: 99%

“…Previous studies have estimated consumption of the Cape fur seal population as the product of population size and the daily prey requirement of an individual of average size, the latter assumed to be a set percentage of body weight (Shaughnessy, 1985;Butterworth et al, 1988;Wickens et al, 1992b), or from an allometric equation for growing marine mammals (Wickens et al, 1992b;Balmelli and Wickens, 1994;Smale et al, 1994;Butterworth et al, 1995). The more refined of these studies incorporated information on the population age structure (Butterworth et al, 1995), or apportioned the total consumption into prey groups using information on diet composition obtained from analyses of stomach contents (David, 1987). However, none of these studies took into account the varying calorific content (or energy density) and the digestive efficiency (also known as the assimilation efficiency) of prey of different species, or variation in the diet composition between locations and over time, as has been demonstrated to be the case for Namibia's seal population (Mecenero et al, 2006a, b).…”

Section: Introductionmentioning

confidence: 99%

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A refined fish consumption model for lactating Cape fur seals (Arctocephalus pusillus pusillus), based on scat analyses

Mecenero

Kirkman

Roux³

2006

ICES Journal of Marine Science

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Mecenero, S., Kirkman, S. P., and Roux, J-P. 2006. A refined fish consumption model for lactating Cape fur seals (Arctocephalus pusillus pusillus), based on scat analyses. e ICES Journal of Marine Science, 63: 1551e1566.A refined fish consumption model for lactating Cape fur seals in Namibia during the eightmonth lactation period, which allows for spatio-temporal variation in the diet as determined by scat analyses, has been developed. Previous estimates of prey consumption by Cape fur seals have been based mostly on coarse diet composition models. Sensitivity analyses showed that the energetic requirement and mass of lactating females (bioenergetic variables), as well as the energetic density of prey (diet variable), contributed most to the uncertainty in consumption estimates. Uncertainty in the remaining input variables had minimal effects on the estimates of food consumption. The consumption of commercial prey (horse mackerel, hake and pelagic fish) was greatest by the colony at Cape Cross. The model estimated that a female of average mass 55 kg ingested, on average, 11% of her body mass per day. This model is easily applied to other age/sex classes of the seal population. It permits improvement of the estimates of prey consumption by seals, which are useful for assessing levels of competitive interactions between seals and fisheries or other predators, or the impacts of seals on prey species.

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Section: Uncertainty Estimatesmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

A refined fish consumption model for lactating Cape fur seals (Arctocephalus pusillus pusillus), based on scat analyses

Mecenero

Kirkman

Roux³

2006

ICES Journal of Marine Science

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“…The life histories of the sexes also differ greatly, with females maturing earlier than males and living longer (Bester 1987;Butterworth et al 1995;Lander 1981;McCann and Doidge 1987;Payne 1977;Trillmich 1987;Warneke and Shaughnessy 1985). The larger males have both a greater diving capacity than females, and generally forage at greater depths, and should be able to capture larger prey more easily (Meynier et al 2008;Page et al 2005b;Page et al 2006;Staniland and Robinson 2008).…”

Section: Introductionmentioning

confidence: 99%

Intraspecific differences in the diet of Antarctic fur seals at Nyrøysa, Bouvetøya

et al. 2010

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Intraspecific differences in the diets of many species of pinnipeds are to be expected in view of the great differences in morphology, life history and foraging behaviour between the sexes of many species. We examined the diet of the Antarctic fur seal population at Bouvetøya, Southern Ocean to assess intersexual differences. This was made possible by the analysis of prey remains extracted from scats and regurgitations collected in areas used primarily by one or the other sex. The results indicate that both males and females feed primarily on Antarctic krill Euphausia superba with several species of fish and squid being taken, likely opportunistically given their prevalence. Significant differences were identified in the frequency of occurrence of otoliths in scats and the percentage numerical abundance of the major fish prey species in the diet. Adult males ate a smaller quantity of fish overall, but ate significantly more of the larger fish species.The greater diving capabilities of males and the fact that they are not limited in the extent of their foraging area by having to return regularly to feed dependant offspring may play a role in the differences found in the diets of males and females. Additionally, females might be more selective, favouring myctophids because they are richer in energy than krill. The absence of major differences in the diet between the sexes at this location is likely due to the high overall abundance of prey at Bouvetøya.

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“…Overexploitation of seals between the 17th and 19th centuries reduced their numbers substantially and caused the extinction of many island colonies. Under controlled harvesting from the beginning of the 20th century, the population increased from <100,000 to around 2 million individuals overall between 1920 and the late-1990s, distributed throughout the SB and NB (Butterworth et al 1995). Sealing stopped in South Africa in 1990 (Wickens et al 1991), but is still practiced in Namibia, at three mainland colonies (Kirkman and Lavigne 2010).…”

Section: Jellyfishes In the Benguelamentioning

confidence: 99%

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

Roux¹,

Lingen²,

Gibbons³

et al. 2013

BMS

125

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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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The effects of future consumption by the Cape fur seal on catches and catch rates of the Cape hakes. 3. Modelling the dynamics of the Cape fur sealArctocephalus pusillus pusillus

Cited by 53 publications

References 6 publications

A refined fish consumption model for lactating Cape fur seals (Arctocephalus pusillus pusillus), based on scat analyses

A refined fish consumption model for lactating Cape fur seals (Arctocephalus pusillus pusillus), based on scat analyses

Intraspecific differences in the diet of Antarctic fur seals at Nyrøysa, Bouvetøya

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

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