Fish Aggregation Research Articles

Remote sensing of environmental indicators of potential fish aggregation: An overview

Sustainable use of marine resources requires effective monitoring and management of the world’s fish stocks. Acoustic and electromagnetic remote sensing techniques are being used to help manage fisheries at sustainable levels, while also guiding fishing fleets to locate fish schools more efficiently. Fish tend to aggregate in ocean areas that exhibit conditions favored by specific fish species. Some of the relevant oceanographic conditions, such as sea surface temperature, ocean color (productivity) and oceanic fronts, which strongly influence natural fluctuations of fish stocks, can be observed and measured by remote sensors on satellites, aircraft and ships. The remotely sensed data are provided in near-real time to help fishermen save fuel and ship time during their search for fish, to modelers who produce fisheries forecasts, and to scientists who help develop strategies for sustainable fisheries management. This article describes how acoustic, optical and radar sensors on ships, satellites and aircraft are used with forecast models to improve the management and harvesting of fisheries resources.

A critique of the ecosystem impacts of drifting and anchored FADs use by purse-seine tuna fisheries in the Western and Central Pacific Ocean

In the Western and Central Pacific Ocean (WCPO), which accounts for over half of world tuna production, purse seine effort and catch on floating objects have increased significantly due to a rapid increase in the use of fixed and free-floating fish aggregation devices (FADs). FAD fishing has had an impact on the current status of the stocks of the three main target tunas in the equatorial WCPO, skipjack (Katsuwonus pelamis), yellowfin (Thunnus albacares) and bigeye (T. obesus). FADs have been shown to influence the behaviour and movement patterns of the three tuna species with the juveniles of each species occupying shallower habitats when associated with FADs. Aggregation of tunas around drifting objects increases their vulnerability to purse seine gear, particularly for juvenile and small size classes. Further to the impacts on the target stocks, the use of FADs has increased the vulnerability of other fishes to the purse seine method, including some shark and billfish species. Given the concern over FAD-related fishing effort on target and bycatch species, there is a need to understand how FAD use affects target and bycatch stocks. Science needs to better support management decisions are highlighted including the need to identify the magnitude of broader community-level affects.

Shifting from marine reserves to maritime zoning for conservation of Pacific bigeye tuna ( Thunnus obesus )

Over 50% of the total bigeye tuna (BET) landed in the Western Central Pacific Ocean is caught incidentally in the purse seine fishery and sold for canning at prices less than US$2/kg. The remainder is landed in longline fisheries directed at BET and sold as fresh or frozen tuna at prices greater than US$10/kg. The combined fishing mortality by all gears will soon reduce the BET biomass in the Pacific Ocean to less than that capable of producing maximum sustainable yield. Closure of the high-seas enclaves in 2009 was hailed as a conservation measure, but was not scientifically evaluated before implementation and appears to have had no beneficial effect on the BET stock. A spatially explicit age-structured ecosystem model, SEAPODYM, is used to simulate alternative area-based fishery management policies to conserve bigeye tuna in the Western Central Pacific Ocean. Closing the high-seas enclaves to purse seine fishing has negligible effect on the BET biomass. Fishery management policies that control mortality on both juveniles and adults, through prohibition of fish aggregation devices in the purse seine fishery and restrictions on longline fishing in spawning areas, are the most efficient conservation policies. Large-scale benefits from bigeye conservation measures will become apparent only in the 2030s, assuming timely implementation and minimal effects of climate change.

