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

Effects of oral laxatives on colonic motor complexes in dogs.

The effect of oral laxatives on the organisation of colonic motor complexes was investigated in four conscious dogs. Six strain gauge transducers were implanted on the colon of each dog. After a control period of two to three hours, dogs were orally dosed with 1, 2, or 4 ml/kg of castor oil, or 0.5 g/kg magnesium citrate. Oral olive oil, 4 ml/kg, was used as control. The recording was continued for another 10 hours or until defecation occurred. Each dog showed spontaneous cyclic bursts of contractions (contractile states) at all recording sites during the control period. Contractile states migrating orad or caudad over at least half the length of the colon were called colonic migrating motor complexes (CMMC). Castor oil and magnesium citrate significantly increased the period of colonic motor complexes, but olive oil had no significant effect. None of the above substances changed the percentage of orad migrating motor complexes, as compared with the control values. Periods in which colonic motor activity was completely absent for at least 60 min over at least three consecutive recording sites occurred more frequently after all of the substances. The occurrence of these periods of inhibition, however, was not a consistent feature and there seemed to be no relationship between the occurrence of inhibitory periods and defecation during the recording period. The dogs defecated within 10 hours after administration of magnesium citrate, 1, 2, and 4 ml/kg of castor oil in 12.5, 25, 37.5, and 88.8% of experiments respectively, but never with olive oil. Defecation was generally accompanied by giant migrating contractions in the colon. We conclude that oral laxatives, magnesium citrate and castor oil have a profound effect on colonic motor complexes and colonic motor activity. The period of CMMC is significantly prolonged after their oral administration because of an increased number of non-migrating motor complexes or periods of inhibition of motor activity