The three-dimensional structure of airborne bird flocks

Summary The three-dimensional structure of flocks of dunlin, Calidris alpina, and starlings, Sturnus vulgaris, was studied while birds were in transit between feeding, loafing and roosting sites. A technique was developed that uses standard photogrammetric methods to determine the three-coordinate position of birds in flocks from stereoscopic pairs of simultaneously exposed photographs. A comparison of nearest neighbour distances indicates that dunlin have a tighter, more compact flock structure than do starlings (Fig. 2; Table 2). Analysis of interbird angles in both the vertical and horizontal planes indicates that each dunlin's nearest neighbour is most likely to be behind and below it. This spatial structure results in areas in which few nearest neighbours occur (e.g., immediately in front and below) (Fig. 3). Flight speeds during transit flights are also presented (Table 4). The spatial structure and behaviour of dunlin and starling flocks appear to be very similar to the structure and behaviour of schools of fish.     

Distance to neighbours influences the trade-off between hiding after disturbance and defending food patches in convict cichlids (<Emphasis Type="Italic">Archocentrus nigrofasciatus</Emphasis>)

Territories are often aggregated. Because of this, distance to neighbours should influence how territory-holders balance safety from predators with the use and defence of resources. I examined the influence of distance to a neighbour on refuge use by pairs of convict cichlids (Archocentrus nigrofasciatus) faced with a conflict between hiding and defending food patches. Neighbours could reduce the rate of intrusions by strangers as a by-product of their own resource defence. This should allow fish with near neighbours to spend more time in the refuge. Neighbours could also steal from patches that are left undefended. This should lead to a reduction in use of the refuge. When one fish was confined to its refuge (so that its patch was undefended), theft by the other increased as inter-patch distance decreased. Distance between patches did not influence the rate of intrusion by non-territorial fish. When both fish defended patches, body mass influenced the effect of inter-patch distance on refuge use. Large fish rarely used the refuge, but small territory-holders spent more time in the refuge when patches were close together, as predicted. However, when one fish was dominant at both patches, distance between patches did not influence refuge use. These results suggest that, despite increased opportunity for theft, there is no realised foraging or defensive benefit to settling near neighbours that are of similar competitive ability.Communicated by J. Krause     

Colonial and single breeding in fieldfares,Turdus pilaris L.: a comparison of nesting success in early and late broods

Summary Pairs of fieldfares breed singly or in colonies. Their breeding season was subdivided into an early part, when the trees were still leafless, and a late part, when the trees had leaves. Early colonial pairs has a significantly higher nesting success than single pairs. However, amongst late broods the nesting success of both categories of pairs was the same. Colonies of only two pairs show a weak trend suggesting that it could be advantageous for early broods to have a neighbour nearby, while the nesting success of late broods seems to be highest if the neighbour is farther away. The number of neighbours is more important than the distance to the nearest neighbour when colonies of different sizes are examined. The nesting success of colonial broods is also influenced by the time of breeding. Success increases with colony size, in early colonies, but the opposite applies for the late breeding season.A little owl was placed near single and colonial nests to show how communal defence contributed to the greater success of early colonial nests. Predators entering a colony risk being contaminated by significantly more faeces due to the higher rate of attacks by fieldfares using aimed defecation near colonial nests and these could affect the predators' flying ability.The possibility of fieldfare colonies serving as information centres for food finding is also discussed.     

Fish school density and volume

All the fish in a school occupy a volume estimated as N·BL³, where N is the number of fish and BL is their mean body length. We present extensive data from our experiments on cruising schools of saithe (Pollachius virens), herring (Clupea harengus) and cod (Gadus morhua) to validate this formula. Two methods of calculating the volumes of schools are described. One method is aggregative and depends on measuring the envelope of free space around a schooling fish, whereas the other is based on the dimensions of the school as a whole. The whole-school method is more reliable since it includes lacunae between the sub-units which exist in schools. For this method, we derive a computation which eliminates bias from outliers. The most realistic theoretical aggregative packing model predicts a volume per fish of 0.6 BL³. In saithe, the envelope of free space is approximately an ellipsoid, which, although it becomes more compressed at higher swimming speeds, yields a volume close to 0.7 BL³. From the whole-school method we calculate average volumes of 1.4 BL³ for saithe and 0.7 BL³ for herring. Increase in swimming speed produces more compact schools in saithe, but changes in arousal level can generate equally large differences. Changes in volume were not adequately explained by changes in nearest neighbour distance, giving support to the whole-school method.     