Effect of current and daylight variations on small-pelagic fish aggregations (Selar crumenophthalmus) around a coastal fish aggregating device studied by fine-scale acoustic tracking

Several pelagic fish species are known to associate with floating objects. However, quantitative information on the main factors that drive this associative behaviour is still lacking. Small pelagic fish offer a particularly interesting case study for this phenomenon, since the small spatial scales involved in their association with floating objects allow experimental data to be collected at a fine scale. Here, we monitored twelve acoustically-tagged bigeye scads, Selar crumenophthalmus (Carangidae), (mean fork length 16.4 cm, SD 2.1 cm) around a floating object moored in shallow water (15 m depth) playing the role of a coastal fish aggregating device (FAD). To quantify the role played by variations in current and daylight, we calculated the speed distribution, pair correlation function and group polarization for the tagged fish hourly, from midday to dusk (13:00–18:00), for varying current strengths and daylight conditions. We found that the current induced a shift in the position of the aggregation upstream of the FAD, at distances that increased with the current strength. We found evidence of an expansion and a higher coordination in the aggregation at dusk, with increasing speed, distance among conspecifics and alignment. We discuss possible scenarios in which group polarization increases at dusk and suggest complementary measurements for future experiments that could confirm our findings.

SENSORY ORGAN DISCOVERY DEMYSTIFIES RORQUAL WHALE FEEDING

![Figure][1] The rorqual whale family is composed of the largest animals on Earth with a unique way of eating their prey: they dive into aggregations of small crustaceans and fish at great depths and lunge with their mouths wide open, engulfing kilograms of fish in a single mouthful. The

Formation and development of the Black Sea-Caspian kilka Clupeonella cultriventris (Clupeidae) in the Rybinsk reservoir

We studied the state and development of the population of the Black Sea-Caspian kilka Clupeonella cultriventris in the Rybinsk reservoir from 1994 to 2010. The population dynamics, the rate of growth of the kilka, as well as its food relationships with other fish species of the pelagial community, were analyzed. It was found that the period of exponential increase in the numbers was replaced by the period of interannual fluctuations. With the development of the population, tendencies for a decrease in the rate of growth of the kilka and aggravation of its food relationships with other species are revealed, which is determined by changes in the structure of pelagic aggregations of fish. One of the most significant environmental factors determining dynamics of the numbers and state of the kilka population in the northernmost part of its modern range is water temperature.

On the use of omnidirectional sonars and downwards-looking echosounders to assess pelagic fish distributions during and after midwater trawling

Abstract Stockwell, J. D., Weber, T. C., Baukus, A. J., and Jech, J. M. 2013. On the use of omnidirectional sonars and downwards-looking echosounders to assess pelagic fish distributions during and after midwater trawling. – ICES Journal of Marine Science, 70:196–203. Small pelagic fish can play an important role in the structure and function of ecosystems, and there is increasing interest in their non-market value. At the scale of fish aggregations, however, the impact of fishing has received relatively little attention, with most effort devoted to impacts of vessel and gear avoidance on stock size estimates. We used concurrent deployment of a downwards-looking echosounder (Simrad ES60 system) and an omnidirectional sonar (Simrad SP90 system) during commercial pairtrawling operations for Atlantic herring (Clupea harengus) in the Gulf of Maine to examine their potential for studying the impacts of fishing on herring aggregations. We compared a number of aggregation metrics to illustrate similarities and differences between the two systems, and then qualitatively examined their properties during and after pairtrawling events to illustrate potential applications. Our results suggest that using both downwards-looking and omnidirectional systems provides complementary information on fish aggregation metrics. Future applications of these systems in before–after–control-impact (BACI) designs may help inform management agencies when evaluating potential impacts of fishing at the time and space scales of pelagic fish aggregations.

Documenting recovery of a spawning aggregation through size frequency analysis from underwater laser calipers measurements

