Gene S. Helfman

Threat-sensitive predator avoidance in damselfish-trumpetfish interactions

SummaryPredatory threat can vary during a predator-prey interaction as an attack escalates or among predators at different times. A Threat-sensitivity hypothesis is presented which predicts that prey individuals will trade-off predator avoidance against other activities by altering their avoidance responses in a manner that reflects the magnitude of the predatory threat. This hypothesis was tested in the field by presenting prey (threespot damselfish, Stegastes planifrons) with models of foraging predators (Atlantic trumpetfish, Aulostomus maculatus). During a presentation, damselfish displayed progressively stronger avoidance as predator models were brought nearer; response waned rapidly once predator models passed overhead. Larger predator models and those oriented in a strike pose evoked stronger avoidance reactions than smaller and non-attacking models, intermediate responses were evoked by size and orientation combinations that were intermediate in threat, and habituation was more common to weakly-threatening presentations. Smaller damselfish showed stronger avoidance of models than did larger damselfish. Nonavoidance activities, such as feeding and territorial defense, were curtailed during presentations or were more common during weakly threatening presentations. Approaches to the models, equated with mobbing, were more common among large damselfish, again reflecting degrees of vulnerability among different size prey individuals. These initial results indicate that damselfish threatened by predators respond in a graded manner that reflects the degree of threat posed by the predator, in accordance with the Threat-sensitivity hypothesis.

Native Invasions, Homogenization, and the Mismeasure of Integrity of Fish Assemblages

Abstract Human activities, particularly habitat destruction and species introductions, are resulting in increased homogenization of once unique biogeographic regions. In the southeastern United States, extensive endemism occurs among highland fish species that have specialized ecologies, are adapted to cool, clear, nutrient-poor conditions, and are sediment-intolerant. Highland streams flow into lower elevation systems, which are often inhabited by more widespread, generalist fish species adapted to warmer, more turbid, fine-sediment-rich, and nutrient-rich conditions. Common land use practices, such as deforestation, degrade stream habitats and reduce habitat diversity, which is often correlated with taxonomic and ecological diversity. Habitat homogenization can thus cause assemblage homogenization via loss of native species and addition of nonindigenous species. However, midpoints in the homogenization process may be characterized by constant or even increased species diversity because generalist, sedim...

Fish Schools: An Asset to Corals

Schools of juvenile haemulid fish feed in sea grass beds at night. By day they rest over coral heads, where they excrete substantial quantities of ammonium and particulate nitrogen and phosphorus into the nutrient-poor waters. The percentages of these nutrients contributed by the fish were comparable to those from other sources. Coral heads with resident fish schools grew faster than those without resident schools, indicating that fish may be more beneficial to the corals than has been assumed.

Social transmission of behavioural traditions in a coral reef fish

Abstract Traditional behaviours involve the non-genetic transmission of social information across age classes or generations. French grunts (Haemulon flavolineatum) exhibit social traditions of daytime schooling sites and twilight migration routes. Individuals transplanted to new schooling sites and allowed to follow residents at the new sites used the new migration routes and returned to the new sites in the absence of resident fish. Control fish with no opportunity to learn showed no such directionality or return. This is the first demonstration of apparent pre-cultural behaviour in free-living fish. Our observations suggest additional classes of behaviour and taxonomic groups in which pre-cultural activities are likely to have evolved.

Safety in numbers : shoal size choice by minnows under predatory threat

SummaryLarger animal groups often provide greater protection from predators. An individual might therefore be expected to join the larger of two groups. To test this, we hypothesized that fathead minnows would choose to associate with the larger of two shoals and that the presence of a predatory largemouth bass would influence their shoal size choice. Individual minnows were presented with a series of choices between two shoal sizes, ranging from 1 to 28 fish, both with and without a predator present. Although responses were highly variable, minnows displayed an ability to choose between shoal sizes even when size differences were small, preferring the larger shoal whenever a size preference was shown. In the presence of a predator, minnows made quicker shoaling decisions and showed a strong tendency to avoid very small shoals.

