Colin G. Attwood

Crossing Invisible Boundaries: the Effectiveness of the Langebaan Lagoon Marine Protected Area as a Harvest Refuge for a Migratory Fish Species in South Africa

The application of no-take areas in fisheries remains controversial. Critics argue that many targeted species are too mobile to benefit from area protection and that no-take areas are only appropriate for resident species. The degree of protection does not depend on the size of the no-take area but rather on the time fish reside inside its boundaries during key life-history events (i.e., spawning) and during periods of peak fishing activity. We evaluated the potential of a small no-take marine protected area (MPA) inside a coastal embayment as a harvest refuge for a mobile, possibly migratory, long-lived fish species. We used acoustic telemetry to track movements of 30 transmitter-tagged white stumpnose (Rhabdosargus globiceps) across and on both sides of the boundary of a small (34 km(2)) no-take area over a full year. Being landlocked on 3 sides, the location of the MPA inside the lagoon made it practical to detect all boundary crossings and to calculate the time individual fish used the MPA. We detected frequent movements across the boundary, with strong seasonal and individual variations. There were significant differences in MPA use patterns between fish from different release areas. The time spent in the MPA by individual fish during summer (mean 50%; max 98%) was out of proportion with the size of that area (4% of total habitat). Summer coincided with peak recreational fishing activity and with the spawning season of this species. The small MPA provided a refuge for a part of the spawning stock of white stumpnose. Our findings suggest that if strategically placed, a small no-take area can be effective in protecting mobile species and that models of spillover from no-take areas should account for seasonal and individual variation in area use and the spatiotemporal distribution of fish and fishers.

A review and tests of hypotheses about causes of the KwaZulu-Natal sardine run

The term ‘sardine run’ is part of the cultural heritage of the South African nation and refers to a natural phenomenon that is well known to the general public but still poorly understood from an ecological perspective. This lack of understanding has stimulated numerous hypotheses, often contradictory, that try to explain why (ultimate factors) and how (proximate factors) the run occurs. Here, we provide a new definition of the term sardine run, review the various hypotheses about the run, and propose ways to test those hypotheses. Where possible, the results of tests that have been conducted thus far are presented and discussed. Our interpretation of the causes is that the sardine run most likely corresponds to a seasonal (early austral winter) reproductive migration of a genetically distinct subpopulation of sardine that moves along the coast from the eastern Agulhas Bank to the coast of KwaZulu-Natal (KZN) as far as Durban and sometimes beyond, in most years if not in every year. This eastward migration is constrained close to the coast by the thermal preference of sardine and the strong and warm offshore Agulhas Current. The run is facilitated by the presence of a band of cooler coastal water and by the occurrence of Natal Pulses and break-away eddies that enable sardine shoals to overcome their habitat restrictions. These enabling mechanisms are most important in the area where the shelf is at its narrowest and feature most prominently off Waterfall Bluff, which has led to the coining of the ‘Waterfall Bluff gateway hypothesis’. Based on the collection of eggs off the KZN coast, sardine remain there for several months and their westward, return migration during late winter to spring is nearly always unnoticeable because it likely occurs at depth as the fish avoid warmer surface waters. Years in which the sardine run is not detected by coastal observers could reflect either its real absence due to high water temperatures and/or other hydrographic barriers, or an eastward migration that is farther offshore and possibly deeper and is enabled by hydrographical anomalies.

Movement pattern and home range of Roman Chrysoblephus laticeps

The movement behaviour of adult Roman Chrysoblephus laticeps (Sparidae) was investigated using mark-and-recapture techniques in the Goukamma Marine Protected Area (GMPA) and the Tsitsikamma National Park (TNP) on the South African temperate south coast. The study was designed to improve the spatial resolution of previous studies on Roman that suggested residence behaviour, but which could not estimate home range size. A combination of conventional barbed dart tags and visible implant fluorescent elastomer (VIFE) tags were used to tag Roman from a skiboat in the GMPA. Roman were tagged with dart tags from the shore in the TNP. Of the recaptures, 61% were within 50m of the tagging position, confirming that Roman are very resident. A few fish, independent of size and sex, moved distances up to 4km. A model was developed to quantify the degree of residence, by accounting for tag loss, mortality and the distribution of recovery effort. The probability of a Roman remaining within an area of 1ha is 0.91 (GMPA) and 0.94 (TNP) — such a small home range suggesting that individual Roman would benefit from small MPAs.

