Vertical Distribution of Denitrification Potential,Denitrifying Bacteria,and Benzoate Utilization in Intertidal Microbial Mat Communities

Aspects of denitrification and benzoate degradation were studied in two estuarine microbial mat communities on the California coast by measuring the depth distributions of potential denitrification rates, genetic potential for denitrification, nitrate concentration, benzoate mineralization rates, total bacterial abundance, and abundance of a denitrifying strain (TBD-8b) isolated from one of the sites. Potential denitrification was detected in microbial mat cores from both Elkhorn Slough and Tomales Bay. Maximum denitrification rates were more than two orders of magnitude higher at Elkhorn Slough (3.14 mmol N m⁻² d⁻¹) than at Tomales Bay (0.02 mmol N m⁻² d⁻¹), and at both sites, the maximum rates occurred in the 0–2 mm depth interval. Ambient pore [NO₃+NO₂] was substantially higher at Elkhorn Slough than at Tomales Bay. Incorporation and mineralization of benzoate was maximal near the mat surface at Elkhorn Slough. The areal rate of benzoate utilization was 1045 nmol C m⁻² d⁻¹, which represented utilization of 70% of the added substrate in 24 h. Total bacterial and TBD-8b abundances were greatest near the surface at both Tomales Bay and Elkhorn Slough, and TBD-8b represented less than 0.2% of the total. Genetic potential for denitrification, quantified by hybridization with a nitrite reductase gene fragment, was present below the mat surface at average levels representing presence of the gene in approximately 10% of the total cells.     

Elevated body temperatures of adult female leopard sharks, Triakis semifasciata, while aggregating in shallow nearshore embayments: Evidence for behavioral thermoregulation?

The leopard shark, Triakis semifasciata, is an abundant nearshore elasmobranch, ranging from Baja California, Mexico to Oregon, USA. Mature female leopard sharks have been observed aggregating in shallow embayments throughout California; however, it is unclear why only females aggregate in these shallow areas. The goal of this study was to determine if mature female leopard sharks selectively occupy the warmest areas of a shallow embayment, if free-ranging leopard sharks' body temperatures are significantly warmer during the day than at night, and to quantify temporal use of these shallow habitats. Visual observations of sharks' fine-scale movements within the shallows of Big Fisherman's Cove Marine Life Refuge (Santa Catalina Island) aggregation site indicated that sharks preferred the warmest areas of the embayment and moved to warmer locations over the course of the day (p < 0.05). Active and passive acoustic tracking, along with archival transponder technology (Vemco: V13, V13-R256, VX32TP-CHAT tags respectively) of 16 sharks caught and tagged within this aggregation were used to monitor core body temperature, swimming depth, and movements. Sharks had significantly higher core body temperatures in the late afternoon (1700 h-2000 h) during the summer, showed increased fidelity to thermal refuges during the day and increased movement away from these refuges at night (χ², p < 0.05). Seasonal variations in warm, shallow water usage were also observed. Elevated core body temperature of mature female leopard sharks using warm shallow embayments will likely augment metabolic and physiological functions such as digestion, somatic growth, and possibly reproduction.     

Habitat Differences in Marine Invasions of Central California

We carried out a two-part investigation that revealed habitat differences in marine invertebrate invasions. First, we compared invasion levels of hard vs soft substrata in Elkhorn Slough, an estuary in Central California, by comparing abundance and richness of native vs exotic species in quantitative samples from each habitat type. Our results revealed that the hard substrata were much more heavily invaded than the soft substrata. Nearly all the hard substrata in Elkhorn Slough, as in most estuaries along the Pacific coast of North America, are artificial (jetties, rip-rap, docks). Some exotic species may by chance be better adapted to this novel habitat type than are natives. Two major vectors responsible for marine introductions, oyster culturing and ship-hull fouling, are also more likely to transport species associated with hard vs soft substrata. Secondly, we compared estuarine and open coast invasion rates. We examined species richness in Elkhorn Slough and adjacent rocky intertidal habitats along the Central California coast. The absolute number of exotic species in the estuary was an order of magnitude higher than along the open coast (58 vs 8 species), as was the percentage of the invertebrate fauna that was exotic (11% vs 1%). Estuaries on this coast are geologically young, heavily altered by humans, and subject to numerous transport vectors bringing invasive propagules: all these factors may explain why they are strikingly more invaded than the open coast. The finding that the more species rich habitat – the open coast – is less invaded is in contrast to many terrestrial examples, where native and exotic species richness appear to be positively correlated at a broad geographic scale.     