Many spawning aggregations of marine fishes have been fished beyond the point of sustainability, leading to increased calls for protection through seasonal and/or site-specific fishery closures. Once a closure has been put in place, monitoring the aggregation is imperative in order to learn whether protection leads to the recovery of the population. Current methods for monitoring the status of spawning aggregations rely largely on counts, either subsample or census, usually combined with capturing a subset of the fish to assess individual traits such as length and weight. Handling fish during the spawning aggregation can be stressful for the fish, and can ultimately lead to decreased spawning success, increased susceptibility to predators, or increased mortality through capture trauma or infection. Here we present a novel analysis for monitoring fish on a spawning aggregation that does not require the capture and handling of fish. Following a recovering aggregation of Nassau grouper (Epinephelus striatus) over seven spawning seasons, we show that length-distribution data can be collected by divers using a video-based system with parallel lasers calibrated to a specific distance apart, and subsequently use those data to monitor changes in the size distribution over time. We detected recruitment of new fish to the grouper spawning aggregation in the fourth year of monitoring. In addition to tracking size distribution trends over time, the length distribution information could be combined with an established length–weight regression and an estimate of total abundance to estimate spawning stock biomass. We qualitatively cross-validate this method with census data to evaluate its effectiveness in monitoring the recovery or decline of aggregating species that can be visually observed.

Impacts of fish aggregation devices on size structures of skipjack tuna Katsuwonus pelamis

Tuna purse seine fisheries target fish aggregated in schools, including free schools that are formed naturally based on fish biology and aggregations associated with natural and/or artificial drifting objects. Using data collected from skipjack tuna (Katsuwonus pelamis) fisheries, we evaluated differences in size structures between drifting-floating-object-associated schools and unassociated schools. We developed a generalized linear model to remove impacts of environmental variables on skipjack size composition. This study indicates that the drifting-floating-object-associated schools tended to have significantly wider size ranges than the unassociated schools. This suggests that unassociated schools were likely formed based on similarity in sizes among individuals within a school while drifting-floating-object-associated schools were probably composed of individuals of large size ranges and their formation was not based on the “size selection” rule. We concluded that the unassociated schools and the drifting-floating-object-associated schools were formed through different mechanisms, and drifting floating objects could aggregate unassociated schools of different size structures. Thus, a large scale of deployment of man-made floating objects might disrupt the spatial aggregation pattern of fish that otherwise tended to school based on their sizes in the absence of floating objects.

The governance of diverse, multi-scale fisheries in which there is a lot to learn

Recent research indicates that fish populations once considered panmictic are in fact meta populations characterized by spatial complexity and comprised of demographically isolated subpopulations adapted to local environmental conditions. A synthesis of the relevance of population structure for fisheries management is provided, using New England's ground fisheries as an example. Federal fisheries management has been ineffective in conserving or restoring ground fish stocks in the region. Implemented in 1977 and based on the understanding of population structure at the time, management has focused on broad-scale restrictions on fishing that fail to protect subpopulations. But even very rigorous broad-scale management in a system with fine-scale ecological structure does little to protect local populations. As local stocks are fished down, fishermen develop strong incentives to locate and efficiently harvest remaining aggregations of fish, resulting in the serial extirpation of local populations. Because both fishing and monitoring occur at a broad-scale, scientific and management feedback regarding the effect of harvesting on fish stocks is ambiguous. Learning about the multi-scale complexity of the ocean is a substantial problem. The problem is not unique to fisheries. The need for cost effective and appropriately scaled knowledge is a problem faced by all forms of social organization. The principles of cost effective, adaptive solutions, drawn from the literature on collective action, are identified. The restoration of ground fish stocks in New England will require reform of fisheries governance towards a multi-scale governance structure, including fine-scale scientific and management institutions that are able to obtain timely and accurate feedback about the effects of fishing at several ecological scales. The failure to address fine-scale population structure in fisheries governance may result in persistent depletion of New England's groundfish stocks.

