Seasonal Aggregation Research Articles

Habitat suitability of Rhincodon typus in three localities of the Gulf of California: Environmental drivers of seasonal aggregations.

The whale shark is an endangered species that usually feeds in coastal areas of highly productive seas such as the Gulf of California, Mexico. This study aims to describe the effect of sea surface temperature, chlorophyll a, bathymetry and slope on the habitat suitability of whale sharks in three important aggregation sites of the Gulf of California. A total of 2396 records of occurrence of whale sharks were obtained from international databases and scientific literature between 1996 and 2018. These records were used for the creation of a species distribution model using MaxEnt for each of the three aggregation sites. The concentration of chlorophyll a explained 71% of the habitat suitability, followed by bathymetry and slope with a combined 17%, and sea surface temperature constituting 10% of the model. Habitat suitability was related to areas where nontargeted fisheries may impact whale sharks through bycatch, entanglement and ship strikes. The implications for the conservation of whale sharks should be considered for management decisions in terms of marine protected areas, fishing refugees or bans, and other regulations regarding fisheries activities.

A matter of diversity? Identifying past hunter-gatherer aggregation camps through data driven analyses of rock art sites

During the last decades, ethnographic observations taught hunter-gatherer archaeologists to incorporate social phenomena into their interpretative models of land uses, including rock art. Although ethnographic analogies always allow only a rough approximation to the past, strategies such as the (seasonal) aggregation and dispersion of San groups, and related exchange systems, have become an integral part of the interpretation of variability in the spatial organisation of sites and the excavated material culture. Regardless of a long-standing dispute about its potential meaning, one widely accepted explanation for making rock art is that it was part of the social contexts of past hunter-gatherer aggregation events. While motifs such as the jointly performed trance dance seem to support this view, methods to identify aggregation and dispersion events within rock art on a larger scale are poorly developed. The objective of this paper is to better understand the organisation of the rock art sites at the Dâureb in Namibia using GIS and computational archaeology in the light of hunter-gatherer aggregation and dispersion strategies.

The influence of zooplankton communities on the feeding behavior of whale shark in Bahia de La Paz, Gulf of California

The whale shark is the world’s largest filter feeding shark and holds international protection as an endangered species. It is known to form seasonal aggregations linked to blooms of zooplanktonic organisms. The aim of this studywas to investigate whether whale shark foraging behaviors are related to the density and/or concentration of available prey items. Zooplankton samples were obtained between October-2016 and March-2017 from Bahia de La Paz in theabsence (n =32) and presence (n = 12) of foraging whale sharks. Results indicated that zooplankton biomass was 1.6 times significantly greater (t = -3.21, p < 0.05) when sharks were observed feeding and the mean number of zooplankton individuals of all taxonomic groups in the presence of feeding whale sharks was 2.5 times greater than in the absence of feeding, although it was not significantly different (t = -1.70, p > 0.05). The dominant taxonomic groups were copepods and chaetognaths with no significant differences found in the mean number of copepods (t = -1.81, p > 0.05), chaetognaths (t = -1.70, p > 0.05), euphausiids (t = -0.12, p > 0.05) and decapods (t = -1.80, p > 0.05), between the presence and absence of feeding whale sharks. In conclusion, whale sharks feeding behaviors occur during higher concentrationsof zooplankton biomass and the species may potentially target dense patches of both copepods and chaetognaths.

Radar-Based Precipitation Climatology in Germany—Developments, Uncertainties and Potentials

Precipitation is a crucial driver for many environmental processes and weather radars are capable of providing precipitation information with high spatial and temporal resolution. However, radar-based quantitative precipitation estimates (QPE) are also subject to various potential uncertainties. This study explored the development, uncertainties and potentials of the hourly operational German radar-based and gauge-adjusted QPE called RADOLAN and its reanalyzed radar climatology dataset named RADKLIM in comparison to ground-truth rain gauge data. The precipitation datasets were statistically analyzed across various time scales ranging from annual and seasonal aggregations to hourly rainfall intensities in regard to their capability to map long-term precipitation distribution, to detect low intensity rainfall and to capture heavy rainfall. Moreover, the impacts of season, orography and distance from the radar on long-term precipitation sums were examined in order to evaluate dataset performance and to describe inherent biases. Results revealed that both radar products tend to underestimate total precipitation sums and particularly high intensity rainfall. However, our analyses also showed significant improvements throughout the RADOLAN time series as well as major advances through the climatologic reanalysis regarding the correction of typical radar artefacts, orographic and winter precipitation as well as range-dependent attenuation.

