Sp Kirkman

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

Trends in the population of Cape fur seals Arctocephalus pusillus pusillus were estimated from counts of pups on aerial photographs of colonies taken between 1972 and 2004 to determine trends in the overall population and subpopulations. Incomplete coverage resulted in missing data in some years. Various methods of determining proxy values for missing data were assessed, and it was concluded that different methods were applicable to Namibian and South African colonies. This reflected variation in trends of pup counts between the countries, which was associated with differences in productivity between the southern and northern Benguela ecosystems. In Namibia, temporal changes in pup numbers were non-linear in some years and there was correspondence in fluctuations at most colonies. This appeared to be on account of an effect of periodic, wide-scale prey shortages that reduced birth rates. There was a northward shift in the distribution of seals in the northern Benguela system. In South Africa, pup counts were much less variable between years, probably on account of a relative stability of food supply. A linear approach was therefore suitable for determining proxy values for missing data at South African colonies. Pup counts suggest that there has been little change in the overall population of the Cape fur seals since 1993, when it was estimated at about two million animals.

St Helena Bay (southern Benguela) then and now: muted climate signals, large human impact

The development of suitable reference states for ecosystem-based management requires documentation of changes in structure and functioning of marine ecosystems, including assessment of the relative importance of bottom-up and top-down processes as drivers of change. We used monitoring data available from St Helena Bay, the most productive bay and an important nursery area situated on the west coast of South Africa, during 1950–2010 to reveal changes in the abiotic and biotic components. St Helena Bay in the 1950s showed similarities to 2000–2010 in terms of wind patterns, hydrology and phytoplankton. Upwelling, oxygen and nutrient concentrations in subthermocline water displayed pronounced decadal-scale variability. Primary production in St Helena Bay is variable, but consistently higher than that on the adjacent Namaqua shelf. Zooplankton size composition and biomass in August have changed markedly since the 1950s. During 2001–2010, mesozooplankton biomass in autumn was considerably lower than in summer, probably due to predation by small pelagic fish. Pelagic fish catch patterns and distributions have altered dramatically. Conservation measures, implemented to reverse past negative human impact, have benefitted marine mammals, the abundance of which has increased in the area, but additional conservation measures are necessary to reverse the decline in African penguins Spheniscus demersus. St Helena Bay shows a muted response to long-term change in the southern Benguela, with marked decadal variability but no clear long-term trend in oceanography and biogeochemistry. Changes in ecosystem boundary conditions and fishing pressure cannot be ignored as important drivers of change in the southern Benguela since the 1950s.

Trends and interventions in large whale entanglement along the South African coast

The major causes of large whale entanglement in South Africa are static fishing gear, especially the type associated with the West Coast rock lobster Jasus lalandii industry, and large-mesh gillnets that are set off the coast of KwaZulu-Natal (KZN) to reduce shark attacks (shark nets). The prevalence of entanglements is seasonal with the peaks in activity coinciding with the breeding migrations of humpback whales Megaptera novaeangliae and southern right whales Eubalaena australis, the two large whale species that are the most prone to entanglement. Generalised linear models with a Poisson or quasi-Poisson distribution were used to describe the relationship between the number of incidents and time. Taking into account the combined length of shark-net installations per year as an offset variable, entanglement of humpback whales in shark nets increased at 15.1% per year (95% CI = 9.5–21.6%) from 1990 to 2009. This is comparable to the rate of increase in the numbers of this species migrating past the KZN coast, between 1988 and 2002 (9–11%). The number of reported incidents of southern right whales entangled in gear other than shark nets also increased between 1990 and 2009. This was accounted for by the increase in numbers of this species in South Africa (7% per year), so in neither case are the two species at increasing risk of individual entanglement, and anthropogenic factors including entanglement do not seem to be affecting the recovery of these whale populations. Nevertheless, there is concern regarding the vulnerability to entanglement of a small assemblage of humpback whales that habitually visits the West Coast in spring and summer. The continued recovery of whale populations is likely to lead to greater levels of anthropogenic interaction and heighten the need for adequate mitigation measures. The KZN Sharks Board and the South African Whale Disentanglement Network (since 2006) have respectively released (disentangled) 81% and 23% of confirmed entangled individuals, and recorded relevant information on entanglement incidents. Such information is critical for developing mitigation measures and monitoring the prevalence of entanglement.