Spatial and temporal extension of wild fish aggregations at Sparus aurata and Thunnus thynnus farms in the north-western Mediterranean

AEI Aquaculture Environment Interactions Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections AEI 2:239-252 (2012) - DOI: https://doi.org/10.3354/aei00042 Spatial and temporal extension of wild fish aggregations at Sparus aurata and Thunnus thynnus farms in the north-western Mediterranean Kathrin Bacher1,*, Ana Gordoa1, Oscar Sagué2 1Centro de Estudios Avanzados de Blanes (CEAB-CSIC), 17300 Blanes, Spain 2Federació Catalana d’Activitats Subaquàtiques, 08930 Sant Adrià del Besòs, Spain *Email: kbacher@ceab.csic.es ABSTRACT: Fish farms cause wild fish to aggregate nearby and thus act to modify their distribution, but the spatial and temporal extent of the attraction effect around farms is still poorly understood. We examined attraction using an integrated approach combining 3 factors: spatial extent, depth and season. We analysed an Atlantic bluefin tuna Thunnus thynnus (ABT) and a gilthead seabream Sparus aurata (SB) farm off the north-eastern coast of Spain which represent different fish farming models in the Mediterranean. Underwater visual counts were conducted from summer 2010 to spring 2011. We recorded 39 species at the SB farm and 17 species at the ABT farm. The analysis of fish abundance and biomass at 3 stations located along a distance gradient from the farm (SB: 0, 30, 200 m; ABT: 0, 100, 200 m) showed significantly higher values next to the cages than at more distant stations, revealing that the spatial extent of the attraction effect was mainly restricted to the cage rather than following a gradient. The intensity of the cage effect varied according to farm, season and depth, reflecting species-specific behaviours. At the SB farm, the cage effect persisted year round, varying with depth, whereas at the ABT farm, the effect was only detected in summer and spring. These distinct spatial and temporal aggregation patterns suggest that implementation of fishery regulatory measures to protect wild fish should be farm-specific. This study emphasizes the importance of using an integrated approach to study wild fish aggregations at farms. KEY WORDS: Sea-cage . Aquaculture . Wild fish . Attraction effect . Sparus aurata . Thunnus thynnus . Mediterranean Full text in pdf format PreviousNextCite this article as: Bacher K, Gordoa A, Sagué O (2012) Spatial and temporal extension of wild fish aggregations at Sparus aurata and Thunnus thynnus farms in the north-western Mediterranean. Aquacult Environ Interact 2:239-252. https://doi.org/10.3354/aei00042 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in AEI Vol. 2, No. 3. Online publication date: June 25, 2012 Print ISSN: 1869-215X; Online ISSN: 1869-7534 Copyright © 2012 Inter-Research.

PERUBAHAN GILLNETTER MENJADI TROLL LINER DI PPN PALABUHANRATU (Changes of Gillnetter into Troll Liner in PPN Palabuhanratu)

<p>ABSTRACT<br />Fishing fleet in Palabuhanratu, especially for big pelagic fish such as tuna has shown increasing significantly since 2008. This is caused by implementation of rumpon (fish aggregating device) technology that makes fishing activities more efficient due to more exactly fishing ground. This condition made some gillnet fisherman modify their boat to troller that considered having high ability to catch tuna. The aimed of the research was to determine parts of gillnetter that can be modified to be operated as troller. The result showed that only upper parts of gillnetter that had significant modification to be operated as troller. Some upper part was reduced, the roof was eliminated and fish hold was added from 2 to 3 rooms. A little addition of buildings was constructed at the bow and new engine mounting was constructed as well to reach appropriate speed.<br />Key words: gillnetter, modification, Palabuhanratu, troller</p><p>-------</p><p><br />ABSTRAK<br />Armada penangkapan ikan pelagis besar, khususnya perikanan tuna, di Palabuhanratu mengalami perkembangan yang sangat pesat sejak tahun 2008. Ini dikarenakan adanya penerapan teknologi rumpon. Penggunaan alat bantu ini menyebabkan biaya operasi penangkapan ikan menjadi rendah, karena daerah penangkapan tuna sudah tertentu, yaitu di sekitar rumpon. Untuk meningkatkan jumlah tangkapan</p>

Effect of kraft lignin on protein aggregation, functional, and rheological properties of fish protein‐based material

