We examined the feeding ecology of whale sharks by analyzing the signature fatty acids of their sub-dermal tissue and those of an extensive set of potential prey collected at Ningaloo Reef, Western ...Australia, in 2013, 2014 and 2015. Sub-dermal tissue of whale sharks was low in lipid content (4.0 mg g−1 dry mass) and dominated by phospholipids (72% of total lipids), with a calculated energy density of 18.7 kJ g−1 dry mass. There was significant intraspecific variability in fatty acid profiles of whale sharks, with cluster analysis identifying 4 distinct groups in 2013 and 5 groups in 2014. As this variability was not related to sex or size-class, we suggest that it may be attributed to differences in the feeding habitats used by these groups of whale sharks. Variation in dietary patterns was also observed between years, likely due to changes in the primary and secondary producers. Examination of food web interactions showed that fatty acid profiles of whale sharks and their presumed prey were significantly different, suggesting that sharks fed over a wide range of habitats, including deep waters. Our findings show that signature fatty acids of sub-dermal tissue can be used to examine broad trophic pathways and to identify spatial and temporal changes in the diet of these large and wide-ranging animals.
Documentation of scarring patterns on marine megafauna provides a means of quantifying the risk of anthropogenic threats that occur in the open ocean, such as ship strike. This study investigated the ...rates and putative sources of scarring of whale sharks Rhincodon typus aggregating at Ningaloo, Western Australia. Identification photos of whale sharks were contributed by tourism operators and research groups over a 6 yr period. Analysis of this database found that 355 (38.8%) of 913 whale sharks individually identified between 2008 and 2013 exhibited some form of scarring. This decreased to 15.9% after the omission of categories of minor scarring (nicks and abrasions). An increase in the number of sharks with lacerations between 2008 and 2013 provides some evidence of increasing boat strikes over this time. However, capture-mark-recapture modelling using the multi-state open robust design found no evidence that major scarring influenced the apparent survival or residency time of whale sharks aggregating at Ningaloo. Although lacerations are a useful indication of the level of threat to whale sharks from boat strike, it cannot necessarily be attributed to boat activity in Ningaloo due to the migratory nature of whale sharks in this aggregation, which commonly venture beyond Australian waters. Close collaboration with whale shark tourism operators proved a vital tool to generate the volume of data required for this assessment, and provides a model for similar studies of other megafauna with an associated tourism industry.
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.
Global declines of shark populations are of concern because of their largely assumed role as moderators of ecosystem function. Without long-term data on movement patterns for many species, it is ...impossible to infer relative extinction risk, which varies as a function of range, dispersal and habitat specificity and use. The past 50 yr of research on coastal sharks has revealed common movement patterns among species. In the horizontal plane, measured home range size generally increases with body size. We demonstrate meta-analytically the effects of increasing body size and monitoring time on home range size. Changes in the extent of horizontal movement might arise from ontogeny, predator avoidance or environmental tolerances. In the vertical plane, movement patterns include oscillatory vertical displacement, surface swimming, diel vertical migration and swimming at depth. These vertical movements are often attributed to foraging or navigation, but have been quantified less than horizontal patterns. Habitat specificity is often correlated with environmental conditions such as depth, salinity, substratum, and in some cases, prey availability. Site fidelity is common in species that use nursery areas. However, fidelity to mating, pupping, feeding and natal sites has only been observed in a few species. To date, few studies have examined habitat partitioning, although some general patterns have emerged: habitats appear to be subdivided by benthos type, prey availability and depth. The conservation of coastal sharks can be facilitated in some cases by the use of marine protected areas, especially for coastal resident species using specific nursery, reproduction or feeding areas. Partial protected-area closures might be effective during aggregation or migration periods to protect older size classes, but these must be applied with other management strategies such as reduced fishing and size or bag limits to protect individuals throughout different life history phases. More long-term research on habitat use, migration patterns and habitat partitioning is essential for developing successful management initiatives for coastal shark populations.
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.
Records of age and growth stored within otoliths were used to compare early life history traits with patterns of light trap catches for the damselfish Stegastes partitus (Poey). Otoliths provided ...strong evidence that fast growing cohorts of S. partitus larvae had higher survivorship than slow growing cohorts. Average growth rates during the larval phase accounted for 83% of the variability in the magnitude of catches in light traps on a monthly basis. This result suggests that fast growing cohorts of larvae contribute more to the replenishment of benthic populations than slow growing cohorts of this species. Multiple regression identified water temperature, rainfall and wind component as important determinants of larval growth, age at capture and monthly catches of this species. These variables accounted for 7 to 36% of the variance in growth rates, while water temperature was moderately correlated (r super(2) = 0.48) with catches. If such correlations between larval growth rates and replenishment are a general phenomenon, then this may provide a simple means of predicting year-class success in a range of reef fishes.
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 tested the hypothesis that fast growing Atlantic cod Gadus morhua survived better than their population of origin in the winters of 1991–1992 and 1992–1993 on the Scotian Shelf (northwest ...Atlantic). Survivors were defined as fish >90 d of age at capture which comprised epibenthic juveniles >20 mm sampled mainly near the bottom. The majority of larvae and juveniles aged ≤90 d at capture were sampled with a midwater trawl and were assumed to be representative of the pelagic population (survivors + non-survivors) from which epibenthic survivors originated. Standard length corrected for shrinkage was linearly correlated to lapillar radius (r2 = 0.97). Individual growth histories were reconstructed from the width of lapillar increments. Selection for fast growth was weak in the winter of 1991–1992 and back-calculated growth and length at age of survivors were not significantly larger than that of the population (repeated-measures MANOVA). In winter 1992–1993, a strong selection for fast growth was evident in late larvae 41 to 80 d old. The divergence in length at age between survivors and the population reached 4 mm at an age of 70 d, corresponding to a 13 d reduction in the duration of the larval phase. Survivors in the winter of 1992–1993 had larger hatch marks than the population, suggesting that the potential for fast growth may be reflected in traits present at hatching. Our results support the hypothesis that fast growth increases the survivorship of Atlantic cod during larval life in the plankton and indicate that the intensity of size-selective mortality may vary considerably from year to year.
Evidence from the wild as to the ecological and evolutionary consequences of top predator depletions remains limited, especially in marine systems. Given the pace and extent of predator loss, an ...understanding of these processes is important. Two sets of adjacent coral reef systems off north-western Australia have similar biological, physical and environmental conditions, but one of the reef systems has been exposed to nearly exclusive commercial fishing of sharks. Across reefs where sharks have been depleted, prey fishes had significantly smaller caudal fins and eyes compared to the reefs with intact shark populations (up to 40 and 46% relative difference in standardized means). These patterns were consistent across 7 teleost prey species (N = 611 individuals) that vary in behavior, diet and trophic guild. We hypothesize that these morphological patterns were primarily driven by differences in shark predation. Morphological differences were not consistent with plausible alternative explanations (habitat complexity, temperature, light, current, food availability, prey targets, competition) as primary drivers. These results provide field evidence of morphological changes in prey potentially due to predator depletions consistent with ecological predictions; specifically, predator loss caused a reduction in the size of prey morphological traits associated with predator detection and evasion. While our analysis cannot differentiate between rapid evolutionary change versus morphological plasticity due to shark depletions, either possible outcome would indicate that predator removals may have profound effects on body shapes of prey communities. This is particularly significant in the case of sharks, given that the consequences of their widespread removal have been a topic of significant speculation, debate and concern.
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