Acoustic and conventional tagging support the growth patterns of grey nurse sharks and reveal their large-scale displacements in the west coast of Australia

The grey nurse shark, Carcharias taurus, is a globally vulnerable coastal species with aggregatory behaviour and low productivity, making it highly susceptible to overfishing. Little is known on the biology and movement for the population along the west coast of Australia. Here we use acoustic telemetry to show that C. taurus can undertake large-scale movements and potentially capitalise on seasonal prey aggregations. Conventional tagging provided evidence to support the growth parameter values used to represent the species’ growth dynamics and considerably extended the species’ maximum observed age. As maximum age is a proxy for productivity, our findings directly inform the recovery plan currently in place for Australian C. taurus.

Reproductive biology, multiple paternity and polyandry of the bull shark Carcharhinus leucas.

To improve understanding of bull shark Carcharhinus leucas reproductive biology, we analysed reproductive traits from 118 bull sharks caught along Reunion Island coasts (Western Indian Ocean), including 16 gravid females. Specific microsatellite loci were used to investigate the frequency of multiple paternity. Males and females reached maturity at c. 234 cm and 257 cm total length (LT ), respectively, and litter sizes ranged from 5 to 14 embryos. Analysis of the 16 litters collected in various months of the year indicated that parturition occurs between October and December, with a size at birth c. 60-80 cm LT and that the gestation period is probably c. 12 months. Assuming a 1 year resting period and a period of sperm storage (4-5 months) between mating (in June-September) and fertilisation, the reproductive cycle of bull sharks at Reunion Island would be biennial. At least 56.25% of the litters investigated were polyandrous, sired by 2-5 males. Several males that each sired several litters conceived during the same or distinct mating seasons were detected, suggesting both a seasonal aggregation of sharks to mate and some male fidelity to mating site. Altogether, these findings provide valuable information for both shark risk management and conservation of the species in the Western Indian Ocean.

Influence of Sea Surface Temperature on Simulated Future Change in Extreme Rainfall in the Asia-Pacific

The changes in precipitation simulated by the 40-model CMIP5 ensemble for the Asia-Pacific region are assessed, focusing on the two periods 1986–2005 and 2080–2099 under the RCP8.5 scenario. The frequency distributions of both daily and monthly mean rain rates at model grid points in each of four seasons are considered. In spatial averages for the land domain and seven selected regions, there is both an increase in the frequency of dry days and a shift towards heavier rain. Three 20-year seasonal aggregate statistics, the mean, the top decile of monthly amounts, and top percentile of daily amounts are assessed. The percentage changes over land are on average 4% higher for monthly extremes than for the mean, and a further 10% higher for the daily extremes. Over Australia and central Indonesia in some seasons the mean decreases but daily extremes increase. In much of Asia the daily extremes increase by 30% or more. For these statistics, there is a range across the ensemble that can often be linked to a pattern of sea surface temperature change that is quantified by a Pacific-Indian Dipole (or difference, PID) index. Correlations with both mean and extreme rain are highly negative over Australia and Indonesia and moderately positive over parts of south and east Asia in some seasons. The mechanism for this is explored through additional simulations and links with water vapour path. The results provide some understanding of the range of projections of future rainfall change based on the CMIP5 results, with some potential for narrowing it.

Seasonal variability in global industrial fishing effort

Human beings are the dominant top predator in the marine ecosystem. Throughout most of the global ocean this predation is carried out by industrial fishing vessels, that can now be observed in unprecedented detail via satellite monitoring of Automatic Identification System (AIS) messages. The spatial and temporal distribution of this fishing effort emerges from the coupled interaction of ecological and socio-economic drivers and can therefore yield insights on the dynamics of both the ecosystem and fishers. Here we analyze temporal variability of industrial fishing effort from 2015-2017 as recorded by global AIS coverage, and differentiated by fishing gear type. The strongest seasonal signal is a reduction of total deployed effort during the annual fishing moratorium on the numerically-dominant Chinese fleet, which occurs during boreal summer. An additional societally-controlled reduction of effort occurs during boreal winter holidays. After accounting for these societal controls, the total deployed effort is relatively invariant throughout the year for all gear types except squid jiggers and coastal purse seiners. Despite constant deployment levels, strong seasonal variability occurs in the spatial pattern of fishing effort for gears targeting motile pelagic species, including purse seiners, squid jiggers and longliners. Trawlers and fixed gears target bottom-associated coastal prey and show very little overall seasonality, although they exhibit more seasonal variation at locations that are further from port. Our results suggest that societal controls dominate the total deployment of fishing effort, while the behavior of pelagic fish, including seasonal migration and aggregation, is likely the most prominent driver of the spatial seasonal variations in global fishing effort.

