Transmission loss patterns from acoustic harassment and deterrent devices do not always follow geometrical spreading predictions

Author Posting. © The Author(s), 2009. This is the author's version of the work. It is posted here by permission of John Wiley & Sons for personal use, not for redistribution. The definitive version was published in Marine Mammal Science 25 (2009): 53-67, doi:10.1111/j.1748-7692.2008.00243.x.Acoustic harassment and deterrent devices have become increasingly popular mitigation tools for negotiating the impacts of marine mammals on fisheries. The rationale for their variable effectiveness remains unexplained but high variability in the surrounding acoustic field may be relevant. In the present study, the sound fields of one acoustic harassment device and three acoustic deterrent devices were measured at three study sites along the Scandinavian coast. Superimposed onto an overall trend of decreasing sound exposure levels with increasing range were large local variations in sound level for all sources in each of the environments. This variability was likely caused by source directionality, inter-ping source level variation and multi-path interference. Rapid and unpredictable variations in the sound level as a function of range deviated from expectations derived from spherical and cylindrical spreading models and conflicted with the classic concept of concentric zones of increasing disturbance with decreasing range. Under such conditions, animals may encounter difficulties when trying to determine the direction to and location of a sound source, which may complicate or jeopardize avoidance responses.The project was funded by the Swedish Fishermen Association, the Swedish Board of Fisheries, Aage V. Jensen Foundations, Danish Forest and Nature Agency, The Nordic Research Council and the Carlsberg Foundation. Additional logistical support was furnished by the Oticon Foundation and Reson A/S. A.D. Shapiro received financial support from the National Defense Science and Engineering Graduate Fellowship and the WHOI Academic Programs Office. 35

Inferring fish escape behaviour in trawls based on catch comparison data: Model development and evaluation based on data from Skagerrak, Denmark

During the fishing process, fish react to a trawl with a series of behaviours that often are species and size specific. Thus, a thorough understanding of fish behaviour in relation to fishing gear and a scientific understanding of the ability of different gear designs to utilize or stimulate various behavioural patterns during the catching process are essential for developing more efficient, selective, and environmentally friendly trawls. Although many behavioural studies using optical and acoustic observation systems have been conducted, harsh observation conditions on the fishing grounds often hamper the ability to directly observe fish behaviour in relation to fishing gear. As an alternative to optical and acoustic methods, we developed and applied a new mathematical model to catch data to extract detailed and quantitative information about species- and size-dependent escape behaviour in towed fishing gear such as trawls. We used catch comparison data collected with a twin trawl setup; the only difference between the two trawls was that a 12 m long upper section was replaced with 800 mm diamond meshes in one of them. We investigated the length-based escape behaviour of cod (Gadus morhua), haddock (Melanogrammus aeglefinus), saithe (Pollachius virens), witch flounder (Glyptocephalus cynoglossus), and lemon sole (Microstomus kitt) and quantified the extent to which behavioural responses set limits for the large mesh panel's selective efficiency. Around 85% of saithe, 80% of haddock, 44% of witch flounder, 55% of lemon sole, and 55% of cod (below 68 cm) contacted the large mesh panel and escaped. We also demonstrated the need to account for potential selectivity in the trawl body, as it can bias the assessment of length-based escape behaviour. Our indirect assessment of fish behaviour was in agreement with the direct observations made for the same species in a similar section of the trawl body reported in the literature

Does soak time influence the effect of artificial light on catch efficiency in snow crab (Chionoecetes opilio) pot fishery?

