Abstract
The methodological approach to data collection and analysis in Mediterranean bottom trawl surveys has changed considerably since their beginnings in the mid-1980s. The introduction of new technologies in surveys greatly improved knowledge on trawl underwater geometry, but also raised a question on the reliability of the data obtained. The most commonly used trawl monitoring systems in the Mediterranean use the constant speed of sound when calculating horizontal distance from one sensor to another. However, the speed of the sound in seawater is not constant because it depends on water temperature, pressure, and salinity. This is known to affect the performance of the horizontally transmitting sonars, and it is reasonable to assume that unless it is properly compensated for, the readings from trawl monitoring systems can be incorrect, resulting in biased swept area estimates, and hence potentially producing bias in abundance estimates. In the knowledge that speed of sound depends on temperature, salinity and pressure, the Del Grosso (1974) equation was used to calculate the compensation coefficient for a series of theoretical depth and temperature data. A simple model is proposed in the current paper for the manual correction of the horizontal net opening and swept area estimates. Temperature and depth data obtained from the MEDATLAS project are used to show seasonal and spatial variation in the near-bottom speed of sound, and the results demonstrate that, unless compensated for properly, the error in the horizontal net spread ranges from –1.5% to 2.9%.
Highlights
Effective and successful fisheries management depends greatly on reliable monitoring of fish stocks (Godø 1998, Legović et al 2010)
A swept area represents the area covered by the trawl while towing and can be calculated by multiplying trawl wing spread by distance covered, while gear is in contact with the seabed
The most common net trawl measurement systems used in the central Mediterranean (SCANMAR pers. comm.), use the constant speed of sound of 1500 m/s in their algorithms when calculating the distance between two sensors
Summary
Effective and successful fisheries management depends greatly on reliable monitoring of fish stocks (Godø 1998, Legović et al 2010). The most common method used for relative biomass estimation is the swept area method (Saville 1977, Sparre and Venema 1998). Relative biomass or catch per unit of area is calculated by dividing catch by swept area (Sparre and Venema 1998) and it is usually expressed in terms of density (e.g. kg km–2). These estimates depend highly on the correct calculation of the swept area, which can be miscalculated if the effective tow duration or wing spread are inaccurate (Godø and Engås 1989). The most common net trawl measurement systems used in the central Mediterranean (SCANMAR pers. comm.), use the constant speed of sound of 1500 m/s in their algorithms when calculating the distance between two sensors
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