Sharks caught in the KwaZulu-Natal bather protection programme,South Africa. 14. The smooth hammerhead shark Sphyrna zygaena (Linnaeus)

The current study provides long-term catch-rate, biological and feeding data for smooth hammerhead sharks, Sphyrna zygaena, caught in South Africa’s KwaZulu-Natal bather protection programme. In total, 2 512 S. zygaena were caught in net installations between 1978 and 2014, and 72 S. zygaena were caught on drumlines between 2007 and 2014. There was no significant log-linear year trend in the net catch rate over time (slope = 0.0054, t = 1.808, p = 0.07). However, there was a significant temporal increase in mean size of the captured sharks (slope = 0.0012, t = 3.502, p < 0.001). A quasi-Poisson generalised additive mixed model showed that increasing latitude, winter months, colder sea temperatures and the deployment of drumlines all had a significant positive effect on the catch rate of sharks in nets. The size frequency of the catch was unimodal, with significantly more females caught in the nets and more males on the drumlines. The majority (93.1%) of all sharks caught were immature and measured between 80 and 120 cm precaudal length. Teleosts and cephalopods dominated the sharks’ diet in terms of all dietary indices. The prey species consumed indicate that immature S. zygaena are feeding primarily within the pelagic zone of shallow coastal habitats.     

Digital waveform codings for ocean acoustic telemetry

M-sequence waveform coding with a single long codeword has been considered as the basis for long-range underwater acoustic telemetry for one user. (An m-sequence is a periodic, binary, linear-law maximal-length sequence. If the span of the law is n, the maximal length L-2ⁿ=1). For a given law, a single m-sequence transmits a maximum of log₂ (L) bits of source information per channel word. To increase the number of bits per word, families of m-sequences and Gold codes are considered and compared to a single m-sequence. A hypothetical idealized multipath channel with added white Gaussian noise is assumed. Coding using families of m-sequences is recommended because it requires a smaller bit-energy-to-noise ratio than other waveform codes to achieve an equivalent codeword error probability     

Structure and dynamics of demersal fish assemblages over three decades (1985–2012) of increasing fishing pressure in Guinea

In a context of growing fishing pressures and recommendations for an ecosystem approach to fisheries, there is a need to monitor changes in fish communities over time. In this study, we analysed data from scientific trawl surveys carried out on the continental shelf off Guinea between 1985 and 2012. We performed factorial analyses and calculated biodiversity indices to characterise the changes in the structure and composition of fish communities that occurred over the 28-year period in this area, particularly given intensive fishing activities. We show that, over the study period, fish communities on the Guinean shelf were structured primarily according to spatial factors, with temporal changes being less pronounced than expected. However, a temporal analysis of biodiversity indices and species dominance showed that the intensification of fishing had significant effects on the general ecological features of the fish assemblages under study. There was a decrease in fish density and in mean trophic level. In addition, there were changes in species dominance, whereby large, slow-growing species with high commercial value were gradually replaced by smaller, fast-growing species of lesser commercial interest. These results from field observations are in line with some conclusions from previous modelling studies in the same geographical area, and provide further support for the hypothesis of a progressive ‘ecosystem effect of fishing’ occurring in Guinean waters.     

Channel capacity in bits per joule

Underwater acoustic telemetry has a total input energy constraint, since the energy is stored in the transmitter's batteries. This work is primarily rephrasing the work in channel capacity in terms ofC_{J}bit/J as compared toCbit/s, to emphasize the energy efficiency and to deemphasize the speed of the telemetry. The energy channel capacityC_{J}, for any waveform channel with well-defined capacityCbit/s at signal powerSwatts, is defined asC/Sbit/J. It is shown that for coherent binary frequency shift keying (BFSK) and waveform channels, the supremum ofC_{J}overSis approached asSapproaches zero. For the Gaussian channel the best coding uses narrow bands with the highest S/N.     

Genomic resources for the spotted ragged-tooth shark Carcharias taurus

Genomic data can be a useful tool in the management and conservation of biodiversity. Here, we report the development of genomic resources for the spotted ragged-tooth shark Carcharias taurus using genome-wide DNA data from Illumina next-generation sequencing. We explored two commonly used genetic marker types: microsatellites and mitochondrial DNA. A total of 4 394 putative microsatellites were identified, of which 10 were tested on 24 individuals and found to have ideal properties for population genetic analyses. Additionally, we reconstructed the first complete mitochondrial genome of a South African spotted ragged-tooth shark, and highlight the most informative gene regions to facilitate future primer design. The data reported here may serve as a resource for future studies and can ultimately be applied in the sustainable conservation and fisheries management of this apex predator.     

