underwater

Calls emitted by the brown meagre Sciaena umbra (L., fam. Sciaenidae) were recorded at the Natural Marine Reserve of Miramare (Trieste, Italy) in seven nocturnal surveys (12-h continuous sampling) during the summer of 2009. Calls consist of pulses, with the main energy content below 2 kHz and mean peak frequency of c. 270 Hz. Pulses were short, with an average duration of 20 ms and a pulse period of 100 ms. Sounds lasted approximately 500 ms. Three types of sound patterns were recognized: irregular (I), regular (R) and the chorus (C). Their acoustic parameters are described showing that I, R and C differ in pulse duration, pulse peak frequency and pulse period. Occurrence of the three call types changes throughout the night: the R pattern occurred mainly at dawn and dusk, C predominated after nightfall, while I calls were produced sporadically during the whole nocturnal period. Our results indicate that S. umbra has a pronounced nocturnal rhythm in vocalizing behaviour and highlight how the diagnostic time–frequency pattern of S. umbra calls can be used to identify the species in the field. Considering that the abundance of S. umbra is currently declining, the information presented here will be relevant in developing non-invasive and low-cost monitoring acoustic systems for managing S. umbra conservation and fishery along the Mediterranean Sea.

The minke whale (Balaenoptera acutorostrata) is a small, elusive baleen whale that is rarely sighted in tropical waters of the North Pacific Ocean. During winter and spring, they produce songs, also known as ‘boings’, that are commonly detected at deep water hydrophones located around the Hawaiian Islands. We acoustically monitored minke whales using a fixed seafloor hydrophone array encompassing a large ( >2000 km2), deep-water area off the island of Kauai. Simultaneous visual-acoustic surveys of the same region were conducted from a quiet motor-sailing vessel. The combination of the towed and fixed hydrophone arrays allowed animals to be localized and tracked in near real-time. Using both methods, we were able to visually confirm the location of a minke whale initially detected and localized using the fixed hydrophone array, and later with the towed hydrophone array. These data are being collected to help validate statistical methods that are being developed to estimate densities of marine mammals using acoustic signals they produce. In a related study, boings recorded in the Hawaiian Islands (central North Pacific) were acoustically characterised and compared to boings recorded in the western and eastern North Pacific. These results are discussed in relation to the behaviour and population biology of this species. We provide recommendations for tracking, monitoring behaviours and estimating the distribution and distribution of these vocally active, but visually elusive whales.

Visual and acoustic surveys have shown that endangered North Atlantic right whales overwinter in the waters of the Northeast US. Detailed knowledge of this population’s distribution is critical to reducing anthropogenic mortality events. Passive acoustic studies have confirmed that Stellwagen Bank National Marine Sanctuary (SBNMS) and Jeffreys Ledge are areas of high baleen whale acoustic activity during the winter. Right whales produce three main types of calls, the up-call, gun-shot and tonal calls. However, no thorough assessment has yet been made of whether this call type is the most appropriate one to use. SBNMS has been part of an ongoing passive acoustic study since 2006 and provides an opportunity for understanding the relationship between up-calls and gun-shots for this region. From December 18th 2007 to May 28th 2008 an array of 10 marine acoustic recording units was deployed in SBNMS. An automated detector was used to determine seasonal occurrence of calls. Gun-shots and upcalls occurred throughout the winter period, with gun-shots peaking in January, and up-calls peaking in December. Comparative analyses of up-calls and gun-shots show that they occur simultaneously around 17:00. Gun-shots primarily tended to occur between 16:00 and 4:00, while up-calls primarily occurred between 11:00 and 18:00. Average gun-shot activity was 2.15 calls/min, while average up-call activity was 3.45 calls/min. Future research should be directed towards assessing occurrence of different call types between seasons, and in the south-east US breeding grounds and northern foraging areas.

Hearing sensitivity and sound characteristics of poikilothermic animals are influenced by ambient temperature. The present study investigates the influence of temperature on the hearing ability and on sound characteristics in the neotropical Raphael catfish Platydoras costatus. Using the auditory evoked potential (AEP) recording technique, the hearing thresholds of eight specimens were tested at six different frequencies from 100 to 4000 Hz. The temporal resolution was determined by analyzing the minimum resolvable click period (0.3 - 5 ms). Hearing sensitivity and sound production were measured after acclimation to 22 °C followed by acclimation to 30 °C for at least three weeks each. Auditory sensitivity increased from 1 to 4 kHz with rising temperature, whereas the temporal resolution showed no change. Catfishes produce stridulation sound by rubbing the pectoral spines in the shoulder girdle during abduction and adduction of pectoral fins. Distress calls were recorded when fish were handheld. The stridulation sounds produced during pectoral fin spine abduction and adduction became shorter at the higher temperature. Maximum and minimum pulse period as well as pulse number did not change with temperature, whereas the dominant frequency of sounds was higher at the higher temperature. Our data indicate that constraints imposed on hearing sensitivity at different temperatures cannot be compensated even by longer acclimation. As sound characteristics also change with temperature, we suggest that the ambient temperature directly affects acoustic communication in the neotropical catfish P. costatus.