Potential latitudinal variation in egg size and number of a geographically widespread reef fish,revealed by common-environment experiments

Variation in maternal reproductive traits was examined in field and reared populations of a geographically widespread reef fish, Pomacentrus coelestis (Pomacentridae), drawn from three different latitudes in Japan. Size-specific clutch size and clutch weight of wild fish increased with increasing latitude. Conversely, latitudinal variation in egg size of wild fish was obscure in same-season comparisons, probably because of the temperature effect on egg size. Common-environment experiments conducted at three temperatures showed that egg size decreased with increasing temperature in all populations. In the experiments, egg size, clutch size and clutch weight differed among populations at all temperatures, showing clear latitudinal clines. Females from low latitude spawned larger eggs at every experimental temperature. Size-specific clutch size and weight were greater in females from high latitude. Thus, the northern fish had a larger reproductive output per spawning and a larger number of smaller eggs in a spawning. Such interpopualtion variation in this fish is likely to be partially genetically based, although environmental effects on the variation cannot be entirely ruled out. This study provides evidence of potential latitudinal variation in the egg size and number in a coastal fish, by common-environment experiments. The close correspondence between latitudes and these maternal reproductive traits may be a consequence of local adaptation.     

Food resource partitioning in a Mediterranean demersal fish assemblage: the effect of body size and niche width

We investigated the effects of body size, feeding strategy and depth distribution on the trophic resource partitioning among the 26 dominant fish consumers in a fish assemblage on the central Mediterranean shelf-break. The fish assemblage was structured in two major trophic guilds: epibenthic and zooplanktonic feeders, according to the position of each predator along the benthos–plankton gradient. Within each main guild, the species were segregated along a prey-size or fish-size gradient into five further guilds. Fish size and prey size were strongly correlated, indicating that the prey-size niche can be well explained by predator size. Fish consumers showed a significant negative correlation between the similarity in prey type and the similarity in depth distribution; most species with similar trophic preferences segregated along the depth dimension. The only predators overlapping in both food and depth preferences were those with a more specialist trophic behavior. These results suggest that fish body size and depth preferences are the two main niche dimensions, explaining a large part of the coexistence between the Mediterranean shelf-break fish consumers.     

Distribution and diet of juvenile Patagonian toothfish on the South Georgia and Shag Rocks shelves (Southern Ocean)

The distribution and diet of juvenile (<750 mm) Patagonian toothfish are described from four annual trawl surveys (2003–2006) around the island of South Georgia in the Atlantic sector of the Southern Ocean. Recruitment of toothfish varies inter-annually, and a single large cohort dominated during the four years surveyed. Most juveniles were caught on the Shag Rocks shelf to the NW of South Georgia, with fish subsequently dispersing to deeper water around both the South Georgia and Shag Rocks shelves. Mean size of juvenile toothfish increased with depth of capture. Stomach contents analysis was conducted on 795 fish that contained food remains and revealed that juvenile toothfish are essentially piscivorous, with the diet dominated by notothenid fish. The yellow-finned notothen, Patagonotothen guntheri, was the dominant prey at Shag Rocks whilst at South Georgia, where P. guntheri is absent, the dominant prey were Antarctic krill and notothenid fish. The diet changed with size, with an increase in myctophid fish and krill as toothfish grow and disperse. The size of prey also increased with fish size, with a greater range of prey sizes consumed by larger fish.     