Assessing potential environmental influences on killer whale (Orcinus orca) distribution patterns in the Bremer Canyon, south-west Australia

ABSTRACTSubmarine canyons play an important role in the regional distribution, abundance and dispersal of marine biota and are increasingly being recognised as geomorphic features of high conservation significance along Australia’s continental margin. Certain canyons have been described as foraging ‘hotspots’ attributable to the high abundance of apex cetacean species aggregating in these areas. Anecdotal evidence of large seasonal aggregations of killer whales in the Bremer Canyon, south-west Australia, has attracted significant research attention in the last decade. To identify important environmental drivers influencing aggregation patterns, a predictive spatial habitat model using the Maxent model was developed based on presence-only whale sighting data. In addition, remotely sensed sea surface temperature and chlorophyll-a concentrations were assessed to investigate the spatio-temporal variation in sea surface conditions. Habitat preference was predicted in areas between canyon heads, with the most influential predictor variables being depth and distance from the continental shelf break. Analysis of remote-sensing data highlighted low localised variability in surface waters and illustrated the seasonal trends of the Leeuwin Current. This study demonstrates the influence of bathymetry and submarine geomorphology on enhanced cetacean abundance and highlights the need for recognition of this potential foraging area in marine reserve planning.

Revealing the deposition of macrophytes transported offshore: Evidence of their long-distance dispersal and seasonal aggregation to the deep sea

The role of coastal macrophyte beds as a carbon sink is under debate. Various studies have provided global estimates of the carbon sequestration and stocks of macrophyte beds; however, the final fate of macrophyte debris exported from coastal beds remains uncertain, and must be determined in order to fully clarify the role of coastal vegetation as a carbon sink. Here we conducted bottom-trawl surveys to investigate the extensive and seasonal aggregation of exported macrophytes on the continental shelf and slope seafloor (40–1,800 m). Sunken macrophytes showed a clear seasonal trend with highest biomasses in summer. This was mainly caused by the most collected macrophyte species Sargassum horneri. Furthermore, we used numerical simulations to verify the link between sea-surface hydrographic condition and seafloor distribution of sunken macrophytes. Our results showed that S. horneri accumulated beneath the Kuroshio Extension current, which is the western boundary current of the North Pacific subtropical gyre. Overall, floating macrophytes that became transported offshore by a stable sea-surface current, such as the western boundary current, constitute an organic carbon pathway from coastal waters to the deep sea. Our findings suggest that these buoyant macrophytes can act as a biological pump to enhance oceanic carbon sequestration.

Short-term movement of a night shark (Carcharhinus signatus) in the western Caribbean with notes on the species’ distribution and threats in the region

Night sharks Carcharhinus signatus have suffered large declines in the western Atlantic Ocean, and while fishing mortality has slowed and populations stabilized in the northwestern Atlantic, they are still captured in high numbers as a targeted and bycatch species in the southern Atlantic Ocean. Considered one of the top five most vulnerable stocks in the Atlantic pelagic longline fishery, few records exist for this large-bodied species in the western Caribbean, and nothing is known about their movements or migration in the region. We found that mature night sharks occur in the western Caribbean from March through November and juveniles contribute to artisanal fisheries in the Guatemalan Caribbean. A seasonal aggregation of night sharks occurs at Glover’s Reef Atoll, Belize, in the summer months, possibly for mating purposes, making this a potentially important site for the species. We also document the first recapture of a tagged night shark in the western Caribbean, a subadult male that was tagged in northern Belize and recaptured after c. 60 days off Havana, Cuba. This recapture suggests that individuals undergo large scale movements, and coupled with known threats from fisheries, a regional approach for this species’ management is favored.

Temporal stability and assignment power of adaptively divergent genomic regions between herring (Clupea harengus) seasonal spawning aggregations.