Movements of brown smoothhounds, <Emphasis Type="Italic">Mustelus henlei</Emphasis>, in Tomales Bay,California

Ultrasonic telemetry was used to analyze the effects of environmental variables on movement directions and movement rates of brown smoothhounds, Mustelus henlei, in Tomales Bay, California. Ultrasonic transmitters were surgically implanted in the peritoneal cavities of one male and five female brown smoothhounds and tracked during the period of 29 June to 15 July 2004. Coarse-scale tracking consisted of locating all tagged individuals multiple times during a single session, while fine-scale tracking consisted of following a single individual continuously during a session. Coarse-scale tracking suggested movement toward the inner bay with incoming and high tides and toward the outer bay with outgoing and low tides (P = 0.01), whereas the diel cycle had no apparent effect on their movement directions. Mean shark movement rate was 0.09 m s⁻¹ (range: 0.01–0.34 m s⁻¹), with diel and tidal cycles both having significant effects on their rates of movement (P = 0.02 and P < 0.01), respectively. We tracked two female sharks on a fine scale over three tracking sessions in July 2004. Both individuals exhibited higher rates of movement during the night compared to the day (P < 0.01). While one shark’s rate of movement was not significantly affected by tidal stage, the other’s was (P < 0.001).     

Flooding regimes increase avian predation on wildlife prey in tidal marsh ecosystems

Within isolated and fragmented populations, species interactions such as predation can cause shifts in community structure and demographics in tidal marsh ecosystems. It is critical to incorporate species interactions into our understanding when evaluating the effects of sea‐level rise and storm surges on tidal marshes. In this study, we hypothesize that avian predators will increase their presence and hunting activities during high tides when increased inundation makes their prey more vulnerable. We present evidence that there is a relationship between tidal inundation depth and time of day on the presence, abundance, and behavior of avian predators. We introduce predation pressure as a combined probability of predator presence related to water level. Focal surveys were conducted at four tidal marshes in the San Francisco Bay, California where tidal inundation patterns were monitored across 6 months of the winter. Sixteen avian predator species were observed. During high tide at Tolay Slough marsh, ardeids had a 29‐fold increase in capture attempts and 4 times greater apparent success rate compared with low tide. Significantly fewer raptors and ardeids were found on low tides than on high tides across all sites. There were more raptors in December and January and more ardeids in January than in other months. Ardeids were more prevalent in the morning, while raptors did not exhibit a significant response to time of day. Modeling results showed that raptors had a unimodal response to water level with a peak at 0.5 m over the marsh platform, while ardeids had an increasing response with water level. We found that predation pressure is related to flooding of the marsh surface, and short‐term increases in sea levels from high astronomical tides, sea‐level rise, and storm surges increase vulnerability of tidal marsh wildlife.     

Changes in trophic level of Squatina guggenheim with increasing body length: relationships with type, size and trophic level of its prey

The occurrence of changes in the trophic level (TL) of sharks with growth has not been quantified until now. Here length-related changes on Squatina guggenheim Marini trophic level were determined, and shifts in type, size and trophic level of its prey were analysed. Sampling took place during five bottom trawl surveys conducted in the Argentine–Uruguayan Common Fishing Zone during spring (December/1995, October/1997) and fall (March/1997, March–April/1998, May–June/1998), using an Engel bottom-trawl net to capture the sharks. Three length groups were defined based on diet composition and using a cluster analysis (group I, 23–60 cm; group II, 61–80 cm; group III, 81–91 cm L _T). An ANOSIM procedure detected significant differences (P < 0.05) in the diet spectrum between the three length groups. The smallest sharks (group I) ingested fish prey ranging from 5 to 21 cm L _T, medium sharks (group II) fed on fish prey between 11 and 35 cm L _T, and largest sharks (group III) preyed on fish between 13 and 40 cm L _T. Diet structure of length groups were discriminated by almost the same prey taxa that characterized them. The increase of S. guggenheim body length promoted a decrease in the relative importance of small pelagic fishes. Contrarily, prey as medium benthopelagic fishes, medium pelagic squid and medium benthopelagic fishes showed an inverse tendency, indicating a broad diet spectrum of adults. Predator-length and prey-length relationship indicated a trend where 44.8% of S. guggenheim diet was integrated by prey <20% of their own body length and 32.8% of their diet was composed by prey >30% of their own length. The increase of mean prey weight was associated with the increase of predator weight and length. Smallest sharks (group I) were identified as secondary consumers (TL < 4) whereas medium sharks (group II) and largest sharks (group III) were placed as tertiary consumers (TL > 4). The study revealed an increase in S. guggenheim TL with shark growth as a consequence of changes on type, size and TL of prey ingested.     