Schooling tendency and foraging benefit in the ocean surgeonfish

Summary Field observations of individuallytagged ocean surgeonfish (Acanthurus bahianus) indicated that schooling provided a foraging benefit: an increase in relative forging time. The magnitude of this increase differed among individuals, primarily due to time budgets of the fish when they were not in schools. The proportion of nonschooling time devoted to foraging was positively correlated with fish size, while the proportion of schooling time that was spent forging was similar for all individuals. Schooling tendency (percentage of time in schools) was positively related to magnitude of foraging benefit and negatively related to fish size. When fish size was held constant by partial correlation, schooling tendency remained positively related to foraging benefit. These results provide evidence that fish which derive a greater foraging benefit from school membership spend more time in schools.     

Growth and ovarian development of Maurolicus muelleri during spring

The size, fecundity and gonad development of Maurplicus muelleri (Gmelin) were studied in Herdlefjorden, western Norway, from January to June 1994. In January, the two age groups formed separate sound-scattering layers (SSL). Juvenile 1-group fish were found to form the upper SSL, while adult 2+ individuals inhabited the lower, these distributions reflecting the different feeding to fitness functions of these two age groups. In June there was only one SSL, and in March and May some 2+ were also found in the upper SSL. Growth of the younger fish to 2+ fish size was observed over the period, and it was found that M. muelleri mature after 1 yr, although size at maturity was lower than found in previous studies. Growth of the 2+ fish changed as the period progressed, to allow gonadal development after a winter of low feeding. Increased investment in development of the gonads was also observed in the 1-group fish over the study period, but substantial maturation only occurred from May to June. Absolute fecundity was size dependent, although high variability was found in the larger size ranges. Oocyte development occurred in discrete batches, although lack of gonadal synchrony between individuals means that spawning is continuous in the population as a whole, once the spawning period begins. Batch fecundity was found to be similar to previous studies, and 2+ fish were found to start spawning earlier, and with an increased intensity, than the 1-group fish.     

Social spacing in the temperate marine goby Coryphopterus nicholsi

Coryphopterus nicholsi is a temperate marine goby that occupies rock rubble in protected subtidal areas along the Pacific coast of NOrth America. In field populations, fish of all sizes and both sexes are found in the same habitat. All observed fish over 25 mm standard length defended space during both the reproductive and the non-reproductive seasons, with territory size being directly correlated with fish size. There was peripheral overlap of territories, particularly between adjacent adult conspecifics of different size classes, but also with smaller fish that had territories centered in the interstices of larger territories. However, zones of overlap were used at different times by space-sharing fish so that defended areas remained temporally discrete. Access to areas of overlap appeared to be determined by dominance rank. More dominant (usually larger) fish used areas of overlap at will; the defense and use of shared areas by less dominant fish was contingent upon the absence of larger fish. Social organization in C. nicholsi appears to be defined by a combination of territorial behavior and dominance relationships among adjacent individuals, permitting the coexistence of all members of the population under circumstances of limited habitat availability.     

Reproductive Success of Captive-Bred Steelhead Trout in the Wild: Evaluation of Three Hatchery Programs in the Hood River

Abstract:  Population supplementation programs that release captive-bred offspring into the wild to boost the size of endangered populations are now in place for many species. The use of hatcheries for supplementing salmonid populations has become particularly popular. Nevertheless, whether such programs actually increase the size of wild populations remains unclear, and predictions that supplementation fish drag down the fitness of wild fish remain untested. To address these issues, we performed DNA-based parentage analyses on almost complete samples of anadromous steelhead ( Oncorhynchus mykiss ) in the Hood River in Oregon (U.S.A.). Steelhead from a supplementation hatchery (reared in a supplementation hatchery and then allowed to spawn naturally in the wild) had reproductive success indistinguishable from that of wild fish. In contrast, fish from a traditional hatchery (nonlocal origin, multiple generations in hatcheries) breeding in the same river showed significantly lower fitness than wild fish. In addition, crosses between wild fish and supplementation fish were as reproductively successful as those between wild parents. Thus, there was no sign that supplementation fish drag down the fitness of wild fish by breeding with them for a single generation. On the other hand, crosses between hatchery fish of either type (traditional or supplementation) were less fit than expected, suggesting a possible interaction effect. These are the first data to show that a supplementation program with native brood stock can provide a single-generation boost to the size of a natural steelhead population without obvious short-term fitness costs. The long-term effects of population supplementation remain untested.     

