Acoustic methods have long been used to rapidly and synoptically survey marine organisms of ecological and economic importance. Direct sampling furnishes biological data such as abundance, biomass, direct measurements of organism size and species identification, but encounters problems such as net avoidance, small sampling volumes, and catch destruction of delicate specimens. Understanding the scattering mechanisms of fish is challenge due to the fact that fish anatomy is complex and the acoustic scattering characteristics are correspondingly complex. For example, (Nash,1987) qualitatively illustrated the influence of the various anatomical components of fish on scattering by performing length-wise acoustic scans of whole fish, dissected swimbladders. The swimbladder is the main control factor of fish¡¦s floating and diving. This research will carry on the measure the target with acoustic characteristic of imitative air swimbladder of Tsuchida Seisakusho. We use the imitative air swimbladder and collocate the Reversed Engineering to measure the Backscattering intensity of the swimbldder which is different medium. We set up experiment in an acoustic water tank of dimension (4mo3.5mo2m) located in National Sun Yat-sen University. The projector and receiver both are transducer of fish finder with directivity and frequency response at 200k Hz. The signal receptor used Data Acquisition System of NI 6129 with sample rate at 800kHz. The target set up of this research is referring to (Timothy K. Stanton,2004), and we use servo motor to control the incidence angle of the target. Preliminary research is pointed out, the target strength will change by the incidence geometry of the target, and the target diameter is the factor of the target strength.

This thesis is concerned with the study on the effect of the swimbladder on hearing sensitivity in Ameiurus nebulosus. The investigation has been performed by means of auditory threshold curves in normal catfish and in catfish in which the resonance of the swimbladder was eliminated. The results were compared. The thesis consists of an introduction to the investigation, followed by a review of the literature in relation to the problem, a description of the material and technique and of the experimental work, the results obtained with a discussion and a summary. / Thesis / Master of Science (MSc)

Batrachoididae, one of the most well-studied soniferous fishes, are typical examples of fish using intrinsic sonic muscles to excite vibration of swim bladder to emit sound. Most fishes possess a single swim bladder including the Batrachoididae. However, the grunting toadfish (Allenbatrachus grunniens) was found to have two separated swim bladders located in the dorsal part of the abdominal cavity; the size of the swim bladders were quite similar. Sonic muscles were firmly attached to the lateral side of each swim bladder. The aims of this study were to (1) investigate the specialized swimbladder morphology and the acoustic signals of the grunting toadfish (A. grunniens), (2) to falsify the hypothesis that this unique form of swim bladder is a synapomorphic character, and species that possess this character are sister groups. The vocal repertoire in grunting toadfish can generally divided into two types ¡V grunt and boatwhistle. Grunts were harmonic signals with shorter call duration, and could be emitted alone as single grunt (i.e., hand-held grunt) or in series (known as grunt train). Boatwhistles were also hamonic but much longer in call duration and usually appeared in succession. A small portion of signals were found to have acoustic beats, which was previously described in the three-spined toadfish (Batrachomoeus trispinosus), which also possess a pair of swim bladders. Therefore, signals with acoustic beats may be a key character for generating sound by two separated swim bladders. Comparing the morphological measurements of swim bladders indicated that there were no significant differences between swim bladders on different sides. However, comparisons between genders showed that the width, thickness, and weight of sonic muscle in females were significantly higher than males. Less wide sonic muscles with shorter sonic muscle fibers may enables the muscle to contract at a higher velocity in male fishes. However, females were found to have thicker sonic muscle, which indicated that the vocal ability in females may be higher than it was expected. The sonic muscle fibers of females have a larger myofibrillar region compared with males, which have a relatively larger central core. These characters may increase the fatigue-resistance of sonic muscle in males, which can contract at a relatively higher rate for a longer duration. Species with two separated swim bladders were found to be sister group in the molecular phylogenetic tree, implying that this specialized morphological character is synapomorphy.