Atlantic herring (Clupea harengus), a vital ecosystem component and target of the largest Northwest Atlantic pelagic fishery, undergo seasonal spawning migrations that result in elusive sympatric population structure. Herring spawn mostly in fall or spring, and genomic differentiation was recently detected between these groups. Here we used a subset of this differentiation, 66 single nucleotide polymorphisms (SNPs) to analyze the temporal dynamics of this local adaptation and the applicability of SNP subsets in stock assessment. We showed remarkable temporal stability of genomic differentiation corresponding to spawning season, between samples taken a decade apart (2005 N = 90 vs. 2014 N = 71) in the Gulf of St. Lawrence, and new evidence of limited interbreeding between spawning components. We also examined an understudied and overexploited herring population in Bras d'Or lake (N = 97); using highly reduced SNP panels (N SNPs > 6), we verified little‐known sympatric spawning populations within this unique inland sea. These results describe consistent local adaptation, arising from asynchronous reproduction in a migratory and dynamic marine species. Our research demonstrates the efficiency and precision of SNP‐based assessments of sympatric subpopulations; and indeed, this temporally stable local adaptation underlines the importance of such fine‐scale management practices.

Seasonal aggregation and diel activity by the sicklefin devil ray Mobula tarapacana off a small, equatorial outcrop of the Mid-Atlantic Ridge.

The present study aimed to analyse occurrence patterns, relative abundance and habitat use by the sicklefin devil ray Mobula tarapacana in the Saint Peter and Saint Paul Archipelago (SPSPA), Brazil, located in the Mid-Atlantic Ridge. Sampling was conducted between December 2008 and December 2012 by surface surveys and tagging with conventional tags and acoustic transmitters. Mobula tarapacana were sighted in the SPSPA year round but the highest frequency of occurrence was observed from January to June, probably due to greater food availability during this season. However, M. tarapacana might also use the area for reproductive purposes based on evidence of mating behaviour. The residency period of tagged rays measured by three autonomous acoustic receivers deployed around SPSPA spanned from 1 to 74 days with most of the detections occurring during daytime. These results suggest that the SPSPA could be an ecologically-important aggregation area for M. tarapacana, possibly providing the species with a feeding or resting habitat while transiting in the equatorial Atlantic Ocean.

Fine-scale movement of juvenile Atlantic sturgeon (Acipenser oxyrinchus oxyrinchus) during aggregations in the lower Saint John River Basin, New Brunswick, Canada

Juvenile Atlantic sturgeon (Acipenser oxyrinchus oxyrinchus) form seasonal aggregations near the salt wedge in their natal river systems. We used an array of Vemco positioning system acoustic receivers to track fine-scale movement within aggregation sites in two rivers in the Saint John River Basin. We used the t-LoCoH convex hull construction algorithm to map space use and aggregation behavior and nonmetric multidimensional scaling to test for differences among rivers, seasons, and photoperiods. Aggregation sites consisted of small core areas, where juvenile Atlantic sturgeon remained for long periods, that are adjacent to foraging grounds. This structure was largely consistent between rivers and seasons. Directional movement within aggregation sites differed between rivers. In areas of high flow velocity, directional movement was parallel to flow and largely restricted to littoral areas, whereas areas of low flow exhibited no distinct patterns in directional movement. This indicates flow may be an important driver of fine-scale distribution within aggregation sites. Studies of fine-scale space use can inform future investigations of rearing capacity, aid in the identification of critical habitat, and inform management decisions.

Acoustic backscatter at a Red Sea whale shark aggregation site

An aggregation of sexually immature whale sharks occurs at a coastal submerged reef near the Saudi Arabian Red Sea coast each spring. We tested the hypothesis that these megaplanktivores become attracted to a prey biomass peak coinciding with their aggregation. Acoustic backscatter of the water column at 120 kHz and 333 kHz –a proxy for potential prey biomass – was continuously measured spanning the period prior to, during, and subsequent to the seasonal whale shark aggregations. No peak in acoustic backscatter was observed at the time of the aggregation. However, we observed a decrease in acoustic backscatter in the last days of deployment, which coincided the trailing end of whale shark season. Organisms forming the main scattering layer performed inverse diel vertical migration, with backscatter peaking at mid-depths during the day and in the deeper half of the water column at night. Target strength analyses suggested the backscatter was likely composed of fish larvae. Subsurface foraging behavior of the whale sharks within this aggregation has not been described, yet this study does not support the hypothesis that seasonal peaks in local whale shark abundance correspond to similar peaks in prey availability.

