Theory predicts that loss of gape-limited sharks should lead to increases in the abundance and biomass of smaller size classes of prey. We used stereo-baited remote underwater video stations ...(stereo-BRUVS) and stereo diver-operated video systems (stereo-DOVS) to characterise the shark and fish assemblages on 2 remote, atoll-like reef systems in northwestern Australia, the Rowley Shoals and the Scott Reefs. Whereas the Rowley Shoals is a marine protected area, sharks have been removed from the Scott Reefs for over 3 centuries. We found that sharks were significantly more diverse, more abundant, larger in size and greater in biomass in the marine reserve relative to the Scott Reefs. Consistent with a priori hypotheses, bony fishes displayed greater species diversity, abundance and biomass where sharks were common relative to the predator-depleted location. The size and trophic structure of bony fish assemblages also differed between locations. Our results provide large-scale evidence consistent with the hypothesis that reef-associated sharks are gape-limited trophic omnivores that impose top-down effects on medium sized (<50 cm), low- to mid-trophic level fishes. On stereo-BRUVS, for example, prey in the 0 to 29.99 cm size class had 203% more biomass at the predator-depleted reef relative to the location where sharks were abundant. As body size is an important determinant of ecological role and fitness in fishes, these findings suggest that the rapid and ongoing loss of sharks from reefs globally may have important implications for reef management and investigations into the effect of fishing on reef systems.
Establishing the ecological role of predators within an ecosystem is central to understanding community dynamics and is useful in designing effective management and conservation strategies. We ...analysed differences in the trophic ecology of four species of reef sharks (
Carcharhinus melanopterus
,
Carcharhinus amblyrhynchos
,
Triaenodon obesus
and
Negaprion acutidens
) at Ningaloo Reef, Western Australia, by analysing tissue stable isotopes (δ
15
N and δ
13
C). We also monitored animals using acoustic telemetry to determine long-term residency patterns in a bay at the southern end of the reef, Coral Bay. Overall, mean δ
13
C was similar among species, ranging between −10.9 and −11.8‰, suggesting a food-web dependency on coastal producers. Classification and regression tree analysis identified an effect of species on δ
15
N that separated
C. amblyrhynchos
and
C. melanopterus
from
N. acutidens
and
T. obesus
. For
C. amblyrhynchos
and
C. melanopterus
, animals were also divided by size classes, with smaller sharks having lower average δ
15
N than larger animals; this suggests that δ
15
N increases with size for these two species. Juvenile
C. melanopterus
, juvenile
N. acutidens
and adult
T. obesus
had trophic levels of 3.7, for juvenile
C. amblyrhynchos
and adult
C. melanopterus
it was 4, and adult
C. amblyrhynchos
had a value of 4.3. Trophic-level estimates for
C. melanopterus
and
C. amblyrhynchos
corroborate previous conclusions based on diet studies. We found no evidence for a difference in isotopic composition between resident and non-resident sharks. The lack of variation in isotopic composition was consistent with high mean residency of these species recorded using acoustic telemetry, which was 79% (±0.09 SE) of days monitored for
T. obesus
, followed by
N. acutidens
(57 ± 19.55%),
C. amblyrhynchos
(54 ± 13%) and
C. melanopterus
(33 ± 8.28%). High δ
13
C composition in reef sharks and long-term residency behaviour suggest that coastal marine reserves might provide effective conservation refuges for some species.
Reproductive philopatry in bull sharks Carcharhinus leucas was investigated by comparing mitochondrial (NADH dehydrogenase subunit 4, 797 base pairs and control region genes 837 base pairs) and ...nuclear (three microsatellite loci) DNA of juveniles sampled from 13 river systems across northern Australia. High mitochondrial and low microsatellite genetic diversity among juveniles sampled from different rivers (mitochondrial φST = 0·0767, P < 0·05; microsatellite FST = −0·0022, P > 0·05) supported female reproductive philopatry. Genetic structure was not further influenced by geographic distance (P > 0·05) or long‐shore barriers to movement (P > 0·05). Additionally, results suggest that C. leucas in northern Australia has a long‐term effective population size of 11 000–13 000 females and has undergone population bottlenecks and expansions that coincide with the timing of the last ice‐ages.