Do inter-colony differences in Cape fur seal foraging behaviour reflect large-scale changes in the northern Benguela ecosystem?

The northern Benguela ecosystem adjoining Namibia has undergone considerable changes in recent decades, with reductions and northwards shifts of key prey species that have had severe implications for marine top predator populations. We investigated how such environmental variability may impact foraging behaviour of the Cape fur seal Arctocephalus pusillus pusillus, using satellite telemetry on animals in northern, central and southern Namibia. We expected that seal foraging behaviour would reflect a gradient of deteriorating feeding conditions from north to south. Results showed that foraging trips were shorter in the central region, where seals fed over the continental shelf, than in the north or south, where seals fed at the shelf edge. However, whereas seals in the north showed strong fidelity to the colony at which they were tagged and to persistent, clustered foraging areas, seals in the south showed weak fidelity both to the colony at which they were tagged and to foraging areas, which were scattered and variable. Seals in the south also tended to migrate northwards to other colonies while concurrently adapting their foraging behaviour to local conditions. Flexible use of foraging space and colonies of Cape fur seals during the three-year period (2002-2004) demonstrates that the species is adapted for variable environments over time and space.

Recolonisation of the Robberg Peninsula (Plettenberg Bay, South Africa) by Cape fur seals

The Cape fur seal Arctocephalus pusillus pusillus colony at Robberg Peninsula, Plettenberg Bay, on the south-east coast of South Africa, was driven to extinction by indiscriminate harvesting by the late 1800s. Seals only began to recolonise this site in the 1990s. This study describes the recolonisation process from 2000 to 2009, exploring both within- and between-year count data of seals using the site. Counts increased over the study period from <300 animals to >3 100. Generalised linear models indicated the importance of year and month in explaining variability in the counts. Withinyear variability in the counts decreased over the study period, which may be related to an increasing proportion of resident (as opposed to transient) seals in the colony. However, the colony is currently still in a transition phase with a low ratio of breeding to non-breeding animals, based on the low numbers of pups born in the colony (currently still <100 per year). The influx of seals to the Robberg area may be associated with shifts in prey availability at the ecosystem level. The colony benefits from the protection afforded by the reserve status of the Robberg Peninsula and the existence of a marine protected area adjacent to it. However, human interference associated with fishing and/or ecotourism on the peninsula may inhibit development into a substantial breeding colony. Potential interventions for the conservation and management of this colony are discussed.

Prioritising range-wide scientific monitoring of the Cape fur seal in southern Africa

The range of the Cape fur seal Arctocephalus pusillus pusillus population largely coincides with the region of the cold, nutrient-rich Benguela Current Large Marine Ecosystem (BCLME) adjoining the west coast of South Africa, Namibia and Angola. Range-wide scientific monitoring of the seal population was initiated in the 1970s to inform on population management questions related to commercial seal harvesting and seal–fishery interactions. Since the 1970s, seal-related management goals have changed, especially in South Africa where seal harvesting ceased in 1990 and government has conformed to scientific advice against the culling of seals for the intended benefit of fisheries. However, renewed impetus has been provided to seal research and monitoring through the expansion of the ‘ecosystem-based management’ concept in the region, as well as improved international cooperation in the management of marine resources. Together with wide-scale ecosystem changes in the marine environment, and forecast effects of global climate changes, this has justified the continuation and improvement of range-wide scientific monitoring of Cape fur seals. We prioritised seal monitoring based on cost, effort, and relevance to monitoring objectives that have been identified for the region, with consideration given to the conservation status of top predators, interspecific and predator-fishery interactions and the potential use of Cape fur seals as indicators of ecosystem health. An integrated approach incorporating a suite of life-history attributes of seals is recommended, useful monitoring tools are discussed and the need for coordinated monitoring effort and standardisation of sampling techniques is emphasised.