Feeding habits of the blue shark, Prionace glauca, and salmon shark, Lamna ditropis, in the transition region of the Western North Pacific

We describe the feeding habits of 70 blue sharks (Prionace glauca) and 39 salmon sharks (Lamna ditropis) caught at 0–7 m depth at night by research drift gillnets in the transition region of the western North Pacific during April–May of 1999 and 2000. Blue sharks of 50–175 cm total length fed on a large variety of prey species, consisting of 24 species of cephalopods and 16 species of fishes. Salmon sharks of 69–157 cm total length fed on a few prey species, consisting of 10 species of cephalopods and one species of fish. Important prey for the blue sharks were large, non-active, gelatinous, meso- to bathypelagic cephalopods (e.g., Chiroteuthis calyx, Haliphron atlanticus, Histioteuthis dofleini and Belonella borealis) and small myctophid fishes. Important prey for the salmon sharks were mid-sized, active, muscular, epi- to mesopelagic squids (e.g. Gonatopsis borealis, Onychoteuthis borealijaponica and Berryteuthis anonychus). Our results suggest that blue sharks feed on cephalopods mainly during the daytime when they descend to deep water. Salmon sharks may feed opportunistically with no apparent diurnal feeding period. Blue sharks and salmon sharks have sympatric distribution in the transition region in spring; they have different feeding habits and strategies that reduce competition for food resources.     

Population abundance, structure and turnover estimates for leopard seals during winter dispersal combining tagging and photo-identification data

Winter dispersal in leopard seals is poorly understood because of its low density in most of its range. By combining photo-identification and tagging data from Bird Island, South Georgia, in mark-recapture models, leopard seal abundance over the winter of 2005 was estimated as 118 (95% CI: 78–179). Seasonal residents arrived earlier and stayed longer around the island (27 days; 95% CI: 23–32) and their numbers were low and stable over the winter Most of the seals (81; 95% CI: 31–130) were young transients, stayed only 1–7 days, and arrived later in the season. This suggests (1) very low predatory pressure upon the locally abundant prey populations; (2) two different patterns of winter movements: a winter migration in adult seals with long-term site fidelity, and large numbers of juveniles in dispersal possibly attracted by locally abundant prey colonies, and potentially influenced by increased environmental stress.     

Spatial ecology of juvenile lemon sharks (<Emphasis Type="Italic">Negaprion brevirostris</Emphasis>) in tidal creeks and coastal waters of Eleuthera,The Bahamas

Fisheries exploitation and habitat alteration are threatening lemon shark (Negaprion bevirostris) populations because they use nearshore regions as nursery sites. As such, there is a need for information on the spatial ecology of juvenile lemon sharks to identify critical habitats that require protection, as well as to understand their basic ecology. The purpose of this study was to determine the habitat preferences and movement patterns of juvenile lemon sharks along a sub-section of coastline characterized by coastal flats and tidal creeks of Eleuthera, The Bahamas. Eleven juvenile lemon sharks (766 ± 127 mm total length; mean±SD) were captured from various tidal creeks within the 23 km study area and were surgically implanted with acoustic transmitters. A series of 27 hydrophone receivers acted as a passive monitoring array to detect tagged individuals as they moved among habitats. Findings suggest that juvenile lemon sharks tagged in this study prefer shallow water habitats within tidal creeks, and typically display high site fidelity with occasional forays to alternate habitats or creeks. In fact, more than 90% of tagged lemon sharks had the greatest percentage of detections located at a receiver at or close to the location where they were tagged. There was no evidence of differences in diel or seasonal movement and habitat use. Knowledge gained from this study will be useful for directing future conservation and management strategies including coastal development plans and marine protected areas.     