AbstractThe main drawbacks of protein‐based bioplastics are a high sensitivity to water, insufficient mechanical properties, and a narrow window of processing conditions. The objective of this work was to study the effect of Kraft lignin (KL) on protein aggregation, functional, and rheological properties of fish protein (FP)‐based bioplastic. FP powder was blended with 30% glycerol and 0–70% KL. Then, blends were thermoformed by compression molding. KL addition increased protein solubility in sodium dodecyl sulfate buffer, indicating a decrease of protein molecular weight. An introduction of KL in protein blend increased mechanical properties and decreased water absorption of materials. KL addition resulted in a decrease in storage modulus in rubbery state of protein blends. It also resulted in a decrease in viscosity of protein blends at processing temperature, as determined by capillary rheometry. Therefore, KL is an alternative additive to enlarge the protein thermal processing window and improve functional properties of FP‐based materials. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013

Species identification in seamount fish aggregations using moored underwater video

Abstract O'Driscoll, R. L., de Joux, P., Nelson, R., Macaulay, G. J., Dunford, A. J., Marriott, P. M., Stewart, C., and Miller, B. S. 2012. Species identification in seamount fish aggregations using moored underwater video. – ICES Journal of Marine Science, 69: 648–659. Acoustic surveys of New Zealand deep-water seamounts often show fish aggregations up to 150 m high on the summit. Although bottom trawls on the seamount slopes catch predominantly orange roughy (Hoplostethus atlanticus), species composition in the midwater plumes is extremely uncertain. In June 2010, moored underwater video cameras were deployed on the summit of the Morgue seamount (summit depth 890 m), a feature that has been closed to fishing since 2001. Cameras and lights were timed to come on for 2 min every 2 h. Fish response to the mooring was monitored using vessel-mounted echosounders. Moored cameras confirmed that orange roughy were present up to 70 m above the seamount summit. Orange roughy made up 97% of the fish identified from the video. Other species observed included smooth oreo (Pseudocyttus maculatus), spiky oreo (Neocyttus rhomboidalis), deep-water dogfish, cardinalfish (Epigonus spp.), and squid. Total along-track backscatter from the plume varied by a factor of 25 over a period of hours. Peak acoustic densities in the plume (equivalent to 20 orange roughy m−3) were an order of magnitude higher than peak visual estimates (0.64 orange roughy m−3), but relative densities between paired video and acoustic observations were generally consistent.

Development and Application of Quality Index Method Scheme in a Shelf‐Life Study of Wild and Fish Farm Affected Bogue (Boops boops, L.)

Quality index method scheme was developed for raw bogue (Boops boops) and evaluated it in a shelf-life study, using samples from wild fish aggregations at fish farms (BF) and from area not influenced by the fish farms (BW). Different environment influenced the shelf life of bogues; thus the maximum ice storage, evaluated from cooked samples, was found to be 17 d for BF and 12 d for BW. The calculated quality indexes (QIs) evolved linearly with storage time on ice (QI(BF) = 0.968x + 0.583, R(2) = 0.947, Std. Err. Est. = 1.41; QI(BW) = 1.212x + 0.474, R(2) = 0.972, Std. Err. Est. = 0.95). The multivariable analysis was used to identify the most effective variables during spoilage evolution. The sum of all demerit points showed higher correlation (R > 0.99) then any single parameters itself, indicating that the individual parameters could not replace the usage of 20 demerit points QIM scoring scheme in sensory assessment. The impact of farming cages was observed in fat (BF > 19%; BW < 2%) and water content (BF = 55%; BW = 77%) of bogues, but also in pH, dielectric properties, thiobarbituric acid index, and volatile amine changes. High correlations of these parameters with storage time and sensory assessment were observed. The catch landings of bogue make this species very important in the Mediterranean fishery production. The effect of finfish farms makes individual fish samples of this species different in size and chemical content, thus changing the rate and in which their postmortem changes occur. The practical use of the article is a new developed and species adopted descriptive scheme for bogue, ready to use for scientific and industrial freshness assessment providing the information on fish quality and its remaining shelf life in ice, taking into account the environmental conditions such as catching ground.

