Individual differences in foraging behavior and cortisol levels in recently emerged brook charr (Salvelinus fontinalis)

Recently emerged brook charr (Salvelinus fontinalis) foraging in still-water pools along the sides of streams tend to be either sedentary, feeding from the lower portion of the water column (a sit-and-wait tactic), or very active, feeding from the upper portion of the water column (an active search tactic). We tested whether the individual differences in foraging behavior were associated with baseline concentrations and responses of cortisol, a steroid hormone linked to personality differences in a variety of animals including fishes. We quantified the proportion of time spent on moving by focal charr in the field and then capturing them. Captured individuals were either (i) sacrificed immediately to quantify baseline cortisol concentrations, (ii) held in an unfamiliar field environment for 15 min and then sacrificed to quantify cortisol concentrations in response to handling and holding in a novel field environment, or (iii) held in an unfamiliar field environment with a white Plexiglas base (stressor) for 15 min to quantify cortisol concentrations in response to a novel object. Eleven statistical models relating cortisol concentrations to the proportion of time individuals spent on moving while searching for prey were compared using multi-model inferencing. Cortisol concentrations were higher for charr that spent a lower proportion of time on moving in the field than for charr that spent a higher proportion of time on moving. For a given proportion of time spent on moving, mean cortisol concentrations between baseline and experimental treatments, our measure of cortisol response, did not differ markedly. Our findings suggest that the foraging tactics displayed by wild brook charr in the field could reflect differences in how individuals perceive their environment.     

Foraging behaviour and brain morphology in recently emerged brook charr, Salvelinus fontinalis

Recently emerged brook charr (Salvelinus fontinalis) foraging in still-water pools along the sides of streams are either active, feeding on insects from the upper portion of the water column away from the stream bank, or sedentary, feeding on crustaceans emerging from the hyporheic zone near the stream bank. We tested whether the frequency of movement displayed by individual brook charr searching for prey in the field was related to the relative volume of the telencephalon, a brain region involved with movement and space use in fishes. Movement of individuals searching for prey was quantified in the field, individuals were captured and volumes of the telencephalon and of the olfactory bulbs, a brain region neighbouring the telencephalon but not implicated in space use, were measured. Individuals with larger telencephalon volumes moved more frequently on average while searching for prey in the field than did individuals with smaller telencephalon volumes. The frequency of movement was unrelated to differences in the volume of the olfactory bulbs, suggesting that the relationship between telencephalon volume and movement was not a consequence of differences in overall brain size. Demonstrating a correlation between foraging behaviour and brain morphology for brook charr exhibiting different foraging tactics suggests that diversification in brain structure and function could be important aspects of the foraging specialization believed to occur during early stages in the evolution and development of resource polymorphisms.     

MHC and fertilization success in the Arctic charr (Salvelinus alpinus)

Genes of the major histocompatibility complex (MHC) are remarkably polymorphic. Several selection mechanisms have been invoked to account for this diversity, including disassortative mating preferences. In addition, eggs may discriminate between sperm based on MHC. To investigate the effects of MHC-genotype on fertilization success, we obtained mature gametes from ripe Arctic charr (Salvelinus alpinus) males and females captured on spawning grounds. The eggs of each female were divided into two batches, and by letting each of 2 males fertilize 1 of the batches, we obtained a total of 36 half-sibling batch-pairs. The semen was diluted to ensure that the two males in each half-sibling batch-pair contributed with the same number of sperm cells. We found that MHC-heterozygous males had significantly higher fertilization success than MHC-homozygous males and neither initial spermatocrit, sperm motility nor swimming velocity co-varied with difference in fertilization success. There was no effect of female genotype or female-male MHC-similarity on fertilization success. However, one MHC-allele was associated with increased fertilization success. It seems plausible that the difference in fertilization success between homo- and heterozygous males may be due to MHC-dependent sperm selection by the ovum.Communicated by M. Abrahams     

