Inferring Fish Escape Behaviour in Trawls Based on Catch Comparison Data: Model Development and Evaluation Based on Data from Skagerrak, Denmark

Ludvig Ahm Krag,Junita Diana Karlsen,Bent Herrmann,Konstantinos I Stergiou

doi:10.1371/journal.pone.0088819

Abstract

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.

Highlights

During the last decade, advanced and sophisticated trawl designs have been developed in an attempt to reduce by-catch in the commercial fishing industry
Extensive research has been focused on understanding fish behaviour in relation to fishing gears to aid the development of more efficient species or size selective fishing gears [1,2,3]
We quantified the length-dependent escape behaviour of five commercial fish species in the mixed demersal fishery in Skagerrak in terms of their contact likelihood in relation to a large mesh panel placed in the aft part of a trawl

Summary

Introduction

During the last decade, advanced and sophisticated trawl designs have been developed in an attempt to reduce by-catch in the commercial fishing industry. In addition to optical or acoustic observations, fish behaviour can be inferred from the catch composition (e.g., by using spatially divided gear designs). We evaluated the effect of inserting a large mesh panel on catch efficiency of five commercial species in the Nephrops (Nephrops norvegicus) directed fishery in Skagerrak off northern Denmark This fishery is conducted in relatively deep waters on muddy grounds where optical observation techniques repeatedly have failed during our prior experiments in this area. We developed a new model to describe and quantify fish behaviour indirectly based on analysis of catch data alone Using this method, we quantified the length-dependent behavioural response for cod, haddock, saithe (Pollachius virens), lemon sole (Microstomus kitt), and witch flounder (Glyptocephalus cynoglossus) in relation to the large mesh panel in the experimental trawl body

Materials and Methods

Results

Discussion