Tidal Influence on Spatial Dynamics of Leopard Sharks, Triakis semifasciata, in Tomales Bay, California

We used ultrasonic telemetry to determine the movement directions and movement rates of leopard sharks, Triakis semifasciata, in Tomales Bay, California. To analyze tide and time of day effects, we surgically implanted transmitters in the peritoneal cavities of one male and five female leopard sharks, which we located during summer for three to five sampling sessions lasting 12 to 24h each. All leopard sharks showed strong movement direction patterns with tide. During incoming tides, sharks moved significantly (p<0.0001) towards the inner bay, apparently to exploit the extensive inner bay muddy littoral zones' food resources. On outgoing tides, sharks showed significant (p<0.0001) movements towards the outer bay. During high tide, there was no discernible pattern to their movements (p=0.092). Shark movement rates were significantly (p<0.0001) greater during dark periods (mean±SE: 10.5±1.0m min^–1), compared with fully lighted ones (6.7±0.5m min^–1). Movement rates of longer sharks tended to be greater than those of shorter ones (range means±SE: 5.8±0.6m min^–1 for the 91cm shark, to 12.8±1.6m min^–1 for the 119cm shark), but the leopard sharks' overall mean movement rate (8.1±0.5m min^–1) was slower than other (more pelagic) sharks.     

Coastal swimming patterns of white sharks (<Emphasis Type="Italic">Carcharodon carcharias</Emphasis>) at Mossel Bay,South Africa

Between June and December 2005, active and passive acoustic telemetry was used to examine fine scale movements of 13 white sharks (Carcharodon carcharias) (ten passive, three active) at Mossel Bay. A total of 24 active trackings (ranging from 2 h to 103 h in duration) were conducted. Patterns of rate of movement (ROM), swimming linearity (LI), swimming bearing, and instantaneous swimming speed (ISS) were assessed. A conversion quotient (Q) of 1.21 between ISS and ROM (10 min sample interval) was calculated suggesting ROM is a good indicator of white shark activity. The mean ROM for tracked sharks was 0.52 m·s⁻¹, with a greatest sustained ROM of 1.33 m·s⁻¹ (4.8 km·h⁻¹). Sharks displayed greatest LI and ROM during directional travels between the three persistent aggregation sites. The majority of the shark movement was, however, non-linear as the sharks repeat patrolled at the three aggregation sites. Two of these sites were not associated with pinniped presence, and sharks typically patrolled back and forth parallel to the shore line at a comparatively low ROM which suggested resting. The third aggregation site was adjacent to Seal Island, and despite low LI, sharks displayed a high ROM, indicating high activity levels. We propose that the high ROM is related to maximising search area when patrolling to hunt Cape fur seals (Arctocephalus p. pusillus).     

Demography and movement patterns of leopard sharks (Triakis semifasciata) aggregating near the head of a submarine canyon along the open coast of southern California, USA

The demography, spatial distribution, and movement patterns of leopard sharks (Triakis semifasciata) aggregating near the head of a submarine canyon in La Jolla, California, USA, were investigated to resolve the causal explanations for this and similar shark aggregations. All sharks sampled from the aggregation site (n?=?140) were sexually mature and 97.1 % were female. Aerial photographs taken during tethered balloon surveys revealed high densities of milling sharks of up to 5470 sharks ha^?1. Eight sharks were each tagged with a continuous acoustic transmitter and manually tracked without interruption for up to 48 h. Sharks exhibited strong site-fidelity and were generally confined to a divergence (shadow) zone of low wave energy, which results from wave refraction over the steep bathymetric contours of the submarine canyon. Within this divergence zone, the movements of sharks were strongly localized over the seismically active Rose Canyon Fault. Tracked sharks spent most of their time in shallow water (≤2 m for 71.0 % and ≤10 m for 95.9 % of time), with some dispersing to deeper (max: 53.9 m) and cooler (min: 12.7 °C) water after sunset, subsequently returning by sunrise. These findings suggest multiple functions of this aggregation and that the mechanism controlling its formation, maintenance, and dissolution is complex and rooted in the sharks’ variable response to numerous confounding environmental factors.     

Insights into Young of the Year White Shark, Carcharodon carcharias, Behavior in the Southern California Bight