Nitrogen dynamics in the Bellingshausen Sea during the Austral spring of 1992

Abstract A series of 15N experiments were conducted in the early summer of 1992 during the course of cruise 198 of R.R.S. Discovery, with a focus on the seasonal ice zone (SIZ) of the Bellingshausen Sea. Nitrogen (NO3, NH4 and urea) uptake was measured over the nominal euphotic zone, and size fractionation experiments of a similar nature were conducted on surface waters. The most productive zone during this study was not associated with stable ice-melt water but with a west-east oriented, haline dominated density front centred around 67.3°S. The pelagic bloom coincident with the area south of the front had a banded structure with chlorophyll a concentrations up to 5 mg m 3. Integrated N uptake rates within the bloom varied between 56 and 34 mmol N m −2 day − , with a decreasing trend from north to south. A similar trend was evident in the |-ratios, where values decreased from 0.6 to 0.3. From size fractionated experiments in the bloom it was found that 82–87% of the chlorophyll a was in the netplankton fraction, which also accounted for 28 to 95% of the N uptake. The bloom was dominated by Porosira glacialis, Coscinodiscus bouvet, Thalassiosira antarctica and Phaeocystis spp. North of the front, integrated N uptake was 25 mmol N m −2 day − (f-ratio 0.28) and 60 to 69% of the uptake in surface waters was in the netplankton fraction. Waters adjacent to the ice-edge exhibited a lower rate of N uptake than the frontal stations (19 mmol N m −2 day −1 ) with an f-ratio of 0.1. Specific rates on N uptake indicated strong bloom viability in an Antarctic context but a lack of response of VN0P 3 to [N0 3 ]. The cross-frontal trends may provide an explanation of bloom development and persistence in the SIZ of the Bellingshausen Sea. Rates of new production in mature waters of the SIZ were comparable to those of previous studies in other regions, but rates measured within the bloom were higher. The rates of new production presented here are the first to have been made in the Bellingshausen Sea and as such can be incorporated in regional and global models relating to the sink of carbon.

Reduction in fecundity and lipids of the copepod Calanus australis (Brodskii) by strongly pulsed upwelling

Gravid female Calanus australis (Brodskii) were starved for varying periods in the laboratory to test the hypothesis that the relatively low Zooplankton biomass in the southern Benguela is caused by a discontinuous, albeit rich, food supply. Egg-production rates and lipid content were monitored for groups of copepods which were starved, fed subsequent to starvation and fed continuously. Average egg production for unstarved females was 27.6 eggs-day−1. Starvation was found to inhibit egg production after 24 h. Cumulative egg production during 5 days of feeding was significantly reduced if starvation was experienced immediately before feeding, being even lower for extended lengths of starvation. The total lipid content of ≈ 17 % of dry mass was reduced to < 4 % following starvation for 9 days. Given a mean dry mass of 184.4 μg, this animal expends 6.4 μg lipid · day−1. It would appear that fecundity is related not only to feeding rate as has been demonstrated but to feeding history also. Phytoplankton blooms in the southern Benguela, usually lasting only 5 days, are interrupted by advection of chlorophyll-free upwelled water. These results support the hypothesis that, although on average food is abundant, starvation experienced during episodic upwelling events may retard the establishment of a large herbivore community.

Dynamics of the Fishery for Galjoen Dichistius Capensis, With an Assessment of Monitoring Methods

The dynamics of the fishery for galjoen Dichistius capensis were investigated at four sites in South Africa from 1987 to 2000. At three sites, which were protected from fishing, Koppie Alleen, Lekkerwater (both in the De Hoop Marine Protected Area) and Tsitsikamma National Park, fishery-independent surveys were conducted. Data for the Cape Peninsula (the fourth site) were obtained from records kept by a recreational angler. Catch per unit effort (cpue) was highest at De Hoop and lowest at Tsitsikamma. At De Hoop, cpue varied significantly among anglers, months, years and gear type. The mortality rate was greater at the exploited site than at protected sites, where mortality rates were taken as estimates of natural mortality. Tag and recovery data were used to estimate density and catchability. Fishing mortality rates were very high, whether inferred from size distributions or from the product of effort counts and the catchability coefficient. Such high rates can be sustained only through refuges. Variation in fishery parameters among habitats and the high variance in cpue data suggest that an effective monitoring programme will need to be intensive. To provide adequate stock assessments, fishery-dependent surveys should be augmented by studies in marine protected areas.