Natal site fidelity by breeding female southern elephant seals in relation to their history of participation in the winter haulout

Of the four types of terrestrial haulout periods undertaken by southern elephant seals Mirounga leonina, only the purpose of the winter haulout is unknown. Returning to a haulout site from distant pelagic foraging grounds bears significant costs in terms of increased energy expenditure, reduced foraging time and increased exposure to predation; therefore, each haulout must serve a purpose. We examined the hypothesis that the winter haulout serves to maintain familiarity with the natal site, thereby increasing site fidelity. To this end, we analysed a long-term mark-recapture dataset for female southern elephant seals at Marion Island, Southern Ocean. Results indicate that, whereas greater natal site fidelity as primiparous females was associated with recorded presence ashore at the study site during the winter haulout as immatures, this was not the case for multiparous females. Furthermore, recorded presence ashore during both the moult haulouts as immatures, and all haulouts as immatures, irrespective of haulout type, was also associated with increased site fidelity. This suggests that any haulout at the natal island as an immature seal, whether for the moult or winter haulout, assists in maintaining site fidelity. Therefore, while the winter haulout facilitates greater natal site fidelity, whether this is the sole reason for this terrestrial period remains uncertain.

Cephalopod diet of the Cape fur seal, Arctocephalus pusillus pusillus , along the Namibian coast : variation due to location

Scats of the Cape fur seal, Arctocephalus pusillus pusillus, were sampled at four mainland colonies, Cape Cross, Atlas Bay,Wolf Bay and Van Reenen Bay, along the Namibian coast over a period of eight years (1994–2001) to assess the diversity and spatial variability in the cephalopod component of the seal diet. Additional scat samples were collected from the Possession Island seal colony (1999–2000) to gain a broader perspective of spatial variation. A uniform and low diversity of cephalopods, only six species amongst all colonies, was identified, indicating that independently the scat sampling method is unsatisfactory for determining species diversity within the diet. Given the ease of scat collection, this method does, however, provide valuable insight into the variability of the most important species in the diet. Ommastrephids dominated the cephalopod component of the diet of seals from Atlas/Wolf Bay and Cape Cross, both in terms of wet mass and numbers. Sepia australis proved to be numerically the most important cephalopod in the diet of seals from Possession Island and Van Reenen Bay, while Octopus magnificus dominated at these colonies in terms of wet mass. Contrary to previous findings it is suggested that seals from Van Reenen Bay and Possession Island forage south of the upwelling cell at Lüderitz (in the southern Benguela ecosystem), while previous evidence of Atlas/Wolf Bay and Cape Cross seals foraging north of this upwelling cell (in the northern Benguela ecosystem) is supported. Prey specimen size differences, within species between colonies, were identified, but lack of cephalopod life history and movement data, and scat sampling biases preclude adequate explanation of these findings, indicating the necessity for further studies.

Spatial characterisation of the Benguela ecosystem for ecosystem-based management

The three countries of the Benguela Current Large Marine Ecosystem (BCLME), namely Angola, Namibia and South Africa, have committed to implementing ecosystem-based management (EBM) including an ecosystem approach to fisheries (EAF) in the region, to put in practice the principles of sustainable development in ocean-related matters. There is also recognition of the need for marine spatial planning (MSP) as a process for informing EBM with regard to the allocation and siting of ocean uses so that ecosystem health is ensured and trade-offs between ecosystem services are appropriately dealt with. Marine spatial planning is both an integrated and an area-based process, and this paper produces a spatial characterisation of the BCLME for achieving a common basis for MSP in the region, focusing on the oceanography, biology and fisheries. Recognising spatial variation in physical driving forces, primary and secondary production, trophic structures and species richness, four different subsystems are characterised: (1) north of the Angola–Benguela Front, (2) from the Angola–Benguela Front to Lüderitz, (3) from Lüderitz to Cape Agulhas, and (4) from Cape Agulhas to Port Alfred on the south-east coast of South Africa. Research and monitoring requirements of relevance for MSP and EBM in the region are identified, focusing on understanding variability and change, including with regard to the boundary areas identified for the system. To this end, 14 cross-shelf monitoring transects are proposed (including seven that are already being monitored) to estimate fluxes of biota, energy and materials within and between the subsystems. The usefulness of models for understanding ecosystem variability and changes is recognised and the need for fine-scale resolution of both sampling and modelling for adequate MSP as input to EBM for the often-conflicting interests of conserving biodiversity, and managing fisheries, recreation, offshore oil and gas exploration and exploitation, offshore mining and shipping routes, is emphasised.

Vagrant Subantarctic fur seals at Bouvetøya

ABSTRACT Three vagrant Subantarctic fur seals, Arctocephalus tropicalis, were seen amongst a colony of Antarctic fur seals, A. gazella, at the isolated subantarctic island, Bouvetøya. Possible sources of the vagrants are populations at either Gough Island or the Prince Edward Islands Archipelago.