Scale-dependent detection of the effects of harvesting a marine fish population

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 444:251-261 (2012) - DOI: https://doi.org/10.3354/meps09434 Scale-dependent detection of the effects of ­harvesting a marine fish population Valerio Bartolino1,2,*, Lorenzo Ciannelli3, Paul Spencer4, Thomas K. Wilderbuer4, Kung-Sik Chan5 1Marine Research Institute, Department of Aquatic Resources, Swedish University of Agricultural Sciences, Lysekil 45321, Sweden 2Regional Climate Group, Department of Earth Sciences, University of Gothenburg, 40530 Gothenburg, Sweden 3College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, Oregon 97331, USA 4Alaska Fisheries Science Center, NOAA, Seattle, Washington 98115, USA 5Department of Statistics and Actuarial Science, University of Iowa, Iowa City, Iowa 52242, USA *Email: valerio.bartolino@slu.se ABSTRACT: Certain ecological processes dominate others at particular scales, and the response of populations to exogenous and endogenous forces are typically scale dependent. In spite of this central role played by scale, the temporal and spatial scales of human impacts on ecosystems and populations remain almost unknown. We applied a multiscale regression analysis to investigate the spatiotemporal scales which characterize the fisheries exploitation of yellowfin sole Limanda aspera in the Bering Sea. We found that harvesting affects the abundance of this species simultaneously at local and regional scales. At the local scale harvesting produced a negative effect on local fish densities, particularly in those areas and seasons of high fish aggregation, when both the vulnerability of the fish and the fishing effort increase. At the regional scale harvesting was characterized by a widespread negative effect on the whole fish population. Our findings demonstrate that (1) detection of the fisheries exploitation effects on natural populations is sensitive to the scale of investigation, (2) fisheries harvesting can simultaneously affect multiple ecological scales which are not linearly correlated with each other. We developed analytical techniques for the detection of scale-dependent processes, which can be readily applied to other systems. Our results provide insights on the risks of extrapolating the effects of harvesting on natural populations across scales, making the issues of scale and space central to the management and conservation of natural populations. KEY WORDS: Spatial distribution · Local exploitation · Regional exploitation · Scale · Yellowfin sole · Bering Sea Full text in pdf format Supplementary material PreviousNextCite this article as: Bartolino V, Ciannelli L, Spencer P, Wilderbuer TK, Chan KS (2012) Scale-dependent detection of the effects of ­harvesting a marine fish population. Mar Ecol Prog Ser 444:251-261. https://doi.org/10.3354/meps09434 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 444. Online publication date: January 10, 2012 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2012 Inter-Research.

Quantifying the Interplay between Environmental and Social Effects on Aggregated-Fish Dynamics

Demonstrating and quantifying the respective roles of social interactions and external stimuli governing fish dynamics is key to understanding fish spatial distribution. If seminal studies have contributed to our understanding of fish spatial organization in schools, little experimental information is available on fish in their natural environment, where aggregations often occur in the presence of spatial heterogeneities. Here, we applied novel modeling approaches coupled to accurate acoustic tracking for studying the dynamics of a group of gregarious fish in a heterogeneous environment. To this purpose, we acoustically tracked with submeter resolution the positions of twelve small pelagic fish (Selar crumenophthalmus) in the presence of an anchored floating object, constituting a point of attraction for several fish species. We constructed a field-based model for aggregated-fish dynamics, deriving effective interactions for both social and external stimuli from experiments. We tuned the model parameters that best fit the experimental data and quantified the importance of social interactions in the aggregation, providing an explanation for the spatial structure of fish aggregations found around floating objects. Our results can be generalized to other gregarious species and contexts as long as it is possible to observe the fine-scale movements of a subset of individuals.

