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Newport Laboratory: Fisheries Behavioral Ecology Program

Understanding Flatfish Behavior Facilitates Gear Design and New Approaches To Reducing Bycatch

hypothetical counter-herding gear, see caption
Figure 1.  Diagram of hypothetical counter-herding gear that would take advantage of differences in behavior to reduce roundfish bycatch.

Understanding Flatfish Behavior Facilitates Gear Design and New Approaches To Reducing Bycatch Flatfish support a variety of targeted fisheries and constitute valuable incidental catch in numerous others, with an estimated 924,615 metric tons (t) harvested in 2003. However, as is becoming the norm, fishers are faced with attempting to exploit particular fish stocks while minimizing the incidental take or bycatch of those that are of little economic value, overexploited, or otherwise subject to catch/landing restrictions.

Gear-based advances in bycatch reduction will likely come through a more thorough understanding of behavioral differences among species. While the behavioral interaction between fish and trawl gear has received focused attention for many commercially important roundfish species, there have been far fewer focused studies examining flatfishes, with most information on flatfish reported more anecdotally alongside more comprehensive data for roundfish.

Unlike baited gears which attract fish, trawls capture fish by taking advantage of avoidance behavior. This is, in essence, antipredator behavior; when fish move away from a trawl door, they are responding as they would to an approaching predator. Flatfish have evolved a highly specialized anti-predator strategy based upon detection minimization. When lying flat on the sediment, their low body profile and ability to cryptically match the color and texture of sediment, as well as bury in it, renders them nearly invisible to approaching predators and/or prey. Upon detecting an approaching predator, flatfish cease movement and are reticent to flee unless the predator gets very close. In contrast, many of the roundfish that are targeted by trawl fisheries (e.g., gadoids and clupeids) are less intimately associated with the seafloor, having evolved strategies where predator avoidance and escape are maximized. Compared to flatfish, they respond to approaching predators at greater distances and have stronger burst and sustained swimming capabilities.

These divergent strategies for dealing with predation threat have consequences for fish behavior relative to trawls throughout the capture process. From a detailed review of relevant literature, the following generalizations about flatfish behavior relative to trawls emerge. Flatfish generally 1) do not respond to the approach of trawl ground gear until it is less than 1 m away, 2) herd close to the bottom where they are primarily influenced by the action of ground gear, 3) herd for only a short while, and 4) remain low to the bottom during net entry and during their passage to the codend.

The tendency of roundfish to exhibit varying degrees of 'rise' as they tire and fall back into the net, has led to the development of multilevel trawls, with horizontal separators and multiple codends, thereby allowing partial segregation of catch by species, as well as trawls with low rise/swept back head ropes and/or large mesh or open top intermediates to facilitate roundfish escape. Similarly, the tendency of most flatfish to remain close to the seafloor during herding has led to experimentation with incorporating gaps in the footrope's center, or along the floor of the net, thereby providing an escape route for flatfish.

An as yet uninvestigated means of reducing roundfish bycatch in directed flatfish fisheries could capitalize upon the longer reactive distances of most roundfish, compared to flatfish, as well as tendency of many roundfish to herd farther off the seafloor. For lack of a better name, this can be referred to as reverse or 'counter-herding'. Conventional flatfish trawls make use of sweeps; two angled stimulus lines which form a V, with the apex of the V pointed toward the net (Fig. 1). This V herds flatfish along the bottom, inward toward the opening of the net.

Counter-herding might be initiated in roundfish by installing a second inverted stimuli Λ approximately a meter or so off the seafloor, positioned between the sweeps, with its apex pointing forward and its open end facing the net mouth. This could be a series of neutral density floats or bobbins of sufficient size that they constitute a prominent visual stimulus. Roundfish species encountering this 'counter- herder' would likely be herded out of the path of the net. Those that do not herd would likely rise into the water column, making it more likely they would pass over the trawl headrope. In contrast, flatfish would likely not respond to the counter-sweep. If some flatfish were herded toward the outside, this would likely be reversed when they came in contact with the tradition sweeps, which, being on the seafloor, would constitute a more immediate perceived threat.

Realization of such a counter-herding device would entail significant engineering challenges. For example, changes in the tension on cables and/or lines of the device would need to be accommodated, since the spread of trawl doors changes with bottom topography and sediment characteristics. Further, the tendency of spheres, or other shapes, to oscillate when towed through the water would need to be considered. Successfully engineered, such a device could potentially be utilized alone, or in combination with the aforementioned selective flatfish trawls and/or other bycatch reduction devices to preclude roundfish from entering the net.

By Clifford Ryer

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