This thesis is concerned with the study on the effect of the swimbbadder on hearing sensitivity in Ameiurus nebulosus nebulosus. The investigation has been performed by means of auditory threshold curves in normal catfish and in catfish in which the resonance of the swim­bladder was eliminated, The results were compared. The thesis consists of an introduction to the investigation, followed by a review of the literature in relation to the problem, a description of the material and technique and of the experimental work, the results obtained with a discussion and a summary. / Thesis / Master of Science (MS)

The carp Cyprinus carpio has a two-chambered swimbladder and excellent hearing. I explored the hypothesis that the anterior chamber, which connects to Weberian ossicles, is adapted for hearing by testing both chambers for material properties. I also determined displacement and auditory responses to mechanical strikes. Wall stress is higher in the posterior, strain in the anterior and modulus lower in the anterior chamber. Strikes increase pressure followed by a variable rebound that rapidly decays. Displacement and sound amplitude increase with hammer force, and amplitude is similar in both chambers for within chamber strikes but lower across chambers. Normalized for equivalent displacement, the anterior chamber produces a more intense sound. Stiffness and damping are greater for the anterior chamber, but sound spectra are similar. More intense sound production per unit of movement, greater damping and higher stiffness for the anterior chamber should all contribute to high-frequency auditory sensitivity.

Acoustic survey techniques are widely used to quantify abundance and distribution of a variety of pelagic fish such as herring (Clupea harengus). The information provided is becoming increasingly important for stock assessment and ecosystem studies, however, the data collected are used as relative indices rather than absolute measures, due to the uncertainty of target strength (TS) estimates. A fish’s TS is a measure of its capacity to reflect sound and, therefore, the TS value will directly influence the estimate of abundance from an acoustic survey. The TS is a stochastic variable, dependent on a range of factors such as fish size, orientation, shape, physiology, and acoustic frequency. However, estimates of mean TS, used to convert echo energy data from acoustic surveys into numbers of fish, are conveniently derived from a single metric - the fish length (L). The TS used for herring is based on TS-L relationships derived from a variety of experiments on dead and caged fish, conducted 25-30 years ago. Recently, theoretical models for fish backscatter have been proposed to provide an alternative basis for exploring fish TS. Another problem encountered during acoustic surveys is the identification of insonified organisms. Trawl samples are commonly collected for identification purposes, however, there are several selectivity issues associated with this method that may translate directly into biased acoustic abundance estimates. The use of different acoustic frequencies has been recognised as a useful tool to distinguish between different species, based on their sound reflection properties at low and high frequencies. In this study I developed theoretical models to describe the backscatter of herring at multiple frequencies. Data collected at four frequencies (18, 38, 120 and 200 kHz) during standard acoustic surveys for herring in the North Sea were examined and compared to model results. Multifrequency backscattering characteristics of herring were described and compared to those of Norway pout, a species also present in the survey area. Species discrimination was attempted based on differences in backscatter at the different frequencies. I examined swimbladder morphology data of Baltic and Atlantic herring and sprat from the Baltic Sea. Based on these data, I modelled the acoustic backscatter of both herring stocks and attempted to explain differences previously observed in empirical data. I investigated the change in swimbladder shape of herring, when exposed to increased water pressures at deeper depths, by producing true shapes of swimbladders from MRI scans of herring under pressure. The swimbladder morphology representations in 3-D were used to model the acoustic backscatter at a range of frequencies and water pressures. I developed a probabilistic TS model of herring in a Bayesian framework to account for uncertainty associated with TS. Most likely distributions of model parameters were determined by fitting the model to in situ data. The resulting probabilistic TS was used to produce distributions of absolute abundance and biomass estimates, which were compared to official results from ICES North Sea herring stock assessment. Modelled backscatter levels of herring from the Baltic Sea were on average 2.3 dB higher than those from herring living in northeast Atlantic waters. This was attributed to differences in swimbladder sizes between the two herring stocks due to the lower salinity Baltic Sea compared to Atlantic waters. Swimbladders of Baltic herring need to be bigger to achieve a certain degree of buoyancy. Morphological swimbladder dimensions of Baltic herring and sprat were found to be different. Herring had a significantly larger swimbladder height at a given length compared to sprat, resulting in a modelled TS that was on average 1.2 dB stronger. Water depth, and therefore the increase in ambient pressure, was found to have a considerable effect on the size and shape of the herring swimbladder. Modelled TS values were found to be around 3 dB weaker at a depth of 50 m compared to surface waters. At 200 m, this difference was estimated to be about 5 dB. The Bayesian model predicted mean abundances and biomass were 23 and 55% higher, respectively, than the ICES estimates. The discrepancy was linked to the depth-dependency of the TS model and the particular size-dependent bathymetric distribution of herring in the survey area.