Geomorphic Characterization of Reef Fish Aggregation Sites in the Upper Florida Keys, USA, Using Single-Beam Acoustics*

The objective of this study was to determine if multiple reef fish aggregation sites in the upper Florida Keys share characteristic geomorphic features. A commercial single-beam acoustic seabed classification system was used to map the seabed in the vicinity of reported or observed black grouper (Mycteroperca bonaci), mutton snapper (Lutjanus analis), and yellowtail snapper (L. chrysurus) aggregation sites. Results showed that drowned, margin-parallel, rocky ridges, known locally as outlier reefs, were features found in proximity to all of the mapped aggregation sites. In particular, three geomorphic characteristics were observed at most sites: a steep slope of the landward boundary of the upper slope terrace, an exposed outlier reef forming the seaward boundary of the upper slope terrace, and at least one other exposed outlier reef on the upper slope terrace. The results suggest that the acoustic-mapping technologies used herein could be valuable when considering the placement of marine reserves.

Lévy flight and Brownian search patterns of a free‐ranging predator reflect different prey field characteristics

1. Search processes play an important role in physical, chemical and biological systems. In animal foraging, the search strategy predators should use to search optimally for prey is an enduring question. Some models demonstrate that when prey is sparsely distributed, an optimal search pattern is a specialised random walk known as a Lévy flight, whereas when prey is abundant, simple Brownian motion is sufficiently efficient. These predictions form part of what has been termed the Lévy flight foraging hypothesis (LFF) which states that as Lévy flights optimise random searches, movements approximated by optimal Lévy flights may have naturally evolved in organisms to enhance encounters with targets (e.g. prey) when knowledge of their locations is incomplete. 2. Whether free-ranging predators exhibit the movement patterns predicted in the LFF hypothesis in response to known prey types and distributions, however, has not been determined. We tested this using vertical and horizontal movement data from electronic tagging of an apex predator, the great white shark Carcharodon carcharias, across widely differing habitats reflecting different prey types. 3. Individual white sharks exhibited movement patterns that predicted well the prey types expected under the LFF hypothesis. Shark movements were best approximated by Brownian motion when hunting near abundant, predictable sources of prey (e.g. seal colonies, fish aggregations), whereas movements approximating truncated Lévy flights were present when searching for sparsely distributed or potentially difficult-to-detect prey in oceanic or shelf environments, respectively. 4. That movement patterns approximated by truncated Lévy flights and Brownian behaviour were present in the predicted prey fields indicates search strategies adopted by white sharks appear to be the most efficient ones for encountering prey in the habitats where such patterns are observed. This suggests that C. carcharias appears capable of exhibiting search patterns that are approximated as optimal in response to encountered changes in prey type and abundance, and across diverse marine habitats, from the surf zone to the deep ocean. 5. Our results provide some support for the LFF hypothesis. However, it is possible that the observed Lévy patterns of white sharks may not arise from an adaptive behaviour but could be an emergent property arising from simple, straight-line movements between complex (e.g. fractal) distributions of prey. Experimental studies are needed in vertebrates to test for the presence of Lévy behaviour patterns in the absence of complex prey distributions.

Colossal Aggregations of Giant Alien Freshwater Fish as a Potential Biogeochemical Hotspot

The ubiquity and fascinating nature of animal aggregations are widely recognised. We report here consistent and previously undocumented occurences of aggregations of a giant alien freshwater fish, the Wels catfish (Silurus glanis). Aggregative groups were on average composed of 25 (±10 SD, ranging from 15 to 44) adults with estimated average total biomass of 651 kg (386 – 1132) and biomass density of 23 kg m−2 (14 – 40). Aggregations always occurred within the same location. No foraging, reproductive or anti-predator behaviour were observed during the aggregations. A mass-balance model estimated that these colossal aggregations of an alien species can locally release, through excretion only, up to 70 mg P m−2 h−1 and 400 mg N m−2 h−1, potentially representing the highest biogeochemical hotspots reported in freshwater ecosystems and another unexpected ecological effect of alien species.