Patterns in the benthic communities on the shelf of the sub-Antarctic Prince Edward Islands

SummaryBenthic communities at the sub-Antarctic Prince Edward Islands were sampled qualitatively with an epi-benthic sled at 57 stations over a five year period. Additional information on these communities was obtained from replicate underwater photographs. In total, 546 macrofaunal species were found. Despite the extreme isolation and geological youth of the islands, there seems to have been a diverse colonisation. A cluster-analysis based on species abundance aggregated most stations into 8 groups. Each of these groups had a unique depth and substratum combination. Thourella variabilis (Octocoralea), and Cabarea darwinii (Bryozoa) characterised deep rocky assemblages and Serpula vermicularis, Lanice conchilega (Polychaeta) and Magellania kerguelenensis (Brachiopoda) characterised soft substrata. Indicator species separating each of the 8 clusters are given. Trends in percentage cover and densities of major taxa with respect to depth and substratum, as shown by photographs, are described. Deep, rocky substrata were dominated by Porifera, Bryozoa and Cnidaria, while soft-substrata were dominated by Polychaeta, Bivalvia and Brachiopoda. Errant forms consisted predominantly of Echinodermata, with Crustacea being important in shallower habitats. Filter-feeders dominated the benthos of the islands in all habitats.

Alternate explanations of the dispersal pattern of galjoen Dichistius capensis

The movement behaviour of galjoen Dichistius capensis (>250mm total length) was studied by using a tag and recapture technique. A total of 25 191 galjoen was tagged at four sites in South Africa. Three of the sites were in fully protected reserves (two in the De Hoop Marine Protected Area, one in the Tsitsikamma National Park), whereas the fourth site was at the Cape Peninsula where fishing is permitted. In all, 2 174 fish were recaptured, some of them several times. The vast majority of fish were caught at the site of release. After correcting for differences in recovery effort between the release sites and other areas, it was estimated that 95% of tagged fish remained at the release site. The remaining 5% moved throughout the South African range of galjoen, which was separated from the Namibian range. There was no evidence to suggest that movement is linked to season, age or sex. Although not territorial, galjoen do hold home ranges, which were estimated to be no larger than 1.38km in extent, but probably much smaller. Two likely models, the polymorphic and the tourist, are advanced to explain the movement behaviour. The polymorphic model is a combination of two movement patterns, one resident and another nomadic. The balance between the two may represent a mixed evolutionary stable strategy. The tourist model does not differentiate between fish. Each fish spends its time at a small number of widely separated sites, moving between them as conditions dictate. The tag and recovery data do not favour one model above the other unequivocally.

Production and standing stocks of the kelp Macrocystis laevis Hay at the Prince Edward Islands, Subantarctic

SummaryThe recently described species Macrocystis laevis Hay is endemic to the Prince Edward Islands. Aerial photographs of Marion Island were used to outline the distribution of the kelp and to assess its cover. M. laevis occurs along the lee shore of the island, between the 5 and 20 m isobaths. Plant densities and gross plant morphology were measured by divers during April/May 1988. Net production was estimated from growth measurements taken in April/May 1988 and 1989 and again during August 1989. The mean biomass of kelp was 0.67 kgC·m−2 within the kelp beds. Net production was estimated at 7.7 gC·m−2·d−1 and 11.5 gC·m−2d−1 during the months of April and August respectively. M. laevis had a uniform frond-length frequency distribution, which suggests that only the oldest fronds are lost by wave action or senescence. Based on calculations for M. laevis and Durvillaea antarctica (the two species making up most of the macrophyte biomass) macrophytes are more productive per unit area than the phytoplankton but contribute less to the seas around the Prince Edward Islands by virtue of their small spatial coverage. Neither of the kelps lose much material as particulate or dissolved organic carbon through fragmentation. The extent of grazing on live M. laevis fronds is unknown, and only D. antarctica contributes to a macrofaunal detrital community. The contribution of M. laevis production to the nearshore ecology of the islands seems limited, as we suspect that almost all of its production is exported to the open ocean pelagic system.

Long-term effects of two sizes of surgically implanted acoustic transmitters on a predatory marine fish (Pomatomus saltatrix)

Fish telemetry is increasingly used to study fish behaviour in marine systems and it is crucial that the tagging does not affect fish behaviour and welfare negatively. Hence, the long-term effects of surgically implanted acoustic telemetry transmitters on survival, tag retention, healing and growth on Pomatomus saltatrix (known as tailor in Australia, elf in South Africa and bluefish in North America) were studied over 144 days. P. saltatrix are well suited for transmitter implants because no tagging-related short- or long-term mortality or transmitter expulsion was recorded. Small transmitters (9 × 28 mm, 3.3 g in water) did not affect the specific growth rate of tagged fish compared with an untagged control group. In contrast, the fish tagged with large transmitters (13 × 50 mm, 6.9 g in water) had a reduced specific growth rate. The small, but not the large transmitter, seemed suitable for tagging P. saltatrix of the body sizes represented in this study (215–621 g). The specific growth rate was negatively affected by increasing the transmitter-mass-in-water to body-mass ratio. It is recommended that this ratio should not exceed 1.4%.