The generation of digital random time histories

“A method to generate digital random time histories is described. A random number sequence is shaped to give the desired spectral density curve. This finite set of numbers is then Inverse Fast Fourier Transformed (IFFT). The result is a pseudo random time history which has given spectral characteristics. An application of this technique is described.”     

ANALYSIS OF POPULATION AGE BALANCE

Single-value indices of population age balance have proved useful. Coulson's index, although an improvement over other measures, is deficient when applied to middle-aged dominant populations and violates normality assumptions required by the technique for deriving the index. An alternative index of age balance was developed to provide more information and flexibility than other measures without losing their primary advantages.     

Multimegameter-range acoustic data obtained by bottom-mountedhydrophone arrays for measurement of ocean temperature

Acoustic signals transmitted from the ATOC source on Pioneer Seamount off the coast of California have been received at various sites around the Pacific Basin since January 1996. We describe data obtained using bottom-mounted receivers, including US Navy Sound Surveillance System arrays, at ranges up to 5 Mm from the Pioneer Seamount source. Stable identifiable ray arrivals are observed in several cases, but some receiving arrays are not well suited to detecting the direct ray arrivals. At 5-Mm range, travel-time variations at tidal frequencies (about 50 ms peak to peak) agree well with predicted values, providing verification of the acoustic measurements as well as the tidal model. On the longest and northernmost acoustic paths, the time series of resolved ray travel times show an annual cycle peak-to-peak variation of about 1 s and other fluctuations caused by natural oceanic variability. An annual cycle is not evident in travel times from shorter acoustic paths in the eastern Pacific, though only one realization of the annual cycle is available. The low-pass-filtered travel times are estimated to an accuracy of about 10 ms. This travel-time uncertainty corresponds to errors in range- and depth-averaged temperature of only a few millidegrees, while the annual peak-to-peak variation in temperature averaged horizontally over the acoustic path and vertically over the upper 1 km of ocean is up to 0.5°C     

Design of periodic signals using FM sweeps and amplitude modulationfor ocean acoustic travel-time measurements

A design procedure for an amplitude-modulated and nonlinear frequency-modulated (AM-NLFM) signal is introduced. The designed signal can drive a given transducer to its peak power to produce a sound pressure waveform into the water with a desired power spectrum and maximum possible energy. The signal can be formed either in the time domain or in the frequency domain. The frequency domain approach gives an output power spectrum precisely identical to a preferred shape. Therefore, the sidelobe levels after matched filtering are not raised by unwanted spectral magnitude ripples which exist when a time domain method is adopted. The absence of spectral ripples is desirable for applications requiring long range transmission and good multipath discrimination capability. An acceptable tradeoff between time resolution and sidelobe levels is achieved by properly choosing the desired power spectral shape. As the time resolution is usually the most critical specification for precision travel-time measurements, it is shown that by sacrificing some of the transducer's output power capability, a waveform with a considerably wider bandwidth can be transmitted, resulting in a significantly enhanced time resolution. A quasi-steady-state (QSS) approximation is used in the signal design, leading to a manageable and intuitive design procedure     

Does one size fit all? An evaluation of length–weight relationships for New Zealand's freshwater fish species

Length–weight relationships are a fundamental tool for assessing populations and communities in fisheries science. Many researchers have collected length–weight data throughout New Zealand, yet parameters describing these relationships remain unpublished for many species of freshwater fish. We compiled 285,124 fish records from researchers and institutions across New Zealand to parameterise length–weight equations, using both power and quadratic models, for 53 freshwater species belonging to 13 families. The influence of location and sex on length–weight relationships was also assessed. Location, in particular, generated different length–weight relationships for 65% of the species examined. Length–weight equations were validated by comparing predicted weights against independently measured weights from 25 electrofished sites across New Zealand and the equations were highly accurate (R²>0.99). Recommendations are made about how to robustly apply this new resource which should assist freshwater fisheries researchers throughout New Zealand.