Current studies on the ontogenetic development of acoustic communication in the labyrinth fish Trichopsis vittata and the toadfish Halobatrachus didactylus show that the ability to detect conspecific sounds develops during growth. In order to investigate at which stage sound detection starts in an otophysine fish, we investigated the ontogenetic development of auditory sensitivity and vocalizations in the mochokid catfish Synodontis schoutedeni. Mochokids are also called squeakers because they produce broadband stridulation sounds during abduction and adduction of pectoral fin spines. Fish from five different size groups – from 22 mm standard length to 126 mm – were studied. Hearing thresholds were measured between 50 Hz and 6 kHz using the auditory evoked potentials (AEP) recording technique; stridulation sounds were recorded and their sound pressure levels determined. The smallest juveniles showed the poorest hearing abilities of all size groups between 50 Hz and 1000 Hz. At higher frequencies (5 and 6 kHz), hearing sensitivity was lowest in the largest groups. Duration of abduction and adduction sounds, sound pressure level and pulse period increased, while the dominant frequency of sounds decreased with size. Comparisons between audiograms and sound spectra revealed that the most sensitive frequencies correlate with the dominant frequencies of stridulation sounds in all Synodontis size groups and that all specimens are able to detect stridulation sounds. This study demonstrates that the hearing ability in the siluriform S. schoutedeni changes during ontogeny, whereas no changes were observed in two cypriniform species investigated earlier. Furthermore, S. schoutedeni is able to communicate acoustically at all stages of development, which contrasts to prior findings in teleosts.

Otophysine fishes (carps and minnows, catfishes, characins, knifefishes) comprise about 8000 species and are the most successful freshwater fish group. They have evolved mammal-like structures for hearing, namely a chain of tiny ossicles transmitting oscillations from the swimbladder, which acts as a tympanum or ear drum, to the inner ear. These Weberian ossicles enable otophysines to detect sound pressure and extend their hearing range to lower sound intensities and to sound frequencies of up to several kHz. They represent the largest group of hearing specialists. Hearing enhancement by the Weberian ossicles is frequency dependent. Removal of the tripus, the largest ossicle, in goldfish decreased hearing sensitivity as measured by the non-invasive auditory evoked potential (AEP) recording technique from 7 dB at 100 Hz up to 33 dB at 2 kHz. Despite their common trait, otophysines reveal a large diversity in hearing sensitivities both between and within orders. This is linked to swimbladder and ossicle morphology, which are in particular diverse in catfishes. Several families (doradids and ariids) possess large, free, unpaired swimbladders, while others (loricariids) have paired tiny swimbladders totally encapsulated in bone. Investigations of 11 species from 6 families showed that large bladders and ossicles as well as higher ossicle numbers improve hearing ability at higher frequencies. Which ecological or social constraints prompted the evolution of the Weberian apparatus in otophysine species? The similarity of auditory sensitivity between vocalizing and non-vocalizing species indicates that the Weberian apparatus evolved prior or mostly independently from the development of sound-generating mechanisms. Predator and/or prey detection in quiet freshwater habitats were probably the major selective pressures behind this hearing specialization.

Acoustic communication plays an important role in the social behaviours of vocal teleost fishes in the Family Batrachoididae (midshipman and toad fishes). The midshipman and toadfishes have become good models for investigating the neural and endocrine mechanisms of auditory perception and vocal production shared by all vertebrates, in part, because the reproductive success of these batrachoidid fishes is highly dependent on acoustic communication. The plainfin midshipman fish (Porichthys notatus), like other teleost fish, use the saccule as the main acoustic end organ for hearing to detect and locate vocalizing mates that produce multiharmonic advertisement calls during the breeding season. Recent work showed that the frequency sensitivity of the peripheral auditory system changed seasonally with female reproductive state such that reproductive females became better suited than non-reproductive females to encode the higher harmonics of the male’s advertisement call. Approximately one month before the breeding season, females showed peaks in circulating plasma levels of testosterone and 17beta-estradiol, which are now known to induce the female’s summer reproductive auditory phenotype and enhance auditory sensitivity to the dominant higher harmonic components of the male’s mate call. Furthermore, midshipman-specific oestrogen receptor alpha and androgen receptor have been identified in the saccule which now provides additional support for a direct steroid effect on the inner ear. Additional physiological evidence will be presented that suggests that the saccular hair cell receptors are the prime candidate sites for this novel form of steroid-dependent auditory plasticity. I will discuss why this auditory plasticity may represent an adaptation to enhance mate detection and localization during the breeding season.

The calls of Pipa carvalhoi were recorded with a hydrophone and analysed with a sound level meter, and the data compared with those measured in air. Three types of calls can be differentiated in males (females are not able to produce calls):

• the advertisement call which is composed of clicks followed by a series of rapid identical pulses; this series, called buzz by Weygoldt (1976), is named trill here;
•  the encounter call consisting of short trills, and
• a call which is very similar to the encounter call, but preceded occasionally by clicks. It is produced by males in response to amplexus.

The sound pressure level of trills reached 120 dB, that of clicks 90 dB. Corresponding data for the sound measured in air are 62 and 41 dB. Experiments with wall covers suppressing resections suggest that the high dB values measured in normal aquaria are unbiased. Calling activity is high at midnight and in the morning; it increases in the presence of receptive females. In a group, the dominant male calls; however, when a female becomes receptive, other males produce encounter calls.