In the Barents Sea commercial snow crab (Chionoecetes opilio) fishery, an increase in catch efficiency of the conical pots is important for the profitability of the industry. Light emitting diodes (LEDs) have previously been tested for increasing catch efficiency of the snow crab pots. These earlier experiments have shown varying results ranging from large increase in snow crab catches to no significant effect. These experiments have used different pot soaking times; however, the soaking time might affect the impact of LEDs on catch efficiency. In commercial snow crab fishery, the pot soak time is varying which has not been considered in earlier experiments testing the effect of LEDs. Therefore, this study examined whether pot soaking time can explain the observed differences in relative catch efficiency of snow crab pots with and without LEDs with soak times ranging from 2 to 14 days in the Barents Sea snow crab fishery. For target sizes of snow crab (≥95 mm carapace width), results indicated an increase in catch efficiency between 10 and 30% for pots with LEDs with exception of one experiment using six days soak time. However, experimental results were subjected to large uncertainties and, except from one experiment with five days soak time, the estimated increases were nonsignificant. Furthermore, the pot soak time was not found to impact the effect of white LEDs on capture efficiency

One mesh does not fit all: A dual compartment codend provides flexible selectivity opportunities to manage mixed fisheries

Fisheries management is moving from a single-species to an ecosystem-based approach to better balance the technical, biological, and economic aspects of the many mixed-species fisheries in the world. Most mixed fisheries are conducted with trawl gears, which have a hereditary challenge of providing a species and size selectivity that meets the development in management objectives of reducing unwanted catch. When fishing on sympatric species, a codend that allows separation of species into different compartments during towing can provide separated catch fractions with contrasting selectivity opportunities. In a case study, we quantified the processes of vertical separation and length-based selectivity for a dual compartment codend having an upper, large mesh compartment intended for cod (Gadus morhua) and a lower, small mesh compartment intended for Nephrops (Nephrops norvegicus). This allowed identification of which processes to improve to reduce unwanted catch. The gear concept delivered a complex selectivity profile while enabling high flexibility to adjust selectivity, also at sea, and provides a tool for the industry to ensure better compliance with increasing management ambitions. An unusual selectivity curve for cod resulted from the contrast in mesh size between the two compartments. Cod and Nephrops smaller than and just above the minimum conservation reference size (MCRS) were efficiently released through the 120 mm square meshes in the upper and the 60 mm square meshes in the lower compartment, respectively. Cod (47%) had a low probability of escaping when caught in the lower compartment, and Nephrops (9%) were lost when caught in the upper compartment. A compartment in the dual compartment codend may easily be changed at sea to adjust the selectivity according to the mix of species encountered, available quota portfolio, and management regime

Does the efficiency of a counter-herding device depend on seabed contact?

To prevent unwanted fish bycatch from entering the trawl, we modified a counter-herding device, FLEXSELECT, developed for a Northeast Atlantic Nephrops (Nephrops norvegicus) trawl fishery. FLEXSELECT, which was designed to direct fish away from the trawl by means of sweeping ropes in the herding area, has been shown to be highly effective on both roundfish and flatfish species. However, by adding sweeping elements we could be increasing the benthic impact of trawling. Therefore, in this study we investigated if the effect of FLEXSELECT can be maintained when raising the ropes off the seabed. A reduction in fish bycatch could still be achieved, but the effect of FLEXSELECTraised with respect to the previous design was significantly different for four of the seven species analysed. While for some species the effect was lower, it substantially increased for cod (Gadus morhua), making it highly relevant for the Nephrops fisheries, as well as for other fisheries in needs of avoiding cod catches. The results offer insights regarding species behavioural responses to FLEXSELECTraised, to be used for future developments of counter-herding devices in commercial fisheries without the addition of seabed impact to the baseline gears

Mean scores of the quality parameters for four fish species during three steps in the value chain.

<p>n is the number of individuals assessed.</p><p>*p < 0.05,</p><p>**p < 0.01</p><p>Statistical comparisons were made using a proportional odds model, and significance level was calculated using the Wald test.</p

Odds with confidence intervals for obtaining lower quality scores compared with the standard codend for the quality parameters of landed fish caught in the (a) upper and (b) lower codends of the test codend.

<p>The test and standard codends were equal for odds = 1 (dotted line). The results were statistically significant at *p < 0.05 and **p < 0.01 when the confidence intervals did not include 1.</p