Adaptive peaks and alternative foraging tactics in brook charr: evidence of short-term divergent selection for sitting-and-waiting and actively searching

Some recently emerged brook charr (Salvelinus fontinalis) inhabiting still-water pools along the sides of streams are sedentary and eat crustaceans from the lower portion of the water column. Others are more active and eat insects from the upper portion of the water column. We provide evidence that this divergent foraging behavior reflects short-term divergent selection brought about by intraspecific competition in the presence of alternative food sources. Rates of encounters and interactions between individuals were density dependent, and encounter and interaction events were closely timed with prey capture attempts. In addition, aggressive fish made more foraging attempts per minute than nonaggressive fish. Aggressive fish were also either inactive or very active, while nonaggressive fish exhibited intermediate levels of activity. Growth rate potential, an important component of fitness during the early life stages of brook charr, was assessed using tissue concentrations of RNA and found to be highest for sedentary fish and for active fish making frequent foraging attempts, and lower for fish exhibiting intermediate levels of activity. Our findings support contentions that individual behavior plays an important role during initial steps in the evolution of resource polymorphisms. Received: 27 July 1998 / Accepted after revision: 16 November 1998     

Effects of ovarian fluid on sperm velocity in Arctic charr (Salvelinus alpinus)

Numbers of studies in externally fertilizing fish species provide evidence for an effect of ovarian fluid on sperm motility characteristics such as duration of forward mobility, velocity or percent motile sperm cells. Yet, because of variations among females in the quality of their ovarian fluid, such effects might differ between individuals. Additionally, ovarian fluid from different females could also be expected to affect each ejaculate differently, resulting in cryptic female choice. In this study on Artic charr (Salvelnius alpinus), sperm velocity from several males was measured in the diluted ovarian fluid of several females according to a fully balanced crossing design. This design allowed us to estimate variations among females in the effect of their ovarian fluid on the velocity of sperm from different males, and to detect variations among males in the ability of their sperm to swim in ovarian fluid. Sperm velocity was estimated by computer-assisted sperm analysis. Average velocity was found to vary among females, with some females having constantly higher velocity measurements in their ovarian fluid, and among males, indicating that some males had overall faster sperm in ovarian fluid than others. Moreover, variation in sperm velocity was shown to depend on individual female-male interactions. Our results document that females vary in the effect of their ovarian fluid on sperm velocity and that their ovarian fluid may stimulate sperm velocity according to individual characteristics of males. This latter result suggests a potential mechanism for cryptic female choice.

Mating system and individual reproductive success of sympatric anadromous and resident brook charr, <Emphasis Type="Italic">Salvelinus fontinalis</Emphasis>, under natural conditions

Salmonids are known for the occurrence in sympatry of two life-history forms, one that undergoes migration to sea before returning to freshwater to reproduce (anadromous) and one that inhabits freshwater without a migration phase (resident). Whereas one breeding population is often suggested by population genetic studies, mating patterns have rarely been directly assessed, especially when both sexes are found within each life-history form. By using highly polymorphic microsatellite loci and parentage analysis in a natural population of sympatric anadromous and resident brook charr (Salvelinus fontinalis), we found that gene flow occurred between the two forms and was mediated by resident males mating with both resident and anadromous females. Determinants of reproductive success, estimated by the number of surviving juveniles (ages 1 and 2 years), differed between the sexes. No strong evidence of the influence of size on individual reproductive success was found for males, whereas larger females (and hence most likely to be anadromous) were more successful. The higher individual reproductive success of anadromous fish compared to residents was mainly explained by this higher reproductive success of anadromous females. We suggest that resident males adopt a “sneaking” reproductive tactic as a way of increasing their reproductive success by mating with females of all sizes in all habitats. The persistence of the resident tactic among females may be linked to their advantage in accessing spatially constrained spawning areas in small tributary streams unavailable to larger females.     