Trophic relations of the cephalopod Martialia hyadesi (Teuthoidea: Ommastrephidae) at the Antarctic Polar Front,Scotia Sea

Samples of the squid Martialia hyadesi were collected aboard two Japanese squid-jigging vessels carrying out commercial fishing trials at the Antarctic Polar Frontal Zone, north Scotia Sea, in February 1989. The dissected stomachs of 61 specimens were classified according to fullness and the contents were examined visually. Identifiable food items included fish sagittal otoliths, crustacean eyes, the lappets on euphausiid first antennule segments and cephalopod sucker rings. The most frequent items in the squid's diet were the myctophid fishes Krefftichthys anderssoni and Electrona carlsbergi, the euphausiid Euphausia superba and a hyperiid amphipod, probably Themisto gaudichaudi. A small proportion of the sample had been feeding cannibalistically. Total lengths of the fish prey were estimated from sagittal otolith size using published relationships. All fish were relatively small; 7 to 35% of squid mantle-length. However, it is possible that some heads of larger fish are discarded by the squid and so are not represented by otoliths in the stomach contents. Over the size range of squid in the sample there was no relationship between size of fish prey and size of squid. Similarly, when the squid sample was divided into groups according to prey categories: crustaceans, crustaceans+fish, fish, cephalopod, there was no evidence that dietary preference was related to squid size. The prevalence of copepod-feeding myctophids in the diet of this squid, which is itself a major prey item of some higher predators in the Scotia Sea, suggests that a previously unrecognised food chain: copepod-myctophid-M. hyadesi-higher predator, may be an important component of the Antarctic oceanic ecosystem.     

Some aspects of recolonization of coral rocks in Eilat (Gulf of Aqaba) by fish populations after poisoning

As a result of an accident, a limited strip of the coast of Eilat (Gulf of Aqaba, Red-Sea) was affected by poison that killed all fishes. The area was observed monthly for the following year, to study the recovery of fish populations. The study also included observations on growth rate of fish and size of populations. Recovery of fish populations was complete 10 to 12 months after the poisoning. It is believed that this is due to the relatively small size of the contaminated area and the survival of most invertebrates that constitute an important part of the biotope of the fish.     

Shifts in a Pacific Ocean Fish Assemblage: the Potential Influence of Exploitation

Abstract:  As in many regions of the world, marine fishes and invertebrates along the Pacific coast of the United States have long been subjected to overexploitation. Despite this history, however, we lack basic information on the current status of many fishes along this coastline. We used data from a quarter century of fishery-independent, coast-wide trawl surveys to study systematically the demersal fish assemblages along the U.S. Pacific coast. We documented fundamental shifts in this fish assemblage. Average fish size, across a diversity of species, has declined 45% in 21 years. There have been major shifts in the constituent species of the assemblage, with some species achieving annual population growth rates of >10% and others declining in excess of 10% per year. Annual rate of change in population size appeared to be a function of life history interacting with fishing pressure. Negative trends in population size were particularly apparent in rockfish ( Sebastes spp.). However, across all taxa examined, trends in population size were associated with size of maturity, maximum size, and growth rate. Trends in population size were associated inversely with harvest levels, but stocks that mature late tended to decline faster than would be predicted by catch rates alone. Our results are disquieting because they raise the possibility that fishing-induced phase shifts in fish communities may affect the recovery of fishes, even after the implementation of severe fishing restrictions.     