A young of the year female white shark, Carcharodon carcharias, was tagged with a pop-up satellite archival tag off Southern California in early June of 2000. The tag was recovered after 28 days, and records of temperature, depth and light intensity were extracted. Depth and temperature records indicate a number of interesting behaviors, including a strong diurnal pattern. At night the shark remained in the top 50 m, often making shallow repetitive vertical excursions. Most dives below the mixed layer were observed during the day, 91% of which occurred from 05:00 to 21:00 h, with depths extending to 240 m. Many of the dives exhibited secondary vertical movements that were consistent with the shark swimming at the bottom (at depths from 9 to 165 m) where it was most likely foraging. The white shark experienced dramatic and rapid changes in temperature, and demonstrated a considerable tolerance for cold waters. Temperatures ranged from 9°C to 22°C, and although 89% of the total time was spent in waters 16–22°C, on some days the small shark spent as much as 32% of the time in 12°C waters. The deep dives into cold waters separate the white sharks from mako sharks, which share the California Bight nursery ground but appear to remain primarily in the mixed layer and thermocline. Movement information (derived from light-based geolocation, bottom depths and sea surface temperatures) indicated that the white shark spent the 28 days in the Southern California Bight, possibly moving as far south as San Diego, California. While the abundance and diversity of prey, warm water and separation from adults make this region an ideal nursery ground, the potential for interaction with the local fisheries should be examined.     

Regulation of body temperature in the white shark, Carcharodon carcharias

Stomach temperatures of three white sharks, Carcharodoncarcharias, (one reported previously and two new individuals) were intermittently recorded by acoustic telemetry at the South Farallon Islands, central California. Temperature profiles of the water column were obtained concurrently. Stomach temperatures were elevated over ambient water temperatures by as much as 14.3 °C. Stomach temperatures varied within a narrow range while ambient water temperature fluctuated over a much larger range, showing that this species regulates its body temperature. These data, in combination with previous work on the physiology and anatomy of white sharks, indicate that the white shark is endothermic. It appears that the heat retention system in lamnid sharks has allowed them to inhabit cold water and remain active predators of swift and agile prey. Accepted: 17 February 1997     

The influence of tidal cycles and freshwater inflow on the distribution and movement of an estuarine resident fish Acanthopagrus butcheri

Movement patterns and habitat utilization by black bream Acanthopagrus butcheri (Sparidae), an estuarine resident species, were investigated using acoustic telemetry in a small estuary on the east coast of Tasmania, Australia. Thirty‐four adult A. butcheri were tracked for periods of up to 187 days between August 2005 and January 2006. Although able to tolerate a wide range of salinities, the fish spent most of the time within the upper and middle regions of the estuary, where brackish conditions dominated. The species exhibited extensive movements linked to tidal cycles, with small‐scale upstream movements during incoming tides and downstream movements during out going tides. The extent of these movements was positively correlated with the tidal height difference between consecutive tidal peaks and troughs. Freshwater inflows and resultant changes in salinity also significantly influenced distribution and movement patterns. Fish moved downstream during the periods of heavy inflows, returning upstream as salinities increased to c. >10. During the peak of spawning period (November to December) fish moved into the upper region of the estuary, where they aggregated to spawn. Periodic increases in freshwater discharge, however, resulted in fish leaving the spawning grounds and moving downstream. Towards the end of the spawning season (January), the fish became more dispersed throughout the entire estuarine system.     

Measuring sediment accretion in early tidal marsh restoration

Sediment accretion is a critical indicator of initial progress in tidal marsh restoration. However, it is often difficult to measure early deposition rates, because the bottom surface is usually obscured under turbid, tidally-influenced waters. To accurately measure early sediment deposition in marshes, we developed an echosounder system consisting of a specialized acoustic profiler, differential global positioning system unit, and laptop computer mounted on a shallow-draft boat. We conducted a bathymetry survey at the Tubbs Setback tidal restoration site on San Pablo Bay, California, along north–south transects at 25-m intervals. Horizontal position was recorded within 1 m each second and water depth to 1 cm every 0.05 s. Bottom elevations were adjusted for tidal height with surveyed tide gages. We created detailed bathymetric maps (grid cell size: 12.5 m × 12.5 m) by interpolation with inverse distance weighting. During the third year after restoration, sediment accretion averaged 57.1 ± 1.1 cm and the estimated sediment gain was 132,900 m³. The mean difference between the elevations from the bathymetry system and the 9 sediment pins was 2.0 ± 1.0 cm. The mean difference of the intersection points of east–west and north–south survey transects was 2.1 ± 0.2 cm, which provided a measure of repeatability with changing water levels. Our echosounder system provided accurate and repeatable measurements of sediment accretion of a recently restored tidal wetland, and this system proved to be a viable tool for determining sediment deposition in marshes and assessing early restoration progress.     

