Bycatch In Gillnet Fisheries Research Articles

Differences in the response of a striped dolphin (Stenella coeruleoalba) and a harbour porpoise (Phocoena phocoena) to an acoustic alarm

Small cetacean bycatch in gillnet fisheries may be reduced by deterring odontocetes from nets acoustically. However, different odontocete species may respond differently to acoustic signals from alarms. Therefore, in this study a striped dolphin and a harbour porpoise were subjected simultaneously to sounds produced by the XP-10 experimental acoustic alarm. The alarm produced 0.3s tonal signals randomly selected from a set of 16 with fundamental frequencies between 9 and 15kHz, with a constant pulse interval of 4.0s (duty cycle 8%) and a Source Level range of 133-163dB re 1muPa (rms). The effect of the alarm was judged by comparing the animals' respiration rate and position relative to the alarm during test periods with those during baseline periods. As in a previous study on two porpoises with the same alarm, the porpoise in the present study reacted strongly to the alarm by swimming away from it and increasing his respiration rate. The striped dolphin, however, showed no reaction to the active alarm. Based on harbour porpoise audiograms and the specific audiogram of the striped dolphin in the present study, and the low background noise levels during the experiment, both animals must have heard the alarm signals clearly. This study indicates that cetacean species are not equally sensitive to human-made noise disturbance. Therefore, source levels of acoustic alarms should be adapted to the species they are supposed to deter. In addition, alarms should be tested on each odontocete species for which they are intended to reduce bycatch.

Target strengths of two gillnets and predictions of biosonar detection ranges

Small cetaceans are often caught as bycatch in gillnet fisheries. One potential method that has been shown to significantly reduce harbor porpoise incidental take is the use of barium sulfate enhanced gillnets. One hypothesis states barium sulfate nets reduce bycatch because they are acoustically more reflective than monofilament nets, thus easier to detect by echolocating odontocetes. This premise remains untested. To determine acoustic reflectivity, this study compared the target strength of a barium sulfate gillnet with a comparable monofilament gillnet at 0, 10, 20, 30, and 40. At normal incidence (0) the monofilament target strength measured slightly greater than the barium sulfate net and biosonar detection ranges are estimated to be greater for the monofilament. As the angle of incidence increased from normal to 20, the barium sulfate net echo strength remained the same while the nylon net echo strength decreased significantly at 10. Biosonar detection ranges are estimated to be greater for the barium sulfate net at increased angles. Echolocating odontocetes may utilize acoustic reflections of both nets but due to the weak target strength of both nets, other mechanisms may also mitigate reduced odontocete bycatch. Current psychoacoustic research is investigating the net detection ranges of captive bottlenose dolphins.

Abundance of harbour porpoise and other cetaceans in the North Sea and adjacent waters

Summary The status of small cetaceans in the North Sea and adjacent waters has been of concern for many years. Shipboard and aerial line transect surveys were conducted to provide accurate and precise estimates of abundance as a basis for conservation strategy in European waters. The survey, known as SCANS (Small Cetacean Abundance in the North Sea), was conducted in summer 1994 and designed to generate precise and unbiased abundance estimates. Thus the intensity of survey was high, and data collection and analysis methods allowed for the probability of detection of animals on the transect line being less than unity and, for shipboard surveys, also allowed for animal movement in response to the survey platform. Shipboard transects covered 20 000 km in an area of 890 000 km2. Aerial transects covered 7000 km in an area of 150 000 km2. Three species dominated the data. Harbour porpoise Phocoena phocoena were encountered throughout the survey area except in the Channel and the southern North Sea. Whitebeaked dolphin Lagenorhynchus albirostris and minke whale Balaenoptera acutorostrata were found mainly in the north‐western North Sea. Phocoena phocoena abundance for the entire survey area was estimated as 341 366 [coefficient of variation (CV) = 0·14; 95% confidence interval (CI) = 260 000–449 000]. The estimated number of B. acutorostrata was 8445 (CV = 0·24; 95% CI 5000–13 500). The estimate for L. albirostris based on confirmed sightings of this species was 7856 (CV = 0·30; 95% CI = 4000–13 000). When Atlantic whitesided dolphin Lagenorhynchusacutus and Lagenorhynchus spp. sightings were included, this estimate increased to 11 760 (CV = 0·26; 95% CI 5900–18 500). Shortbeaked common dolphin Delphinus delphis were found almost exclusively in the Celtic Sea. Abundance was estimated as 75 450 (CV = 0·67; 95% CI = 23 000–149 000). Current assessments and recommendations by international fora concerning the impact on P. phocoena of bycatch in gillnet fisheries in the North Sea and adjacent waters are based on these estimates.

THE EFFECTS OF ACOUSTIC ALARMS ON THE BEHAVIOR OF HARBOR PORPOISES (PHOCOENA PHOCOENA) IN A FLOATING PEN

AbstractIn an attempt to test the effectiveness of sounds in deterring harbor porpoises from nets and reducing porpoise bycatch in gill net fisheries, two harbor porpoises, kept in a large floating pen at Neeltje Jans, The Netherlands, were subjected to 3 different underwater sounds. The effect of each sound was judged by comparing the animals' behavior during a 15‐min test period with that during a 15‐min baseline period immediately before the test and a 15‐min recovery period immediately after the test. The effects of the alarms were quantified as the distance between the porpoises' surfacings and the alarm and the animals' respiration rates. Each alarm was tested in two positions in the pen. The behavior observed was related to the sound‐pressure‐level distribution in the pen. All three alarms: the standard Dukane alarm (a commercially available alarm with a regular pulse interval of 4.3 sec used to deter dolphins from fishing nets), the random Dukane alarm (the same alarm with random pulse interval of between 2 and 30 sec), and the “bird alarm” (a sound from a generator) resulted in increases in both the distance of the animals' surfacings from the alarms and their respiration rates. The standard Dukane alarm and the bird alarm were more effective than the random Dukane alarm in inducing the animals to swim away from the sound source.