Less is more: social learning of predator recognition requires a low demonstrator to observer ratio in Arctic charr (Salvelinus alpinus)

Reintroduction programs for conservation purposes suffer from low post-release survival of captive-bred animals, especially so with fish reintroductions. High mortality is most often due to weakened antipredator skills, which could be enhanced through direct experience with predators and social learning. As yet, the relative efficiency of these learning methods has not been tested, and the significance of the factors determining how efficiently antipredator responses are transmitted socially from experienced (demonstrator) to predator naive (observer) individuals is largely unknown. Using hatchery-bred stock of the endangered Saimaa Arctic charr (Salvelinus alpinus), we show for the first time that: (1) contrary to theoretical predictions, acquired predator recognition was only socially transmitted from predator-experienced to predator-naive fish in groups with a smaller proportion of experienced than naive individuals, and that (2) socially acquired avoidance of the predator odors was as strong as in those fish conditioned with a combination of odors and predator exposure. We conclude that using social transmission procedures in training fish for reintroduction programs may provide considerable ecological, economical and ethical advantages.Communicated by J. Krause     

Food selection by laboratory-reared larvae of the scaled sardine Harengula pensacolae (Pisces,Clupeidae) and the bay anchovy Anchoa mitchilli (Pisces,Engraulidae)

Food selection by laboratory-reared larvae of scaled sardines Harengula pensacolae, and bay anchovies Anchoa mitchilli, was compared. Natural plankton was fed to the larvae during the 22 days following hatching. Food levels in the rearing tanks were maintained at an average of 1,600 to 1,800 potential food organisms per liter. Larvae of both species selected as food copepod nauplii, copepodites, and copepods; initial feeding was on organisms of 50 to 75 body width. Larvae of H. pensacolae averaged 4.2 mm in total length at hatching and those of A. mitchilli about 2.0 mm. H. pensacolae larvae grew about 1.0 mm per day and A. mitchilli 0.70 mm per day. The mean number of food organisms in each digestive tract was greater in H. pensacolae than in A. mitchilli, and the difference in number increased as the larvae grew. Average size of food organisms eaten increased for both species with growth, because of selection by the larvae; the average size of copepodites and copepods in digestive tracts increased at a faste rate in H. pensacolae than A. mitchilli. A. mitchilli longer than 8 mm did not eat copepod nauplii.Contribution No. 170, Bureau of Commercial Fisheries Tropical Atlantic Biological laboratory, Miami, Florida 33149, USA.     

The interplay between foraging mode, habitat structure, and predator presence in antlions

Antlion larvae are sand-dwelling insect predators, which ambush small arthropod prey while buried in the sand. In some species, the larvae construct conical pits and are considered as sit-and-wait predators which seldom relocate while in other species, they ambush prey without a pit but change their ambush site much more frequently (i.e., sit-and-pursue predators). The ability of antlion larvae to evade some of their predators which hunt them on the sand surface is strongly constrained by the degree of sand stabilization or by sand depth. We studied the effect of predator presence, predator type (active predatory beetle vs. sit-and-pursue wolf spider), and sand depth (shallow vs. deep sand) on the behavioral response of the pit building Myrmeleon hyalinus larvae and the sit-and-pursue Lopezus fedtschenkoi larvae. Predator presence had a negative effect on both antlion species activity. The sit-and-wait M. hyalinus larvae showed reduced pit-building activity, whereas the sit-and-pursue L. fedtschenkoi larvae decreased relocation activity. The proportion of relocating M. hyalinus was negatively affected by sand depth, whereas L. fedtschenkoi was negatively affected also by the predator type. Specifically, the proportion of individual L. fedtschenkoi that relocated in deeper sand was lower when facing the active predator rather than the sit-and-pursue predator. The proportion of M. hyalinus which constructed pits decreased in the presence of a predator, but this pattern was stronger when exposed to the active predator. We suggest that these differences between the two antlion species are strongly linked to their distinct foraging modes and to the foraging mode of their predators. Reut Loria and Inon Scharf contributed equally to the paper.     