Diel and tidal changes in the distribution and feeding habits of Japanese temperate bass Lateolabrax japonicus juveniles in the surf zone of Ariake Bay

Japanese temperate bass Lateolabrax japonicus juveniles recruit to the surf zone and grow by feeding on commonly occurring coastal copepods. However, little is known about diel and tidal patterns in their migration and feeding habits. We sampled wild juveniles during the neap and spring tides, over periods of 24 h, with small seine nets in the sand flat of the eastern part of Ariake Bay, Kyushu, western Japan. In both the neap and spring tides, abundance of juveniles significantly increased during the daytime, being highest around the time of high tide. The relative gut fullness indices of juveniles drastically increased in the morning (0700–0900) and during the flood tide in the daytime, while major prey copepod (Paracalanus spp.) density in the ambient water was relatively constant. We summarized that L. japonicus juveniles would migrate to the surf zone after sunrise to feed on copepods, and then emigrate from the surf zone after sunset. The migratory behavior of L. japonicus juveniles would be influenced by light (daytime) and feeding activity influenced by both light (morning) and tidal condition (flood tide). The intertidal region of the tidal flat was recognized to be one of the important habitats for L. japonicus during their early life history.     

Major and Trace Elements in Zooplankton from the Northern Gulf of California During Summer

We report the distribution of major and trace element concentrations in epipelagic zooplankton collected in the Northern Gulf of California in August 2003. The Bray–Curtis index defined three element assemblages in zooplankton: (1) major metals, which included only two elements, Na (3.6–17.0%) and Ca (1.0–4.8%). Na had its highest concentrations in the shallow tidally mixed Upper Gulf, where high salinity, temperature, and zooplankton biomass (dominated by copepods) prevailed. Ca showed its highest concentrations south of Ballenas Channel, characterized by tidal mixing and convergence-induced upwelling, indicated by low sea-surface temperature, salinity, and zooplankton biomass; (2) Six biological essential elements, like Fe (80–9,100 mg kg⁻¹) and Zn (20–2,570 mg kg⁻¹), were detected in high concentrations in zooplankton collected near Guaymas Basin, which had high surface temperature and chlorophyll a concentrations. (3) Metals of terrigenous origin, such as Sc (0.01–1.4 mg kg⁻¹) and Th (0.03–2.3 mg kg⁻¹), and redox-sensitive metals, like Co (3–23.8 mg kg⁻¹); this was the assemblage with the largest number of elements (15). Both types of elements of assemblage 3 had maximum concentrations in the cyclonic eddy that dominates the summer circulation in the Northern region. We concluded that sediment resuspension by tidal mixing in the Upper Gulf, upwelling south of Ballenas Channel, and the cyclonic eddy were key oceanographic features that affected the element concentrations of epipelagic zooplankton in the Northern Gulf of California. Oceanographic mechanisms such as these may contribute to element incorporation in marine organisms in other seas.     

Distribution and movement of domestic rainbow trout, <Emphasis Type="Italic">Oncorhynchus mykiss</Emphasis>, during pulsed flows in the South Fork American River,California

We tracked the movements of ten small (SL = 25.5–31.0 cm) and ten large (SL = 32.0–38.5 cm) radio-tagged domestic rainbow trout (Oncorhynchus mykiss) in response to frequent pulsed releases of water in the South Fork American River (California) from July to October 2005. In week one all the small trout moved less than 1 km upstream or downstream of their release sites. Four small trout moved 1–3 km upstream or downstream of their release sites in the following 8 weeks. Seven out of ten large trout moved downstream after their release. In subsequent weeks most large trout showed smaller upstream and downstream movements, and were observed between 1 km upstream and 8 km downstream of their release sites. Our results suggest that domestic rainbow trout with SL > 25 cm are not forced downstream by daily pulsed flow increases from 5 to over 40 m³s⁻¹.     

Effects of environmental conditions on predator–prey interactions between white sharks (Carcharodon carcharias) and Cape fur seals (Arctocephalus pusillus pusillus) at Seal Island, South Africa

Effects of environmental factors on frequency and success rate of 2,546 natural predatory attacks by white sharks, Carcharodon carcharias, on Cape fur seals, Arctocephalus pusillus pusillus, were studied over an 8-year period at Seal Island, South Africa. Attacks occurred primarily during winter months (June–August). Attack frequency increased significantly during northerly winds, during high tides, and within 400 m of the island, but predatory success rate decreased with proximity to the island. Attacks occurred over a depth range of 5–31 m, with significantly more occurring at depths of 26–30 m. Attack frequency and success rate increased significantly at low light levels. These results are compared with published effects of environmental factors on white shark predation frequency at the Farallon Islands, California, and discussed in terms of the Predation Cycle. Suggestions for future work at this site are offered.