A Bayesian modelling method for post-processing daily sub-seasonal to seasonal rainfall forecasts from global climate models and evaluation for 12 Australian catchments

Abstract. Rainfall forecasts are an integral part of hydrological forecasting systems at sub-seasonal to seasonal timescales. In seasonal forecasting, global climate models (GCMs) are now the go-to source for rainfall forecasts. For hydrological applications however, GCM forecasts are often biased and unreliable in uncertainty spread, and calibration is therefore required before use. There are sophisticated statistical techniques for calibrating monthly and seasonal aggregations of the forecasts. However, calibration of seasonal forecasts at the daily time step typically uses very simple statistical methods or climate analogue methods. These methods generally lack the sophistication to achieve unbiased, reliable and coherent forecasts of daily amounts and seasonal accumulated totals. In this study, we propose and evaluate a Rainfall Post-Processing method for Seasonal forecasts (RPP-S), which is based on the Bayesian joint probability modelling approach for calibrating daily forecasts and the Schaake Shuffle for connecting the daily ensemble members of different lead times. We apply the method to post-process ACCESS-S forecasts for 12 perennial and ephemeral catchments across Australia and for 12 initialisation dates. RPP-S significantly reduces bias in raw forecasts and improves both skill and reliability. RPP-S forecasts are also more skilful and reliable than forecasts derived from ACCESS-S forecasts that have been post-processed using quantile mapping, especially for monthly and seasonal accumulations. Several opportunities to improve the robustness and skill of RPP-S are identified. The new RPP-S post-processed forecasts will be used in ensemble sub-seasonal to seasonal streamflow applications.

Seasonal aggregations of sharks near coastal power plants in Israel: an emerging phenomenon

Seasonal aggregations of sharks near coastal power plants in Israel: an emerging phenomenon

Can't we all just get along? Weaponry standardization and sharing at the Agate Basin site

This study explores weaponry design as it relates to the organization of technology at the Agate Basin site. Specifically, I explore potential sharing of weaponry elements during communal hunts, and the implications of such sharing as they pertain to overall technological organization of Agate Basin hunting groups. K-means cluster analysis is utilized to determine whether hafted-area morphologies on Agate Basin points are standardized and consistent with expectations we might have if sharing of weaponry elements was planned for in the preparation for a communal hunt. I argue that standardization and sharing of weapons was a reliable organizational technique potentially employed by Paleoindian hunters during seasonal aggregations that served to secure critical resources during a time when failure during a hunt would prove catastrophic.

Assessing Fishing Experts' Knowledge to Improve Conservation Strategies for an Endangered Grouper in the Southwestern Atlantic

The Atlantic goliath grouper (Epinephelus itajara) is a critically endangered reef fish that has suffered from overfishing. In Brazil, the species has been under a fishing ban since 2002; however, there are no evidences of population recovery. In this context, data to help improve management programs are sorely needed. We assessed fisheries landings and interviewed key informants to investigate fishing, habitat use, and occurrence of goliath grouper in Abrolhos Bank, eastern Brazil. In mangroves, longline was the main fishing gear used to catch the species. Informants reported higher abundances of goliath grouper in red mangroves (Rizophora mangle) within shallow and complex channels. In marine reefs, the species was caught mainly through spearfishing. Goliath grouper occurrence was associated with coral and rocky reefs and was described as an aggregation site of up to 20 individuals. Informants also reported a decline in goliath grouper abundance across all habitats, primarily due to overfishing. Aggregation and nursery sites were mapped and described as essential for goliath grouper conservation. In Brazil, a scarcely enforced fishing moratorium alone is insufficient to guarantee the recovery of goliath grouper populations. Besides increasing the moratorium enforcement, we suggest implementing no-take zones, which encompass both nursery habitats and seasonal aggregation sites described in this study.

Feeding the world's largest fish: highly variable whale shark residency patterns at a provisioning site in the Philippines.

Provisioning wildlife for tourism is a controversial yet widespread practice. We analysed the residency patterns of juvenile whale sharks (Rhincodon typus) in Oslob, Philippines, where provisioning has facilitated a large shark-watching operation since 2011. We identified 208 individual sharks over three years, with an average of 18.6 (s.d. = 7.8, range = 6–43) individuals sighted per week. Weekly shark abundance varied seasonally and peak-season abundance (approx. May–November) increased across years. Whale sharks displayed diverse individual site visitation patterns ranging from a single visit to sporadic visits, seasonal residency and year-round residency. Nine individuals became year-round residents, which represents a clear response to provisioning. The timing of the seasonal peak at Oslob did not align with known non-provisioned seasonal aggregations elsewhere in the Philippines, which could suggest that seasonal residents at Oslob exploit this food source when prey availability at alternative sites is low. Since prolonged residency equates to less time foraging naturally, provisioning could influence foraging success, alter distributions and lead to dependency in later life stages. Such impacts must be carefully weighed against the benefits of provisioning (i.e. tourism revenue in a remote community) to facilitate informed management decisions.