Winter mortality, growth, and behavior of young-of-the-year of four coastal fishes in New Jersey (USA) waters

Winter mortality has been hypothesized to select for large body size in young-of-the-year (YOY) fishes, yet substantiation of winter mortality and its cause(s) are available for few estuarine or marine species. We examined seasonal length distributions of wild populations of four common marine species, black sea bass (Centropristis striata), tautog (Tautoga onitis), cunner (Tautogolabrus adspersus), and smallmouth flounder (Etropus microstomus), and mortality (i.e., frequency of death), growth, and behavior of their YOY in the laboratory at ambient winter temperatures (mean 7°C, range 2-13°C) during a 135-day period (December 1992 through mid-April 1993) to establish potential causes of their mortality in the field. Young-of-the-year black sea bass experienced 100% mortality when water temperatures decreased to 2-3°C in February, emphasizing the importance of winter emigration from estuaries in this southern species. The low mortality of two labrid species, YOY tautog (14%) and YOY cunner (3%), was consistent with their northern distribution and year-round occurrence in estuarine and nearshore coastal waters. Laboratory mortality of YOY smallmouth flounder (33%) was higher for small (<35 mm total length) fish, suggesting that this small species may experience high winter mortality in estuaries and nearshore coastal waters. Seasonal differences in fish length result potentially from several mechanisms (e.g., mortality and/or migration) that are difficult to assess, but our laboratory experiments suggest that seasonal temperature changes cause size-specific mortality of YOY smallmouth flounder and offshore migration of YOY black sea bass.     

Innate antipredator responses of Arctic charr (<Emphasis Type="Italic">Salvelinus alpinus</Emphasis>) depend on predator species and their diet

The ability to discriminate between more dangerous and less dangerous predators can have serious fitness advantages for fish juveniles. This is especially true for hatchery-reared fish young used for stocking, because their post-release mortality is often much higher than that of wild-born conspecifics. We tested whether two coexisting fish predators and their different diets induce innate behavioral responses in predator-naive Arctic charr (Salvelinus alpinus) young originating from an endangered hatchery-bred population used for re-introductions. We predicted the antipredator responses of charr to be stronger towards chemical cues of brown trout (Salmo trutta) and pikeperch (Stizostedion lucioperca) than towards odorless control water. More pronounced antipredator behavior was predicted in treatments with predators fed on charr than when their diet consisted of another sympatric salmonid, European grayling (Thymallus thymallus), or when they were food-deprived. The Arctic charr young showed strong antipredator responses in all brown trout treatments, whereas odors of the less likely predator pikeperch were avoided with conspecific diet only. Freezing was the most sensitive antipredator behavior, as it was completely absent in control treatments. We found considerable individual variation in the amount and strength of antipredator responses. Although almost half of the charr failed to show antipredator behavior towards the piscivores, those with the innate ability showed highly sensitive recognition of predator odors. Our results indicate that the innate antipredator behavior of the juvenile fish is already finely tuned to respond specifically to chemical cues from different fish predators and even their diets.Communicated by J. Krause     

Aggregation and foraging behavior of whirligig beetles (Gyrinidae)

Summary Whirligig beetles aggregate in the daytime into dense single-and multispecies groups (rafts) of hundreds or thousands of individuals. On the 22km shoreline of Lake Itasca in northern Minnesota, these aggregations were on the average 0.8 km apart, and they were usually found day after day in the same ocations.Most beetles apparently do not home to the aggregation of their origin after dispersing at night because (a) the species composition of some aggregations changed greatly, and (b) paint-marked beetles (Dineutus horni) moved overnight from one aggregation as far as 4km, joining 11 of the 14 large (>300 beetles) D. horni groups on the lake.Throughout the night, the largest concentrations of beetles remained within 100m of the diurnal aggregation sites. Beetles reconvened into the compact rafts before daybreak, in part by following each other in sometimes long single files or trains. Their forward motion stopped after they joined large number of other beetles. We infer that following behavior enables those individuals that have dispersed from their original aggregations (during their nocturnal foraging) to find and join other aggregations before daylight.Naive fish ate the beetles despite their noxious secretions. However, fish living near rafting sites and feeding on insects on the water surface in daylight should soon learn to avoid the beetles. The rafting sites would then become safe places. We observed fish attacking only those beetles that had been either dispersed from their rafts or released into open water away from raft sites in the daytime. We speculate that the evolutionary significance of the aggregation behavior is related to predator (fish) avoidance.     