The ontogeny of twilight migration patterns in grunts (Pisces: Haemulidae)

Abstract The development of dusk and dawn migratory behaviours was investigated in French and white grunts ( Haemulon flavolineatum and H. plumieri ) at St. Croix, U.S. Virgin Islands. Four juvenile stages were recognized: small, medium, transitional, and large. Each stage differed in age, size, coloration, habitat preference, diel foraging patterns, and twilight migratory behaviour. Comparisons between medium (15 to 30 mm long, 30 to 50 days old) and large juveniles (40 to 120 mm, 80 to 700 days) were emphasized. Medium juveniles occurred in small, diurnally feeding groups near sea urchins in the sand halo around a reef. Group composition varied during the day. They migrated at 15 min after sunset, moving hesitantly from halo to grassbed. Migration routes remained constant over a month, but differed over two years. Large juveniles fed nocturnally, formed daytime resting schools over coral heads, and migrated at 25 min after sunset. Compared with medium fish, resting site constancy was greater during a day, migration activities were significantly less variable, and migration routes remained relatively constant for two years. Ontogenetic differences in constancy of daytime school locales and migration routes may result from learning, facilitated perhaps by greater overlap of age cohorts in large juveniles. Other differences in behavioural variability may result from ontogenetic development of the visual apparatus, plus stabilizing selection due to greater predation on smaller, behaviourally variable fish.

School fidelity in fishes: the yellow perch pattern

School fidelity concerns the cohesiveness of social aggregations over time, as measured by the regularity or repeatability with which known individuals re-occur as members of a school. School fidelity was measured by divers who observed yellow perch (Perca flavescens) marked with individually recognizable tags in Cazenovia Lake, New York. Yellow perch were facultative schoolers, with individual differences in schooling tendency (measured as percentage of time in schools) accounting for different numbers of observations of schooled versus solitary fish. The greatest co-occurrence was found between fish with strong schooling tendencies, but within this group co-occurrence was random. Home ranges overlapped greatly between individuals, yet co-occurrence was more a function of schooling tendency than of overlapping home ranges. Individuals with overlapping home ranges but with weak schooling tendencies seldom co-occurred. Home range size increased directly with schooling tendency: the distances that individuals moved apparently depended on the movements of the schools they joined, with strongly schooling fish remaining in schools longer. Individuals joined and left schools frequently; this also reflects a lack of fidelity between fish. Fish schools that form primarily for predator avoidance may show higher levels of fidelity than do schools that form for foraging or hydrodynamic efficiency.

Size, age and sex of american eels in a Georgia river

Abstract Seasonal collections of American eels Anguilla rostrata were made with baited traps at an estuarine and a freshwater site in the Altamaha River, Georgia. Estuarine animals were on average shorter, lighter, and younger than freshwater animals, although growth was faster in the estuary. Both sites were characterized by greater weight gains in spring through fall and lesser weight gains during late winter. Males constituted 36% of sexually differentiated American eels in the estuarine population but only 6% in the freshwater population. Estuarine males grew more slowly than estuarine females. Males and females differentiated sexually at the same lengths and ages, but matured at younger ages in the estuary. Sexually undifferentiated and differentiated fish overlapped considerably in length, weight, and age at both sites. Glass eels were captured in late winter; they were 49–56 mm long and 250–300 days old. Maturing silver eels were caught during late winter-early spring and were 353–587 mm long and 3...

Mode Selection and Mode Switching in Foraging Animals

Publisher Summary The objectives of this chapter are to (1) consider evidence suggesting that animals from a variety of taxa choose among alternative modes while foraging, dependent on proximate characteristics of prey or other ecological conditions— that is, (“mode switching”), (2) consider generalities in the behavior of apparent mode switchers, (3) discuss the application of a cost-benefit approach to understanding the ultimate causes of mode switching, in terms of both optimal foraging theory (OFT) and other decision-based approaches, and (4) suggest shortcomings in existing theory and ways in which future research might explore the validity and extent of the phenomenon.

Ultrasonic Telemetry of American Eel Movements in a Tidal Creek

Abstract Movements and distribution of eight immature American eels Anguilla rostrata were monitored periodically with location-only ultrasonic transmitters for 5 days in Fridaycap Creek, a Georgia estuarine stream. Fish were concentrated within 25 m of the mouths of feeder creeks. Movements were usually with tidal currents and were restricted to 400 m of the 1,000-m study section. Moves greater than 50 m usually involved round trips from and to a point of origin. Limited movements and regions of concentrated activity may be evidence of home ranges within the creek. Daytime movements were restricted to the main creek channel, shorelines being preferred during high tides and the midchannel during low tides. At night, fish were generally near mouths of feeder creeks during low tides; they often moved up feeder creeks and even onto the marsh with a flood tide, returning to the feeder mouth as the tide ebbed. Tendency to move with tidal currents, activity region sizes, and types of habitats frequented were al...