Reef manta rays Mobula alfredi are large filter-feeding elasmobranchs that are undergoing substantial population declines on a global scale. In order to effectively conserve and manage populations, ...it is crucial that the drivers of their occurrence are defined and that key aggregation areas for this species are identified and protected. Here, we used passive acoustic telemetry to monitor and assess the movement ecology of M. alfredi in the remote Amirante Islands, Republic of Seychelles. Acoustic transmitters were externally deployed on M. alfredi at D’Arros Island (n = 42) and movement data retrieved from an array of 70 acoustic receivers deployed throughout the Amirantes between November 2013 and October 2017. Individuals were detected year-round, with a peak in detections occurring between November and April coinciding with the arrival and departure of the north-west monsoon. Individuals were most likely to be detected within the array during the day, at low wind speeds, and when water temperatures were approximately 28°C. Additionally, individuals were more likely to be detected during a new moon, when the tidal range was at its highest, and on the slack of high tide. M. alfredi travelled widely within the Amirantes, with larger individuals travelling greater distances per day than smaller individuals and juveniles. The majority of detections (89%) were recorded within 2.5 km of the shoreline of D’Arros Island and the neighbouring St. Joseph Atoll, highlighting the importance of these sites to M. alfredi in the Amirante Islands, and supporting the proposed development of a marine protected area at this location.
We compared catches of settlement-stage reef fishes in light traps attached to under-water speakers playing reef sounds with those of silent traps during a summer recruitment season at Lizard Island, ...Great Barrier Reef, Australia. Of the total 40191 reef fishes we collected, significantly more (67%; Wilcoxon and Binomial tests: p< 0.001) appeared in the traps with broadcast reef noise. Traps deployed with speakers consistently caught a greater diversity of species (Wilcoxon test: p< 0.001, total 81 vs 68) than did silent traps. This study provides a clear demonstration that the settlement-stages of a broad range of families of coral reef fishes are attracted to reef sounds.
The idea that the presence of sharks impacts the behavior of mesopredatory reef fishes is controversial and lacks clear evidence at reef‐wide scales. We compared the abundance and behavior of these ...reef fishes in response to the presence of reef sharks using Baited Remote Underwater Video System (BRUVS) deployments in two adjacent reef systems where sharks have either been exclusively targeted by fishing or protected by a no‐take marine reserve. For a subset of videos, we also compared the behavior of mesopredatory reef fishes immediately before and after the appearance of sharks in the video. On reefs where sharks were more abundant, mesopredatory fishes spent less time swimming in midwater (i.e., away from shelter) and guarding bait compared to reefs where sharks have been selectively removed. The same responses occurred after the appearance of sharks in the video. Reactions to sharks varied both in strength and type among species of mesopredator and were mediated by the availability of shelter on the reef and, for one species, by the levels of activity of the reef sharks. In contrast, we did not find that the presence of sharks influenced the abundance of mesopredators at either reef system across hour‐long videos or immediately before and after a shark appeared in the video. Collectively, our findings show that the presence of sharks reduces the propensity of mesopredatory fish to engage in potentially risk‐prone behaviors over large spatial scales and that these interactions are mediated by the behavioral characteristics of both predators and prey, and the environment in which they co‐occur. Our results are consistent with the idea that sharks as predators or larger competitors initiate changes in the behavior of mesopredatory reef fishes likely to affect trophic structuring within coral reef ecosystems.
Predation on parasites is an important ecological process, but few experimental studies have examined the long-term impacts on the prey. Cleaner fish prey upon large numbers and selectively feed on ...the larger individuals of the ectoparasitic stage of gnathiid isopods. Removal of cleaner fish Labroides dimidiatus for 1.5–12.5 years negatively affects coral reef fishes, but the mechanism is unclear. A reduction in local parasite populations or the size of individual parasites would benefit all susceptible fishes. We tested whether cleaner presence reduces local gnathiid populations using 18 patch-reefs distributed between two sites (both at Lizard Island, Great Barrier Reef) which were maintained cleaner-free or undisturbed for 12 years. Using emergence traps (1 m²), free-living gnathiid stages were sampled before and after cleaner fish were removed during the day and night, up to 11 times over the course of the experiment. There were effects of the removal in the predicted direction, driven largely by the response at one site over the other involving 200% more gnathiids, but manifested only in the daytime sampling after 4 months. There was also a main effect (36%) for the shared sample dates at both sites after 12 years. Gnathiid size occasionally differed with cleaner presence, but in no consistent way over time. Contrary to our predictions, changes in free-living gnathiid population numbers and their size structure rarely reflected the changes in fish populations and individuals observed on cleaner-free reefs. Therefore, evidence that this predator alone regulates gnathiids remains limited, suggesting other contributing processes are involved.
Bigger is better Meekan, M. G.; Vigliola, L.; Hansen, A. ...