Eggs and early larvae of Thrissocles species (Engraulidae: Pisces)

Eggs of Thrissocles species are found in surface plankton in the Ernakulam Channel (Cochin Harbour)during February to May 1967. The eggs hatch within 24 h. Empty egg shells have characteristic apertures, through which the embryos have emerged; yolk is resorbed 36 h after hatching.Larvae (36 to 72 h groups) assemble at the lighted region of the aquaria during day-time and scatter to different levels at night. Larvae older than 72 h show no inclination to assemble as before. All larvae died between 96 and 110 h after hatching. Many batches of eggs were reared in the same medium, and all of them behaved as described. The results indicate that the right type of food was available in the aquaria for larvae up to a period of 72 h. The volume of water also appears to have a bearing upon the survival rate and longevity of the larvae since, in small aquaria, more larvae died at an early stage.     

Energy budgets,risk and foraging preferences in dark-eyed juncos (Junco hyemalis)

Summary Juncos' preferences for constant versus variable food rewards were tested in three series of aviary experiments. Deprivation and feeding rates were varied across the three treatments, but the mean of the variable reward equalled the constant reward in every experiment. When the birds gained energy faster than required to meet all 24-h costs, they preferred the constant reward. When the birds' energy intake was less than the minimally required rate, they preferred the variable reward. When energy intake just balanced average daily costs, the birds preferred the constant reward or were indifferent, and their response depended on the particular mean-variance combination presented.     

Parental care sex differences in the brown bullhead,Ictalurus nebulosus (Pisces,Ictaluridae)

Summary Males were the principal care-givers in the brown bullhead, Ictalurus nebulosus. Direct observations on individually tagged adults in nature revealed that males attended broods at closer distances than females and were the most frequent lone care-givers. However, females often participated in parental care and in some cases assumed the typical male role in the absence of the male. In the presence of her mate, females generally performed different care-giving functions than males. Females attended broods at a distance and chased other fishes more frequently than males. Males remained directly over the brood guarding. A series of mated pairs in each of three reproductive conditions were captured and examined. Different pairs were compared between reproductive conditions for body weight, body condition, gonadal-somatic index, and a gut-contents index. The inital costs of reproduction, as measured by weight loss, condition change, and gonadal-somatic index change, were more severe in females than males. During breeding and subsequent care-giving, males did little or no feeding whereas females increased their feeding during the same time period. However, males appeared to sustain only minor weight and condition losses due to care-giving. I suggest that males were the principal care-givers because the net benefits of remaining at the nest were greater than the net benefits of leaving. Males had little opportunity to mate with more than one female each season, and offspring had little chance of survival in the absence of a care-giving adult. Considerable variation in female behavior occurred which suggests that the net benefits of care-giving were nearly balanced with the net benefits of leaving the brood. The relative importance of feeding versus care-giving by females may vary with slight differences between females and the behavior of their mates as care-givers.Present address: Department of Zoology, Ohio State University, 1735 Neil Avenue, Columbus, Ohio 43210, USA     

Otolith record of age, growth, and ontogeny in larval and pelagic juvenile Stephanolepishispidus (Pisces: Monacanthidae)