Documenting Loss of Large Trophy Fish from the Florida Keys with Historical Photographs

Abstract:  A loss of large vertebrates has occurred in aquatic and terrestrial ecosystems, but data to measure long-term population changes are sparse. Historical photographs provide visual and quantitative evidence of changes in mean individual size and species composition for groups of marine fish that have been targeted by sport fishing. I measured such trends for 13 groups of recreationally caught "trophy" reef fish with photographs taken in Key West, Florida, from 1956 to 2007. The mean fish size declined from an estimated 19.9 kg (SE 1.5) to 2.3 kg (SE 0.3), and there was a major shift in species composition. Landings from 1956 to 1960 were dominated by large groupers ( Epinephelus spp.), and other large predatory fish were commonly caught, including sharks with an average length of just <2 m. In contrast, landings in 2007 were composed of small snappers ( Lutjanus spp. and Ocyurus chrysurus ) with an average length of 34.4 cm (SE 0.62), and the average length of sharks declined by more than 50% over 50 years. Major declines in the size of fish caught were not reflected in the price of fishing trips, so customers paid the same amount for a less-valuable product. Historical photographs provide a window into a more pristine coral reef ecosystem that existed a half a century ago and lend support to current observations that unfished reef communities are able to support large numbers of large-bodied fish.     

Some notes on the population dynamics of the monogenean gill parasite Gastrocotyle indica

The fish Caranx kalla of different size groups (2 to 18 cm body length) is present during various calender months of the year off the Madras coast (S. India). C. kalla displays a definite progressive shift in growth during successive months; the fish grows from 2 cm (September) to about 18 cm (August) over a period of 12 months. The highest (47%) or lowest (11%) incidence of a monogenean gill parasite Gastrocotyle indica has been observed in the C. kalla size groups 2.0 to 3.9 cm or 16.0 to 17.9 cm; however, the maximum specific density of 10.9 parasite/infested fish was observed in the size group 14.0 to 15.9 cm. Small fish (2.0 to 3.9 cm) carry only larvae (58/100 fish); larger hosts (4.0 to 5.9 cm) larvae (45/100 fish) and juveniles (8/100 fish); individuals of the 6.0 to 15.9 cm size group harbour larval, juvenile and adult stages of G. indica. Fishes of more than 16 cm carry only adult parasites. The level of parasitization is higher on the male hosts (30% incidence; 5 parasites/infested male) than on the females (25% incidence; 4 parasites/infested female). The highest incidences of larval, juvenile and adult G. indica occur in September, February, and May, respectively; the parasite seems to live for 1 year. The host requires a period of about 8 months to acquire immunity against infestation by G. indica or copepod parasites.     

Ecological and social determinants of spacing behaviour in desert baboon groups

This paper investigates the determinants of individual spacing behaviour in a desert baboon population (Papio cynocephalus ursinus). Patterns of neighbour proximity and neighbour density were examined among adults in four groups under different ecological and social conditions (through instantaneous sampling during focal follows). Initial analysis of these data shows that (1) the use of vertical substrates (refuges such as tall trees and cliff faces) can confound patterns of spacing, and (2) individual differences in spacing can depend on the spatial scale over which it is measured. To minimise these substrate and scale effects, this analysis focuses on animals which are off refuges and examines spacing behaviour through its underlying statistical `dimensions' (identified through factor analysis). Analysis of these dimensions indicates that sex, group size, activity-habitat and female reproductive state can all have independent effects on spacing: (1) males are more dispersed than females in smaller groups, (2) male and female dispersion increases with time spent in foraging habitats, and (3) female dispersion is reduced during lactation. According to the hypotheses tested, these results indicate that feeding competition only affects spacing behaviour during foraging while predation risk plays little or no role in spacing. Most aspects of spacing behaviour are best explained by male reproductive strategies and their social repercussions. Received: 25 May 1998 / Accepted after revision: 18 July 1998     

Foraging behavior in early Atlantic cod larvae (Gadus morhua) feeding on a protozoan (Balanionsp.) and a copepod nauplius (Pseudodiaptomussp.)