Marine ecology. Progress series (Halstenbek),
07/2006, Letnik:
317
Journal Article
Recenzirano
Odprti dostop
A cohort of the fast-growing spratSpratelloides graciliswas sampled during late-larval, juvenile and adult life-history phases using light traps on the North West Shelf of Western Australia. Otoliths ...from 154 larvae, juveniles and adults that hatched during a 20 d window were analysed to produce back-calculated daily records of size-at-age and growth rate. These traits were compared among sequential samples using repeated-measures MANOVAs (multivariate analyses of variance) to determine whether selective mortality occurred in the cohort. Late-stage larvae in our catches averaged 23 ± 3 d of age and were 22 ± 2 mm standard length (SL), while juveniles averaged 47 ± 6 d of age and 36 ± 6 mm SL. We found that individuals that survived the larval stage to become juveniles underwent strong selective mortality. This selective mortality acted to preferentially remove fish that were slow-growing and/or relatively small members of the cohort. The size variation on which this selection acted was present at hatching and propagated by growth during the larval stage. Size at hatching is principally determined by egg size, implying that maternal contributions had an important influence on the outcome of selective events. We found no evidence of selective mortality operating during the transition of juvenile sprats to adulthood. Adults averaged 78 ± 6 d of age and 44 ± 5 mm SL. Log-linear analyses indicated that the cohort underwent 8.6% mortality mm–1SL and had a daily mortality rate of 3.7% between larval and adult stages. Given an average linear growth rate of 0.96 mm d–1during the larval phase, this suggests that selective mortality based on size (bigger-is-better) was approximately twice as important as mortality due to age differences (stage duration) among members of the cohort.
Whale sharks (Rhincodon typus) are thought to aggregate in nearshore waters around Christmas Island (105°37'E, 10o29'S) to consume the marine larvae of the endemic red land crab (Gecarcoidea ...natalis). However, there have been no direct observations of sharks feeding on crab larvae. Whale shark faeces were analysed using genetic testing to confirm the presence of crab larvae in their diet. Primers were designed for amplifying two Gecarcoidea natalis mitochondrial small-subunit (mtSSU) rDNA regions. Gel electrophoresis of polymerase chain reaction (PCR) products amplified from whale shark faecal DNA produced bands of the expected size for G. natalis templates. Specificity of both primer sets for G. natalis mtSSU rDNA was expected to be high from comparisons with mtSSU rDNA regions from closely related crabs and we confirmed their specificity empirically. The amplification of fragments from faecal DNA of the same size as those produced from G. natalis DNA indicates that the whale shark had been feeding on G. natalis and that enough of the crab DNA survived digestion to be detected by these PCRs. Our study provides further evidence that aggregations of whale sharks in coastal waters occur in response to ephemeral but predictable increases in planktonic prey.
Mutualisms are pivotal in shaping ecological communities. Iconic images of cleaner fish entering the mouths of predatory fish clients to remove ectoparasites epitomize their mutual benefit. ...Experimental manipulations of cleaner wrasse reveal declines in fish size and growth, and population abundance and diversity of client fishes in the absence of cleaner wrasse. Fishes grow more slowly and are less abundant and diverse on reefs without cleaner wrasse, both for larger species that are regularly cleaned and have high ectoparasite loads (“attractive species”), and for those smaller species that are rarely cleaned and are rarely infested with parasites (“unattractive species”). We therefore considered whether these previously observed declines in individual and population parameters on reefs without cleaners were related to increased ectoparasite infestation using an attractive species (
Hemigymnus melapterus
, Labridae) and an unattractive species (
Pomacentrus amboinensis
, Pomacentridae). Traps with these fish as a form of bait were deployed to sample blood-sucking gnathiid ectoparasites (Gnathiidae: Isopoda) on reefs from which cleaners (
Labroides dimidiatus
, Labridae) have been removed for 13 yr. Cleaner fish could not enter traps to access the clients/hosts, but gnathiids could enter the traps to infest hosts; thus, this method sampled the indirect effect of cleaners on gnathiid infestation of fish. Infestation was higher on reefs without cleaners than on those with them. The effect was only detected during the daytime when cleaners are active and only on the attractive species (
H. melapterus
). Thus, cleaner presence indirectly reduced fish exposure to parasites in a species that is highly susceptible to parasites, but not in one that is rarely infested with parasites. This suggests that cleaner presence indirectly reduces exposure of a common fish species to harmful parasites, which may explain some observed benefits in fishes at this location.
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