Juveniles of the planehead filefish Stephano-lepishispidus (Pisces: Monacanthidae) (Linnaeus, 1766) are a major component of the Sargassum spp. community, yet little is known of their ecology. In this study, the otolith record of age, growth, and ontogeny in S. hispidus was examined. Juveniles caught off Beaufort Inlet, North Carolina (USA) on 30 June 1996 were marked with alizarin complexone and reared in a flow-through, outdoor tank for up to 19 days. Examination of marked otoliths at several time intervals showed that increment formation was not significantly different than one increment per day, and thus, increment number was used to estimate age. Depth-distribution, morphology, and meristics of larvae and juveniles collected (1990–1992) between Cape Romain, South Carolina, and Cape Hatteras, North Carolina, were examined to identify the timing of the larval to juvenile transition. All indicators suggested the transition occurred between 17 and 20 days. Mean otolith increment widths exhibited a marked change at about 20 days, coinciding with the timing of the larval to juvenile transition and a change in the depth distribution from bottom to surface waters. Increment width of individual juveniles, however, did not exhibit the same pattern; only 40% conformed to the pattern identified for all fish. Thus, the record of the larval to juvenile transition is clear at the population level, but unresolved at the individual level. Received: 1 November 1999 / Accepted: 18 December 2000     

Foraging areas and foraging behavior in the notch-eared bat,Myotis emarginatus (Vespertilionidae)

Summary Field observations in a maternity colony of Myotis emarginatus (Vespertilionidae) were made during the summers of 1986 and 1987 in southern Germany. The nursery colony consisted of about 90 adult and 30 juvenile bats which roosted in a dimly lit and relatively cool church attic. Telemetry data from six adult M. emarginatus disclosed that some individuals also use secondary day roosts in trees or small buildings located close to their foraging areas. During the night, radiotagged individuals spent most of the time on the wing in forested areas (Fig. 2). Stationary bouts lasted no longer than 63 min. Individual bats returned to the same foraging areas on consecutive nights. All major foraging areas were situated in or at the fringes of forests, at distances as far as 10 km from the nursery roost. During commuting flights to the forests, M. emarginatus avoided open fields and preferred flight paths which offered cover such as orchards, hedges, overhanging foliage along creeks, etc. On the way to the forests, the bats started to forage within buildings, in open spaces where aggregations of insects were present, and around or within the foliage of various types of trees at the level of tree tops or the upper third of the foliage. At these transient foraging areas close to the maternity roost, M. emarginatus displayed flexible foraging strategies: (1) They gleaned prey (mainly flies and spiders) from the substrate, (2) seized insects in aerial pursuit, and (3) occasionally hovered in front of foliage and walls.Our observations confirm the conclusion from morphometric data on the wings that M. emarginatus is a predominantly gleaning bat and contradict the suggestion that it makes only brief flights of short distances. On the contrary, our field data suggest that M. emarginatus spends most of the night on the wing and commutes over distances of at least 10 km. Offprint requests to: D. Krull     

Allozyme variation in global populations of striped mullet, Mugil cephalus (Pisces: Mugilidae)

The striped mullet, Mugil cephalus Linnaeus, 1758, is one of the few species of marine shore fish with a worldwide circumtropical distribution. Because of this distribution and the dependency of M. cephalus on coastal waters during various phases of its life cycle, as well as nearshore living habits, questions have been raised regarding levels of genetic divergence and gene flow among transoceanic populations. To cast more light on this, allozyme variation at 27 presumptive gene loci was investigated in ten globally diverse populations. The observed heterozygosity ranged from 0.018 (Hawaii) to 0.081 (Florida), averaging 0.050. The proportion of polymorphic loci showed a similar trend. Several populations were characterised by fixed allelic differences. Estimated gene diversities were very high, the allele frequency variation among populations was found to be 68%; genetic distances reached 0.242, with an average of 0.117. Estimated rates of gene flow were high among Mediterranean populations (Nm = 7.26), and between Mediterranean and East Atlantic populations (Nm= 2.86), but extremely low between non-contiguous populations within the Indian, Pacific and Atlantic Oceans, where Nm ranged from 0.03 to 0.05. Received: 7 April 1997 / Accepted: 4 February 1998