Food limitation is likely to be a source of mortality for fish larvae in the first few weeks after hatching. In the laboratory, we analyzed all aspects of foraging in cod larvae (Gadus morhua Linnaeus) from 5 to 20 d post-hatching using protozoa (Balanion sp.) and copepod nauplii (Pseudodiaptomus sp.) as prey. A camera acquisition system with two orthogonal cameras and a digital image analysis program was used to observe patterns of foraging. Digitization provided three-dimensional speeds, distances, and angles for each foraging event, and determined prey and fish larval head and tail positions. Larval cod swimming speeds, perception distances, angles, and volumes increased with larval fish size. Larval cod swam in a series of short intense bursts interspersed with slower gliding sequences. In 94% of all foraging events prey items were perceived during glides. Larval cod foraging has three possible outcomes: unsuccessful attacks, aborted attacks, and successful attacks. The percentage of successful attacks increased with fish size. In all larval fish size classes, successful attacks had smaller attack distances and faster attack speeds than unsuccessful attacks. Among prey items slowly swimming protozoans were the preferred food of first-feeding cod larvae; larger larvae had higher swimming speeds and captured larger, faster copepod nauplii. Protozoans may be an important prey item for first-feeding larvae providing essential resources for growth to a size at which copepod nauplii are captured. Received: 20 April 1999 / Accepted: 12 January 2000     

Lag Effects in the Impacts of Mass Coral Bleaching on Coral Reef Fish, Fisheries, and Ecosystems

Abstract:  Recent episodes of coral bleaching have led to wide-scale loss of reef corals and raised concerns over the effectiveness of existing conservation and management efforts. The 1998 bleaching event was most severe in the western Indian Ocean, where coral declined by up to 90% in some locations. Using fisheries-independent data, we assessed the long-term impacts of this event on fishery target species in the Seychelles, the overall size structure of the fish assemblage, and the effectiveness of two marine protected areas (MPAs) in protecting fish communities. The biomass of fished species above the size retained in fish traps changed little between 1994 and 2005, indicating no current effect on fishery yields. Biomass remained higher in MPAs, indicating they were effective in protecting fish stocks. Nevertheless, the size structure of the fish communities, as described with size-spectra analysis, changed in both fished areas and MPAs, with a decline in smaller fish (<30 cm) and an increase in larger fish (>45 cm). We believe this represents a time-lag response to a reduction in reef structural complexity brought about because fishes are being lost through natural mortality and fishing, and are not being replaced by juveniles. This effect is expected to be greater in terms of fisheries productivity and, because congruent patterns are observed for herbivores, suggests that MPAs do not offer coral reefs long-term resilience to bleaching events. Corallivores and planktivores declined strikingly in abundance, particularly in MPAs, and this decline was associated with a similar pattern of decline in their preferred corals. We suggest that climate-mediated disturbances, such as coral bleaching, be at the fore of conservation planning for coral reefs.     

Effects of fish feeding by snorkellers on the density and size distribution of fishes in a Mediterranean marine protected area

Although there is a great deal of evidence to show that supplementary feeding by humans in terrestrial environments causes pronounced changes in the distribution and behaviour of wild animals, at present very little is known about the potential for such effects on marine fish. This study evaluated the consequences of feeding by snorkellers on fish assemblages in the no-take area of the Ustica Island marine protected area (MPA; western Mediterranean) by (1) determining if reef fish assemblage structure is affected in space and time by tourists feeding the fish; (2) assessing the effects of feeding on the abundance of the most common fish species; and (3) assessing the effects of feeding on the size structure of the two most numerically dominant ones. In particular, we hypothesised that both the abundance and the size structure of some fish species would increase at the study site following supplementary feeding, since the additional food provided by humans would make the site more appealing to them. Fish feeding influenced the fish assemblages within the Ustica MPA, and significant spatio-temporal changes occurred. While fish feeding appeared to have no effect on the ornate wrasse Thalassoma pavo, there was a noticeable increase in the number of Oblada melanura and Epinephelus marginatus in the impacted location after feeding. It is very likely that aggregations of fishes that evolve as a result of fish feeding by the public may have negative effects on local populations of fishes and invertebrates that make up their prey. Recreational use of coastal areas and MPAs is increasing elsewhere, making fish feeding a generalised human activity. Accurate information about its effect on the fish assemblage is essential to make responsible management decisions.Communicated by R. Cattaneo-Vietti, Genova