Factors influencing growth of the African penguin colony at Boulders,South Africa, 1985–1999

This paper reports on growth of the Boulders colony of African penguins Spheniscus demersus from inception in 1985 to the present. More than 900 pairs now breed there. Growth of the colony slowed in 1995 and 1996 and reversed in 1998, coinciding with periods of low abundance of Cape anchovy Engraulis capensis off South Africa. In December 1996, penguins were excluded from a portion of land where they had formerly bred. They responded by increasing the density of their nests in other areas and expanding their area of breeding longshore. These patterns indicate that food and not space are currently controlling colony growth rate. Much of the colony growth probably results from immigration of first-time breeders from other colonies. Of immigrants, 70–80% may be from Dyer Island to the south-east, where numbers of penguins have decreased. Boulders also is frequently visited by penguins from other colonies, and by rehabilitated birds.     

Population structure and growth of polydorid polychaetes that infest cultured abalone Haliotis midae

Polydorid polychaetes can infest cultured abalone thereby reducing productivity. In order to effectively control these pests, their reproductive biology must be understood. The population dynamics and reproduction of polydorids infesting abalone Haliotis midae from two farms in South Africa is described using a length-based, age-structured model. Shells were infested mainly by introduced Boccardia proboscidea. Polydora hoplura and Dipolydora capensis were also present but in numbers too few to identify factors influencing infestation. At both farms, B. proboscidea lived for a minimum of 12 months. Growth rate, size at maturity, maximum size, infestation intensity, recruitment, percentage of the population brooding and mortality appear to be affected by abalone feeding regime and water temperature, and these factors need to be considered in controlling infestation. Brooders and recruits were present throughout the year, but increased significantly during mid- to late winter/early spring when water temperature and day length increased. Treatment measures should therefore be implemented throughout the year but with increased effort when water temperature increases.     

Biological parameters reflecting the current state of the exploited pink dentex Dentex gibbosus (Pisces: Sparidae) population off the Canary Islands

A total of 639 pink dentex Dentex gibbosus was collected in Canary Islands waters between April 1991 and September 1993. Total lengths ranged from 14,2 to 95,2 cm. Females dominated smaller size-classes and males the larger ones. The species was characterized by protogynous hermaphroditism. The overall ratio of males to females was 1 : 1,45. The reproductive period extended from April to September, spawning peaking in June/July. The total lengths at 50% maturity were 34,7 cm for females and 38,6 cm for males. The length-mass relationship for the whole sample can be described by the parameters a = 0,01014 and b = 3,0812. Fish aged 0–16 years were present in the samples. The parameters of the Von Bertalanffy growth equation were: L _∞ = 101,2 cm, k = 0,149·year^?1, and t ₀ = ?0,111 years. The rates of total mortality Z and natural mortality M were 0,57 and 0,28·year^?1 respectively. Rates of fishing mortality F and exploitation E were 0,29 and 0,51·year^?1 respectively. The estimated length at first capture (LC ₅₀) was 17,8 cm total length.     

Selected fishery and population parameters of eight shore-angling species in the Tsitsikamma National Park no-take marine reserve

An eight-year research angling dataset collected between February 1998 and December 2005 in the Tsitsikamma National Park marine protected area (MPA), along the south-eastern Cape coast of South Africa, was examined to provide estimates of important fishery and population parameters for eight important shore-angling species. Five different estimates of natural mortality (M), and the coefficients of variation (CVs) of catch per unit effort (CPUE) and size measurements were calculated. There was no consistency in estimates of M between methods, highlighting the difficulty in obtaining reliable estimates of this parameter for per-recruit assessments. The CVs of CPUE measurements were generally higher than those for size measurements. Estimates obtained for the most abundant species, blacktail Diplodus capensis, with a mean CPUE of 0.252 fish h^–1, yielded the lowest CVs for CPUE (0.806) and size (0.130) measurements. Therefore, blacktail is considered the preferred indicator species for making comparisons with estimates obtained for shore-angling fish in exploited areas and other MPAs along the South African coastline. The large inter annual variations in mean CPUE and size for all species indicated that a minimum assessment period of four years is required to obtain reliable estimates.     

Seabird bycatch by tuna longline fisheries off southern Africa, 1998–2000

The incidental mortality of seabirds in tuna longline fisheries is estimated for the continental South African Exclusive Economic Zone (EEZ). Fishery observers accompanied 13 fishing trips and observed 108 sets (143 260 hooks) during the period 1998–2000. Despite most lines being set at night, seabird bycatch rates were high, with a mean of 1.6 birds killed per 1 000 hooks. Japanese vessels (1% effort observed) had a higher bycatch rate (2.6 birds per 1 000 hooks, range per trip 0.1–5.4) than South African vessels (0.8, range 0.0–4.3; 17% effort observed), possibly as a result of gear differences. Bird bycatch differed regionally in relation to the numbers of birds attending vessels. In international waters off the Northern Cape and southern Namibia, where there are few birds, only one bird was caught on 93 600 hooks (0.01 birds per 1 000 hooks). Shy Thalassarche cauta, black-browed T. melanophris and yellow-nosed T. chlororhynchos albatrosses, and white-chinned petrels Procellaria aequinoctialis were killed most frequently. Based on 1998–1999 fishing effort, simple extrapolation suggests that 19 000–30 000 seabirds are killed annually in South Africa's EEZ, of which 70% are albatrosses. Confidence in these estimates is low, given the small proportion of effort observed, but it is clear that urgent steps are needed to reduce seabird bycatch within South African waters.     

Impact of predation by Cape fur seals Arctocephalus pusillus pusillus on Cape gannets Morus capensis at Malgas Island,Western Cape,South Africa

Cape fur seals Arctocephalus pusillus pusillus were estimated to kill some 6 000 Cape gannet Morus capensis fledglings around Malgas Island in the 2000/01 breeding season, 11 000 in 2003/04 and 10 000 in 2005/06. This amounted to about 29%, 83% and 57% of the overall production of fledglings at the island in these breeding seasons respectively. Preliminary modelling suggests this predation is not sustainable. There was a 25% reduction in the size of the colony, the second largest of only six extant Cape gannet colonies, between 2001/02 and 2005/06. There has been a large increase in predation by Cape fur seals on seabirds around southern African islands since the mid-1980s, coincidental with both an increase in the seal population, altered management of the islands and an altered distribution of prey for gannets and seals. At Malgas Island, most gannet fledglings were killed between 10:00 and 18:00, the period when most are in the water around the island, from mid-January to mid-March, the main fledging period. The Cape gannet is classified as Vulnerable.